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Chapter 8 The space radiation environment
INSTABILITIES IN SILICON DEVICES Silicon Passivation and Related Instabilities G. Barbottin and A. Vapaille (Editors) © 1999 Elsevier Science B.V. All rights reserved.
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CHAPTER 8
THE SPACE RADIATION ENVIRONMENT
byD.BRAUNIGd)
Key items Flux Fluence Dose Linear energy transfer Solar radiation Cosmic rays Van Allen belts Cut-ofF mechanisms Environment models Mission modeling
(1) Hahn-Meitner-Institut Berlin GmbH Department AT Glienicker Str. 100 14109 Berlin, Germany
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D, Brdunig
Abstract of Chapter 8: The space radiation environment In many civilian and military applications, electronic components must operate in a radiation environment, and thus may experience instability phenomena. In Section 1 of this chapter, we introduce the parameters which enable us: to fully characterize the particles encountered in a radiation environment and to describe their effects on matter. We then describe the two basic constituents of the environment encountered in space: solar particles and cosmic rays. Particles can be emitted by the sun, either continuously (solar wind) or sporadically (solar flares). The rest of the galaxy behaves also as a continuous source of radiation (the cosmic rays). In Section 3 we show that there exists around the Earth, zones where particles are preferentially trapped (the so-called Van Allen belts). The Earth's magnetic field is responsible for this trapping. This field also shields our planet by filtering out most of the solar and galactic particles. In the last section we show that, given some hypotheses, the radiation environment of the Earth can be modeled. The models enable us to predict the fluxes and the energy spectrum of the particles that a spacecraft will encounter during a mission in space and thus help us compute the probability that a "Single Event Phenomenon" occur or that a component fail. We finally give a few examples illustrating the use of these models. Resume du chapitre 8 : Le rayonnement spatial Dans de nombreuses applications civiles et militaires, des composants electroniques sont amenes a fonctionner dans un environnement radiatif et peuvent devenir instables ou etre endommages. Dans la premiere section de ce chapitre, nous rappelons les parametres qui permettent de caracteriser : les particules rencontrees dans un environnement donne, et leur action sur la matiere. Nous decrivons ensuite les deux sources du rayonnement spatial : le soleil et la galaxie. Le soleil est a I 'origine de particules emises en continu (vent solaire) ou de maniere sporadique (lors des eruptions solaires). Le reste de la galaxie contribue a un fond continu de rayonnement (le rayonnement cosmique galactique). Dans la section 3, nous montrons qu 'il existe autour de la terre des zones de piegeage privilegiees pour les particules, encore appelees ceintures de Van Allen. Le champ magnetique terrestre est a Vorigine de ce piegeage. Ce champ joue par ailleurs le role d'un ecran protegeant la terre de la plupart des particules galactiques et solaires. Nous montrons dans la derniere section que, moyennant certaines hypotheses, I'environnement radiatif terrestre peut etre modelise. Les mode les permettent de predire les flux et les energies des particules qu 'un vehicule spatial va rencontrer et aident ainsi a chiffrer la probabilite qu 'un "evenement singulier" ou qu 'une panne se produise. Nous donnons enfin quelques exemples qui illustrent I 'application de ces modeles. Zusammenfassung zu Kapitel 8: Die Weltraumstrahlung In zahlreichen zivilen und militdrischen Anwendungen miissen elektronische Bauelemente in strahlungsbelasteter Umgebung arbeiten und dabei Instabilitdtsprobleme erfahren. In Abschnitt 1 dieses Kapitels werden die Parameter eingefuhrt, die es uns ermoglichen, sowohl die Eigenschaften der daran beteiligten Strahlungsspezies als auch ihre Wechselwirkung mit dem Material vollstdndig zu beschreiben. Anschliessend werden die zwei grundsdtzlichen Arten der im Weltraum anzutreffenden Strahlung beschrieben: die solare und die kosmische Strahlung. Teilchen konnen von der Sonne abgestrahlt werden, sowohl kontinuierlich (solarer Wind) oder sporadisch (SonnenausbrUche) oder der Weltraum selbst ist eine Quelle kontinuierliche Strahlung (kosmische Strahlung) Im Abschnitt 3 wird beschrieben, dafi im Umfeld der Erde Gebiete existieren, in denen geladene Teilchen vorzugsweise eingefangen werden (die sogenannten Van Allen-GUrtel). HierfUr ist das Erdmagnetfeld verantwortlich. Dieses Magnetfeld wirkt fUr einen grojien Teil der kosmischen und solaren Strahlung als Abschirmung der Erde. Im Abschnitt 4 wird gezeigt, dafi die Strahlungsumgebung der Erde, unter Beriicksichtigung einiger Hypothesen, modelliert werden kann. Diese Modelle gestatten die Vorhersage von FlUssen und energetischer Verteilung der Strahlung fur eine spezifische Mission und damit die Belastung der elektronischen Komponenten an Bord eines Raumfahrzeugs. Dies wird an einigen Beispielen demonstriert.
The Space Radiation Environment Chapter 8: The space radiation environment Table of contents List of symbols and abbreviations used 1. Introduction 2. Definitions and units 2.1. Definitions related to particle properties 2.1.1. Flux 2.1.2. Fluence 2.1.3. Energy and momentum 2.1.4. Species 2.2. Definitions related to particle-matter interactions 2.2.1. Exposure 2.2.2. Radiation-absorbed dose 2.2.3. Dose equivalent 2.2.4. Linear Energy Transfer 2.3. Additional definition 3. The space environment 3.1. Introduction 3.2. Solar radiation 3.3. Galactic cosmic rays 3.4. Trapped radiation in the Earth's magnetosphere 3.4. L A brief description of the Earth's magnetic field 3.4.2. The trapping and filtering properties of the magnetosphere 3.4.3. Some properties of the Van Allen Belts 3.4.4. Two anomalous regions: The SAA and the Horns 3.5. Cut-off mechanisms for protons and heavy ions 3.6. Role of the orbit characteristics 4. A survey of environment modeling and mission modeling 4.1. Modeling the Earth's radiation environment 4.2. Examples and mission considerations 5. Summary and conclusion Acknowledgements List of references
527
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D. Brdunig
LIST OF SYMBOLS AND ABBREVIATIONS USED
/\
~
Atomic mass (e.g. for carbon, A = 12)
B
Tesla
Strength of the magnetic field
c
m.s-1
Velocity of light (c = 2.99792 x 108 m. s-1)
E
eV, keV, MeV
Energy
L
-
Mc lUwain's parameter
mo
kg
Rest mass of a particle
M
Tesla. cm3, A.m2
Momentum of the Earth's magnetic dipole
P
MV/amu
Rigidity of a particle
P
kg.m.s-1
Momentum of a particle
R(0)
cm, m
Distance away from the center of the Earth (measured in the plane of the magnetic equator)
R(X)
cm, m
Distance away from the center of the Earth (measured at geomagnetic latitude X)
R.
Unit of Earth's radius (Re = 6.371 10^ cm)
Z
Atomic number
X
m
Photon wavelength Geomagnetic latitude
0(E)
cm"2.s"^MeV-l
Differential flux of particles
0(>E)
cm'^.s'l
Integral flux of particles with energy greater than E
4)(E)
cm'2.MeV-l
Differential fluence of particles
0(>E)
cm'2
Integral fluence of particles with energy greater than E
amu
Atomic mass unit
LEO
Low Earth Orbit
LET
Linear Energy Transfer
SAA
South Atlantic Anomaly
The Space Radiation Environment
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1. INTRODUCTION Pieces of electronic equipment are increasingly used in hostile environments, which may cause acute functionality or reliability problems. The most commonly encountered hostile environments, where radiation endangers electronics, are the following ones: Space. Nuclear reactors. Medical therapies. Material testing. Semiconductor processing (ion implants. X-ray lithography, plasma etching, etc. Nuclear waste processing (robotics). High-energy particle accelerators. Nuclear warfare. Of all these radiation environments only one will be described in some detail in this chapter, namely that encountered by spacecraft electronics. The reason we chose to treat only this example is that the application of electronics in space is one of the most demanding tasks in an engineering sense. Moreover, the way radiation and reliability aspects are dealt with for space applications can also be applied to other areas. Most particles encountered in space are also encountered in other environments: electrons, protons, photons, heavy ions, fission particles. Neutrons are very rare in space (beyond the upper atmospheric layers) and their effects will therefore only be mentioned very briefly in this chapter. We indicate next, in Tab. 8.1, for the types of radiation environments in which electronic components and equipment are currently used, the types of radiation (particles and photons) encountered, as well as their energy range. The ranges indicated are those of concem for hardened electronics but actual ranges may be much greater. The aim of this short chapter is mostly to provide the reader with an overview of the radiation environment encountered in space. More specialized chapters can be found in other chapters of this book. • How particles and photons interact with matter is reviewed in Chap. 9. • How radiation interacts with the materials making up silicon devices and how it affects device functioning is reviewed in Chap. 10. • Some of the defects (natural or radiation-induced) encountered in silica and the role they play in radiation-induced phenomena are described in Chap. 11. • Finally, how high-energy ions trigger so-called "Single Event Phenomena" in highly integrated silicon devices is detailed in Chap. 12.
D. Braunig
530
These five chapters should give engineers and researchers alike a good basis to understand the major aspects of radiation-induced instabilities in electronic components. For even more detailed descriptions of the natural radiation environment and of radiationcomponent and radiation-system interactions, the reader is referred to five recently published books [39-43]. Particular application in radiation environment Space electronics
Reactor engineering
1 Medical instrumentation 1 Material characterization 1 Semiconductor processing 1 Robotics
1 High-energy accelerators
1 Military equipment
Types of radiation encountered Electrons Protons Ions Quanta Fast neutrons Thermal neutrons Electrons X-Quanta X-rays Various types Various types Depends on application, (mostly X,-Quanta in reactor engineering) Electrons Protons Heavy ions 7-Quanta Compton electrons Fast neutrons
Energy range of concern 0.5-10 MeV 5-450 MeV up to a few GeV 0.1-22 MeV 0.1 to 14 MeV 0.026 eV 1 MeV (not significant) 0.1 to 10 MeV < 0.5 MeV From below 1 eV to 1 MeV From below 1 eV to 1 MeV
1
0.1 to 10 MeV 1 GeV and more 1 GeV and more 1 to 1000 MeV 1 MeV (average energy) From 1 keV to 1 MeV (not significant) From thermal to 14 MeV |
Tab, 8.1 - Radiation environments, types of radiation encountered and energy ranges of concern,
2. DEFINITIONS AND UNITS To be able to describe a given radiation environment and to quantify its effects on electronics, we must first introduce new sets of definitions and units specific to this field: • the first group of definitions is associated with the properties of the particle itself, • the second group describes the interactions of the particle with matter, • a third group describes the source of the particles. 2.1. DEFINITIONS RELATED TO PARTICLE PROPERTIES
2.1.1. Flux Flux O designates the number of particles or photons flowing through, or impinging upon a unit area per unit time. It is expressed in [cm'^.s'l].
The Space Radiation Environment
531
In the case of photons, the term flux is often called intensity. When the particles, or the photons, are distributed in energy, in other words, when there exists an energy spectrum, one distinguishes between integral flux O (> E) and differential flux O (E). The integral flux is the total flux of those particles whose energy is above a given energy level E, whereas the differential flux is the rate of change of the flux with energy, at a specified energy E. Therefore, the integral flux is obtained by integrating the differential flux above energy E, which can be written : 0{>E) = \0{E)'dE
(8.1)
E
Because of this definition, an integral flux is always a decreasing function of energy. Fluxes of space particles are usually described as "omnidirectional", (which means that particles often come from all directions) and are usually considered as isotropic. For isotropic fluxes, it is sometimes useful to introduce the concept of flux per unit solid angle dQ (expressed in steradian). The flux can then be expressed as a number of particles per unit area, unit time and unit solid angle. For isotropic fluxes, unit [cm-^.s-^] is equal to [471 cm-2.s"l.sterl]. In space, fluxes are generally expressed as an average over a period of time and over a given orbit rather than representing an instantaneous value. Let us note, however, that the electron and the proton fluxes of the Van Allen belts (see § 3.4.2) are not onmidirectional but spiral around the magnetic field lines (as illustrated in Fig. 8.8). This peculiarity can be very important in hardening satellite systems. 2.1.2, Fluence Fluence O is the number of particles flowing through, or impinging upon a unit area, over a longer period of time. It is a time-integrated flux and it is expressed in [cm-^]. Just as we did when defining fluxes, we distinguish between "integral", "differential" and "solid-angleresolved" fluences. In Fig.8.1 the differential and integral proton fluences encountered for a specific space mission have been plotted versus energy. 2.1.3. Energy and momentum The energy E of a particle or a photon is usually expressed in units of eV, keV or MeV. Another common expression is often used, namely energy per nucleon expressed in [MeV/amu], where amu is the atomic mass unit of the energetic particle (e.g. A(H) = 1, A(He) = 4, A(C) = 12). These definitions apply to the kinetic energy of the particle. For instance, 1 MeV expresses the kinetic energy an elementary charge gains in an electrostatic potential of 1 MV. Let us note that a particle should be considered as relativistic when its kinetic energy approaches the energy equivalent of its rest mass (e.g. electrons whose energy approaches 0.511 MeV or ions whose energy, expressed in GeV, approaches - A).
532
D. Braunig Integral and Differential Ruence (LEO,cinxilar,800kn%98^ 1.00E+13
1.00E+12 jDifferemiai hiuenoe in p^cnrr^zsmnev | ^
IH^^
1 '"^*.»^_^
1.00E+11 lU
^H
O 2 1.00E+10
^Kji
^^ "-^'^^^
1.00E-H)9
y-
—p^ 1.00E408
'—
\
Integral Ruenoe in p/an^2Br
1.00E+07 1.00E-01
I.OOE+OO
1.00E+01
1.00Ef02
PROTON ENERGY in MeV
Fig. 8.1 - The integral and differential proton fluences encountered for a circular orbit (800 km, inclination 98°), are plotted versus energy.
The momentum p of a particle is given by p=mv, where m is the mass of the particle and v its velocity. There exists a relativistic relationship linking energy and momentum, expressed as : P'C^^E'{E
+ 2'mo'C^)
(8.2)
where nio is the rest mass of the particle and c is the velocity of light. The energy of a photon is given by E = hv = hc/A., where h is Planck constant, v is the frequency of the photon and X is its wavelength. The momentum of a photon is given by: p = hv/c. 2.1A. Species The term "radiation species" includes photons and particles. Photons may differ in energy, whereas particles may differ in energy, mass and charge state. It is sometimes useful to identify a particle by its name, mass, number of charges within the nucleus and its charge state. For instance in expression 2 He
, He stands for Helium, A is the atomic mass (e.g. 4),
Z is the charge of the nucleus in units of elementary charge and 2+ indicates a doubly charged state. When an atom is totally ionized, and this is a common occurrence in space, then its charge Z is approximately equal to A/2 for all species except for Hydrogen, where Z/A equals 1. Some properties of particles commonly encountered in space are summarized in Tab. 8.2 below.
The Space Radiation Environment Species
Mass
Atomic mass
533 Rest mass
1 Electrons
9.1M0-31kg
0.000549
0.511 MeV
1 Protons
1.672-10-27 kg
1.007277
938.259 MeV
Neutrons
1.675-10-27 kg
1.008665
939.553 MeV
« 6.710-27 kg
4.0026
= 4GeV
1 Helium ions
Tab, 8.2 - Some properties ofparticles commonly encountered in space. 2.2. DEFINITIONS RELATED TO PARTICLE-MATTER INTERACTIONS
To properly describe how energy is transferred from an energetic particle to an absorbing material, intemational definitions have been introduced. Some of the definitions and units used in this chapter are briefly described next. A much more thorough description can be found in Chap. 9. 2.2.1. Exposure The exposure (E) describes the amount of ionization charges generated by a radiation species per unit mass of standard atmosphere. Exposure is expressed in units of Coulomb per mass [C/kg]. An older unit is the Roentgen (= 2.56-10-4 C/kg). 2.2.2. Radiation-absorbed dose The dose (D) is the energy absorbed per unit mass of the material being irradiated (e.g. silicon). A dose of 1 Joule per kilogram is called a Gray (Gy). A unit still commonly used in the literature is the rad, which is just 10"^ Gray. The nature of the material being irradiated should always be mentioned, e.g. 1 Gy(Si). For high-energy charged particles, most of the absorbed energy is consumed in ionization phenomen. There results that the terms "dose" and "ionizing dose" are sometimes used interchangeably. This approximation is of course not valid for neutral particles such as neutrons, or for low-energy ions. 2.2.3. Dose equivalent The dose equivalent (H) has the same meaning as the radiation-absorbed dose except that it introduces a qualifying and weighting factor q to account for the fact that human tissue responds differently to different ionizing radiation. The unit of dose equivalent is the Sievert (Sv). It has the same dimension as the radiation-absorbed dose since we can write: H = q.D, where q is equal: to 1 for electrons and y 's, to 10 for neutrons and protons and even to 20 for a's from radionuclides. The older unit of dose equivalent is the rem, which corresponds to an absorbed dose of one rad.
534
D. Brdunig
2.2.4. Linear Energy Transfer Other definitions are needed to express the amount of energy transferred per unit pathlength. The electronic stopping power dE/dx is a ratio which describes the energy which a charged particle loses through ionization phenomena when travelling through a material (e.g. silicon). The Linear Energy Transfer (LET) is just a different term for electronic stopping power with a dimension of MeV/|im. (The electronic stopping power is sometimes normalized to the specific mass of the absorbing material (i.e. dE/pdx), in which case it is expressed in MeV.cm^/g). The energy lost through ionization phenomena is calculated by using the well-known Bethe-Bloch equation. The slowing down due to ionization is accompanied by the generation of electron-hole pairs. The effects of such a generation will be covered in more detail in Chaps. 9, 10, 11 and 12. The LET value of a particle depends both on its atomic mass and on its energy. Thus, for a flux of various particles of various energies, we are led to define a LET spectrum, giving the number of ions corresponding to a given LET. Some preliminary examples of LET spectra will be given in § 4.2 of this chapter. 2.3. ADDITIONAL DEFINITION
Activity of a radioactive substance The activity (A) of a radioactive source is defined as the number of transitions occurring per second. The unit of activity is called the Bequerel (Bq) whose dimension is [s"l]. An older unit is the Curie (Ci), which equals 3.7-10^0 per second. 3. THE SPACE ENVIRONMENT 3.1. INTRODUCTION
The Earth is part of our Sun-dominated planetary system and it travels through the space of our galactic system. Due to its own magnetic field, the Earth can be considered, in a first approximation as a magnetic dipole. There exists, however, a number of interdependencies which link the Sun's activity, the galactic radiation and the shape and magnitude of the geomagnetic field. The Earth is, to a large extent, protected from outside radiation by its magnetic field. This field acts as a shield against the solar wind (see below) and creates a cavity called the geomagnetic cavity, illustrated in Fig. 8.2. The field deflects also the cosmic rays, i.e. energetic ions coming from outer space, as we shall see in § 3.5. Spacecrafts (satellites, launchers, space stations, space probes, etc.) are exposed to the following types of radiation: • • • •
solar radiation, mainly solar flare protons and solar wind particles, galactic cosmic rays, protons and electrons trapped in the radiation belts, Bremsstrahlung due to the scattering of energetic electrons in the spacecraft material.
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535
MAGNETOPAUSe
DAYSIDECUSP
DAYSIDECUSP
MAGNETOPAUSE CURRENTS
PLASMASPHERE
Fig. 8.2. - Illustration of the geomagnetic cavity (for more detail see [11]).
The origin of these contributions - except for Bremsstrahlung - will be discussed in the following paragraphs in more detail. Models currently used to quantify the amount of radiation received by a spacecraft will be presented in Sect. 4 and some examples will be given. 3-2. S O L A R RADIATION
The solar radiation encountered in space originates from the solar wind and from solar flare events. The solar wind is due to the constant emission, by the sun, of low-energy particles consisting of protons (91.2%), singly and doubly charged He ions and a very small amount of electrons whose energy is less than 500 eV. Because of the Earth's magnetic field, solar wind particles cannot come close to the Earth except through the polar regions [1], Solar Flares are bursts of radiation strongly correlated with the cycles of sun spot activity at the Sun's surface. Due to their large fluxes and high energies, the particles emitted during a solar flare constitute a severe problem for a spacecraft but on a probabilistic basis only. For instance, an intense period of solar activity occurred in March 1991 and triggered a sequence of major effects, including the generation of a second inner radiation belt around the Earth, functional anomalies in satellites, and power surges in electrical power grids on the ground [2]. The cycle of growing and vanishing sun spot groups on the Sun's surface has a duration of approximately 22 years, with a reversal of the magnetic fields at the Sun's surface every 11 years. During this cycle, fluctuations of the magnetic fields take place in the upper layer of the solar atmosphere. A consequence of these fluctuations is an outpouring of X-rays, radio waves and UV-radiation as well as an emission of solar wind plasma and energetic particles. These photons and particles are emitted during solar flares and solar storms and their fluxes vary over several orders of magnitude (see for instance [3]).
536
D.
Brdunig
There are anomalous large (AL) solar flares, which are rare, last for periods of up to 8 hours and contribute most of the high-energy solar particles. Ordinary (OR) solar flares occur very frequently, last for a few minutes and do not contribute significantly to the overall solar flux.
I
r
1955
n—r
1965
1980 August 1972
Fig..8.3 - Illustration of solar cycles. Both the number of sun spots (—) and the proton fluences due to solar flares (histograms) are represented (see text) [39].
Both AL events and OR events are illustrated in Fig. 8.3 for the last three complete cycles (19, 20, 21). The continuous line of Fig. 8.3 is a plot of the number of solar spots observed during these cycles. The peaks correspond to the fluence of emitted particles (mostly protons) observed during AL or OR flares. Seen from the vicinity of the Earth, one distinguishes between simultaneous and delayed solar events, depending on the velocity of the individual constituents. The first group (travel time 8.3 minutes) consists of visible light, radio waves, UV-light and X-rays, while the second group (travel time 20 minutes to 50 hours) consists of solar flare and solar storm particles. When entering the Earth's magnetosphere, the charged particles produce large disturbances of the geomagnetic field lines. The amount of exposure of a spacecraft to solar radiation is of course strongly dependent on the orbital parameters of the craft. The ranges in flux and energy of the particles encountered in the solar wind and in solar flares is illustrated in Fig. 8.11. 3.3. GALACTIC COSMIC RAYS
There exists a galactic cosmic radiation in deep space. It is distributed uniformly and is considered as omnidirectional, although for Low Earth Orbits (LEO), the shadow of the Earth plays a significant role in its distribution. The main constituents of this radiation are protons (85%), He ions (14%) and about 1% heavier nuclei. The abundance of heavy nuclei is similar to the universal distribution of elements.
537
The Space Radiation Environment
However, hydrogen and helium have a smaller relative abundance in the galactic cosmic rays whereas the nuclei of the iron group as well as lithium, beryllium, boron and other nuclei with an odd Z number are enhanced in flux, as visible in Fig. 8.4 [5].
o
> 0^
Element-atomic number Fig. 8.4 - Relative abundance of the elements contributing to cosmic rays compared to universal abundance.
I > Fig. 8.5 - The differential fluxes of the most abundant constituents of cosmic rays are plotted versus energy, (after [5]).
in
10^
10^
10^
Kinetic energy in [MeV/amu]
10^
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D. Brdunig
The energy of cosmic rays extends over an extremely vast range, from 10 to 10^^ MeV. The differential energy spectra of the cosmic ray ions in the neighborhood of the Earth exhibit a maximum at 1 GeV/nucleon. Outside the geomagnetic field, the flux of cosmic ray particles is approximately 4 nuclei per cm^ per second, mainly consisting of protons [4]. Because of the interactions with the solar wind and with the interplanetary magnetic field, this flux is reduced at the lower energy end and its fluctuations are related to the solar activity. Figure 8.5 presents the differential spectrum of four of the most abundant nuclei [6, 7]. The reason for the increase in H and He ion fluxes at low energies is due to the spallation of heavy ions into lighter ones. The integral flux of cosmic rays is represented as a function of energy in Fig. 8.13. 3.4. TRAPPED RADIATION IN THE EARTH'S MAGNETOSPHERE
3.4.1. A brief description of the Earth's magnetic field In a first approximation, one can consider that the Earth acts a magnetic dipole (at least within a distance of about 5 Earth radii). The dipole momentum [M] expressed in [Tesla.cm^] can be written^^^: M = /?f • 0304 • 7 0 " "^ = 7M -10^^ T.cm^
(8.3)
where R^ is the Earth's radius, which equals 6.371.10^ cm, [8]. The magnetic field and the associated field lines can be described by the following set of equations using polar coordinates:
5a)=
M ^4-3cos^ (X)
• R{0f
and
(8.4)
cos^ (X)
2 R{X)----R(0)-cos (X)
(8.5 a)
or
L{X)^
R{X) Re
= Rio) ——cos 2(,^ U)
(8.5 b)
Re
Here B{X) is the magnitude of the magnetic field, which remains tangent to a field line, as a function of A., X is the geomagnetic latitude and R(0) is the distance at which the field line intersects the equatorial plane. This allows an R(A.) representation of the geomagnetic properties of the Earth, as illustrated in Fig. 8.6. Parameter L was introduced by McHlwain in his B-L space [9, 10]. L is a reduced form of the locus of the magnetic field lines and is equal to the geocentric distance at which the shell intersects the magnetic equator. It is measured in units of Earth radii Re. ^^^ In the MKS unit system, M = 8.27.1022 A.m^.
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539
North
0 1 South
2
3 4 5 Distance in Re
Fig. 8.6 - Illustration of the geomagnetic field lines and ofparameter used to describe them.
At constant L, the value of B varies from a minimum at the magnetic equator to a maximum where the field lines intersect the surface of the Earth. The B-L space is a fixed and idealized coordinate system which does not take into account any temporal change. 3.4.2. The trapping and filtering properties of the magnetosphere The shape of the magnetic field lines has several important consequences which we detail next [11, 12]: • the geomagnetic field acts as a filter for certain types of charged particles entering the Earth's magnetosphere, • there are allowed cavities where particles are trapped and, conversely, forbidden zones where particles of different properties are rejected, • the polar regions are more easily accessible than the equatorial plane for particles of identical properties, • the properties of the geomagnetic filter function enable electrons and protons to form a stable and structured radiation belt, where the abundance of these particles is significantly greater than outside the magnetosphere, • the same filter function rejects a large portion of heavy charged ions whose energy is insufficient to penetrate the inner regions of the magnetosphere (for more details see § 3.5),
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D, Brdunig
• the stable trapping of electrons, protons and light ions results in a structured set of radiation belts (Van Allen belts), illustrated in Fig. 8.7, which is stationary and whose fluctuations in density and shape is mainly caused by variations in solar activities. Thanks to this trapping and filtering effect, the Earth is relatively protected from outside radiation phenomena. ^
Solar wind Geomagnetic tail
Radiation belts
Fig. 8.7 - Cross-sectional view of the geomagnetic cavity featuring: the solar wind and the associated shock wave, the magnetopause and the radiation (Van Allen) belts (after [42],
3.43. Some properties of the Van Allen Belts The shape of the geomagnetic field and especially the regions of maximum magnetic field strength near the geomagnetic North and South poles determine the motion of the trapped particles. The particles of velocity v are submitted to a force given by F = q v A B . They spiral around and move along the magnetic field lines and are reflected at "conjugate mirror points", as illustrated in Fig. 8.8. The location of these mirror points on a field line depends on the energy of the particle itself. Moreover, the particles drift in a direction perpendicular to the field lines. As a result of their opposite charges, electrons drift Eastward whereas protons drift Westward (azimuthal drift). Typical features of the motion of energetic particles at low altitudes are shown in Tab. 8.3. Particle
Gyration radius
Gyration period
Bounce period
Azimuthal drift period
velocity p=v/c
10 MeV protons
50 km
0.01s
0.5 s
4min
0.2
1 MeV electrons
0.6 km
10|LIS
0.1s
lOmin
0.9
Tab. 3 ' Some characteristics of the motion of particles trapped at low altitudes (^1.5 R^)
The Space Radiation Environment
541
Magnetic field line
Fig. 8.8 - Spiralling and bouncing behaviour of electrons and protons within the geomagnetic field.
The trapping zones are structured into an "inner" and an "outer" belt as illustrated by Figs. 8.9 a, b, c. Protons are entirely trapped within the inner belt and possess energies ranging from 0.5 MeV to more than 400 MeV. In the equatorial plane, their energy decreases conversely with the radial distance from the Earth as shown in Fig.8.9 a. Electrons have energies of up to 7 MeV in the outer belt with fluxes exceeding those of the inner belt (E < 5 MeV). Due to the action of the Sun we have to distinguish between the day and night sides. There are strong variations in shape and population of the belts including diurnal cycles and the 11-year solar cycle as shown in Tab. 8.4 for low altitudes. The outer Van Allen radiation belt typically extends from the plasmasphere to somewhat beyond the geosynchronous altitude, depending on the geomagnetic activity. SOLAR MAX
SOLAR MIN
1
Electron intensities
higher
lower
1
Proton intensities
lower
higher
Tab. 4 - Influence of the solar activity on low-altitudes particles fluxes.
3.4.4. Two anomalous regions: The SAA and the Horns For Low Earth Orbits (LEO) (typically above 500 km) there exist two important contributions to the overall radiation level due to the shape of the geomagnetic field lines: the South Atlantic Anomaly (SAA) and the polar Horns [11]. The SAA is due to the fact that the geomagnetic axis is titled by approximately 11° off the Earth's rotation axis and is displaced by about 500 km towards the Westem Pacific as illustrated in Fig. 8.10. This results in stronger field values at LEOs and correspondingly higher electron and proton fluxes in this geographic region (above Brazil). This is of particular significance for space electronics orbiting on an LEO with a large inclination since the radiation flux is larger by several orders of magnitude in this region compared to other regions of the mission. An illustrative example was given by the Hubble space telescope.
542
D . Brdunig Geostationary orbit Large space telescope ^4
•o
"£ o Q. LU
O "Q. O to 10
20
30
altitude in 10'km
a) electrons
geomagnetic axis
>so(Qr min ' solar max
protons
geomagnetic axis 4
b) Electrons with E > 1 MeV
c) Protons with E > 10 MeV
Fig. 8.9 - Illustration of the Van Allen belts, a) Schematic cross section of the Van Allen Belts in the equatorial plane. Several integrated fluxes of electrons and protons are plotted as a function of altitude. The proton distribution in the inner zone and the dual distribution of the electrons (inner and outer zones) are clearly visible. An LEO and a geostationary orbit are indicated (after [37]). b) Set of isoflux contours describing the trapping belts for electrons of energy > I MeV. c) Idem for protons of energy > 10 MeV.
The Space Radiation Environment
543
Before the 1993 retrofitting of the telescope electronics, every time the spacecraft went through the intense radiation belts of the SAA, the memories suffered from enhanced dark currents and their contents had to be corrected. The Homs at mid-latitude levels are tails of the outer electron belt coming very close to the Earth because of the geomagnetic field. Figures 8.9 a and b illustrate this feature very clearly as well as Fig. 8.12 based on the application of the AE-2 model [13].
Fig. 8.10 - Origin and illustration of the South Atlantic Anomaly (after [42]).
3.5. CUT-OFF MECHANISMS FOR PROTONS AND HEAVY IONS
The penetration of charged particles into the geomagnetic space, in particular in the case of solar flare protons and cosmic heavy ions, is hampered by the shielding properties of the Earth's magnetic field. The more energy a particle possesses, the closer to the Earth it can get. A quantitative description of these "cut-off mechanisms uses a specific term, the rigidity P, whose value determines how easily a charged particle is reflected by the geomagnetic field [8]. The rigidity P of a (relativistic) particle of energy E is given by:
P = pcA/Z
= —ylE^-\'2'm^'C^'E
(8.7)
In this expression m^ is the rest mass of the particle, A is the atomic mass unit (amu), Z is the charge of the nucleus and E is the energy of the particle expressed in MeV. The dimensions of P are [MV/amu]. In the case of protons, ratio A/Z equals 1. For heavier ions, A/Z equals 2 or is very close to it. For low-energy particles, the rigidity is proportional to E^^^ whereas in case of high energy it is proportional to E. We can calculate the distance (L) in units of Earth radii, at which an incoming particle of rigidity P (expressed in MV/amu) and moving in a meridian plane can come close to the Earth [42]. It yields: L=
15.9 10^
cos X
(8.8)
D. Braunig
544
The rigidity concept [14, 15] is used as well to describe the flux spectra of solar protons, for instance as: ^p{>P)
where Op
(8.9)
= ^Pf)-exp\
is the total proton flux for an event expressed in [# protons/cm^] and PQ is
characteristic of a particular solar event. As seen from Fig. 8.11, protons can reach a region corresponding to a geosynchronous orbit (i.e. L = 6 to 7) only if their energy exceeds 87 MeV (for ions 23 MeV/amu). Particles can reach Low Earth Orbits (LEO) only if their energies are greater than 3 GeV for protons, and 1 GeV/amu for ions. For LEO, in addition, atmospheric effects must be considered. But for a polar orbit of nearly 90° inclination, a much lower energy is needed to reach these regions. 12
III
10
H L 6
m\ IONS 10
a)
100
1000
Energy (in MeV for protons and MeV/amu for ions)
23 MeV/n 12 MeV/n
Fig. 8,11 - Illustration of the cut-off mechanism, a) illustrates the penetration depth of solar and cosmic particles in L coordinates (in units of Earth radii) as a function of energy (in units of MeV/amu (ions) and MeV (protons)), b) is a visualization of that cut-off effect giving the total energy required to penetrate the magnetosphere, (after [11]).
The Space Radiation Environment
545
3.6. ROLE OF THE ORBIT CHARACTERISTICS
Due to the shielding behaviour of the geomagnetic field, how satellite electronics will be affected depends on the orbit used. For instance, on a geosynchronous orbit (i.e. at an altitude of about 36 000 km), the main constituents of the encountered radiation are electrons in the 0.5 to 8 MeV range. Therefore, a complete shielding of the equipment is very difficult to achieve due to the generation of Bremsstrahlung by these high-energy electrons. Since the conmiunication satellites which are on such orbits have a mission duration of more than 7 years, the (total) absorbed dose can be high and the electronic components must be able to withstand these high accumulated dose values. Solar flares must also be taken into account on a statistical basis. For Low Earth Orbits (LEO), i.e. for altitudes ranging from 200 to 800 km, one must distinguish between equatorial and polar orbits. Because of the configuration of the geomagnetic field lines, particles encountered on polar orbits are primarily protons and highly energetic ions. Overall, the level of radiation encountered depends strongly on the altitude of the spacecraft since only a few hundred kilometers in altitude may cause large differences in flux due to the steep increase in proton flux at the tail of the inner Van Allen belt. Only a very accurate knowledge of the energy and flux distribution of the radiation species permits a reliable prediction of the radiation threat. Models currently available to describe the environment are discussed next. Let us note that an accurate prediction must also take into account the shielding effect of the equipment itself. 4. A SURVEY OF ENVIRONMENT MODELING AND MISSION MODELING The modeling of the radiation environment is important because it enables: 1) a calculation of the fluxes encountered by the spacecrafts, and 2) a forecast of the type of problems the electronic parts (especially the semiconductor components) will experience during a mission. 4.1.
MODELBSG THE E A R T H ' S RADIATION ENVIRONMENT
From the beginning of unmanned spaceflights, experimental data were gathered to get a comprehensive picture of the Earth's radiation belts. Due to limitations in measurement equipment and theoretical modeling, the description of the constituents of the Earth's environment and of their distribution had to be continuously improved. Today, the descriptions of the environment are still "stationary" ones, with a few exceptions, and for example do not illustrate the temporal changes due to variations in solar activities. Up to now, 8 models describing the Earth's environment have been successively established for electrons (AE-) as well as for protons (AP-). They are called AE-1, AE-2, AE-3, AE-4, AE-5, AE-5 projected (AE-5P), AE-6, AE-8 and AP-1, AP-2, AP>3, AP-4, AP-5, AP-6, AP-7, AP-8. Following the NASA-Publication NSSDCAVDC- A- R&S 91-29 by James I. Vette [13], a short tabular description of the individual models is given next (see Tab. 8.5). Figure 8.12 gives an illustration of data gathered in the AE-2 model.
546
D, Brdunig
The CREME model was developed at the Naval Research Lab to have a tool for predicting Single Event Upsets caused by cosmic rays. A series of reports [31 - 34] entitled "Cosmic Ray Effects on Microelectronics (CREME)" linked the Linear Energy Transfer (LET) of ions and its spectrum (number of ions versus LET) to the SEU sensitivity of individual integrated circuits. The CREME model describes the space environment and the radiation-component interactions leading to SEU and contains estimates of individual ion fluxes from protons up to Uranium^^\ It also takes into account reductions in flux intensity due to solar modulation, geomagnetic cut-off and the Earth's shadowing effect. The transport code includes heavy ion fragmentation and ionization losses for each element individually.
Fig, 8.12 - Illustration of the electron belts. Isoflux lines have been computed (using the AE-l model), for electrons of energy > 1 MeV.
90 80 70 60 50 40 30 20 K) 0 10 20 30 40 50 60 70 80 90 NORTH GEOGRAPHIC LATITUDE SOUTH
SOLPRO [35] is a simple statistical model predicting solar flares and is based entirely on satellite spectral measurements of the 20th solar cycle. Figure 8.13 sunmiarizes and illustrates the relative importance of each radiation species encountered in space.
(2)
Ions with atomic masses larger than that of Fe play no significant part.
The Space Radiation Environment
547
Van Allen electron belt Model
Range [Re]
Energy range [MeV]
AE-1
1.2-3
0.3-7
Omnidir. Integral Flux
[17]
AE-2
1.1-6.3
0.04-7
Solmax, SAA and Homs
[18]
AE-3
6.6
0.01-5
GEO, SOLMAX, SOLMIN
[19]
AE-4
3.0-11.0
0.04-4.85
Time var. SOLMIN, SOLMAX
[20-21]
1
AE-5
1.4-2.8
0.04-4
Inner zone, atmospheric cut-off
[22 - 23]
1
AE-6
1.2-11
0.04-5
Decay times, Starfish
[24]
No release
[25]
The most advanced model yet
[26]
AE-7 AE-8
1.2-4/3.011/2.4-3
0.04-7
Remarks
References
Van Allen proton belt Range [Re]
Energy range [MeV]
AP-1
1.17-3.15
30-50
Integral flux, solar cycle var.
[17]
AP-2
1.17-3.51
15-30
Integral flux, solar cycle var.
[17]
AP-3
1.17-2.9
>50
Integral flux, solar cycle var.
[17]
AP-4
1.17-4.6
4.0-15.0
Integral flux, solar cycle var.
[17]
AP-5
1.2-6.0
0.1-4
Variation, differential flux
[27]
1.2-4
4-40.0
Improved data
[28]
AP-7
1.15-3.0
50-500
Improved data
[29]
AP-8
1.15-6.6
0.1-400
Static model with solar cycle
[30]
Model
|AP-6
Remarks
References
Solar flares and cosmic rays I
Model SOLPRO
1CREME
Remarks
References
Solar Flares
[35]
Cosmic Rays
[31 - 34]
Tab. 8.5 - Models currently (1995) available describing trapped particles, solar protons and cosmic rays. 4.2.
EXAMPLES AND MISSION CONSIDERATIONS
To illustrate the possibilities of modeling, we present next three examples which have been calculated using a software called Space Radiation [36], based on the CREME model. These examples are: the geosynchronous orbit in the best and the worst-case parking position, the impact of solar activity for a circular LEO and the influence of orbit inclination on the LET spectrum for an LEO.
D, Brdunig
548
Solar wind protons 10
10
C/5
10
Trapped electrons
q
I—peak
Geostationary 10
Y Trapped Protons-peak
Solar protons Major flare
X! US
10
7-Year average
Galactic Cosmic rays
10-^
10^
10
10
1
10
10
10
10
Particle energy E [MeV] Fig. 8.13 - Integral spectra of particles encountered in space [37].
In Fig. 8.14 the electron fluence per cm^ and per day as a function of electron energy is shown for two locations, which represent the worst and the best-case conditions for a geosynchronous orbit. The best-case location is at L = 7,70° West, and the worst-case is found for L = 6.6, 160° West. The radiation exposures between the two locations differ by about one order of magnitude for intermediate energies (because of the altitude, the protons play no part and only electrons contribute to the absorbed dose).
The Space Radiation Environment
1.00E+13
^ -d
1.00E+12
e
1.00E+11
W A N
^
1.00E+10 1.00E+09 1.00E-I-08
J3
1.00E-I-07 1.00E+06 1.00E+05 c3 ;-4
<
1
geosynchronous Orbit
•e-
X
549
^
IN
^ ^
1
^ [
1 1
P
1
|WORSTCASE:160"W,L=6.6|
£>.>—
|BESTCASE:70*W.L=7|
I —
^
1
)
N
i
1.CX)E+04 2
3
4
Energy [MeV]
Fig. 8.14 - The (averaged) integral electronfluxesdetected on a geosynchronous orbits for best and worst-case parking positions^ versus energy (after [36]).
In Fig. 8.15 the daily averaged integral electron flux encountered on a circular orbit (600 km) with a 60° inclination, for low solar activity, is compared to the electron flux encountered for maximum solar activity. The electron integral fluence (per cm2) averaged over a period of one day shows a pronounced difference, particularly at low energies as already indicated in Tab. 8.4, because of an enhanced electron fluence for Solar Max periods. Finally, the dependence of LET spectra encountered for LEO's on the inclination angle of the orbit are indicated in Fig. 8.16 as an example which will be described in more detail in Chap. 10. Whereas the fluences for low (BO'') and medium (60°) inclinations are quite similar, the polar orbit (90°) exhibits much more particles per unit area and unit time due to the weaker shielding capabilities of the geomagnetic field at that location. The sharp cut-off at about 500 MeV.cm^/g reflects the strong contribution of protons in the low LET range, since their LET never exceeds this value. 5. SUMMARY AND CONCLUSION More and more items of electronic equipement are being used in space aboard spacecrafts. While in space, they are submitted to a wide array of particles, such as: electrons and protons of the Van Allen Belts, protons and heavy ions emitted during solar flares and protons and heavy ions which make up the cosmic rays. The performance of on-board electronic components and equipment may degrade quickly if precautions are not taken.
550
D. Brdunig
Fig. 8.15 Daily averaged integral electron flux encountered on a circular orbit (600 km, 60°) versus energy (after [36]).
• -e-
1E-^02 1E-02
lE-OI
lE-^OO
1E^01
Energy [MeV] 1.00E+07
q ^ W
90° 1.00E+06 60°
A
-eo
|30 ol
1.00E+05
13 ^S
N
1.00E+04 r r\ rVsh1- /en/ Pir OliOuiai v^i uii ^\j\M/wr\iiiy
Q 1.00E+03 1.00E+00
1.00E+01
1.00E+02
1.00E+03
LET [MeV.cm2.g-i] Fig.8. 16 Daily integralfluenceof particles encountered on a circular orbit (600 km) for different inclinations versus LET. (after [36]).
The Space Radiation Environment
551
The need for an accurate knowledge of the Earth's radiation environment has thus become imperative. Computer models of environment, such as AP-8 for protons and AE-8 for electrons, developed by NASA, have been widely used to predict the radiation threat for a given space mission. A severe disadvantage of these models is that they describe a stationary situation that does not correctly reflect the real situation. Therefore activities are underway at NASA and ESA to study the dynamic behaviour of our radiation environment and to improve the accuracy and the reliability of the predictions. ACKNOWLEDGEMENTS The author wishes to thank his colleagues at the Hahn-Meitner-Institut for fruitful discussions and the editors G. Barbottin and A. Vapaille for their thorough review. LIST OF REFERENCES [I] King J.H., J. Spacecraft 11, No. 6, (1974), p.401. [2] Shea M.A.,D.F. Smart, J.H. Allen and D.C. Wilkinson, "Spacecraft Problems in Association with Episodes of Intense Solar Activity and Related Terrestrial Phenomena During March 1991", lEEE-NS 39,6 (1992), p. 1754. [3] Adams J.H. Jr and A. Gelman, IEEE NS-31,6, (1986), p.l212. [4] Byrne F.N.," A Survey of Solar Flare Phenomena", Space Set. Rev. 3,(1964), p.319. [5] Petersen E.L., "Basic concepts on Single Event Upsets" lEEE-NSREC Short Course, July 1983, Gatlinburg/Tennessee. [6] Adams J.H. Jr., "The Natural Environment Inside Spacecraft", lEEE-NS 29,6, (1982), p. 2095. [7] Adams J.H. Jr., "The Variability of Single Event Upsets Rates in the Natural Environment", lEEE-NS 30, 6,(1983), p.4475. [8] Stormer C , Polar Aurora, Oxford University Press, London,1955. [9] Mclllwain C.E." Magnetic Coordinates" in Radiation Trapped in the Earth Magnetic Field, Ed. B.M. McCormac (Reidel,Holland), (1966), p. 45. [10] Mclllwain C.E., J. Geophys. Res. 66, No.ll, November 1961, p. 3681. [II] Stassinopoulos E.G., " Radiation Environments of Space", lEEE-NSREC: Short Course, Reno, Nevada, July 16, 1990. [12] Haffner J.W., Radiation and Shielding in Space, Academic Press, 1967. [13] Vette J.I.," The NASA/National Space Science Data Center Trapped Radiation Environment Model Program (1964-1991)", NSSDCAVDC-A-R&S 91-29, National Space Science Data Center, Greenbelt,Maryland, November 1991. [14] Rosen A., "The Dynamics of the Outer Zone", Space Science, Ed.D.P. Le Galley (Wiley, New York), (1963), p. 275. [15] Webber W.R., "Solar Flare Proton Data", Nucleonics 21, (1963),p. 154. [16] Nichols D.K., "Trends in Electronic Parts Susceptibility to Single Event Upset Space Station Environment", JPL D-2767, September 1985. [17] Vette J.I.,"Models of the Trapped Radiation Environment", Vol.1: Inner Protons and Electrons NASA SP3024, Vol.1, 1966. [18] Vette J.I., A.B.Lucero and J.A.Wright, "Models of the Trapped Radiation Environment", Vol.2: Inner and Outer Zone Electrons, NASA SP-3024, Vol.2, 1966. [19] Vette J.I., A.B. Lucero and J.A.Wright, "Models of the Trapped Radiation Environment", Vol.3: Electrons at Synchronous Altitudes, NASA SP-3024, Vol.3, 1966.
552
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Brdunig
[20] Singley G.W. and J.I. Vette, "The AE-4 Model of the Outer Radiation Zone Electron Environment", NASA GSFC National Space Science Data Center, NSSDC 72-06, August 1972. [21] Singley G.W. and J.I. Vette, "A Model Environment for Outer Zone Electrons", NASA GSFC National Space Science Data Center, NSSDC 72-13, December 1972. [22] Teague, M.J. and J.I.Vette, "The Inner Zone Model AE-5 ", NASA GSFC National Space Science Data Center, NSSDC 72-10, November 1972. [23] Teague, M.J. and J.I.Vette, "The Use of the Inner Zone Electron Model AE-5 and Associated Computer Programs", NASA GSFC National Space Science Data Center, NSSDC 72-11, November 1972. [24] Teague M.J., K.W.Chan and J.I.Vette "AE-6: A Model Environment for Trapped Electrons for Solar Minimum", NSSDCAVDC-A-R&S 76-04, National Space Science Data Center, Greenbelt, Maryland, March 1976. [25] Chan K.W., M.J.Teague, N.J.Schofield and J.I.Vette," Modeling of Electron Time Variations in the Radiative Belts", American Geophysical Union Geophysical Monograph 21, Quantitative Modeling of Magnetospheric Processes, June 1978. [26] Vette J.I.," The AE-8 Trapped Electron Model Environment", NSSDCAVDC-A- R&S 91-24, National Space Science Data Center, Greenbelt,Maryland, November 1991. [27] King J.H.," Models of the Trapped Radiation Environment", Vol.4: Low Energy Protons, NASA SP- 3024, Vol.4, 1967. [28] Lavine J.P. and J.I.Vette," Models of the Trapped Radiation Environment", Vol.5: Inner Belt Protons, NASA SP-3024, Vol.5, 1969. [29] Lavine J.P. and J.I.Vette," Models of the Trapped Radiation Environment", Vol.6: High Energy Protons NASA SP-3024, Vol.6, 1970. [30] Sawyer D.M. and J.I.Vette," AP-8 Trapped Proton Environment for Solar Maximum and Solar Minimum",NSSDC 76-06, National Space Science Data Center, Greenbelt, Maryland, December 1976. [31] Adams, J.H., R.Silberberg and C.H.Tsao, "Cosmic Ray Effects on Microelectronics",Part l:The Near Earth Particle Environment", NRL Memorandum Report 4506, August 25,1981. [32] Adams J.R.," Cosmic Ray Effects on Microelectronics", Part 4, NRL Memorandum Report 5901, December 31, 1986. [33] Adams J.H., J.R.Letaw and D.F.Smart," Cosmic Ray Effects on Microelectronics", Part 2: The Geomagnetic Cutoff Effects, NRL Memorandum Report 5099, May 1983. [34] Tsao C.H., J.H.Silberberg, J.H.Adams and J.R.Letaw," Cosmic Ray Effects on Microelectronics", Part 3: Propagation of Cosmic Rays in the Atmosphere, NRL Memorandum Report 5402, August 1984. [35] Stassinopoulos E.G., "SOLPRO: A Computer Code to Calculate Probabilistic Energetic Solar Proton Fluences", Rep. NSSDC No. 75-11, National Space Science Data Center, Greenbelt, MD, April 1975 [36] SPACE RADL\TION, Users Manual, DOS-Version 2.0,Trademark of Severn Communications Corporation, 1992. [37] Holme-Siedle A. and Adams L. The Radiation Design Handbook, ESA PSS-01-609, Draft, 1989 [38] McGuire at al. "Solar flare particle fluence during solar cycles 10-20 and 21", 18th Int. Ray Conf., Vol.4,1983, p.66 Recent books on the effects of radiation on electronic components [39] [40] [41] [42]
Ma and Dressendorfer, Ionizing Radiation effects in MOS Devices and Circuits, Wiley, 1989. Messenger and Ash, The effects of Radiation on Electronic Systems, Van Nostrand, 1992. Holmes - Siedle and Adams, Handbook of Radiation Effects, Oxford, 1993. Boudenot J.D., L'environnement spatial. Collection "Que sais-je ?", Presses Universitaires de France, 1995 (in French) [43] Messenger and Ash, Single Event Phenomena, Chapman & Hall, 1997
525
CHAPTER 8
THE SPACE RADIATION ENVIRONMENT
byD.BRAUNIGd)
Key items Flux Fluence Dose Linear energy transfer Solar radiation Cosmic rays Van Allen belts Cut-ofF mechanisms Environment models Mission modeling
(1) Hahn-Meitner-Institut Berlin GmbH Department AT Glienicker Str. 100 14109 Berlin, Germany
526
D, Brdunig
Abstract of Chapter 8: The space radiation environment In many civilian and military applications, electronic components must operate in a radiation environment, and thus may experience instability phenomena. In Section 1 of this chapter, we introduce the parameters which enable us: to fully characterize the particles encountered in a radiation environment and to describe their effects on matter. We then describe the two basic constituents of the environment encountered in space: solar particles and cosmic rays. Particles can be emitted by the sun, either continuously (solar wind) or sporadically (solar flares). The rest of the galaxy behaves also as a continuous source of radiation (the cosmic rays). In Section 3 we show that there exists around the Earth, zones where particles are preferentially trapped (the so-called Van Allen belts). The Earth's magnetic field is responsible for this trapping. This field also shields our planet by filtering out most of the solar and galactic particles. In the last section we show that, given some hypotheses, the radiation environment of the Earth can be modeled. The models enable us to predict the fluxes and the energy spectrum of the particles that a spacecraft will encounter during a mission in space and thus help us compute the probability that a "Single Event Phenomenon" occur or that a component fail. We finally give a few examples illustrating the use of these models. Resume du chapitre 8 : Le rayonnement spatial Dans de nombreuses applications civiles et militaires, des composants electroniques sont amenes a fonctionner dans un environnement radiatif et peuvent devenir instables ou etre endommages. Dans la premiere section de ce chapitre, nous rappelons les parametres qui permettent de caracteriser : les particules rencontrees dans un environnement donne, et leur action sur la matiere. Nous decrivons ensuite les deux sources du rayonnement spatial : le soleil et la galaxie. Le soleil est a I 'origine de particules emises en continu (vent solaire) ou de maniere sporadique (lors des eruptions solaires). Le reste de la galaxie contribue a un fond continu de rayonnement (le rayonnement cosmique galactique). Dans la section 3, nous montrons qu 'il existe autour de la terre des zones de piegeage privilegiees pour les particules, encore appelees ceintures de Van Allen. Le champ magnetique terrestre est a Vorigine de ce piegeage. Ce champ joue par ailleurs le role d'un ecran protegeant la terre de la plupart des particules galactiques et solaires. Nous montrons dans la derniere section que, moyennant certaines hypotheses, I'environnement radiatif terrestre peut etre modelise. Les mode les permettent de predire les flux et les energies des particules qu 'un vehicule spatial va rencontrer et aident ainsi a chiffrer la probabilite qu 'un "evenement singulier" ou qu 'une panne se produise. Nous donnons enfin quelques exemples qui illustrent I 'application de ces modeles. Zusammenfassung zu Kapitel 8: Die Weltraumstrahlung In zahlreichen zivilen und militdrischen Anwendungen miissen elektronische Bauelemente in strahlungsbelasteter Umgebung arbeiten und dabei Instabilitdtsprobleme erfahren. In Abschnitt 1 dieses Kapitels werden die Parameter eingefuhrt, die es uns ermoglichen, sowohl die Eigenschaften der daran beteiligten Strahlungsspezies als auch ihre Wechselwirkung mit dem Material vollstdndig zu beschreiben. Anschliessend werden die zwei grundsdtzlichen Arten der im Weltraum anzutreffenden Strahlung beschrieben: die solare und die kosmische Strahlung. Teilchen konnen von der Sonne abgestrahlt werden, sowohl kontinuierlich (solarer Wind) oder sporadisch (SonnenausbrUche) oder der Weltraum selbst ist eine Quelle kontinuierliche Strahlung (kosmische Strahlung) Im Abschnitt 3 wird beschrieben, dafi im Umfeld der Erde Gebiete existieren, in denen geladene Teilchen vorzugsweise eingefangen werden (die sogenannten Van Allen-GUrtel). HierfUr ist das Erdmagnetfeld verantwortlich. Dieses Magnetfeld wirkt fUr einen grojien Teil der kosmischen und solaren Strahlung als Abschirmung der Erde. Im Abschnitt 4 wird gezeigt, dafi die Strahlungsumgebung der Erde, unter Beriicksichtigung einiger Hypothesen, modelliert werden kann. Diese Modelle gestatten die Vorhersage von FlUssen und energetischer Verteilung der Strahlung fur eine spezifische Mission und damit die Belastung der elektronischen Komponenten an Bord eines Raumfahrzeugs. Dies wird an einigen Beispielen demonstriert.
The Space Radiation Environment Chapter 8: The space radiation environment Table of contents List of symbols and abbreviations used 1. Introduction 2. Definitions and units 2.1. Definitions related to particle properties 2.1.1. Flux 2.1.2. Fluence 2.1.3. Energy and momentum 2.1.4. Species 2.2. Definitions related to particle-matter interactions 2.2.1. Exposure 2.2.2. Radiation-absorbed dose 2.2.3. Dose equivalent 2.2.4. Linear Energy Transfer 2.3. Additional definition 3. The space environment 3.1. Introduction 3.2. Solar radiation 3.3. Galactic cosmic rays 3.4. Trapped radiation in the Earth's magnetosphere 3.4. L A brief description of the Earth's magnetic field 3.4.2. The trapping and filtering properties of the magnetosphere 3.4.3. Some properties of the Van Allen Belts 3.4.4. Two anomalous regions: The SAA and the Horns 3.5. Cut-off mechanisms for protons and heavy ions 3.6. Role of the orbit characteristics 4. A survey of environment modeling and mission modeling 4.1. Modeling the Earth's radiation environment 4.2. Examples and mission considerations 5. Summary and conclusion Acknowledgements List of references
527
528
D. Brdunig
LIST OF SYMBOLS AND ABBREVIATIONS USED
/\
~
Atomic mass (e.g. for carbon, A = 12)
B
Tesla
Strength of the magnetic field
c
m.s-1
Velocity of light (c = 2.99792 x 108 m. s-1)
E
eV, keV, MeV
Energy
L
-
Mc lUwain's parameter
mo
kg
Rest mass of a particle
M
Tesla. cm3, A.m2
Momentum of the Earth's magnetic dipole
P
MV/amu
Rigidity of a particle
P
kg.m.s-1
Momentum of a particle
R(0)
cm, m
Distance away from the center of the Earth (measured in the plane of the magnetic equator)
R(X)
cm, m
Distance away from the center of the Earth (measured at geomagnetic latitude X)
R.
Unit of Earth's radius (Re = 6.371 10^ cm)
Z
Atomic number
X
m
Photon wavelength Geomagnetic latitude
0(E)
cm"2.s"^MeV-l
Differential flux of particles
0(>E)
cm'^.s'l
Integral flux of particles with energy greater than E
4)(E)
cm'2.MeV-l
Differential fluence of particles
0(>E)
cm'2
Integral fluence of particles with energy greater than E
amu
Atomic mass unit
LEO
Low Earth Orbit
LET
Linear Energy Transfer
SAA
South Atlantic Anomaly
The Space Radiation Environment
529
1. INTRODUCTION Pieces of electronic equipment are increasingly used in hostile environments, which may cause acute functionality or reliability problems. The most commonly encountered hostile environments, where radiation endangers electronics, are the following ones: Space. Nuclear reactors. Medical therapies. Material testing. Semiconductor processing (ion implants. X-ray lithography, plasma etching, etc. Nuclear waste processing (robotics). High-energy particle accelerators. Nuclear warfare. Of all these radiation environments only one will be described in some detail in this chapter, namely that encountered by spacecraft electronics. The reason we chose to treat only this example is that the application of electronics in space is one of the most demanding tasks in an engineering sense. Moreover, the way radiation and reliability aspects are dealt with for space applications can also be applied to other areas. Most particles encountered in space are also encountered in other environments: electrons, protons, photons, heavy ions, fission particles. Neutrons are very rare in space (beyond the upper atmospheric layers) and their effects will therefore only be mentioned very briefly in this chapter. We indicate next, in Tab. 8.1, for the types of radiation environments in which electronic components and equipment are currently used, the types of radiation (particles and photons) encountered, as well as their energy range. The ranges indicated are those of concem for hardened electronics but actual ranges may be much greater. The aim of this short chapter is mostly to provide the reader with an overview of the radiation environment encountered in space. More specialized chapters can be found in other chapters of this book. • How particles and photons interact with matter is reviewed in Chap. 9. • How radiation interacts with the materials making up silicon devices and how it affects device functioning is reviewed in Chap. 10. • Some of the defects (natural or radiation-induced) encountered in silica and the role they play in radiation-induced phenomena are described in Chap. 11. • Finally, how high-energy ions trigger so-called "Single Event Phenomena" in highly integrated silicon devices is detailed in Chap. 12.
D. Braunig
530
These five chapters should give engineers and researchers alike a good basis to understand the major aspects of radiation-induced instabilities in electronic components. For even more detailed descriptions of the natural radiation environment and of radiationcomponent and radiation-system interactions, the reader is referred to five recently published books [39-43]. Particular application in radiation environment Space electronics
Reactor engineering
1 Medical instrumentation 1 Material characterization 1 Semiconductor processing 1 Robotics
1 High-energy accelerators
1 Military equipment
Types of radiation encountered Electrons Protons Ions Quanta Fast neutrons Thermal neutrons Electrons X-Quanta X-rays Various types Various types Depends on application, (mostly X,-Quanta in reactor engineering) Electrons Protons Heavy ions 7-Quanta Compton electrons Fast neutrons
Energy range of concern 0.5-10 MeV 5-450 MeV up to a few GeV 0.1-22 MeV 0.1 to 14 MeV 0.026 eV 1 MeV (not significant) 0.1 to 10 MeV < 0.5 MeV From below 1 eV to 1 MeV From below 1 eV to 1 MeV
1
0.1 to 10 MeV 1 GeV and more 1 GeV and more 1 to 1000 MeV 1 MeV (average energy) From 1 keV to 1 MeV (not significant) From thermal to 14 MeV |
Tab, 8.1 - Radiation environments, types of radiation encountered and energy ranges of concern,
2. DEFINITIONS AND UNITS To be able to describe a given radiation environment and to quantify its effects on electronics, we must first introduce new sets of definitions and units specific to this field: • the first group of definitions is associated with the properties of the particle itself, • the second group describes the interactions of the particle with matter, • a third group describes the source of the particles. 2.1. DEFINITIONS RELATED TO PARTICLE PROPERTIES
2.1.1. Flux Flux O designates the number of particles or photons flowing through, or impinging upon a unit area per unit time. It is expressed in [cm'^.s'l].
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531
In the case of photons, the term flux is often called intensity. When the particles, or the photons, are distributed in energy, in other words, when there exists an energy spectrum, one distinguishes between integral flux O (> E) and differential flux O (E). The integral flux is the total flux of those particles whose energy is above a given energy level E, whereas the differential flux is the rate of change of the flux with energy, at a specified energy E. Therefore, the integral flux is obtained by integrating the differential flux above energy E, which can be written : 0{>E) = \0{E)'dE
(8.1)
E
Because of this definition, an integral flux is always a decreasing function of energy. Fluxes of space particles are usually described as "omnidirectional", (which means that particles often come from all directions) and are usually considered as isotropic. For isotropic fluxes, it is sometimes useful to introduce the concept of flux per unit solid angle dQ (expressed in steradian). The flux can then be expressed as a number of particles per unit area, unit time and unit solid angle. For isotropic fluxes, unit [cm-^.s-^] is equal to [471 cm-2.s"l.sterl]. In space, fluxes are generally expressed as an average over a period of time and over a given orbit rather than representing an instantaneous value. Let us note, however, that the electron and the proton fluxes of the Van Allen belts (see § 3.4.2) are not onmidirectional but spiral around the magnetic field lines (as illustrated in Fig. 8.8). This peculiarity can be very important in hardening satellite systems. 2.1.2, Fluence Fluence O is the number of particles flowing through, or impinging upon a unit area, over a longer period of time. It is a time-integrated flux and it is expressed in [cm-^]. Just as we did when defining fluxes, we distinguish between "integral", "differential" and "solid-angleresolved" fluences. In Fig.8.1 the differential and integral proton fluences encountered for a specific space mission have been plotted versus energy. 2.1.3. Energy and momentum The energy E of a particle or a photon is usually expressed in units of eV, keV or MeV. Another common expression is often used, namely energy per nucleon expressed in [MeV/amu], where amu is the atomic mass unit of the energetic particle (e.g. A(H) = 1, A(He) = 4, A(C) = 12). These definitions apply to the kinetic energy of the particle. For instance, 1 MeV expresses the kinetic energy an elementary charge gains in an electrostatic potential of 1 MV. Let us note that a particle should be considered as relativistic when its kinetic energy approaches the energy equivalent of its rest mass (e.g. electrons whose energy approaches 0.511 MeV or ions whose energy, expressed in GeV, approaches - A).
532
D. Braunig Integral and Differential Ruence (LEO,cinxilar,800kn%98^ 1.00E+13
1.00E+12 jDifferemiai hiuenoe in p^cnrr^zsmnev | ^
IH^^
1 '"^*.»^_^
1.00E+11 lU
^H
O 2 1.00E+10
^Kji
^^ "-^'^^^
1.00E-H)9
y-
—p^ 1.00E408
'—
\
Integral Ruenoe in p/an^2Br
1.00E+07 1.00E-01
I.OOE+OO
1.00E+01
1.00Ef02
PROTON ENERGY in MeV
Fig. 8.1 - The integral and differential proton fluences encountered for a circular orbit (800 km, inclination 98°), are plotted versus energy.
The momentum p of a particle is given by p=mv, where m is the mass of the particle and v its velocity. There exists a relativistic relationship linking energy and momentum, expressed as : P'C^^E'{E
+ 2'mo'C^)
(8.2)
where nio is the rest mass of the particle and c is the velocity of light. The energy of a photon is given by E = hv = hc/A., where h is Planck constant, v is the frequency of the photon and X is its wavelength. The momentum of a photon is given by: p = hv/c. 2.1A. Species The term "radiation species" includes photons and particles. Photons may differ in energy, whereas particles may differ in energy, mass and charge state. It is sometimes useful to identify a particle by its name, mass, number of charges within the nucleus and its charge state. For instance in expression 2 He
, He stands for Helium, A is the atomic mass (e.g. 4),
Z is the charge of the nucleus in units of elementary charge and 2+ indicates a doubly charged state. When an atom is totally ionized, and this is a common occurrence in space, then its charge Z is approximately equal to A/2 for all species except for Hydrogen, where Z/A equals 1. Some properties of particles commonly encountered in space are summarized in Tab. 8.2 below.
The Space Radiation Environment Species
Mass
Atomic mass
533 Rest mass
1 Electrons
9.1M0-31kg
0.000549
0.511 MeV
1 Protons
1.672-10-27 kg
1.007277
938.259 MeV
Neutrons
1.675-10-27 kg
1.008665
939.553 MeV
« 6.710-27 kg
4.0026
= 4GeV
1 Helium ions
Tab, 8.2 - Some properties ofparticles commonly encountered in space. 2.2. DEFINITIONS RELATED TO PARTICLE-MATTER INTERACTIONS
To properly describe how energy is transferred from an energetic particle to an absorbing material, intemational definitions have been introduced. Some of the definitions and units used in this chapter are briefly described next. A much more thorough description can be found in Chap. 9. 2.2.1. Exposure The exposure (E) describes the amount of ionization charges generated by a radiation species per unit mass of standard atmosphere. Exposure is expressed in units of Coulomb per mass [C/kg]. An older unit is the Roentgen (= 2.56-10-4 C/kg). 2.2.2. Radiation-absorbed dose The dose (D) is the energy absorbed per unit mass of the material being irradiated (e.g. silicon). A dose of 1 Joule per kilogram is called a Gray (Gy). A unit still commonly used in the literature is the rad, which is just 10"^ Gray. The nature of the material being irradiated should always be mentioned, e.g. 1 Gy(Si). For high-energy charged particles, most of the absorbed energy is consumed in ionization phenomen. There results that the terms "dose" and "ionizing dose" are sometimes used interchangeably. This approximation is of course not valid for neutral particles such as neutrons, or for low-energy ions. 2.2.3. Dose equivalent The dose equivalent (H) has the same meaning as the radiation-absorbed dose except that it introduces a qualifying and weighting factor q to account for the fact that human tissue responds differently to different ionizing radiation. The unit of dose equivalent is the Sievert (Sv). It has the same dimension as the radiation-absorbed dose since we can write: H = q.D, where q is equal: to 1 for electrons and y 's, to 10 for neutrons and protons and even to 20 for a's from radionuclides. The older unit of dose equivalent is the rem, which corresponds to an absorbed dose of one rad.
534
D. Brdunig
2.2.4. Linear Energy Transfer Other definitions are needed to express the amount of energy transferred per unit pathlength. The electronic stopping power dE/dx is a ratio which describes the energy which a charged particle loses through ionization phenomena when travelling through a material (e.g. silicon). The Linear Energy Transfer (LET) is just a different term for electronic stopping power with a dimension of MeV/|im. (The electronic stopping power is sometimes normalized to the specific mass of the absorbing material (i.e. dE/pdx), in which case it is expressed in MeV.cm^/g). The energy lost through ionization phenomena is calculated by using the well-known Bethe-Bloch equation. The slowing down due to ionization is accompanied by the generation of electron-hole pairs. The effects of such a generation will be covered in more detail in Chaps. 9, 10, 11 and 12. The LET value of a particle depends both on its atomic mass and on its energy. Thus, for a flux of various particles of various energies, we are led to define a LET spectrum, giving the number of ions corresponding to a given LET. Some preliminary examples of LET spectra will be given in § 4.2 of this chapter. 2.3. ADDITIONAL DEFINITION
Activity of a radioactive substance The activity (A) of a radioactive source is defined as the number of transitions occurring per second. The unit of activity is called the Bequerel (Bq) whose dimension is [s"l]. An older unit is the Curie (Ci), which equals 3.7-10^0 per second. 3. THE SPACE ENVIRONMENT 3.1. INTRODUCTION
The Earth is part of our Sun-dominated planetary system and it travels through the space of our galactic system. Due to its own magnetic field, the Earth can be considered, in a first approximation as a magnetic dipole. There exists, however, a number of interdependencies which link the Sun's activity, the galactic radiation and the shape and magnitude of the geomagnetic field. The Earth is, to a large extent, protected from outside radiation by its magnetic field. This field acts as a shield against the solar wind (see below) and creates a cavity called the geomagnetic cavity, illustrated in Fig. 8.2. The field deflects also the cosmic rays, i.e. energetic ions coming from outer space, as we shall see in § 3.5. Spacecrafts (satellites, launchers, space stations, space probes, etc.) are exposed to the following types of radiation: • • • •
solar radiation, mainly solar flare protons and solar wind particles, galactic cosmic rays, protons and electrons trapped in the radiation belts, Bremsstrahlung due to the scattering of energetic electrons in the spacecraft material.
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535
MAGNETOPAUSe
DAYSIDECUSP
DAYSIDECUSP
MAGNETOPAUSE CURRENTS
PLASMASPHERE
Fig. 8.2. - Illustration of the geomagnetic cavity (for more detail see [11]).
The origin of these contributions - except for Bremsstrahlung - will be discussed in the following paragraphs in more detail. Models currently used to quantify the amount of radiation received by a spacecraft will be presented in Sect. 4 and some examples will be given. 3-2. S O L A R RADIATION
The solar radiation encountered in space originates from the solar wind and from solar flare events. The solar wind is due to the constant emission, by the sun, of low-energy particles consisting of protons (91.2%), singly and doubly charged He ions and a very small amount of electrons whose energy is less than 500 eV. Because of the Earth's magnetic field, solar wind particles cannot come close to the Earth except through the polar regions [1], Solar Flares are bursts of radiation strongly correlated with the cycles of sun spot activity at the Sun's surface. Due to their large fluxes and high energies, the particles emitted during a solar flare constitute a severe problem for a spacecraft but on a probabilistic basis only. For instance, an intense period of solar activity occurred in March 1991 and triggered a sequence of major effects, including the generation of a second inner radiation belt around the Earth, functional anomalies in satellites, and power surges in electrical power grids on the ground [2]. The cycle of growing and vanishing sun spot groups on the Sun's surface has a duration of approximately 22 years, with a reversal of the magnetic fields at the Sun's surface every 11 years. During this cycle, fluctuations of the magnetic fields take place in the upper layer of the solar atmosphere. A consequence of these fluctuations is an outpouring of X-rays, radio waves and UV-radiation as well as an emission of solar wind plasma and energetic particles. These photons and particles are emitted during solar flares and solar storms and their fluxes vary over several orders of magnitude (see for instance [3]).
536
D.
Brdunig
There are anomalous large (AL) solar flares, which are rare, last for periods of up to 8 hours and contribute most of the high-energy solar particles. Ordinary (OR) solar flares occur very frequently, last for a few minutes and do not contribute significantly to the overall solar flux.
I
r
1955
n—r
1965
1980 August 1972
Fig..8.3 - Illustration of solar cycles. Both the number of sun spots (—) and the proton fluences due to solar flares (histograms) are represented (see text) [39].
Both AL events and OR events are illustrated in Fig. 8.3 for the last three complete cycles (19, 20, 21). The continuous line of Fig. 8.3 is a plot of the number of solar spots observed during these cycles. The peaks correspond to the fluence of emitted particles (mostly protons) observed during AL or OR flares. Seen from the vicinity of the Earth, one distinguishes between simultaneous and delayed solar events, depending on the velocity of the individual constituents. The first group (travel time 8.3 minutes) consists of visible light, radio waves, UV-light and X-rays, while the second group (travel time 20 minutes to 50 hours) consists of solar flare and solar storm particles. When entering the Earth's magnetosphere, the charged particles produce large disturbances of the geomagnetic field lines. The amount of exposure of a spacecraft to solar radiation is of course strongly dependent on the orbital parameters of the craft. The ranges in flux and energy of the particles encountered in the solar wind and in solar flares is illustrated in Fig. 8.11. 3.3. GALACTIC COSMIC RAYS
There exists a galactic cosmic radiation in deep space. It is distributed uniformly and is considered as omnidirectional, although for Low Earth Orbits (LEO), the shadow of the Earth plays a significant role in its distribution. The main constituents of this radiation are protons (85%), He ions (14%) and about 1% heavier nuclei. The abundance of heavy nuclei is similar to the universal distribution of elements.
537
The Space Radiation Environment
However, hydrogen and helium have a smaller relative abundance in the galactic cosmic rays whereas the nuclei of the iron group as well as lithium, beryllium, boron and other nuclei with an odd Z number are enhanced in flux, as visible in Fig. 8.4 [5].
o
> 0^
Element-atomic number Fig. 8.4 - Relative abundance of the elements contributing to cosmic rays compared to universal abundance.
I > Fig. 8.5 - The differential fluxes of the most abundant constituents of cosmic rays are plotted versus energy, (after [5]).
in
10^
10^
10^
Kinetic energy in [MeV/amu]
10^
538
D. Brdunig
The energy of cosmic rays extends over an extremely vast range, from 10 to 10^^ MeV. The differential energy spectra of the cosmic ray ions in the neighborhood of the Earth exhibit a maximum at 1 GeV/nucleon. Outside the geomagnetic field, the flux of cosmic ray particles is approximately 4 nuclei per cm^ per second, mainly consisting of protons [4]. Because of the interactions with the solar wind and with the interplanetary magnetic field, this flux is reduced at the lower energy end and its fluctuations are related to the solar activity. Figure 8.5 presents the differential spectrum of four of the most abundant nuclei [6, 7]. The reason for the increase in H and He ion fluxes at low energies is due to the spallation of heavy ions into lighter ones. The integral flux of cosmic rays is represented as a function of energy in Fig. 8.13. 3.4. TRAPPED RADIATION IN THE EARTH'S MAGNETOSPHERE
3.4.1. A brief description of the Earth's magnetic field In a first approximation, one can consider that the Earth acts a magnetic dipole (at least within a distance of about 5 Earth radii). The dipole momentum [M] expressed in [Tesla.cm^] can be written^^^: M = /?f • 0304 • 7 0 " "^ = 7M -10^^ T.cm^
(8.3)
where R^ is the Earth's radius, which equals 6.371.10^ cm, [8]. The magnetic field and the associated field lines can be described by the following set of equations using polar coordinates:
5a)=
M ^4-3cos^ (X)
• R{0f
and
(8.4)
cos^ (X)
2 R{X)----R(0)-cos (X)
(8.5 a)
or
L{X)^
R{X) Re
= Rio) ——cos 2(,^ U)
(8.5 b)
Re
Here B{X) is the magnitude of the magnetic field, which remains tangent to a field line, as a function of A., X is the geomagnetic latitude and R(0) is the distance at which the field line intersects the equatorial plane. This allows an R(A.) representation of the geomagnetic properties of the Earth, as illustrated in Fig. 8.6. Parameter L was introduced by McHlwain in his B-L space [9, 10]. L is a reduced form of the locus of the magnetic field lines and is equal to the geocentric distance at which the shell intersects the magnetic equator. It is measured in units of Earth radii Re. ^^^ In the MKS unit system, M = 8.27.1022 A.m^.
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539
North
0 1 South
2
3 4 5 Distance in Re
Fig. 8.6 - Illustration of the geomagnetic field lines and ofparameter used to describe them.
At constant L, the value of B varies from a minimum at the magnetic equator to a maximum where the field lines intersect the surface of the Earth. The B-L space is a fixed and idealized coordinate system which does not take into account any temporal change. 3.4.2. The trapping and filtering properties of the magnetosphere The shape of the magnetic field lines has several important consequences which we detail next [11, 12]: • the geomagnetic field acts as a filter for certain types of charged particles entering the Earth's magnetosphere, • there are allowed cavities where particles are trapped and, conversely, forbidden zones where particles of different properties are rejected, • the polar regions are more easily accessible than the equatorial plane for particles of identical properties, • the properties of the geomagnetic filter function enable electrons and protons to form a stable and structured radiation belt, where the abundance of these particles is significantly greater than outside the magnetosphere, • the same filter function rejects a large portion of heavy charged ions whose energy is insufficient to penetrate the inner regions of the magnetosphere (for more details see § 3.5),
540
D, Brdunig
• the stable trapping of electrons, protons and light ions results in a structured set of radiation belts (Van Allen belts), illustrated in Fig. 8.7, which is stationary and whose fluctuations in density and shape is mainly caused by variations in solar activities. Thanks to this trapping and filtering effect, the Earth is relatively protected from outside radiation phenomena. ^
Solar wind Geomagnetic tail
Radiation belts
Fig. 8.7 - Cross-sectional view of the geomagnetic cavity featuring: the solar wind and the associated shock wave, the magnetopause and the radiation (Van Allen) belts (after [42],
3.43. Some properties of the Van Allen Belts The shape of the geomagnetic field and especially the regions of maximum magnetic field strength near the geomagnetic North and South poles determine the motion of the trapped particles. The particles of velocity v are submitted to a force given by F = q v A B . They spiral around and move along the magnetic field lines and are reflected at "conjugate mirror points", as illustrated in Fig. 8.8. The location of these mirror points on a field line depends on the energy of the particle itself. Moreover, the particles drift in a direction perpendicular to the field lines. As a result of their opposite charges, electrons drift Eastward whereas protons drift Westward (azimuthal drift). Typical features of the motion of energetic particles at low altitudes are shown in Tab. 8.3. Particle
Gyration radius
Gyration period
Bounce period
Azimuthal drift period
velocity p=v/c
10 MeV protons
50 km
0.01s
0.5 s
4min
0.2
1 MeV electrons
0.6 km
10|LIS
0.1s
lOmin
0.9
Tab. 3 ' Some characteristics of the motion of particles trapped at low altitudes (^1.5 R^)
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541
Magnetic field line
Fig. 8.8 - Spiralling and bouncing behaviour of electrons and protons within the geomagnetic field.
The trapping zones are structured into an "inner" and an "outer" belt as illustrated by Figs. 8.9 a, b, c. Protons are entirely trapped within the inner belt and possess energies ranging from 0.5 MeV to more than 400 MeV. In the equatorial plane, their energy decreases conversely with the radial distance from the Earth as shown in Fig.8.9 a. Electrons have energies of up to 7 MeV in the outer belt with fluxes exceeding those of the inner belt (E < 5 MeV). Due to the action of the Sun we have to distinguish between the day and night sides. There are strong variations in shape and population of the belts including diurnal cycles and the 11-year solar cycle as shown in Tab. 8.4 for low altitudes. The outer Van Allen radiation belt typically extends from the plasmasphere to somewhat beyond the geosynchronous altitude, depending on the geomagnetic activity. SOLAR MAX
SOLAR MIN
1
Electron intensities
higher
lower
1
Proton intensities
lower
higher
Tab. 4 - Influence of the solar activity on low-altitudes particles fluxes.
3.4.4. Two anomalous regions: The SAA and the Horns For Low Earth Orbits (LEO) (typically above 500 km) there exist two important contributions to the overall radiation level due to the shape of the geomagnetic field lines: the South Atlantic Anomaly (SAA) and the polar Horns [11]. The SAA is due to the fact that the geomagnetic axis is titled by approximately 11° off the Earth's rotation axis and is displaced by about 500 km towards the Westem Pacific as illustrated in Fig. 8.10. This results in stronger field values at LEOs and correspondingly higher electron and proton fluxes in this geographic region (above Brazil). This is of particular significance for space electronics orbiting on an LEO with a large inclination since the radiation flux is larger by several orders of magnitude in this region compared to other regions of the mission. An illustrative example was given by the Hubble space telescope.
542
D . Brdunig Geostationary orbit Large space telescope ^4
•o
"£ o Q. LU
O "Q. O to 10
20
30
altitude in 10'km
a) electrons
geomagnetic axis
>so(Qr min ' solar max
protons
geomagnetic axis 4
b) Electrons with E > 1 MeV
c) Protons with E > 10 MeV
Fig. 8.9 - Illustration of the Van Allen belts, a) Schematic cross section of the Van Allen Belts in the equatorial plane. Several integrated fluxes of electrons and protons are plotted as a function of altitude. The proton distribution in the inner zone and the dual distribution of the electrons (inner and outer zones) are clearly visible. An LEO and a geostationary orbit are indicated (after [37]). b) Set of isoflux contours describing the trapping belts for electrons of energy > I MeV. c) Idem for protons of energy > 10 MeV.
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543
Before the 1993 retrofitting of the telescope electronics, every time the spacecraft went through the intense radiation belts of the SAA, the memories suffered from enhanced dark currents and their contents had to be corrected. The Homs at mid-latitude levels are tails of the outer electron belt coming very close to the Earth because of the geomagnetic field. Figures 8.9 a and b illustrate this feature very clearly as well as Fig. 8.12 based on the application of the AE-2 model [13].
Fig. 8.10 - Origin and illustration of the South Atlantic Anomaly (after [42]).
3.5. CUT-OFF MECHANISMS FOR PROTONS AND HEAVY IONS
The penetration of charged particles into the geomagnetic space, in particular in the case of solar flare protons and cosmic heavy ions, is hampered by the shielding properties of the Earth's magnetic field. The more energy a particle possesses, the closer to the Earth it can get. A quantitative description of these "cut-off mechanisms uses a specific term, the rigidity P, whose value determines how easily a charged particle is reflected by the geomagnetic field [8]. The rigidity P of a (relativistic) particle of energy E is given by:
P = pcA/Z
= —ylE^-\'2'm^'C^'E
(8.7)
In this expression m^ is the rest mass of the particle, A is the atomic mass unit (amu), Z is the charge of the nucleus and E is the energy of the particle expressed in MeV. The dimensions of P are [MV/amu]. In the case of protons, ratio A/Z equals 1. For heavier ions, A/Z equals 2 or is very close to it. For low-energy particles, the rigidity is proportional to E^^^ whereas in case of high energy it is proportional to E. We can calculate the distance (L) in units of Earth radii, at which an incoming particle of rigidity P (expressed in MV/amu) and moving in a meridian plane can come close to the Earth [42]. It yields: L=
15.9 10^
cos X
(8.8)
D. Braunig
544
The rigidity concept [14, 15] is used as well to describe the flux spectra of solar protons, for instance as: ^p{>P)
where Op
(8.9)
= ^Pf)-exp\
is the total proton flux for an event expressed in [# protons/cm^] and PQ is
characteristic of a particular solar event. As seen from Fig. 8.11, protons can reach a region corresponding to a geosynchronous orbit (i.e. L = 6 to 7) only if their energy exceeds 87 MeV (for ions 23 MeV/amu). Particles can reach Low Earth Orbits (LEO) only if their energies are greater than 3 GeV for protons, and 1 GeV/amu for ions. For LEO, in addition, atmospheric effects must be considered. But for a polar orbit of nearly 90° inclination, a much lower energy is needed to reach these regions. 12
III
10
H L 6
m\ IONS 10
a)
100
1000
Energy (in MeV for protons and MeV/amu for ions)
23 MeV/n 12 MeV/n
Fig. 8,11 - Illustration of the cut-off mechanism, a) illustrates the penetration depth of solar and cosmic particles in L coordinates (in units of Earth radii) as a function of energy (in units of MeV/amu (ions) and MeV (protons)), b) is a visualization of that cut-off effect giving the total energy required to penetrate the magnetosphere, (after [11]).
The Space Radiation Environment
545
3.6. ROLE OF THE ORBIT CHARACTERISTICS
Due to the shielding behaviour of the geomagnetic field, how satellite electronics will be affected depends on the orbit used. For instance, on a geosynchronous orbit (i.e. at an altitude of about 36 000 km), the main constituents of the encountered radiation are electrons in the 0.5 to 8 MeV range. Therefore, a complete shielding of the equipment is very difficult to achieve due to the generation of Bremsstrahlung by these high-energy electrons. Since the conmiunication satellites which are on such orbits have a mission duration of more than 7 years, the (total) absorbed dose can be high and the electronic components must be able to withstand these high accumulated dose values. Solar flares must also be taken into account on a statistical basis. For Low Earth Orbits (LEO), i.e. for altitudes ranging from 200 to 800 km, one must distinguish between equatorial and polar orbits. Because of the configuration of the geomagnetic field lines, particles encountered on polar orbits are primarily protons and highly energetic ions. Overall, the level of radiation encountered depends strongly on the altitude of the spacecraft since only a few hundred kilometers in altitude may cause large differences in flux due to the steep increase in proton flux at the tail of the inner Van Allen belt. Only a very accurate knowledge of the energy and flux distribution of the radiation species permits a reliable prediction of the radiation threat. Models currently available to describe the environment are discussed next. Let us note that an accurate prediction must also take into account the shielding effect of the equipment itself. 4. A SURVEY OF ENVIRONMENT MODELING AND MISSION MODELING The modeling of the radiation environment is important because it enables: 1) a calculation of the fluxes encountered by the spacecrafts, and 2) a forecast of the type of problems the electronic parts (especially the semiconductor components) will experience during a mission. 4.1.
MODELBSG THE E A R T H ' S RADIATION ENVIRONMENT
From the beginning of unmanned spaceflights, experimental data were gathered to get a comprehensive picture of the Earth's radiation belts. Due to limitations in measurement equipment and theoretical modeling, the description of the constituents of the Earth's environment and of their distribution had to be continuously improved. Today, the descriptions of the environment are still "stationary" ones, with a few exceptions, and for example do not illustrate the temporal changes due to variations in solar activities. Up to now, 8 models describing the Earth's environment have been successively established for electrons (AE-) as well as for protons (AP-). They are called AE-1, AE-2, AE-3, AE-4, AE-5, AE-5 projected (AE-5P), AE-6, AE-8 and AP-1, AP-2, AP>3, AP-4, AP-5, AP-6, AP-7, AP-8. Following the NASA-Publication NSSDCAVDC- A- R&S 91-29 by James I. Vette [13], a short tabular description of the individual models is given next (see Tab. 8.5). Figure 8.12 gives an illustration of data gathered in the AE-2 model.
546
D, Brdunig
The CREME model was developed at the Naval Research Lab to have a tool for predicting Single Event Upsets caused by cosmic rays. A series of reports [31 - 34] entitled "Cosmic Ray Effects on Microelectronics (CREME)" linked the Linear Energy Transfer (LET) of ions and its spectrum (number of ions versus LET) to the SEU sensitivity of individual integrated circuits. The CREME model describes the space environment and the radiation-component interactions leading to SEU and contains estimates of individual ion fluxes from protons up to Uranium^^\ It also takes into account reductions in flux intensity due to solar modulation, geomagnetic cut-off and the Earth's shadowing effect. The transport code includes heavy ion fragmentation and ionization losses for each element individually.
Fig, 8.12 - Illustration of the electron belts. Isoflux lines have been computed (using the AE-l model), for electrons of energy > 1 MeV.
90 80 70 60 50 40 30 20 K) 0 10 20 30 40 50 60 70 80 90 NORTH GEOGRAPHIC LATITUDE SOUTH
SOLPRO [35] is a simple statistical model predicting solar flares and is based entirely on satellite spectral measurements of the 20th solar cycle. Figure 8.13 sunmiarizes and illustrates the relative importance of each radiation species encountered in space.
(2)
Ions with atomic masses larger than that of Fe play no significant part.
The Space Radiation Environment
547
Van Allen electron belt Model
Range [Re]
Energy range [MeV]
AE-1
1.2-3
0.3-7
Omnidir. Integral Flux
[17]
AE-2
1.1-6.3
0.04-7
Solmax, SAA and Homs
[18]
AE-3
6.6
0.01-5
GEO, SOLMAX, SOLMIN
[19]
AE-4
3.0-11.0
0.04-4.85
Time var. SOLMIN, SOLMAX
[20-21]
1
AE-5
1.4-2.8
0.04-4
Inner zone, atmospheric cut-off
[22 - 23]
1
AE-6
1.2-11
0.04-5
Decay times, Starfish
[24]
No release
[25]
The most advanced model yet
[26]
AE-7 AE-8
1.2-4/3.011/2.4-3
0.04-7
Remarks
References
Van Allen proton belt Range [Re]
Energy range [MeV]
AP-1
1.17-3.15
30-50
Integral flux, solar cycle var.
[17]
AP-2
1.17-3.51
15-30
Integral flux, solar cycle var.
[17]
AP-3
1.17-2.9
>50
Integral flux, solar cycle var.
[17]
AP-4
1.17-4.6
4.0-15.0
Integral flux, solar cycle var.
[17]
AP-5
1.2-6.0
0.1-4
Variation, differential flux
[27]
1.2-4
4-40.0
Improved data
[28]
AP-7
1.15-3.0
50-500
Improved data
[29]
AP-8
1.15-6.6
0.1-400
Static model with solar cycle
[30]
Model
|AP-6
Remarks
References
Solar flares and cosmic rays I
Model SOLPRO
1CREME
Remarks
References
Solar Flares
[35]
Cosmic Rays
[31 - 34]
Tab. 8.5 - Models currently (1995) available describing trapped particles, solar protons and cosmic rays. 4.2.
EXAMPLES AND MISSION CONSIDERATIONS
To illustrate the possibilities of modeling, we present next three examples which have been calculated using a software called Space Radiation [36], based on the CREME model. These examples are: the geosynchronous orbit in the best and the worst-case parking position, the impact of solar activity for a circular LEO and the influence of orbit inclination on the LET spectrum for an LEO.
D, Brdunig
548
Solar wind protons 10
10
C/5
10
Trapped electrons
q
I—peak
Geostationary 10
Y Trapped Protons-peak
Solar protons Major flare
X! US
10
7-Year average
Galactic Cosmic rays
10-^
10^
10
10
1
10
10
10
10
Particle energy E [MeV] Fig. 8.13 - Integral spectra of particles encountered in space [37].
In Fig. 8.14 the electron fluence per cm^ and per day as a function of electron energy is shown for two locations, which represent the worst and the best-case conditions for a geosynchronous orbit. The best-case location is at L = 7,70° West, and the worst-case is found for L = 6.6, 160° West. The radiation exposures between the two locations differ by about one order of magnitude for intermediate energies (because of the altitude, the protons play no part and only electrons contribute to the absorbed dose).
The Space Radiation Environment
1.00E+13
^ -d
1.00E+12
e
1.00E+11
W A N
^
1.00E+10 1.00E+09 1.00E-I-08
J3
1.00E-I-07 1.00E+06 1.00E+05 c3 ;-4
<
1
geosynchronous Orbit
•e-
X
549
^
IN
^ ^
1
^ [
1 1
P
1
|WORSTCASE:160"W,L=6.6|
£>.>—
|BESTCASE:70*W.L=7|
I —
^
1
)
N
i
1.CX)E+04 2
3
4
Energy [MeV]
Fig. 8.14 - The (averaged) integral electronfluxesdetected on a geosynchronous orbits for best and worst-case parking positions^ versus energy (after [36]).
In Fig. 8.15 the daily averaged integral electron flux encountered on a circular orbit (600 km) with a 60° inclination, for low solar activity, is compared to the electron flux encountered for maximum solar activity. The electron integral fluence (per cm2) averaged over a period of one day shows a pronounced difference, particularly at low energies as already indicated in Tab. 8.4, because of an enhanced electron fluence for Solar Max periods. Finally, the dependence of LET spectra encountered for LEO's on the inclination angle of the orbit are indicated in Fig. 8.16 as an example which will be described in more detail in Chap. 10. Whereas the fluences for low (BO'') and medium (60°) inclinations are quite similar, the polar orbit (90°) exhibits much more particles per unit area and unit time due to the weaker shielding capabilities of the geomagnetic field at that location. The sharp cut-off at about 500 MeV.cm^/g reflects the strong contribution of protons in the low LET range, since their LET never exceeds this value. 5. SUMMARY AND CONCLUSION More and more items of electronic equipement are being used in space aboard spacecrafts. While in space, they are submitted to a wide array of particles, such as: electrons and protons of the Van Allen Belts, protons and heavy ions emitted during solar flares and protons and heavy ions which make up the cosmic rays. The performance of on-board electronic components and equipment may degrade quickly if precautions are not taken.
550
D. Brdunig
Fig. 8.15 Daily averaged integral electron flux encountered on a circular orbit (600 km, 60°) versus energy (after [36]).
• -e-
1E-^02 1E-02
lE-OI
lE-^OO
1E^01
Energy [MeV] 1.00E+07
q ^ W
90° 1.00E+06 60°
A
-eo
|30 ol
1.00E+05
13 ^S
N
1.00E+04 r r\ rVsh1- /en/ Pir OliOuiai v^i uii ^\j\M/wr\iiiy
Q 1.00E+03 1.00E+00
1.00E+01
1.00E+02
1.00E+03
LET [MeV.cm2.g-i] Fig.8. 16 Daily integralfluenceof particles encountered on a circular orbit (600 km) for different inclinations versus LET. (after [36]).
The Space Radiation Environment
551
The need for an accurate knowledge of the Earth's radiation environment has thus become imperative. Computer models of environment, such as AP-8 for protons and AE-8 for electrons, developed by NASA, have been widely used to predict the radiation threat for a given space mission. A severe disadvantage of these models is that they describe a stationary situation that does not correctly reflect the real situation. Therefore activities are underway at NASA and ESA to study the dynamic behaviour of our radiation environment and to improve the accuracy and the reliability of the predictions. ACKNOWLEDGEMENTS The author wishes to thank his colleagues at the Hahn-Meitner-Institut for fruitful discussions and the editors G. Barbottin and A. Vapaille for their thorough review. LIST OF REFERENCES [I] King J.H., J. Spacecraft 11, No. 6, (1974), p.401. [2] Shea M.A.,D.F. Smart, J.H. Allen and D.C. Wilkinson, "Spacecraft Problems in Association with Episodes of Intense Solar Activity and Related Terrestrial Phenomena During March 1991", lEEE-NS 39,6 (1992), p. 1754. [3] Adams J.H. Jr and A. Gelman, IEEE NS-31,6, (1986), p.l212. [4] Byrne F.N.," A Survey of Solar Flare Phenomena", Space Set. Rev. 3,(1964), p.319. [5] Petersen E.L., "Basic concepts on Single Event Upsets" lEEE-NSREC Short Course, July 1983, Gatlinburg/Tennessee. [6] Adams J.H. Jr., "The Natural Environment Inside Spacecraft", lEEE-NS 29,6, (1982), p. 2095. [7] Adams J.H. Jr., "The Variability of Single Event Upsets Rates in the Natural Environment", lEEE-NS 30, 6,(1983), p.4475. [8] Stormer C , Polar Aurora, Oxford University Press, London,1955. [9] Mclllwain C.E." Magnetic Coordinates" in Radiation Trapped in the Earth Magnetic Field, Ed. B.M. McCormac (Reidel,Holland), (1966), p. 45. [10] Mclllwain C.E., J. Geophys. Res. 66, No.ll, November 1961, p. 3681. [II] Stassinopoulos E.G., " Radiation Environments of Space", lEEE-NSREC: Short Course, Reno, Nevada, July 16, 1990. [12] Haffner J.W., Radiation and Shielding in Space, Academic Press, 1967. [13] Vette J.I.," The NASA/National Space Science Data Center Trapped Radiation Environment Model Program (1964-1991)", NSSDCAVDC-A-R&S 91-29, National Space Science Data Center, Greenbelt,Maryland, November 1991. [14] Rosen A., "The Dynamics of the Outer Zone", Space Science, Ed.D.P. Le Galley (Wiley, New York), (1963), p. 275. [15] Webber W.R., "Solar Flare Proton Data", Nucleonics 21, (1963),p. 154. [16] Nichols D.K., "Trends in Electronic Parts Susceptibility to Single Event Upset Space Station Environment", JPL D-2767, September 1985. [17] Vette J.I.,"Models of the Trapped Radiation Environment", Vol.1: Inner Protons and Electrons NASA SP3024, Vol.1, 1966. [18] Vette J.I., A.B.Lucero and J.A.Wright, "Models of the Trapped Radiation Environment", Vol.2: Inner and Outer Zone Electrons, NASA SP-3024, Vol.2, 1966. [19] Vette J.I., A.B. Lucero and J.A.Wright, "Models of the Trapped Radiation Environment", Vol.3: Electrons at Synchronous Altitudes, NASA SP-3024, Vol.3, 1966.
552
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Brdunig
[20] Singley G.W. and J.I. Vette, "The AE-4 Model of the Outer Radiation Zone Electron Environment", NASA GSFC National Space Science Data Center, NSSDC 72-06, August 1972. [21] Singley G.W. and J.I. Vette, "A Model Environment for Outer Zone Electrons", NASA GSFC National Space Science Data Center, NSSDC 72-13, December 1972. [22] Teague, M.J. and J.I.Vette, "The Inner Zone Model AE-5 ", NASA GSFC National Space Science Data Center, NSSDC 72-10, November 1972. [23] Teague, M.J. and J.I.Vette, "The Use of the Inner Zone Electron Model AE-5 and Associated Computer Programs", NASA GSFC National Space Science Data Center, NSSDC 72-11, November 1972. [24] Teague M.J., K.W.Chan and J.I.Vette "AE-6: A Model Environment for Trapped Electrons for Solar Minimum", NSSDCAVDC-A-R&S 76-04, National Space Science Data Center, Greenbelt, Maryland, March 1976. [25] Chan K.W., M.J.Teague, N.J.Schofield and J.I.Vette," Modeling of Electron Time Variations in the Radiative Belts", American Geophysical Union Geophysical Monograph 21, Quantitative Modeling of Magnetospheric Processes, June 1978. [26] Vette J.I.," The AE-8 Trapped Electron Model Environment", NSSDCAVDC-A- R&S 91-24, National Space Science Data Center, Greenbelt,Maryland, November 1991. [27] King J.H.," Models of the Trapped Radiation Environment", Vol.4: Low Energy Protons, NASA SP- 3024, Vol.4, 1967. [28] Lavine J.P. and J.I.Vette," Models of the Trapped Radiation Environment", Vol.5: Inner Belt Protons, NASA SP-3024, Vol.5, 1969. [29] Lavine J.P. and J.I.Vette," Models of the Trapped Radiation Environment", Vol.6: High Energy Protons NASA SP-3024, Vol.6, 1970. [30] Sawyer D.M. and J.I.Vette," AP-8 Trapped Proton Environment for Solar Maximum and Solar Minimum",NSSDC 76-06, National Space Science Data Center, Greenbelt, Maryland, December 1976. [31] Adams, J.H., R.Silberberg and C.H.Tsao, "Cosmic Ray Effects on Microelectronics",Part l:The Near Earth Particle Environment", NRL Memorandum Report 4506, August 25,1981. [32] Adams J.R.," Cosmic Ray Effects on Microelectronics", Part 4, NRL Memorandum Report 5901, December 31, 1986. [33] Adams J.H., J.R.Letaw and D.F.Smart," Cosmic Ray Effects on Microelectronics", Part 2: The Geomagnetic Cutoff Effects, NRL Memorandum Report 5099, May 1983. [34] Tsao C.H., J.H.Silberberg, J.H.Adams and J.R.Letaw," Cosmic Ray Effects on Microelectronics", Part 3: Propagation of Cosmic Rays in the Atmosphere, NRL Memorandum Report 5402, August 1984. [35] Stassinopoulos E.G., "SOLPRO: A Computer Code to Calculate Probabilistic Energetic Solar Proton Fluences", Rep. NSSDC No. 75-11, National Space Science Data Center, Greenbelt, MD, April 1975 [36] SPACE RADL\TION, Users Manual, DOS-Version 2.0,Trademark of Severn Communications Corporation, 1992. [37] Holme-Siedle A. and Adams L. The Radiation Design Handbook, ESA PSS-01-609, Draft, 1989 [38] McGuire at al. "Solar flare particle fluence during solar cycles 10-20 and 21", 18th Int. Ray Conf., Vol.4,1983, p.66 Recent books on the effects of radiation on electronic components [39] [40] [41] [42]
Ma and Dressendorfer, Ionizing Radiation effects in MOS Devices and Circuits, Wiley, 1989. Messenger and Ash, The effects of Radiation on Electronic Systems, Van Nostrand, 1992. Holmes - Siedle and Adams, Handbook of Radiation Effects, Oxford, 1993. Boudenot J.D., L'environnement spatial. Collection "Que sais-je ?", Presses Universitaires de France, 1995 (in French) [43] Messenger and Ash, Single Event Phenomena, Chapman & Hall, 1997