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CRRES in review: space weather and its effects on technology
Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1709 – 1721
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CRRES in review: space weather and its e+ects on technology D.H. Brautigam ∗ Air Force Research Laboratory, Space Vehicles Directorate, Hanscom AFB, MA 01731, USA
Abstract The Combined Release and Radiation E+ects Satellite (CRRES) was a unique mission which has provided valuable lessons about the space radiation environment and its e+ects on technological systems in space. The tremendous value of CRRES resides in the fact that it 2ew not only the technologies to be space-tested, but also an extensive suite of instruments to accurately specify the damage-causing radiation environment. CRRES was launched into a geosynchronous transfer orbit on July 25, 1990, 1 yr following the maximum of solar cycle 22, and returned data for approximately 14 months. It was 7rst exposed to 8 months of a relatively quiet magnetosphere, followed by a very active 6 month interval initiated by the much documented March 1991 storm. This large magnetic storm, accompanied by a solar proton event, was responsible for creating a temporary proton and electron belt within the typically benign slot region. The dynamic radiation environment was responsible for a number of observable total dose e+ects, charging=discharging phenomena, single event e+ects, and assorted anomalies within the spacecraft instrumentation and technology tested. This exceptional combination of technologies, instrumentation, and dynamic radiation environment will be reviewed, with attention given to lessons learned and how CRRES has changed c 2002 Published by Elsevier Science Ltd. All rights our perspective on magnetospheric radiation hazards. Crown Copyright reserved. Keywords: CRRES; Radiation belts; Radiation environment; Radiation e+ects on electronics
1. Introduction In the nearly 10 years since the Combined Release and Radiation E+ects Satellite (CRRES) was launched on July 25, 1990, “space weather” has become a much more widely used term among satellite mission planners, operators and design engineers. Space weather may be de7ned as the highly variable space radiation environment which originates at the Sun’s surface, and extends to and includes the Earth’s magnetosphere. Although space weather is driven by the Sun, its e+ects ultimately reach the surface of the Earth and produce adverse e+ects on satellite operations, space and ground-based communication and navigation systems, high altitude manned 2ight, and electrical power grid systems. The CRRES mission began in response to the increasing frequency of single event upsets (SEUs) recorded by many of ∗
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the operational satellites being 2own. At the time (as is still the case) most satellites lacked the environmental sensors to con7dently establish a causal relation between the environment and the reported anomalies. CRRES aimed to rectify this situation by launching a test bed of state-of-the-art technologies along with a suite of particle detectors capable of measuring the radiation environment. Speci7cally, the space radiation e+ects component of the CRRES program, space radiation e+ects (SPACERAD), was designed to (Gussenhoven and Mullen, 1993): (1) measure SEUs and total dose degradation of microelectronic technologies; (2) make in situ measurements of the radiation environment to be correlated with the measured responses of the various onboard technologies; (3) measure the radiation response and annealing characterizations of parts identical to those scheduled for space2ight; (4) model the response of selected devices to the total dose, dose rate, and SEU e+ects anticipated in space;
c 2002 Published by Elsevier Science Ltd. All rights reserved. 1364-6826/02/$ - see front matter. Crown Copyright PII: S 1 3 6 4 - 6 8 2 6 ( 0 2 ) 0 0 1 2 1 - 9
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(5) develop algorithms to relate space performance of microelectronic components to ground test procedures; (6) update existing radiation ground test guidelines required to more accurately simulate the space behavior of microelectronic devices; (7) space-qualify advanced microelectronic technologies for use in operational DoD satellite systems; and (8) update the static radiation belt models and develop the 7rst dynamic models of the high-energy particle populations. Three engineering experiments, including the microelectronics package (MEP), the internal discharge monitor (IDM), and the high-eGciency solar cell panel (HESP), were 2own to quantify the space radiation e+ects on various technologies. The suite of environmental instruments included a number of detectors designed to measure charged particle populations ranging from thermal plasma to 10 MeV electrons and several hundred MeV=Q ions, as well as electric and magnetic 7eld sensors (Gussenhoven et al., 1985; Vampola, 1992). The research papers generated by a satellite mission of this complexity, cover a wide range of topics from the impact of space radiation on microelectronics devices to the theoretical modeling of radiation belt dynamics. Since the lessons reaped from the mission are of such broad scope, the intent here is not to provide an exhaustive summary of all the detailed knowledge gained, but to present an overview of the mission, highlight some of its key 7ndings, and point to the original papers where the interested reader may 7nd complete details. A brief pre-launch history of CRRES is presented in Section 2. Section 3 surveys the general features of the electron and proton population, and then discusses e+orts to model the dynamics and to create statistical models for the engineering community. Section 4 introduces the three engineering experiments and highlights key lessons gleaned from them. Finally, concluding remarks are presented in Section 5.
2. Pre-launch history The nearly decade long evolution of the CRRES program, from conception to launch, is well documented by Liebowitz (1992), and is brie2y summarized here. CRRES grew out of the research and development Radiation Satellite (RADSAT) program 7rst conceived in 1981 by the Air Force Geophysical Laboratory, now the Air Force Research Laboratory. RADSAT’s primary goal was to make extensive measurements of the charged particle environment in order to improve existing models, with a secondary goal to measure the degradation and soft error rate of various devices in space. In 1982, the Air Force RADSAT program was transformed to the joint Department of Defense (DoD)=NASA mission named CRRES. NASA managed the chemical release component of the mission, and the Air Force managed
the SPACERAD component. Initially, CRRES was scheduled to be launched from the Space Shuttle. As of late 1985, the delivery of the payload was scheduled for August 1986 with a shuttle launch date of July 1987. When the shuttle Challenger exploded in January 1986, the CRRES program was thrown into disarray. With large uncertainties and delays looming, support for ground testing and SEU modeling was halted by the end of 1986. With the Space Shuttle schedule derailed, the decision was made to launch CRRES on an Atlas=Centaur, with a new launch date set for June 1990. This launch delay had a negative impact on one of the primary mission goals which was to space-qualify and model the radiation e+ects on DoD state-of-the-art space system technologies. Since the payload was virtually in place by 1986, and new technologies were emerging at a rapid pace, the delay meant that dated technologies would be 2own. CRRES was 7nally launched on July 25, 1990 from Cape ◦ Canaveral into a highly elliptical orbit with an 18 inclination, a 350 km perigee and 36 000 km apogee. Its orbital period of ∼10 h enabled CRRES to pass through the heart of the inner and outer radiation belts 4 times per day as illustrated in Fig. 1. The ground command station permanently lost communication with CRRES on October 12, 1991, as the result of premature battery failure. This abbreviated lifetime negatively impacted the mission in two ways. One, the mission goal to obtain a statistically signi7cant data set of particle 2uxes for model development was compromised, and two, many devices which were designed against high accumulated doses did not receive suGcient dose to test their limitations. Despite these shortcomings, the CRRES mission returned a number of invaluable lessons that will be discussed in the remainder of this paper. 3. Space radiation environment Underestimating the space radiation environment in system design phase can lead to degraded system performance and=or early system failure. In the opposite extreme, overestimation can lead to excessive shielding weight, reduced capability, and increased cost. The successful design of cheaper and smaller, yet reliable and survivable space systems, cannot be reached without improved speci7cation of the radiation environment in which they must operate. A primary goal of the CRRES=SPACERAD mission was thus to improve existing models of the radiation environment. Fig. 2 presents the solar–terrestrial activity during the CRRES epoch within the context of the prevailing solar cycle. Solar cycle 22 spans the interval September 1986 to May 1996, and following a relatively short rise time (∼3 yr) in comparison to its declining phase (∼4:5 yr), solar activity reaches its maximum in July 1989. Relative to sun spot number (panel a), solar activity remains at a plateau from mid-1989 through the end of 1992 (∼2:5 yr), with a corresponding minimum in the galactic cosmic ray background
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Fig. 1. Depiction of CRRES orbit traversing the inner and outer radiation belts as rendered by CRRESPRO and CRRESELE, respectively. The CRRESPRO and CRRESELE models are implemented within the Air Force space environment model package AF-Geospace (Ginet et al., 1995).
Fig. 2. Indices (7-day averages) for solar cycle 22: (a) sun spot number, (b) Ap index, (c) geosynchronous ¿ 10 MeV proton 2ux (cm2 s)−1 measured from GOES 7, and (d) geosynchronous ¿ 2 MeV electron 2ux (cm2 s)−1 from GOES 7. The CRRES epoch is marked by the heavy solid bar.
(Mullen and Ray, 1994). The 14-month CRRES mission is positioned within this window of high solar activity, and saw several periods of peak magnetospheric activity as measured by the Ap index (panel b). Using the ¿ 10 MeV proton 2ux measured at geosynchronous (panel c) as a measure of the frequency of solar proton events (SPE), it can be seen that the predominance of SPEs were clustered within the broad solar maximum plateau and that CRRES was exposed to a sizable fraction of these. It is also noted that relative to the last few solar cycle maxima (not shown), there were signi7cantly more SPE, though fewer magnetic storm sudden commencements (SESC, 1994). The ¿ 2 MeV electron 2ux measured at geosynchronous (panel d) is inversely related to solar activity (sunspot number), with a clear rise in intensity throughout the declining phase towards solar min-
imum. The average intensity for the ∼2 yr approaching solar minimum is a factor of ∼10 higher than that near solar maximum, although the sharp peak seen during the CRRES epoch associated with the March 1991 event clearly runs counter to this trend. 3.1. Survey of environment Although the CRRES environment sensor suite includes instruments designed to measure particle populations ranging from the lowest energy plasmas to energetic heavy ions, the main focus here will be on electrons from ∼0:5 to ∼10 MeV and protons from ∼1 to ∼100 MeV. Figs. 3 and 4 provide a survey of the 1:6 MeV electron 2ux and 57 MeV proton 2ux, respectively. The electron 2ux is pro-
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Fig. 3. Mission survey of 1:6 MeV electron 2ux (cm2 s sr keV)−1 plotted versus L and orbit number.
Fig. 4. Mission survey of 57:0 MeV proton 2ux (cm2 s sr MeV)−1 plotted versus L and orbit number.
vided by the high-energy electron 2uxmeter (HEEF) which measures energies between 0.8 and 8:0 MeV (Dichter et al., 1993), and the proton 2ux is provided by the Proton Telescope (PROTEL) which measures energies between 1 and 100 MeV (Violet et al., 1993). Quite evident in these 7gures is the major recon7guration of the radiation belts which occurred during orbit 588, on March 24, 1991. A large SPE was in progress when the arrival of a very large interplanetary shock suddenly compressed the magnetosphere resulting in the formation of a durable secondary electron and proton belt. Modeling e+orts to explain this sudden recon7guration are discussed in Section 3.3. Magnetic storms of this magnitude and impact (super storms) are a relatively rare event, occurring perhaps once per solar cycle (Bell et al., 1997). This super storm is the signature event of the CRRES mission because of its magnitude (minimum Dst ∼− 300) and the creation of the secondary proton belt (Mullen et al., 1991; Blake et al., 1992a, b) and electron belt (Blake et al.,
1992a, b; Vampola and Korth, 1992). The secondary proton belt was formed between L = 1:8 to 2:6RE and was predominantly ¿ 20 MeV protons. When this belt was 7rst created, the 57 MeV proton 2ux peak was at L∼2:6 RE but ∼40 days later had moved inwards to ∼2:34RE while decaying in intensity (Mullen et al., 1991). The energy at which the 2uxes peaked in the newly created electron belt was inferred from observed drift echoes to be ∼15 MeV. Unfortunately, there were no instruments onboard which were designed to make direct 2ux measurements of electrons in this energy range. Estimates of the inwards cross-L di+usion rates of the secondary electron belt are similar to those found for the 7ssion electrons following the Star7sh nuclear detonation (Korth and Vampola, 1994). The existence of these secondary electron and proton belts, formed in a typically benign region of space, have obvious implications in the assessment of radiation hazards on space systems, and should be included in any future radiation belt model. However, because of the in-
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frequency of these secondary belt formations, they will have to be treated in terms of occurrence probabilities as well as in terms of intensity decay time as a function of energy and altitude. As seen in Fig. 3, the outer zone electrons are a much more dynamic population than the inner belt, with frequent episodic events on the time scale of weeks. Also apparent is a relatively long-term (months) variation, with moderate 2ux intensities during the 7rst ∼300 orbits (∼4 months), followed by a very weak period of ∼250 orbits, and a 7nal very active period of ∼500 orbits (∼6 months) initiated by the March 1991 super storm. Not indicated by this plot is the spectral variation which is important for understanding degradation e+ects on microelectronic devices behind various shielding thicknesses. The average electron spectrum hardened following the March 1991 storm (orbit 588) as indicated in Fig. 5 where cumulative LOLET, HILET, and total dose behind 82:5 mils Al shielding (top) and 232:5 mils Al shielding (bottom), as measured by the space radiation dosimeter (SRD) (Gussenhoven et al., 1992), is plotted. For the thinner shielding, the LOLET dose (primarily from ¿1 MeV electrons) dominated the HILET dose (from ¿20 MeV protons) throughout the mission. However, behind the thicker shielding, the LOLET dose (primarily from ¿2:5 MeV electrons) sharply rose following the super storm and soon exceeded the HILET dose (from ¿35 MeV protons). The signi7cance of the spectral hardening will be alluded to in Section 4. Also highlighted in Fig. 4 are instances of solar proton penetration into the inner magnetosphere. The trace of 57 MeV solar proton 2ux (solid vertical blue streaks from L=7 downwards) during several orbits (588– 600) throughout the March 1991 storm indicates that the protons penetrated down to L∼2:6 where they merged with the newly formed belt. Other signi7cant solar proton events were observed during June 1991 (orbits 757–825) at this energy, with penetration down to L∼3:5. A similar plot for 10:7 MeV protons (Gussenhoven et al., 1994a) show numerous proton events where the penetration cuto+s are typically around 4 to 4:5RE . De7ning a solar proton event as a time interval when the 2ux of ¿10 MeV protons at geosynchronous (GOES 7) exceeds 10 p=cm2 =sr=s, permits identi7cation of 15 SPE during the CRRES mission. The depth to which energetic solar protons penetrate the magnetosphere has traditionally been calculated using Stormer theory which assumes charged particle motion in a dipole magnetic 7eld (Rossi and Olbert, 1970), and predicts that 26 MeV solar protons will penetrate to L∼7RE (Gussenhoven et al., 1994a). These predictions were generally expected to hold not only under quiet magnetic conditions, but also during storm conditions during which the total electric potential imposed across the magnetosphere (∼200 kV) remains small relative to the energy of the solar protons (¿10 MeV). CRRES observations showed that solar protons penetrated to well below the cuto+s predicted by Stormer theory for every large solar event, as well as many smaller ones. CRRES observations have demon-
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strated that 1 to 100 MeV solar protons can typically penetrate to L∼4, or as low as L∼2:5, with the cuto+ depth being a function of not only particle energy, but also storm-speci7c dynamics, thus invalidating attempts to 7x the penetration boundary for protons in this energy range (Gussenhoven et al., 1994a). Some of the events in which solar protons penetrate deep into the magnetosphere are associated with newly trapped populations with peak intensities at L = 2:3 to 4RE (depending on particle energy) and which persist from days to months. As pointed out earlier in relation to the March 1991 storm, once these transient belts are formed, subsequent radial transport may further move their peak to lower L (Mullen et al., 1991). Observation coupled with theoretical modeling (discussed in the following section) has led to the conclusion that solar protons play a more direct role in the trapped populations of the inner magnetosphere than previously thought. 3.2. Dynamic modeling Simulations of the March 1991 event show excellent agreement with the observed formation of the secondary electron belt (Li et al., 1993) and proton belt (Hudson et al., 1995, 1997, 1998). In these simulations, a model electromagnetic pulse accelerates a speci7ed initial particle distribution to the observed energies. The electron simulation shows that an initially trapped distribution speci7ed out to L = 10 is rapidly accelerated inwards, with electrons which drift in phase with the incident pulse reaching maximum energy. The simulation 7nds the peak of the distribution for the newly formed belt occurs at L∼2:85 (2.55) for ∼10 (20) MeV electrons. The simulations show that a major di+erence between the dynamics of the electron and proton belt formation is that solar protons, and not initially trapped protons, act as the primary source for the newly formed proton belt. Although the March 1991 storm was the most dramatic event of the CRRES mission, many episodes of rapid enhancements and gradual depletions of electron 2ux are clearly evident throughout the mission (Fig. 3). The rapid injection of ¿100 keV electrons deep into the inner magnetosphere has been observed for quite some time, yet modeling the radiation belt dynamics has typically been accomplished through gradual radial di+usion over months time scale, where the boundary conditions on 2ux and the di+usion rates are held 7xed. Brautigam and Albert (2000) have shown that the dynamics of 200 keV to ¡ 1 MeV electrons throughout an isolated storm in October 1990 can be reasonably explained in terms of a simple radial di+usion model using time-dependent outer boundary conditions (scaled by geosynchronous data) and Kp -dependent radial di+usion coeGcients spanning two orders of magnitude. For ¿1 MeV electrons, it appears that in addition to radial di+usion, some other non-adiabatic process must be assumed to explain the gradual acceleration of ¿1 MeV electrons well inside geosynchronous. An important point
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Fig. 5. Total, LoLET, and HiLET dose (krads in silicon) versus orbit number measured by the Space Radiation Dosimeter behind 82:5 mils Al shielding (top) and 232:5 mils Al shielding (bottom).
highlighted by this particular storm, is that although geosynchronous data is critical as an outer boundary condition, it is not a reliable monitor of the dynamics in the heart of the outer zone. Although the storm recovery phase 2uxes of ¿1 MeV electrons at geosynchronous show no appreciable increase relative to pre-storm levels, the 2uxes at L∼4 show a factor of ∼10 increase. The 1–100 MeV trapped proton population observed by CRRES has also been modeled using a radial di+usion equation with additional terms representing Coulomb collisions, charge exchange, and cosmic ray albedo neutron decay (Albert et al., 1998; Albert and Ginet, 1998). For these energies, the inner proton belt for L 6 1:7 represents a very stable steady state condition, remaining una+ected even by the large March 1991 event. The region for 1:7 ¡ L ¡ 3:0, however, is a much more dynamic region and departs dramatically from a steady state solution both before as well as after the March storm. 3.3. Statistical models The standard radiation belt models which have been used by the engineering community since the 1960s are the NASA solar minimum and maximum models (Ga+ey and Bilitza, 1994) for protons (most current versions, AP8-min, -max) and electrons (most current versions, AE8-min, -max). One of the primary goals of the CRRES mission was to evaluate and improve upon existing radiation belt models. Towards this end, the Air Force created a set of models mapped to the (L; B=B0 ) magnetic coordinate grid , including CRRESRAD (Gussenhoven et al., 1992) based on dose measurements from the Space Radiation Dosimeter, CRRESPRO (Gussenhoven et al., 1993) based on PROTEL proton 2ux, and CRRESELE (Brautigam et al.,1992; Brautigam
Fig. 6. Equatorial omnidirectional 2ux (cm2 s MeV)−1 of 41 MeV protons plotted as a function of L. Fluxes are from the CRRESPRO quiet and active model, and the NASA AP8-max model.
and Bell, 1995) based on HEEF electron 2ux. These have been summarized by Gussenhoven et al. (1996a, b). Despite the fact that the premature failure of CRRES compromised the statistical basis for the new set of models, the results from CRRES have highlighted the de7ciencies of the old NASA models (Gussenhoven et al., 1994b, 1996a, b, c), as have results from other more recent spacecraft (Daly et al., 1996). The CRRES models have also made important steps toward establishing the next generation of engineering models. The NASA solar maximum models AP8-max and AE8-max have been compared with CRRESPRO (Fig. 6) and CRRESELE (Fig. 7), respectively. Because 2uxes in the radiation belt models depend on a number of variables
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Fig. 7. Equatorial omnidirectional 2ux (cm2 s keV)−1 of 1:6 MeV electrons plotted as a function of L. Fluxes are from CRRESELE and NASA AE8-max models. Included from CRRESELE, are 2ux pro7les representing the mission average (Ave) and three out of the six levels of activity parameterized by the Ap 15 index (Low, Ap 15 = 5–7.5; Moderate, Ap 15 = 10–15; High, Ap 15 = 25–55).
(such as L; B=B0 , energy, pitch angle, and magnetic activity), it is diGcult, even when discussing omnidirectional 2uxes, to summarize the di+erences. Sample comparisons are presented here, and the interested reader may 7nd more details in the reviews by Gussenhoven et al. (1994b; 1996a, b, c). CRRESPRO represents the proton population in two distinct con7gurations, one de7ned by the pre-super storm environment (quiet model) and the second de7ned by the post-super storm environment (active model) which includes the secondary proton belt. Model 2uxes from the CRRESPRO quiet and active model for 41 MeV protons are compared to AP8-max in Fig. 6. At the peak of the inner proton belt (L∼1:6), AP8 is within a factor of ∼2 lower than CRRESPRO (which give the same values for both quiet and active models). The quiet CRRESPRO model shows lower 2uxes than AP8 over most of the region for L ¿1:8. The most signi7cant di+erence between CRRESPRO and AP8 is for L ¿1:8, where the active CRRESPRO model 2uxes are more than a factor of 10 higher at the peak of the newly formed secondary proton belt (L∼2:3). CRRESELE attempts to accommodate the dramatic variation seen in the outer zone electrons by introducing a number of quasi-static models in place of the single static solar maximum model o+ered by AE8-max. These quasi-static models are parameterized by the 15-day running average (with 1 day lag) Ap 15. The equatorial 2uxes for 1:6 MeV electrons from a sampling of the six Ap15-parameterized models are presented in Fig. 7. The 2ux pro7le for the lowest activity model (smallest Ap15) peaks at L∼4:5. As Ap 15 increases for more active models, the position of the pro7le peak moves inwards and increases in intensity. The pro7le at
highest activity is peaked at L∼3:3, where its 2ux intensity is three orders of magnitude greater than that of the lowest activity pro7le. Although the orbital electron 2ux pro7les vary dramatically (Fig. 3), a CRRES mission average of the environment is included in CRRESELE for a direct comparison to AE8-max which is a long-term average over solar maximum. As evident in Fig. 7, the 1:6 MeV electron 2ux from AE8-max is a factor of ∼10 higher than the CRRES average at near geosynchronous altitudes, with CRRESELE 2uxes being slightly higher for L∼3. CRRESELE represents a 7rst step in resolving the diGculties of representing an extremely dynamic population with a practical set of models. One diGculty is that the activity dependence of electron 2ux (independent of the chosen activity parameter) is both energy-dependent and L-dependent. A second diGculty, as has been suggested by Daly et al. (1999), is that the correlation between a speci7c activity parameter may vary with solar cycle phase. CRRESRAD includes quiet and active models de7ned in the same way as in CRRESPRO, and their comparison with the dose predicted by NASA models (NASA model 2uences must be propagated through shielding to yield dose) lead to conclusions consistent with those drawn from the direct comparison of CRRES and NASA 2uences. The difference of orbit dose between those predicted by NASA and CRRES models depend upon the particular orbit, shielding thickness, and magnetospheric activity level (Gussenhoven et al., 1994b). The reasons for the orbital dependence include the NASA models’ omission of the secondary proton and electron peaks in the slot region (L = 1:8—2.5), and also that AE8-max includes higher low-energy elec-
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Fig. 8. Sample single event e+ects (SEE) map giving contours of relative probability for SEE occurrence at 1000 –1050 km altitude.
tron 2uxes for L ¿3. The model di+erences also depend on shielding thickness because NASA spectral shapes are softer than those measured by CRRES which results in an overestimation of dose for thin shielding and an underestimation of dose for thick shielding. Finally, the comparison between NASA and CRRES dose predictions is activity dependent because whereas NASA has a single model to represent solar maximum, CRRES represents the same era by both a quiet and an active model. In addition to the CRRES trapped radiation dose and 2ux models, the CRRES=SPACERAD Heavy Ion Model of the Environment (CHIME) and the Single Event E+ects (SEE) maps have also been developed. CHIME gives di+erential energy 2ux and LET spectra for cosmic rays including all stable elements with energies from 10 MeV=nucleon to 60 GeV=nucleon, as a function of solar modulation for the interval 1970 to 2010 (Chenette et al., 1994). Cosmic ray sources include the galactic and anomalous cosmic ray components, as well as various solar 2are models. The SEE maps (Mullen et al., 1998) were created from CRRES=PROTEL data and the low altitude APEX satellite dosimeter data (Gussenhoven et al., 1995), and give the relative distribution of the ¿50 MeV proton population, which has been shown to be a good indicator of SEE-prone regions (Mullen et al., 1995). The SEE maps, which are mapped in geographic latitude and longitude, and extend in altitude from 350 km to 14 000 km, provide an accurate spatial distribution of the relative probability for space systems to su+er anomaly-producing SEEs. A sample SEE map is provided in Fig. 8, giving the relative probability for SEE occurrence at 1000 –1050 km altitude. At this altitude, the South Atlantic Anomaly (SAA) region is broadened considerably relative to that at lower altitudes.
4. Engineering results A primary objective of CRRES was to measure the performance of various technologies when exposed to the harsh space radiation environment. This objective was met by two approaches, including an extensive survey of reported anomalies from all onboard instruments (Violet and Frederickson, 1993), as well as individual analyses from the suite of engineering experiments. Here, an anomaly is de7ned as any observed instrumental response which is not programmed or commanded. Most environment-induced spacecraft anomalies may be traced to single event e+ects (SEE), di+erential surface charging, deep dielectric charging, or total dose e+ects. The history of anomalies from each instrument was correlated with the history of CRRES measurements of low-energy plasma, high-energy particles, and electromagnetic 7elds. It was found that as a group, the 674 reported anomalies correlate well with high levels of high energy electrons, implying that deep dielectric discharging was the major cause of CRRES anomalies (Violet and Frederickson, 1993). The engineering experiments were designed to quantify the deleterious e+ects of the radiation environment on various technologies which are critical to spacecraft systems. The three engineering experiments 2own on CRRES include the microelectronics package, the IDM and the HESP. 4.1. MEP The MEP is a radiation hard microprocessor (SA3000) controlled device test system which measures space radiation e+ects on multiple samples of over 60 device types for a total of ∼400 individual test devices (Mullen and Ray, 1993). The devices populate three levels of circuit boards,
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and are thus protected by a range of e+ective shielding thicknesses. Approximately 37 device types (248 individual test devices) are tested for single event upsets (SEUs), and the e+ects of total dose degradation are tested on ∼23 device types (220 individual test devices). All devices are of early 1980’s vintage and are from a variety of vendors (Liebowitz, 1992). Reports of observed SEU rates in the various CRRES= MEP test devices, and comparison with various SEU predictive algorithms, is well documented (McNulty et al., 1991; Campbell, 1991; Campbell et al., 1992; Petersen and Adams, 1992; Weatherford et al., 1993; Reed et al., 1994; Mullen and Ray, 1994; Smith, 1994; Petersen, 1997). Of the ∼37 device types available for SEU testing, several did not upset suGciently often enough (or at all) to be useful in mapping out the regions of space where SEUs occurred (Campbell, 1991). The main emphasis in studying the SEU data is to gain an understanding of the relative levels to which cosmic rays, trapped protons, and energetic solar protons contribute to SEUs. As shown in Fig. 9 (Mullen and Ray, 1994), the predominance of SEUs are found in the heart of the inner radiation belt. The peak occurrence frequency is seen at L∼1:5 which corresponds to the peak of ∼50 MeV protons. The shoulder of the main SEU distribution occurs at L∼2:2 and corresponds to the peak of the secondary proton belt formed at the commencement of the March 1991 storm (see Fig. 4). Even though the total number of SEUs from SPEs may be small averaged over a long period of time due to their relative infrequency, a single SPE can create greater numbers of SEUs than those from cosmic rays. Given the relatively low numbers of SEUs directly attributed to cosmic rays (L ¿3), they should be easily handled with error detection and correction (EDAC) codes and careful assignment of address space in the RAMs (Mullen and Ray, 1994). However, for satellites which traverse the inner proton belt (L ¡ 3) or that may be exposed to high-energy solar protons for extended periods of time, the frequency of SEUs and multiple SEUs is suGciently great that EDAC may not be adequate and SEU immune devices should be considered (Mullen and Ray, 1994). A number of lessons were learned from the total dose studies from the MEP (Mullen et al., 1992; Ray et al., 1992a, b; Zimmerman and Ray, 1994). Unfortunately, there was insuGcient total dose accumulated over the 14 months of the mission to a+ect any changes in the operating parameters of the radiation hardened parts, though the more radiation sensitive devices yielded signi7cant results. To answer questions regarding the accuracy of PMOS dosimeter total dose measurements in space, Ray et al. (1992a) compared the total dose measured by a number of P-channel Metal Oxide Semiconductor (PMOS) dosimeters interspersed throughout all three boards of the MEP with that measured by the Space Radiation Dosimeter (SRD). They found that PMOS dosimeters can be used as an accurate, low power, inexpensive way to measure local total dose up to their point of saturation of 30 krads(Si) virtually anywhere onboard the
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spacecraft. The total dose measured by the PMOS dosimeters (behind 75 mils Al equivalent shielding) was less than 10% higher than that measured by the more accurate SRD (behind 82:5 mils Al equivalent shielding). The devices have a non-linear response at the higher limit of their measurable total dose levels which can be corrected by ground calibrations. The relatively small thermal e+ects can be compensated for using data from thermistors placed near the PMOS dosimeters (Ray et al., 1992a). A di+erent study on a chip custom designed for the MEP which used pMOSFETS as on-chip dosimeters yielded similar results, with in situ space dose-degradation factors giving good agreement with ground test results (Soli et al., 1992). The MEP included some individual device radiation shielding packages called RADPAKs comprised of both high and low Z materials to reduce both direct radiation and bremsstrahlung. The RADPAKs were used to spot shield non-radiation-hardened RAMs, and showed that spot shielding can be a very e+ective method of protecting chips from total dose and may permit the use of more commercial o+-the-shelf (COTS) in space (Mullen and Ray, 1993). Another mission goal was to compare the ground-based dose tests of speci7c devices with their identical counterpart’s space-based performance. The on-orbit total dose effects of various types of HEXFETS, Octal Latches, RAMs, and PROMs (Ray et al., 1992b) and six types of 4007 devices (Zimmerman and Ray, 1994) were compared using the MIL-STD-883 Method 1019.2 60 Co test performance of like devices. Given the part-to-part variability inferred from the ground tests on the HEXFETS, the agreement between space and ground test results for this device type were excellent. Of the four device types studied, the CMOS PROMs showed the largest discrepancy between space degradation and ground test results. As a function of dose exposure, the power supply current drawn by the PROMs in space increased 3 times faster than that observed in ground tests. While results from many part types showed that degradation from the low dose rate in space compared reasonably with the results from high dose rate 60 Co testing, other part types showed signi7cant di+erences. The fact that not all part types degraded the same in space as in ground testing, serves as a forceful reminder that conservative satellite design requires substantial radiation dose design margins. Unfortunately, plans to thoroughly evaluate the changes incorporated into the then new MIL-STD-883 Method 1019.2, which addresses dose rate e+ects in more detail than the previous test standards, was compromised because of the vintage of the parts 2own and because of the fact that the unexpectedly short duration of the mission precluded gathering the necessary statistics of device performance in space. 4.2. IDM The IDM was designed to measure the electrostatic discharge pulse rates when various con7gurations of dielectric materials commonly utilized on spacecraft are exposed
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Fig. 9. Distribution of single event upsets (SEUs=bit=day) as a function of L, for 23 of the most sensitive devices on the microelectronic package (MEP) over the entire CRRES mission (orbits 3-1067).
to space radiation. All aspects of the IDM experiment have been amply documented, including design description (Coakley et al., 1985), detailed analyses of the results (Frederickson et al., 1991, 1992, 1993), and theoretical modeling for interpreting and estimating the pulsing results (Frederickson, 1993, 1996a, b). Electrons which are suGciently energetic to penetrate satellite surfaces and deposit charge in dielectric materials will accumulate and lead to high voltages and large electric 7elds. When these electric 7elds reach a threshold, the dielectric breaks down and generates an electrostatic discharge which can couple with nearby circuitry and lead to spacecraft anomalies. IDM pulse rates were analyzed with respect to electron 2uxes measured by HEEF and provided strong evidence that energetic electrons are the primary cause of deep dielectric electrostatic discharges. Fig. 10 shows the correspondence between the integral 2uence of ¿0:85 MeV electrons and the total number of discharges from all samples observed per orbit. It is important to realize that the correlation between electron 2uence and pulsing was restricted to integral 2uences, and does not consider the e+ect of variations in the spectral shape of the electron population. As illustrated by Fig. 5 (discussed in Section 3.1) the electron spectrum hardened following the March 1991 storm (orbit 589). Thus, not only did the integral electron 2uence throughout the second half of the mission exceed that during the 7rst half, but the spectral hardness also increased. The following summarizes the understanding of ESD pulses gained from the IDM experiment (Frederickson et al., 1991, 1992, 1996a, b): (a) the average pulse rate is approximately proportional to the integral electron 2ux for energies ¿300 keV; (b) pulses do not correlate with instantaneous electron 2ux;
(c) pulses result from breakdown of electric 7elds in dielectric materials, and these electric 7elds decay slowly so pulsing may occur long after electron 2uxes diminish; (d) thicker dielectric samples pulsed more frequently, as did those with more exposed surface area; (e) small ESD pulses occur more frequently than large pulses and each dielectric sample produces a range of pulse sizes; (f) the pulse size is limited by the dielectric material properties and geometry, as well as by the electron 2ux spectrum and intensity; and (g) contrary to what was once believed, a minimum electron 2uence following an ESD is not necessarily required to trigger a subsequent ESD. 4.3. HESP The HESP experiment compared the performance of gallium arsenide germanium (GaAs=Ge) and silicon (Si) solar cells in terms of cell output parameters (current, voltage, maximum power), and also examined the performance of the GaAs=Ge cells as a function of coverglass thickness and material (Ray et al., 1993). The GaAs=Ge cells generally outperformed the Si cells. Solar cell degradation showed good correlation with changes in measured proton 2uences, though the high-energy electron 2uence levels did not appear to e+ect solar cell performance. The e+ectiveness of various annealing techniques on GaAs solar cells was also examined, and it was found that forward bias annealing is not only the simpler, but also the more e+ective and cheaper annealing method compared to constant and pulsed heating biasing techniques for cells under relatively thin cover glass.
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Fig. 10. Electron 2uence per orbit (top) and electrostatic discharge pulses (bottom) summed over all samples, plotted versus orbit number. Fluence is that which is estimated to impinge upon the dielectric samples by propagating ¿0:85 MeV electron 2uence, as measured by HEEF, through the sample cover plates (0:02 cm Al).
5. Conclusion Improving our understanding of space weather and its e+ects on systems in space—whether they be astronauts working on the ISS, space–based systems providing critical communication=navigation links with Earth-based systems, or onboard computers choreographing close encounters with planetary neighbors—is critical as we expand our state-of-the-art technology base into space. As this review was being written, NASA announced (Associated Press, 1999) that “Four hours before Galileo was scheduled to pass 186 miles above Io , Jupiter’s radiation triggered a fault in the onboard computer’s memory, shutting down all nonessential operations aboard the craft.” Fortunately, this temporary glitch in a memory chip was not a system-threatening crisis, and NASA’s engineer team was able to resolve the diGculty by uploading new commands to reprogram Galileo’s computer system. However, the event only underscores the vulnerability of space systems to the hazards of space radiation, a fact which satellite designers and operators must remain vigilant to. CRRES was a unique mission speci7cally designed to address a multitude of issues regarding space weather and its impact on technology. By combining an extensive suite of engineering experiments with an array of environment-measuring instruments, CRRES has provided an unprecedented opportunity to study the performance of a wide range of technologies over a large dynamic range of the space environment. CRRES has led the way into a new era of radiation belt dynamic analyses, and spurred on the investigations towards the next generation of space radiation models. However, this is only a beginning, as there
remains much to accomplish. The mechanism for accelerating the outer zone MeV electrons continues to be a hotly researched topic, and the frequency of super storms such as the March 1991 event which created secondary belts remains uncertain. It is clear that it is now time to supercede the two-state (solar minimum and maximum) static NASA radiation models with a generation of models which provide more detailed information about the huge dynamic range of radiation intensities. This will require greatly expanding the available database in spatial, temporal, and energy coverage, which can only be accomplished by making the inclusion of high quality environmental sensors on spacecraft more routine. And 7nally, the microelectronic technologies which 2ew and were tested on CRRES were early to mid 1980s vintage. As technology continues to advance, there remains the need to insure performance parameters are well understood for reliable space 2ight, and this will require a continuing program of space-quali7cation 2ights coupled to a rigorous ground-testing and modeling program. Although CRRES has accomplished much towards understanding the e+ects of space-weather, on-going programs are required to answer forthcoming questions necessary to establish and maintain a reliable technological presence in space.
Acknowledgements The author acknowledges the work performed by the entire CRRES team which made the mission possible, as well as all the authors whose original work has been reviewed here. Special thanks goes to Alan Ling, Dan Madden, Ernie Holeman, and Kevin Ray for helping with the 7gures.
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References Albert, J.M., Ginet, G.P., 1998. CRRES observations of radiation belt protons: 2. time-dependent radial di+usion. Journal of Geophysical Research 103, 14,865–14,877. Albert, J.M., Ginet, G.P., Gussenhoven, M.S., 1998. CRRES observations of radiation belt protons: 1. data overview and steady state radial di+usion. Journal of Geophysical Research 103, 9261–9273. Associated Press, 1999. Engineers salvage Io mission. Foster’s Daily Democrat, November 27. Bell, J.T., Gussenhoven, M.S., Mullen, E.G., 1997. Super storms. Journal of Geophysical Research 102, 14,189–14,198. Blake, J.B., Kolasinski, W.A., Filius, R.W., Mullen, E.G., 1992a. Injection of electrons and protons with energies of tens of MeV into L ¡ 3 on March 24, 1991. Geophysical Research Letters 19, 821. Blake, J.B., Gussenhoven, M.S., Mullen, E.G., Filius, R.W., 1992b. Identi7cation of unexpected space radiation hazard. IEEE Transactions on Nuclear Science 39, 1761–1764. Brautigam, D.H., Albert, J.M., 2000. Radial di+usion analysis of outer radiation belt electrons during the October 9, 1990 magnetic storm. Journal of Geophysical Research 105, 291–309. Brautigam, D.H., Bell, J.T., 1995. CRRES electron omnidirectional 2ux models and CRRESELE utility. IEEE Nuclear and Plasma Sciences Society, pp. 90. Brautigam, D.H., Gussenhoven, M.S., Mullen, E.G., 1992. Quasi-static model of outer zone electrons. IEEE Transactions on Nuclear Science 39 (6), 1797–1803. Campbell, A.B., 1991. SEU 2ight data from the CRRES MEP. IEEE Transactions on Nuclear Science 38 (6), 1647–1654. Campbell, A., McDonald, P., Ray, K., 1992. Single event upset rates in space. IEEE Transactions on Nuclear Science 39 (6), 1828–1835. Chenette, D.L., Chen, J., Clayton, E., Guzik, T.G., Wefel, J.P., Garcia-Munoz, M., Lopate, C., Pyle, K.R., Ray, K.P., Mullen, E.G., Hardy, D.A., 1994. The CRRES=SPACERAD heavy ion model of the environment (CHIME) for cosmic ray and solar particle e+ects on electronic and biological systems in space. IEEE Transactions on Nuclear Science 41 (6), 2332–2339. Coakley, P.G., Treadaway, M.J., Robinson Jr., P.A., 1985. Low 2ux laboratory test of the internal discharge monitor (IDM) experiment intended for CRRES. IEEE Transactions on Nuclear Science 32 (6), 4066–4072. Daly, E.J., Lemaire, J., Heynderickx, D., Rodgers, D.J., 1996. Problems with models of the radiation belts. IEEE Transactions on Nuclear Science 43 (2), 403–415. Daly, E.J., Buhler, P., Kruglanski, M., 1999. Observations of the outer radiation belt with REM and comparisons with models, personal communication. Dichter, B.K., Hanser, F.A., Sellers, B., Hunerwadel, J.L., 1993. High energy electron 2uxmeter. IEEE Transactions on Nuclear Science 40 (2), 242–245. Frederickson, A.R., 1993. Radiation-induced voltage on spacecraft internal surfaces. IEEE Transactions on Nuclear Science 40 (6), 1547–1554. Frederickson, A.R., 1996a. Upsets related to spacecraft charging. IEEE Transactions on Nuclear Science 43 (2), 426–441. Frederickson, A.R., 1996b. Method for estimating spontaneous pulse rate for insulators inside spacecraft. IEEE Transactions on Nuclear Science 43 (6), 2778–2782.
Frederickson, A.R., Mullen, E.G., Brautigam, D.H., Kerns, K.J., Robinson, P.A., Holeman, E.G., 1991. Radiation-induced insulator discharge pulses in the CRRES Internal Discharge Monitor satellite experiment. IEEE Transactions on Nuclear Science 38 (6), 1614–1621. Frederickson, A.R., Holeman, E.G., Mullen, E.G., 1992. Characteristics of spontaneous electrical discharging of various insulators in space radiation. IEEE Transactions on Nuclear Science 39 (6), 1773–1782. Frederickson, A.R., Mullen, E.G., Kerns, K.J., Robinson, P.A., Holeman, E.G., 1993. The CRRES IDM spacecraft experiment for insulator discharge pulses. IEEE Transactions on Nuclear Science 40 (2), 233–241. Ga+ey Jr., G.D., Bilitza, D., 1994. NASA=National Space Science Data Center trapped radiation models. Journal of Spacecraft and Rockets 31, 172–176. Ginet, G.P., Biasca, R., Tautz, M., 1995. PL-Geospace: three-dimensional visualization of the dynamic space environment. IEEE Nuclear and Plasma Sciences Society, pp. 91–92. Gussenhoven, M.S., Mullen, E.G., 1993. Space radiation e+ects program: an overview IEEE Transactions on Nuclear Science 40 (2), 221–227. Gussenhoven, M.S., Mullen, E.G., Sagalyn, R.C., 1985. CRRES=SPACERAD experiment descriptions. AFGL-TR85-0017, Air Force Geophysics Laboratory, Hanscom, AFB, MA. Gussenhoven, M.S., Mullen, E.G., Sperry, M., Kerns, K.J., Blake, J.B., 1992. The e+ect of the March 1991 storm on accumulated dose for selected satellite orbits: CRRES dose models IEEE Transactions on Nuclear Science 39 (6), 1765–1772. Gussenhoven, M.S., Mullen, E.G., Violet, M.D., Hein, C., Bass, J., Madden, D., 1993. CRRES high energy proton 2ux maps. IEEE Transactions on Nuclear Science 40 (6), 1450–1457. Gussenhoven, M.S., Mullen, E.G., Violet, M.D., 1994a. Solar particle events as seen on CRRES. Advances in Space Research 14 (10), 619–629. Gussenhoven, M.S., Mullen, E.G., Brautigam, D.H., 1994b. Near-earth radiation model de7ciencies as seen on CRRES. Advances in Space Research 14 (10), 927–941. Gussenhoven, M.S., Mullen, E.G., Hardy, D.A., Madden, D., Holeman, E., Delorey, D., Hanser, F., 1995. Low altitude edge of the inner radiation belts: dose models from the APEX satellite IEEE Transactions on Nuclear Science 43, 2035–2042. Gussenhoven, M.S., Mullen, E.G., Brautigam, D.H., 1996a. Improved understanding of the Earth’s radiation belts from the CRRES satellite. IEEE Transactions on Nuclear Science 43 (2), 353–368. Gussenhoven, M.S., Mullen, E.G., Brautigam, D.H., 1996b. Phillips laboratory space physics division radiation models. In: Lemaire, J.F., Heynderickz, D., and Baker, D.N. (Eds.), Radiation Belts: Models and Standards, Geophysical Monograph, American Geophysical Union, Washington DC, USA, Vol. 97. American Geophysical Union, pp. 93–101. Gussenhoven, M.S., Mullen, E.G., Brautigam, D.H., Holeman, E., Jordan, C., Hanser, F., Dichter, B., 1996c. Preliminary comparison of dose measurements on CRRES to NASA model predictions. IEEE Transactions on Nuclear Science 38 (6), 1655–1662. Hudson, M.K., Kotelnikov, A.D., Li, X., Roth, I., Temerin, M., Wygant, J., Blake, J.B., Gussenhoven, M.S., 1995. Simulation
D.H. Brautigam / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1709 – 1721 of proton radiation belt formation during the March 24, 1991 SSC. Geophysical Research Letters 22, 291. Hudson, M.K., Elkington, S.R., Lyon, J.G., Marchenko, V.A., Roth, I., Temerin, M., Blake, J.B., Gussenhoven, M.S., Wygant, J.R., 1997. Simulation of radiation belt formation during storm sudden commencements. Journal of Geophysical Research 102 (A7), 14,087–14,102. Hudson, M.K., Marchenko, V.A., Roth, I., Temerin, M., Blake, J.B., Gussenhoven, M.S., 1998. Radiation belt formation during storm sudden commencements and loss during main phase. Advances in Space Research 21 (4), 597–607. Korth, A., Vampola, A.L., 1994. Cross-L motion of a relativistic electron belt formed in the slot region. Journal of Geophysical Research 99 (A7), 13,529–13,535. Li, X., Roth, I., Temerin, M., Wygant, J., Hudson, M.K., Blake, J.B., 1993. Simulation of the prompt energization and transport of radiation belt particles during the March 24, 1991 SSC. Geophysical Research Letters 20, 2423. Liebowitz, R.P., 1992. A history of the space radiation e+ects (SPACERAD) program for the joint USAF=NASA CRRES mission. PL-TR-92-2071, Phillips Laboratory, AFMC, Hanscom, AFB, MA. McNulty, P.J., Beauvais, W.J., Abdel-Kader, W.G., El-Teleaty, S.S., Mullen, E.G., Ray, K.P., 1991. Test of SEU algorithms against preliminary CRRES satellite data. IEEE Transactions on Nuclear Science 38 (6), 1642–1646. Mullen, E.G., Ray, K.P., 1993. Microelectronics test package (MEP). IEEE Transactions on Nuclear Science 40 (2), 228–232. Mullen, E.G., Ray, K.P., 1994. Microelectronics e+ects as seen on CRRES. Advances in Space Research 14 (10), 797–807. Mullen, E.G., Gussenhoven, M.S., Ray, K., Violet, M., 1991. A double-peaked inner radiation belt: cause and e+ect as seen on CRRES. IEEE Transactions on Nuclear Science 38 (6), 1713–1717. Mullen, E.G., Gussenhoven, M.S., Brautigam, D.H., Frederickson, A.R., 1992. Review of CRRES radiation belt measurements and engineering experiment results. SAE Technical Paper Series, No. 921373, 22nd International Conference on Environmental Systems, Society of Automotive Engineers. Mullen, E.G., Ray, K.P., Koga, R., Holeman, E.G., Delorey, D.E., 1995. SEU results from the Advanced Photovoltaic and Electronics Experiments (APEX) Satellite. IEEE Transactions on Nuclear Science 42 (6), 1988–1994. Mullen, E.G., Ginet, G., Gussenhoven, M.S., Madden, D., 1998. SEE relative probability maps for space operations. IEEE Transactions on Nuclear Science 45 (6), 2954–2963. Petersen, E.L., 1997. Predictions and observations of SEU rates in space. IEEE Transactions on Nuclear Science 44 (6), 2174–2187.
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Petersen, E.L., Adams, J.H., 1992. Comparison of SEU prediction techniques. IEEE Transactions on Nuclear Science 39 (6), 1836–1839. Ray, K.P., Mullen, E.G., Stapor, W.J., Circle, R.R., McDonald, P.T., 1992a. CRRES dosimetry results and comparisons using the space radiation dosimeter and P-channel MOS dosimeters. IEEE Transactions on Nuclear Science 39 (6), 1846–1850. Ray, K.P., Mullen, E.G., Bradley, T.E., Zimmerman, D.M., Du+, E.A., 1992b. A comparison between 60 Co ground tests and CRRES space 2ight data. IEEE Transactions on Nuclear Science 39 (6), 1851–1858. Ray, K.P., Mullen, E.G., Trumble, T.M., 1993. Results from the high eGciency solar panel experiment 2own on CRRES. IEEE Transactions on Nuclear Science 40 (6), 1505–1511. Reed, R.A., McNulty, P.J., Beauvais, W.J., Abdel-Kader, W.G., Stassinopoulos, E.G., Barth, J.C., 1994. A simple algorithm for predicting proton SEU rates in space compared to the rates measured on the CRRES satellite. IEEE Transactions on Nuclear Science 41 (6), 2389–2395. Rossi, B., Olbert, S., 1970. Introduction to the Physics of Space. McGraw-Hill, New York. Smith, E.C., 1994. E+ects of realistic satellite shielding on SEE rates. IEEE Transactions on Nuclear Science 41 (6), 2396–2399. Soli, G.A., Blaes, B.R., Buehler, M.G., Ray, K.P., Lin, Y.S., 1992. CRRES Microelectronic test chip orbital data-II. IEEE Transactions on Nuclear Science 39 (6), 1846–1850. Space Environment Services Center (SESC), Preliminary Report and Forecast of Solar Geophysical Data, U.S. Department of Commerce, SESC, Boulder, CO, SESC PRF 936, 941, 945, 946, 950, 962, 1993–1994. Vampola, A.L., 1992. Combined release and radiation e+ects satellite. Journal of Spacecraft and Rockets 29 (4), 592f. Vampola, A.L., Korth, A., 1992. Electron drift echoes in the inner magnetosphere. Geophysical Research Letters 19, 625–628. Violet, M.D., Frederickson, A.R., 1993. Spacecraft anomalies on the CRRES satellite correlated with the environment and insulator samples. IEEE Transactions on Nuclear Science 40 (6), 1512–1520. Violet, M.D., Lynch, K., Redus, R., Riehl, K., Boughan, E., Hein, C., 1993. Proton Telescope (Protel) on the CRRES spacecraft. IEEE Transactions on Nuclear Science 40 (2), 242–245. Weatherford, T.R., McDonald, P.T., Campbell, A.B., Langworthy, J.B., 1993. SEU rate prediction and measurement of GaAs SRAMS onboard the CRRES satellite. IEEE Transactions on Nuclear Science 40 (6), 1463–1470. Zimmerman, D.M., Ray, K.P., 1994. Total-dose correlation of 4007 devices 2own on the CRRES MEP experiment. IEEE Transactions on Nuclear Science 41 (6), 2605–2612.
www.elsevier.com/locate/jastp
CRRES in review: space weather and its e+ects on technology D.H. Brautigam ∗ Air Force Research Laboratory, Space Vehicles Directorate, Hanscom AFB, MA 01731, USA
Abstract The Combined Release and Radiation E+ects Satellite (CRRES) was a unique mission which has provided valuable lessons about the space radiation environment and its e+ects on technological systems in space. The tremendous value of CRRES resides in the fact that it 2ew not only the technologies to be space-tested, but also an extensive suite of instruments to accurately specify the damage-causing radiation environment. CRRES was launched into a geosynchronous transfer orbit on July 25, 1990, 1 yr following the maximum of solar cycle 22, and returned data for approximately 14 months. It was 7rst exposed to 8 months of a relatively quiet magnetosphere, followed by a very active 6 month interval initiated by the much documented March 1991 storm. This large magnetic storm, accompanied by a solar proton event, was responsible for creating a temporary proton and electron belt within the typically benign slot region. The dynamic radiation environment was responsible for a number of observable total dose e+ects, charging=discharging phenomena, single event e+ects, and assorted anomalies within the spacecraft instrumentation and technology tested. This exceptional combination of technologies, instrumentation, and dynamic radiation environment will be reviewed, with attention given to lessons learned and how CRRES has changed c 2002 Published by Elsevier Science Ltd. All rights our perspective on magnetospheric radiation hazards. Crown Copyright reserved. Keywords: CRRES; Radiation belts; Radiation environment; Radiation e+ects on electronics
1. Introduction In the nearly 10 years since the Combined Release and Radiation E+ects Satellite (CRRES) was launched on July 25, 1990, “space weather” has become a much more widely used term among satellite mission planners, operators and design engineers. Space weather may be de7ned as the highly variable space radiation environment which originates at the Sun’s surface, and extends to and includes the Earth’s magnetosphere. Although space weather is driven by the Sun, its e+ects ultimately reach the surface of the Earth and produce adverse e+ects on satellite operations, space and ground-based communication and navigation systems, high altitude manned 2ight, and electrical power grid systems. The CRRES mission began in response to the increasing frequency of single event upsets (SEUs) recorded by many of ∗
Tel.: +1-781-377-3890; fax: +1-781-377-3160. E-mail address: [email protected] (D.H. Brautigam).
the operational satellites being 2own. At the time (as is still the case) most satellites lacked the environmental sensors to con7dently establish a causal relation between the environment and the reported anomalies. CRRES aimed to rectify this situation by launching a test bed of state-of-the-art technologies along with a suite of particle detectors capable of measuring the radiation environment. Speci7cally, the space radiation e+ects component of the CRRES program, space radiation e+ects (SPACERAD), was designed to (Gussenhoven and Mullen, 1993): (1) measure SEUs and total dose degradation of microelectronic technologies; (2) make in situ measurements of the radiation environment to be correlated with the measured responses of the various onboard technologies; (3) measure the radiation response and annealing characterizations of parts identical to those scheduled for space2ight; (4) model the response of selected devices to the total dose, dose rate, and SEU e+ects anticipated in space;
c 2002 Published by Elsevier Science Ltd. All rights reserved. 1364-6826/02/$ - see front matter. Crown Copyright PII: S 1 3 6 4 - 6 8 2 6 ( 0 2 ) 0 0 1 2 1 - 9
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(5) develop algorithms to relate space performance of microelectronic components to ground test procedures; (6) update existing radiation ground test guidelines required to more accurately simulate the space behavior of microelectronic devices; (7) space-qualify advanced microelectronic technologies for use in operational DoD satellite systems; and (8) update the static radiation belt models and develop the 7rst dynamic models of the high-energy particle populations. Three engineering experiments, including the microelectronics package (MEP), the internal discharge monitor (IDM), and the high-eGciency solar cell panel (HESP), were 2own to quantify the space radiation e+ects on various technologies. The suite of environmental instruments included a number of detectors designed to measure charged particle populations ranging from thermal plasma to 10 MeV electrons and several hundred MeV=Q ions, as well as electric and magnetic 7eld sensors (Gussenhoven et al., 1985; Vampola, 1992). The research papers generated by a satellite mission of this complexity, cover a wide range of topics from the impact of space radiation on microelectronics devices to the theoretical modeling of radiation belt dynamics. Since the lessons reaped from the mission are of such broad scope, the intent here is not to provide an exhaustive summary of all the detailed knowledge gained, but to present an overview of the mission, highlight some of its key 7ndings, and point to the original papers where the interested reader may 7nd complete details. A brief pre-launch history of CRRES is presented in Section 2. Section 3 surveys the general features of the electron and proton population, and then discusses e+orts to model the dynamics and to create statistical models for the engineering community. Section 4 introduces the three engineering experiments and highlights key lessons gleaned from them. Finally, concluding remarks are presented in Section 5.
2. Pre-launch history The nearly decade long evolution of the CRRES program, from conception to launch, is well documented by Liebowitz (1992), and is brie2y summarized here. CRRES grew out of the research and development Radiation Satellite (RADSAT) program 7rst conceived in 1981 by the Air Force Geophysical Laboratory, now the Air Force Research Laboratory. RADSAT’s primary goal was to make extensive measurements of the charged particle environment in order to improve existing models, with a secondary goal to measure the degradation and soft error rate of various devices in space. In 1982, the Air Force RADSAT program was transformed to the joint Department of Defense (DoD)=NASA mission named CRRES. NASA managed the chemical release component of the mission, and the Air Force managed
the SPACERAD component. Initially, CRRES was scheduled to be launched from the Space Shuttle. As of late 1985, the delivery of the payload was scheduled for August 1986 with a shuttle launch date of July 1987. When the shuttle Challenger exploded in January 1986, the CRRES program was thrown into disarray. With large uncertainties and delays looming, support for ground testing and SEU modeling was halted by the end of 1986. With the Space Shuttle schedule derailed, the decision was made to launch CRRES on an Atlas=Centaur, with a new launch date set for June 1990. This launch delay had a negative impact on one of the primary mission goals which was to space-qualify and model the radiation e+ects on DoD state-of-the-art space system technologies. Since the payload was virtually in place by 1986, and new technologies were emerging at a rapid pace, the delay meant that dated technologies would be 2own. CRRES was 7nally launched on July 25, 1990 from Cape ◦ Canaveral into a highly elliptical orbit with an 18 inclination, a 350 km perigee and 36 000 km apogee. Its orbital period of ∼10 h enabled CRRES to pass through the heart of the inner and outer radiation belts 4 times per day as illustrated in Fig. 1. The ground command station permanently lost communication with CRRES on October 12, 1991, as the result of premature battery failure. This abbreviated lifetime negatively impacted the mission in two ways. One, the mission goal to obtain a statistically signi7cant data set of particle 2uxes for model development was compromised, and two, many devices which were designed against high accumulated doses did not receive suGcient dose to test their limitations. Despite these shortcomings, the CRRES mission returned a number of invaluable lessons that will be discussed in the remainder of this paper. 3. Space radiation environment Underestimating the space radiation environment in system design phase can lead to degraded system performance and=or early system failure. In the opposite extreme, overestimation can lead to excessive shielding weight, reduced capability, and increased cost. The successful design of cheaper and smaller, yet reliable and survivable space systems, cannot be reached without improved speci7cation of the radiation environment in which they must operate. A primary goal of the CRRES=SPACERAD mission was thus to improve existing models of the radiation environment. Fig. 2 presents the solar–terrestrial activity during the CRRES epoch within the context of the prevailing solar cycle. Solar cycle 22 spans the interval September 1986 to May 1996, and following a relatively short rise time (∼3 yr) in comparison to its declining phase (∼4:5 yr), solar activity reaches its maximum in July 1989. Relative to sun spot number (panel a), solar activity remains at a plateau from mid-1989 through the end of 1992 (∼2:5 yr), with a corresponding minimum in the galactic cosmic ray background
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Fig. 1. Depiction of CRRES orbit traversing the inner and outer radiation belts as rendered by CRRESPRO and CRRESELE, respectively. The CRRESPRO and CRRESELE models are implemented within the Air Force space environment model package AF-Geospace (Ginet et al., 1995).
Fig. 2. Indices (7-day averages) for solar cycle 22: (a) sun spot number, (b) Ap index, (c) geosynchronous ¿ 10 MeV proton 2ux (cm2 s)−1 measured from GOES 7, and (d) geosynchronous ¿ 2 MeV electron 2ux (cm2 s)−1 from GOES 7. The CRRES epoch is marked by the heavy solid bar.
(Mullen and Ray, 1994). The 14-month CRRES mission is positioned within this window of high solar activity, and saw several periods of peak magnetospheric activity as measured by the Ap index (panel b). Using the ¿ 10 MeV proton 2ux measured at geosynchronous (panel c) as a measure of the frequency of solar proton events (SPE), it can be seen that the predominance of SPEs were clustered within the broad solar maximum plateau and that CRRES was exposed to a sizable fraction of these. It is also noted that relative to the last few solar cycle maxima (not shown), there were signi7cantly more SPE, though fewer magnetic storm sudden commencements (SESC, 1994). The ¿ 2 MeV electron 2ux measured at geosynchronous (panel d) is inversely related to solar activity (sunspot number), with a clear rise in intensity throughout the declining phase towards solar min-
imum. The average intensity for the ∼2 yr approaching solar minimum is a factor of ∼10 higher than that near solar maximum, although the sharp peak seen during the CRRES epoch associated with the March 1991 event clearly runs counter to this trend. 3.1. Survey of environment Although the CRRES environment sensor suite includes instruments designed to measure particle populations ranging from the lowest energy plasmas to energetic heavy ions, the main focus here will be on electrons from ∼0:5 to ∼10 MeV and protons from ∼1 to ∼100 MeV. Figs. 3 and 4 provide a survey of the 1:6 MeV electron 2ux and 57 MeV proton 2ux, respectively. The electron 2ux is pro-
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Fig. 3. Mission survey of 1:6 MeV electron 2ux (cm2 s sr keV)−1 plotted versus L and orbit number.
Fig. 4. Mission survey of 57:0 MeV proton 2ux (cm2 s sr MeV)−1 plotted versus L and orbit number.
vided by the high-energy electron 2uxmeter (HEEF) which measures energies between 0.8 and 8:0 MeV (Dichter et al., 1993), and the proton 2ux is provided by the Proton Telescope (PROTEL) which measures energies between 1 and 100 MeV (Violet et al., 1993). Quite evident in these 7gures is the major recon7guration of the radiation belts which occurred during orbit 588, on March 24, 1991. A large SPE was in progress when the arrival of a very large interplanetary shock suddenly compressed the magnetosphere resulting in the formation of a durable secondary electron and proton belt. Modeling e+orts to explain this sudden recon7guration are discussed in Section 3.3. Magnetic storms of this magnitude and impact (super storms) are a relatively rare event, occurring perhaps once per solar cycle (Bell et al., 1997). This super storm is the signature event of the CRRES mission because of its magnitude (minimum Dst ∼− 300) and the creation of the secondary proton belt (Mullen et al., 1991; Blake et al., 1992a, b) and electron belt (Blake et al.,
1992a, b; Vampola and Korth, 1992). The secondary proton belt was formed between L = 1:8 to 2:6RE and was predominantly ¿ 20 MeV protons. When this belt was 7rst created, the 57 MeV proton 2ux peak was at L∼2:6 RE but ∼40 days later had moved inwards to ∼2:34RE while decaying in intensity (Mullen et al., 1991). The energy at which the 2uxes peaked in the newly created electron belt was inferred from observed drift echoes to be ∼15 MeV. Unfortunately, there were no instruments onboard which were designed to make direct 2ux measurements of electrons in this energy range. Estimates of the inwards cross-L di+usion rates of the secondary electron belt are similar to those found for the 7ssion electrons following the Star7sh nuclear detonation (Korth and Vampola, 1994). The existence of these secondary electron and proton belts, formed in a typically benign region of space, have obvious implications in the assessment of radiation hazards on space systems, and should be included in any future radiation belt model. However, because of the in-
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frequency of these secondary belt formations, they will have to be treated in terms of occurrence probabilities as well as in terms of intensity decay time as a function of energy and altitude. As seen in Fig. 3, the outer zone electrons are a much more dynamic population than the inner belt, with frequent episodic events on the time scale of weeks. Also apparent is a relatively long-term (months) variation, with moderate 2ux intensities during the 7rst ∼300 orbits (∼4 months), followed by a very weak period of ∼250 orbits, and a 7nal very active period of ∼500 orbits (∼6 months) initiated by the March 1991 super storm. Not indicated by this plot is the spectral variation which is important for understanding degradation e+ects on microelectronic devices behind various shielding thicknesses. The average electron spectrum hardened following the March 1991 storm (orbit 588) as indicated in Fig. 5 where cumulative LOLET, HILET, and total dose behind 82:5 mils Al shielding (top) and 232:5 mils Al shielding (bottom), as measured by the space radiation dosimeter (SRD) (Gussenhoven et al., 1992), is plotted. For the thinner shielding, the LOLET dose (primarily from ¿1 MeV electrons) dominated the HILET dose (from ¿20 MeV protons) throughout the mission. However, behind the thicker shielding, the LOLET dose (primarily from ¿2:5 MeV electrons) sharply rose following the super storm and soon exceeded the HILET dose (from ¿35 MeV protons). The signi7cance of the spectral hardening will be alluded to in Section 4. Also highlighted in Fig. 4 are instances of solar proton penetration into the inner magnetosphere. The trace of 57 MeV solar proton 2ux (solid vertical blue streaks from L=7 downwards) during several orbits (588– 600) throughout the March 1991 storm indicates that the protons penetrated down to L∼2:6 where they merged with the newly formed belt. Other signi7cant solar proton events were observed during June 1991 (orbits 757–825) at this energy, with penetration down to L∼3:5. A similar plot for 10:7 MeV protons (Gussenhoven et al., 1994a) show numerous proton events where the penetration cuto+s are typically around 4 to 4:5RE . De7ning a solar proton event as a time interval when the 2ux of ¿10 MeV protons at geosynchronous (GOES 7) exceeds 10 p=cm2 =sr=s, permits identi7cation of 15 SPE during the CRRES mission. The depth to which energetic solar protons penetrate the magnetosphere has traditionally been calculated using Stormer theory which assumes charged particle motion in a dipole magnetic 7eld (Rossi and Olbert, 1970), and predicts that 26 MeV solar protons will penetrate to L∼7RE (Gussenhoven et al., 1994a). These predictions were generally expected to hold not only under quiet magnetic conditions, but also during storm conditions during which the total electric potential imposed across the magnetosphere (∼200 kV) remains small relative to the energy of the solar protons (¿10 MeV). CRRES observations showed that solar protons penetrated to well below the cuto+s predicted by Stormer theory for every large solar event, as well as many smaller ones. CRRES observations have demon-
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strated that 1 to 100 MeV solar protons can typically penetrate to L∼4, or as low as L∼2:5, with the cuto+ depth being a function of not only particle energy, but also storm-speci7c dynamics, thus invalidating attempts to 7x the penetration boundary for protons in this energy range (Gussenhoven et al., 1994a). Some of the events in which solar protons penetrate deep into the magnetosphere are associated with newly trapped populations with peak intensities at L = 2:3 to 4RE (depending on particle energy) and which persist from days to months. As pointed out earlier in relation to the March 1991 storm, once these transient belts are formed, subsequent radial transport may further move their peak to lower L (Mullen et al., 1991). Observation coupled with theoretical modeling (discussed in the following section) has led to the conclusion that solar protons play a more direct role in the trapped populations of the inner magnetosphere than previously thought. 3.2. Dynamic modeling Simulations of the March 1991 event show excellent agreement with the observed formation of the secondary electron belt (Li et al., 1993) and proton belt (Hudson et al., 1995, 1997, 1998). In these simulations, a model electromagnetic pulse accelerates a speci7ed initial particle distribution to the observed energies. The electron simulation shows that an initially trapped distribution speci7ed out to L = 10 is rapidly accelerated inwards, with electrons which drift in phase with the incident pulse reaching maximum energy. The simulation 7nds the peak of the distribution for the newly formed belt occurs at L∼2:85 (2.55) for ∼10 (20) MeV electrons. The simulations show that a major di+erence between the dynamics of the electron and proton belt formation is that solar protons, and not initially trapped protons, act as the primary source for the newly formed proton belt. Although the March 1991 storm was the most dramatic event of the CRRES mission, many episodes of rapid enhancements and gradual depletions of electron 2ux are clearly evident throughout the mission (Fig. 3). The rapid injection of ¿100 keV electrons deep into the inner magnetosphere has been observed for quite some time, yet modeling the radiation belt dynamics has typically been accomplished through gradual radial di+usion over months time scale, where the boundary conditions on 2ux and the di+usion rates are held 7xed. Brautigam and Albert (2000) have shown that the dynamics of 200 keV to ¡ 1 MeV electrons throughout an isolated storm in October 1990 can be reasonably explained in terms of a simple radial di+usion model using time-dependent outer boundary conditions (scaled by geosynchronous data) and Kp -dependent radial di+usion coeGcients spanning two orders of magnitude. For ¿1 MeV electrons, it appears that in addition to radial di+usion, some other non-adiabatic process must be assumed to explain the gradual acceleration of ¿1 MeV electrons well inside geosynchronous. An important point
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Fig. 5. Total, LoLET, and HiLET dose (krads in silicon) versus orbit number measured by the Space Radiation Dosimeter behind 82:5 mils Al shielding (top) and 232:5 mils Al shielding (bottom).
highlighted by this particular storm, is that although geosynchronous data is critical as an outer boundary condition, it is not a reliable monitor of the dynamics in the heart of the outer zone. Although the storm recovery phase 2uxes of ¿1 MeV electrons at geosynchronous show no appreciable increase relative to pre-storm levels, the 2uxes at L∼4 show a factor of ∼10 increase. The 1–100 MeV trapped proton population observed by CRRES has also been modeled using a radial di+usion equation with additional terms representing Coulomb collisions, charge exchange, and cosmic ray albedo neutron decay (Albert et al., 1998; Albert and Ginet, 1998). For these energies, the inner proton belt for L 6 1:7 represents a very stable steady state condition, remaining una+ected even by the large March 1991 event. The region for 1:7 ¡ L ¡ 3:0, however, is a much more dynamic region and departs dramatically from a steady state solution both before as well as after the March storm. 3.3. Statistical models The standard radiation belt models which have been used by the engineering community since the 1960s are the NASA solar minimum and maximum models (Ga+ey and Bilitza, 1994) for protons (most current versions, AP8-min, -max) and electrons (most current versions, AE8-min, -max). One of the primary goals of the CRRES mission was to evaluate and improve upon existing radiation belt models. Towards this end, the Air Force created a set of models mapped to the (L; B=B0 ) magnetic coordinate grid , including CRRESRAD (Gussenhoven et al., 1992) based on dose measurements from the Space Radiation Dosimeter, CRRESPRO (Gussenhoven et al., 1993) based on PROTEL proton 2ux, and CRRESELE (Brautigam et al.,1992; Brautigam
Fig. 6. Equatorial omnidirectional 2ux (cm2 s MeV)−1 of 41 MeV protons plotted as a function of L. Fluxes are from the CRRESPRO quiet and active model, and the NASA AP8-max model.
and Bell, 1995) based on HEEF electron 2ux. These have been summarized by Gussenhoven et al. (1996a, b). Despite the fact that the premature failure of CRRES compromised the statistical basis for the new set of models, the results from CRRES have highlighted the de7ciencies of the old NASA models (Gussenhoven et al., 1994b, 1996a, b, c), as have results from other more recent spacecraft (Daly et al., 1996). The CRRES models have also made important steps toward establishing the next generation of engineering models. The NASA solar maximum models AP8-max and AE8-max have been compared with CRRESPRO (Fig. 6) and CRRESELE (Fig. 7), respectively. Because 2uxes in the radiation belt models depend on a number of variables
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Fig. 7. Equatorial omnidirectional 2ux (cm2 s keV)−1 of 1:6 MeV electrons plotted as a function of L. Fluxes are from CRRESELE and NASA AE8-max models. Included from CRRESELE, are 2ux pro7les representing the mission average (Ave) and three out of the six levels of activity parameterized by the Ap 15 index (Low, Ap 15 = 5–7.5; Moderate, Ap 15 = 10–15; High, Ap 15 = 25–55).
(such as L; B=B0 , energy, pitch angle, and magnetic activity), it is diGcult, even when discussing omnidirectional 2uxes, to summarize the di+erences. Sample comparisons are presented here, and the interested reader may 7nd more details in the reviews by Gussenhoven et al. (1994b; 1996a, b, c). CRRESPRO represents the proton population in two distinct con7gurations, one de7ned by the pre-super storm environment (quiet model) and the second de7ned by the post-super storm environment (active model) which includes the secondary proton belt. Model 2uxes from the CRRESPRO quiet and active model for 41 MeV protons are compared to AP8-max in Fig. 6. At the peak of the inner proton belt (L∼1:6), AP8 is within a factor of ∼2 lower than CRRESPRO (which give the same values for both quiet and active models). The quiet CRRESPRO model shows lower 2uxes than AP8 over most of the region for L ¿1:8. The most signi7cant di+erence between CRRESPRO and AP8 is for L ¿1:8, where the active CRRESPRO model 2uxes are more than a factor of 10 higher at the peak of the newly formed secondary proton belt (L∼2:3). CRRESELE attempts to accommodate the dramatic variation seen in the outer zone electrons by introducing a number of quasi-static models in place of the single static solar maximum model o+ered by AE8-max. These quasi-static models are parameterized by the 15-day running average (with 1 day lag) Ap 15. The equatorial 2uxes for 1:6 MeV electrons from a sampling of the six Ap15-parameterized models are presented in Fig. 7. The 2ux pro7le for the lowest activity model (smallest Ap15) peaks at L∼4:5. As Ap 15 increases for more active models, the position of the pro7le peak moves inwards and increases in intensity. The pro7le at
highest activity is peaked at L∼3:3, where its 2ux intensity is three orders of magnitude greater than that of the lowest activity pro7le. Although the orbital electron 2ux pro7les vary dramatically (Fig. 3), a CRRES mission average of the environment is included in CRRESELE for a direct comparison to AE8-max which is a long-term average over solar maximum. As evident in Fig. 7, the 1:6 MeV electron 2ux from AE8-max is a factor of ∼10 higher than the CRRES average at near geosynchronous altitudes, with CRRESELE 2uxes being slightly higher for L∼3. CRRESELE represents a 7rst step in resolving the diGculties of representing an extremely dynamic population with a practical set of models. One diGculty is that the activity dependence of electron 2ux (independent of the chosen activity parameter) is both energy-dependent and L-dependent. A second diGculty, as has been suggested by Daly et al. (1999), is that the correlation between a speci7c activity parameter may vary with solar cycle phase. CRRESRAD includes quiet and active models de7ned in the same way as in CRRESPRO, and their comparison with the dose predicted by NASA models (NASA model 2uences must be propagated through shielding to yield dose) lead to conclusions consistent with those drawn from the direct comparison of CRRES and NASA 2uences. The difference of orbit dose between those predicted by NASA and CRRES models depend upon the particular orbit, shielding thickness, and magnetospheric activity level (Gussenhoven et al., 1994b). The reasons for the orbital dependence include the NASA models’ omission of the secondary proton and electron peaks in the slot region (L = 1:8—2.5), and also that AE8-max includes higher low-energy elec-
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Fig. 8. Sample single event e+ects (SEE) map giving contours of relative probability for SEE occurrence at 1000 –1050 km altitude.
tron 2uxes for L ¿3. The model di+erences also depend on shielding thickness because NASA spectral shapes are softer than those measured by CRRES which results in an overestimation of dose for thin shielding and an underestimation of dose for thick shielding. Finally, the comparison between NASA and CRRES dose predictions is activity dependent because whereas NASA has a single model to represent solar maximum, CRRES represents the same era by both a quiet and an active model. In addition to the CRRES trapped radiation dose and 2ux models, the CRRES=SPACERAD Heavy Ion Model of the Environment (CHIME) and the Single Event E+ects (SEE) maps have also been developed. CHIME gives di+erential energy 2ux and LET spectra for cosmic rays including all stable elements with energies from 10 MeV=nucleon to 60 GeV=nucleon, as a function of solar modulation for the interval 1970 to 2010 (Chenette et al., 1994). Cosmic ray sources include the galactic and anomalous cosmic ray components, as well as various solar 2are models. The SEE maps (Mullen et al., 1998) were created from CRRES=PROTEL data and the low altitude APEX satellite dosimeter data (Gussenhoven et al., 1995), and give the relative distribution of the ¿50 MeV proton population, which has been shown to be a good indicator of SEE-prone regions (Mullen et al., 1995). The SEE maps, which are mapped in geographic latitude and longitude, and extend in altitude from 350 km to 14 000 km, provide an accurate spatial distribution of the relative probability for space systems to su+er anomaly-producing SEEs. A sample SEE map is provided in Fig. 8, giving the relative probability for SEE occurrence at 1000 –1050 km altitude. At this altitude, the South Atlantic Anomaly (SAA) region is broadened considerably relative to that at lower altitudes.
4. Engineering results A primary objective of CRRES was to measure the performance of various technologies when exposed to the harsh space radiation environment. This objective was met by two approaches, including an extensive survey of reported anomalies from all onboard instruments (Violet and Frederickson, 1993), as well as individual analyses from the suite of engineering experiments. Here, an anomaly is de7ned as any observed instrumental response which is not programmed or commanded. Most environment-induced spacecraft anomalies may be traced to single event e+ects (SEE), di+erential surface charging, deep dielectric charging, or total dose e+ects. The history of anomalies from each instrument was correlated with the history of CRRES measurements of low-energy plasma, high-energy particles, and electromagnetic 7elds. It was found that as a group, the 674 reported anomalies correlate well with high levels of high energy electrons, implying that deep dielectric discharging was the major cause of CRRES anomalies (Violet and Frederickson, 1993). The engineering experiments were designed to quantify the deleterious e+ects of the radiation environment on various technologies which are critical to spacecraft systems. The three engineering experiments 2own on CRRES include the microelectronics package, the IDM and the HESP. 4.1. MEP The MEP is a radiation hard microprocessor (SA3000) controlled device test system which measures space radiation e+ects on multiple samples of over 60 device types for a total of ∼400 individual test devices (Mullen and Ray, 1993). The devices populate three levels of circuit boards,
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and are thus protected by a range of e+ective shielding thicknesses. Approximately 37 device types (248 individual test devices) are tested for single event upsets (SEUs), and the e+ects of total dose degradation are tested on ∼23 device types (220 individual test devices). All devices are of early 1980’s vintage and are from a variety of vendors (Liebowitz, 1992). Reports of observed SEU rates in the various CRRES= MEP test devices, and comparison with various SEU predictive algorithms, is well documented (McNulty et al., 1991; Campbell, 1991; Campbell et al., 1992; Petersen and Adams, 1992; Weatherford et al., 1993; Reed et al., 1994; Mullen and Ray, 1994; Smith, 1994; Petersen, 1997). Of the ∼37 device types available for SEU testing, several did not upset suGciently often enough (or at all) to be useful in mapping out the regions of space where SEUs occurred (Campbell, 1991). The main emphasis in studying the SEU data is to gain an understanding of the relative levels to which cosmic rays, trapped protons, and energetic solar protons contribute to SEUs. As shown in Fig. 9 (Mullen and Ray, 1994), the predominance of SEUs are found in the heart of the inner radiation belt. The peak occurrence frequency is seen at L∼1:5 which corresponds to the peak of ∼50 MeV protons. The shoulder of the main SEU distribution occurs at L∼2:2 and corresponds to the peak of the secondary proton belt formed at the commencement of the March 1991 storm (see Fig. 4). Even though the total number of SEUs from SPEs may be small averaged over a long period of time due to their relative infrequency, a single SPE can create greater numbers of SEUs than those from cosmic rays. Given the relatively low numbers of SEUs directly attributed to cosmic rays (L ¿3), they should be easily handled with error detection and correction (EDAC) codes and careful assignment of address space in the RAMs (Mullen and Ray, 1994). However, for satellites which traverse the inner proton belt (L ¡ 3) or that may be exposed to high-energy solar protons for extended periods of time, the frequency of SEUs and multiple SEUs is suGciently great that EDAC may not be adequate and SEU immune devices should be considered (Mullen and Ray, 1994). A number of lessons were learned from the total dose studies from the MEP (Mullen et al., 1992; Ray et al., 1992a, b; Zimmerman and Ray, 1994). Unfortunately, there was insuGcient total dose accumulated over the 14 months of the mission to a+ect any changes in the operating parameters of the radiation hardened parts, though the more radiation sensitive devices yielded signi7cant results. To answer questions regarding the accuracy of PMOS dosimeter total dose measurements in space, Ray et al. (1992a) compared the total dose measured by a number of P-channel Metal Oxide Semiconductor (PMOS) dosimeters interspersed throughout all three boards of the MEP with that measured by the Space Radiation Dosimeter (SRD). They found that PMOS dosimeters can be used as an accurate, low power, inexpensive way to measure local total dose up to their point of saturation of 30 krads(Si) virtually anywhere onboard the
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spacecraft. The total dose measured by the PMOS dosimeters (behind 75 mils Al equivalent shielding) was less than 10% higher than that measured by the more accurate SRD (behind 82:5 mils Al equivalent shielding). The devices have a non-linear response at the higher limit of their measurable total dose levels which can be corrected by ground calibrations. The relatively small thermal e+ects can be compensated for using data from thermistors placed near the PMOS dosimeters (Ray et al., 1992a). A di+erent study on a chip custom designed for the MEP which used pMOSFETS as on-chip dosimeters yielded similar results, with in situ space dose-degradation factors giving good agreement with ground test results (Soli et al., 1992). The MEP included some individual device radiation shielding packages called RADPAKs comprised of both high and low Z materials to reduce both direct radiation and bremsstrahlung. The RADPAKs were used to spot shield non-radiation-hardened RAMs, and showed that spot shielding can be a very e+ective method of protecting chips from total dose and may permit the use of more commercial o+-the-shelf (COTS) in space (Mullen and Ray, 1993). Another mission goal was to compare the ground-based dose tests of speci7c devices with their identical counterpart’s space-based performance. The on-orbit total dose effects of various types of HEXFETS, Octal Latches, RAMs, and PROMs (Ray et al., 1992b) and six types of 4007 devices (Zimmerman and Ray, 1994) were compared using the MIL-STD-883 Method 1019.2 60 Co test performance of like devices. Given the part-to-part variability inferred from the ground tests on the HEXFETS, the agreement between space and ground test results for this device type were excellent. Of the four device types studied, the CMOS PROMs showed the largest discrepancy between space degradation and ground test results. As a function of dose exposure, the power supply current drawn by the PROMs in space increased 3 times faster than that observed in ground tests. While results from many part types showed that degradation from the low dose rate in space compared reasonably with the results from high dose rate 60 Co testing, other part types showed signi7cant di+erences. The fact that not all part types degraded the same in space as in ground testing, serves as a forceful reminder that conservative satellite design requires substantial radiation dose design margins. Unfortunately, plans to thoroughly evaluate the changes incorporated into the then new MIL-STD-883 Method 1019.2, which addresses dose rate e+ects in more detail than the previous test standards, was compromised because of the vintage of the parts 2own and because of the fact that the unexpectedly short duration of the mission precluded gathering the necessary statistics of device performance in space. 4.2. IDM The IDM was designed to measure the electrostatic discharge pulse rates when various con7gurations of dielectric materials commonly utilized on spacecraft are exposed
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Fig. 9. Distribution of single event upsets (SEUs=bit=day) as a function of L, for 23 of the most sensitive devices on the microelectronic package (MEP) over the entire CRRES mission (orbits 3-1067).
to space radiation. All aspects of the IDM experiment have been amply documented, including design description (Coakley et al., 1985), detailed analyses of the results (Frederickson et al., 1991, 1992, 1993), and theoretical modeling for interpreting and estimating the pulsing results (Frederickson, 1993, 1996a, b). Electrons which are suGciently energetic to penetrate satellite surfaces and deposit charge in dielectric materials will accumulate and lead to high voltages and large electric 7elds. When these electric 7elds reach a threshold, the dielectric breaks down and generates an electrostatic discharge which can couple with nearby circuitry and lead to spacecraft anomalies. IDM pulse rates were analyzed with respect to electron 2uxes measured by HEEF and provided strong evidence that energetic electrons are the primary cause of deep dielectric electrostatic discharges. Fig. 10 shows the correspondence between the integral 2uence of ¿0:85 MeV electrons and the total number of discharges from all samples observed per orbit. It is important to realize that the correlation between electron 2uence and pulsing was restricted to integral 2uences, and does not consider the e+ect of variations in the spectral shape of the electron population. As illustrated by Fig. 5 (discussed in Section 3.1) the electron spectrum hardened following the March 1991 storm (orbit 589). Thus, not only did the integral electron 2uence throughout the second half of the mission exceed that during the 7rst half, but the spectral hardness also increased. The following summarizes the understanding of ESD pulses gained from the IDM experiment (Frederickson et al., 1991, 1992, 1996a, b): (a) the average pulse rate is approximately proportional to the integral electron 2ux for energies ¿300 keV; (b) pulses do not correlate with instantaneous electron 2ux;
(c) pulses result from breakdown of electric 7elds in dielectric materials, and these electric 7elds decay slowly so pulsing may occur long after electron 2uxes diminish; (d) thicker dielectric samples pulsed more frequently, as did those with more exposed surface area; (e) small ESD pulses occur more frequently than large pulses and each dielectric sample produces a range of pulse sizes; (f) the pulse size is limited by the dielectric material properties and geometry, as well as by the electron 2ux spectrum and intensity; and (g) contrary to what was once believed, a minimum electron 2uence following an ESD is not necessarily required to trigger a subsequent ESD. 4.3. HESP The HESP experiment compared the performance of gallium arsenide germanium (GaAs=Ge) and silicon (Si) solar cells in terms of cell output parameters (current, voltage, maximum power), and also examined the performance of the GaAs=Ge cells as a function of coverglass thickness and material (Ray et al., 1993). The GaAs=Ge cells generally outperformed the Si cells. Solar cell degradation showed good correlation with changes in measured proton 2uences, though the high-energy electron 2uence levels did not appear to e+ect solar cell performance. The e+ectiveness of various annealing techniques on GaAs solar cells was also examined, and it was found that forward bias annealing is not only the simpler, but also the more e+ective and cheaper annealing method compared to constant and pulsed heating biasing techniques for cells under relatively thin cover glass.
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Fig. 10. Electron 2uence per orbit (top) and electrostatic discharge pulses (bottom) summed over all samples, plotted versus orbit number. Fluence is that which is estimated to impinge upon the dielectric samples by propagating ¿0:85 MeV electron 2uence, as measured by HEEF, through the sample cover plates (0:02 cm Al).
5. Conclusion Improving our understanding of space weather and its e+ects on systems in space—whether they be astronauts working on the ISS, space–based systems providing critical communication=navigation links with Earth-based systems, or onboard computers choreographing close encounters with planetary neighbors—is critical as we expand our state-of-the-art technology base into space. As this review was being written, NASA announced (Associated Press, 1999) that “Four hours before Galileo was scheduled to pass 186 miles above Io , Jupiter’s radiation triggered a fault in the onboard computer’s memory, shutting down all nonessential operations aboard the craft.” Fortunately, this temporary glitch in a memory chip was not a system-threatening crisis, and NASA’s engineer team was able to resolve the diGculty by uploading new commands to reprogram Galileo’s computer system. However, the event only underscores the vulnerability of space systems to the hazards of space radiation, a fact which satellite designers and operators must remain vigilant to. CRRES was a unique mission speci7cally designed to address a multitude of issues regarding space weather and its impact on technology. By combining an extensive suite of engineering experiments with an array of environment-measuring instruments, CRRES has provided an unprecedented opportunity to study the performance of a wide range of technologies over a large dynamic range of the space environment. CRRES has led the way into a new era of radiation belt dynamic analyses, and spurred on the investigations towards the next generation of space radiation models. However, this is only a beginning, as there
remains much to accomplish. The mechanism for accelerating the outer zone MeV electrons continues to be a hotly researched topic, and the frequency of super storms such as the March 1991 event which created secondary belts remains uncertain. It is clear that it is now time to supercede the two-state (solar minimum and maximum) static NASA radiation models with a generation of models which provide more detailed information about the huge dynamic range of radiation intensities. This will require greatly expanding the available database in spatial, temporal, and energy coverage, which can only be accomplished by making the inclusion of high quality environmental sensors on spacecraft more routine. And 7nally, the microelectronic technologies which 2ew and were tested on CRRES were early to mid 1980s vintage. As technology continues to advance, there remains the need to insure performance parameters are well understood for reliable space 2ight, and this will require a continuing program of space-quali7cation 2ights coupled to a rigorous ground-testing and modeling program. Although CRRES has accomplished much towards understanding the e+ects of space-weather, on-going programs are required to answer forthcoming questions necessary to establish and maintain a reliable technological presence in space.
Acknowledgements The author acknowledges the work performed by the entire CRRES team which made the mission possible, as well as all the authors whose original work has been reviewed here. Special thanks goes to Alan Ling, Dan Madden, Ernie Holeman, and Kevin Ray for helping with the 7gures.
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