Energetic particles in the environment of the Earth's magnetosphere

Energetic particles in the environment of the Earth’s magnetosphere E. MOBIUS Max-Planck-Institut

fur Physik und Astrophysik, Institut fur Extratcrrestrische Garching bei Miinchen. F.R.G.

Physik,

D-8046

Abstract-This review summarizes the work in the field of magnetosphertc energetic particles during the years 198771989. Out of a wealth of contributions it concentrates on a few topics. First it follows the path of ions extracted out of the polar ionosphere and their acceleration parallel and perpendicular to the magnetic field, as well as their subsequent transport into the equatorial magnetosphere and tail region. Then it focuses on acceleration of ions in the magnctotail and the related characteristics in the boundary layers including consequences for current substorm modeling. In the ring current region. the AMPTE and VIKING missions have made possible detailed studies of charge state and pitch-angle distributions as well as their variations during magnetospheric storms and substorms, from which conclusions on the transport and loss processes can be drawn. Recently, observations ofenergetic particles from orbiting nuclear reactors in the magnetosphere, which can be used as tracers for these satellites, have been made public. However. missions with an increased this may also constitute a serious background problem for future y-astronomy sensitivity of the instrumentation. Finally, leading beyond the boundaries of the magnetosphere. attention is drawn to the still ongoing debate on the source of energetic particles upstream of the Earth’s bow-shock and the respective importance of particle leakage and/or acceleration at the magnetospheric boundaries.

l encrgctic ions as a tool to identify the modes and propagation properties of magnetospheric ULF waves (TAKAHASHI ct al.. 1988 ; LIN rt c/l., 1988) ; l leakage of ions across the magnetopause and boundary layer physics (e.g. CURRAPI’ and GOERT~. 1989 ; SIRECK et al.. 1987a) ; l entry of solar particles into the magnetosphere via the magnctotail (VAN ALL.~;N c’t rrl.. 1987).

1. INTRODUCTION

This

review

on magnetospheric

energetic

particles

is

Review presented at the General Assembly of the LAGA in Exeter 1989. Therefore, it is basically restricted to the work done during the last two years (I 987-l 989) ; likewise the referencing in the paper. with a few exceptions in order to demonstrate where the current work tics in. For this report such particle distributions have been defined as ‘energetic particle distributions’ which arc ‘suprathermal within their original environment. Since more than I00 articles related to the subject have been published during this period, the review will be selective and cannot give credit to the full width and depth of the work accomplished in this period. The paper concentrates on a few major topics, which have received the attention of several scientific groups, and seem to show significant progress over the last two years, although this judgcment may show the author’s personal bias. A brief list of further subjects of interest is as follows : based

on a Reporter

l counterstreaming electrons at the geomagnetic equator from the aurora1 zone (KLUMPAR CJI ~1.. 1988) ; l observation of intense trapped electron fluxes at geosynchronous altitudes compatible with the Kenncl Petschek trapping limit (DAVIDSON et al., 1988) ; l relativistic electrons as evidence for repeated adiabatic compression acceleration in the magnetosphere (BAKER et rd.. 1989) ;

The paper is organized as a tour through the magnetosphere, starting with the extraction of ions from the polar ionosphere, and their acceleration and transport into the magnetosphere in Section 2. The ion acceleration in the magnetotail and the related magnctic field topology during magnetospheric substorms are treated in Section 3. With the availability of comprehensive data sets from AMPTE and VIKING, the ring current dynamics has attracted quite considerable attention. The related extensive studies of charge state and pitch-angle distributions in the ring current, as well as their variations during magnetospheric storms and substorms. are covered in Section 4. Section 5 is devoted to the discovery and interpretation of a class of artificial particle populations, that is, cncrgctic particles from orbiting nuclear reactors in the magnetosphcrc. the information on which was classified until very recently. Finally. Section 6 alludes to the ongoing debate on the source of energetic particles upstream of the Earth’s bow-shock and the related questions of particle leakage andjor acceleration at magnctospheric boundaries.

I I69

1170

E. Mosrus

2. EXTRACTION

OF IONS FROM THE IONOSPHERE

AND THEIR TRANSPORT

INTO THE MAGNETOSPHERE

The polar ionosphere is known to be one of the major source regions for the magnetospheric particle population (see, for example. recent reviews by CHAPPELL, 1988; PETERSON, 1988). The transport of ions out of the ionosphere into the magnetosphere depends on the scale height of the different species, the degree of ionization and finally requires significant energization of the ions both parallel and perpendicular to the magnetic field. The variability of the ionospheric source has been investigated with GEOS data at geosynchronous altitude over a solar cycle (STOKHOLM et al., 1989). The abundance of ions such as O’, O++ and He+ vary significantly with the solar F10.7-cm radio flux. which is indicative of the ionizing solar UV radiation (Fig. 1). This correlation is obvious in both 27 day variations (with a response time of -2 days) and variations over the solar cycle. It is interpreted in terms of enhanced heating and ionization of the upper atmosphere The variation of the source with the UV flux can

be observed directly in the O+ density in upflowing ion beams. This abundance variation has an additional consequence for the acceleration in electric fields parallel to the magnetic field : while the energy is the same for H+ and O+ (defined by the potential drop along the field line) during solar maximum, that is. when the oxygen abundance is large, the oxygen ions gain significantly more energy than do protons during solar minimum (COLLIN rt ul., 1987; REIFF et LJ~., 1988), when oxygen is a minor constituent, thus reducing the velocity difference between O+ and H+ ions. The additional energy gain by 0’ is attributed to an energy transfer from the H+ ions due to a two-stream instability which is preferentially excited for low O+ abundances. Under certain conditions the parallel acceleration is not sufficient to extract ions from the ionosphere and additional energization perpendicular to the magnetic field is needed. Well-known examples for the coexistence of energization in both directions are the conic ion distributions. These events have been discussed in terms of resonant interactions with ion cyclotron turbulence, non-resonant interactions with

He++

E m 10-Z

I

‘.

10-3 F,0.7cm

Line

7.

100

200

10-3’‘1 70

300

’

100

200

F,0,7 cm Line

F,o.7 cm 1 i ne

F,0,7 cm Line

F,o.7 cm Line

300

I

10

? 5

d)

He+ ’

z

T-----I

+1 F,0,7cm l-‘ig. 1. Correlation

Line

of the abundances of various ions at geostationary cm solar radio flux (from STOKHOLM E/

orbit with the intensity 1989).

cd.,

of the F10.7.

Energetic

particles

lower-hybrid turbulence, or as acceleration in oblique double-layers. Recently, PETERSONet al. (1988) have carried out a search for clear cases of transverse ion acceleration with the simultaneous presence of strong low-frequency waves in the mid-altitude aurora1 zone. In their study they found no example for a clear correlation of conic distributions and enhanced low-frcquency waves, although some cases may be considered as compatible with this view. However, such an cxampie was reported by AKIIRE et al. (1988) at the equatorward boundary of the cusp using VIKING data. From the observed wave intensities the heating rate was estimated to be z 30 eV s- ‘. The occurrence of tonics with simultaneous enhanced wave activity at the cusp boundary was confirmed by PETERSONet ul. (I 989) using DE data. Because of the uncertainties involved in the measurements, the observed heating rates are still consistent with both possibilities, resonant and non-resonast heating mechanisms, such that a decision on the mechanism is not possible. The transport of ions out of the polar magnetosphere into the magnetotail and the equatorial magnetosphere was addressed in a series of papers (SAUVAUD and DELCOURT. 1987; DELCOURT et al., 1988, 1989), using numerical studies of ion trajectories in three-dimensional magnetic and electric field models with the following results : l very low-energy ions (< IOeV) are still gravitationally bound to the ionosphere ; l ions with somewhat higher energies precipitate either into the magnetopause or into the opposite polar region ; l only particles with energies above - lOOeV, up to several kcV (ion beams or tonics), exhibit several bounces in the magnetic field indicating a trapped particle distribution. By drifting in the magnetospheric electric fields they gain energy and their trajectories are shifted to lower latitudes.

In a comparison

with DE I data,

DELCOURT et al.

(I 988) show that by using the measured

ionospheric outflow they can model the basic structures of the observed trapped distribution fairly well. However, a quantitative comparison is still difficult in view of the time variability of the effects, which is not included in the model. In particular, for residence times of > 2 h the ions seem to experience additional scattering and loss processes. In addition, transverse particle acceleration upon injection may account for other deviations from the experimental results. The basic tools to combine the ionospheric source with the observations in the tailward and equatorial

in the magnetosphere

1171

magnetosphere seem to be available. but they may have to be extended by including the time variability on various time scales. With such advanced diagnostics, it will be very fruitful to attack correlative measurements with simultaneous data in the polar region, and at various locations in the tail and the equatorial magnetosphere.

3. ION ACCELERATION

IN THE MAGNETOTAiL

The measurement of energetic particle distributions in the plasma sheet and at its boundary has long been used as an efficient tool to study the particle acceleration in, and the topology and dynamics of, the magnetotail. Using the data sets of the ISEE satellites, CHRISTON et al. (1988) surveyed the distribution of the plasma sheet ions in the wide energy range from 50eV to 1 MeV. As a main characteristic they found a representation by a n--distribution, that is. a Maxwellian of a few keV at low energies ( - < IO keV) and a power-law spectrum at the high energy end. Since these results have been obtained with instrumentation which does not differentiate between the ion species, an assessment on the possible contribution of minor species to the spectra is made. The authors conclude that the mimicking of a power-law by the overlay of several exponential spectra can be excluded. Temporal changes of the distributions are observed almost simultaneously throughout the full energy range, indicating that the relaxation time of the ion distribution in response to heating or cooling is short compared with the relevant temporal changes of the external parameters. On the other hand, WILLIAMS et ul. (1988a) report events in the plasma sheet when low energy ions with typical flow velocities of ~40& 600 kms- ’ are observed simultaneously with isotropic high-energy distributions. This seems to indicate that eventually distributions from different locations are mixed, before an efficient interaction leading to relaxation can take place. The observation of exponential energy spectra and an exponential flux increase ordered in energy/charge for individual ion species in the suprathermal energy range up to 230 keV/e in the course of substorm events by MOBIUS et d. ( 1987) seems to contradict the results of CHRISTON c’t a/. (1988). However, the latter observations were restricted to active times in the plasma sheet, when an exponential spectral dependence seems to extend to higher energies than during quiet times. These observations could be reproduced by SACHSENWEGBR et ul. (1989) (see Fig. 2) in a simulation of the ion acceleration in a two-dimensional rcconncction configuration starting with the

E. M~~RICJS

1172

AMPTEBULEICA 104

0 He+

to-

,

1

3

-AO+ 103-

II--___!_ _ 0

I

XHf I + He*+

+112+

___-~L

.I.

120

energy/charge

240 [keV/e]

240

10°0U

energy/charge

[keV/e]

Fig. 2. Left : ratio of the fluxes of various ion species after and before substorm onset on 8 April 1985, vs energy/charge obtained with the AMPTE/SULEICA instrument. Right : flux ratios vs energy/charge from test particle calculations in a time-dependent resistive MHD model of reconnection in the magnetotail (adapted from SACHSENWEGEK etnl..1989).

observed old plasma sheet population, which was approximated by a bi-Maxwellian. The significant flux increases observed at high energies for all species up to the mass of oxygen could only be obtained by the assumption of a relatively large electric field in the tail ( z I mV m ‘) along the neutral line and a dawn-dusk extent of the acceleration region of up to 40 R,.. The ions show the behavior of conventional current sheet acceleration, which was recently reviewed (c.g. SPEISEK. 1987). and showed no sign of stochasticity in the vicinity of the neutral line. The ordering of the spectra in E/Q rather than in E/M as expected for current sheet acceleration can be attributed to the increase of the reconnection electric field with time. This view of the ion acceleration seems to be in accordancc with the findings by BWHN~R and ZELENYI (1987) and ZEL~NYI et (I/. (1988), who emphasize that, in particular, high-energy ions perform quasiadiabatic trajectories, which means that the longitudinal invariant of motion is kept constant or that the gyro-radius of the ions is large compared with the typical curvature of the magnetic field in the neutral sheet, while the classical adiabatic motion requires the opposite condition. Chaotic orbits. which would lead to a stochastic redistribution of the pitch-angles, arc seen for the transition between both regimes. Using the transition to chaotic orbits for thermal electrons as a condition, which would be a possible mechanism for a local resistivity, B~~CHNERand ZELENYI ( 1987) derive a distance for the distant neutral line in the tail of XOP120 R,,. consistent with observations reported from ISEE-3. It should be mentioned hem that the simulations by

SACHSENWEGERet ul. (I 989) cannot produce isotropic ion distributions; the simulated signatures were similar to the highly anisotropic distributions seen at the plasma sheet boundary rather than the more isotropic ones in the central plasma sheet. In their study of quasi-adiabatic particle acceleration, ZELENYI et al. (1988) point out that after acceleration the ions reach the boundary layer and by convection of the flux tubes may finally fill the plasma sheet proper. However, the problem of isotropization of the ion distributions remains unsolved. The models discussed above are based on a quasi-stationary MHD reconnection scenario, which could be a possible reason for these diffcrences between models and observations. AMBROSIAWO ct rd. (1988) have studied the effects of turbulent reconnection on particle acceleration. Their main arguments are that turbulent reconnection could produce stronger electric fields and provide a scattering mechanism for the ions, which seems to be necessary to explain the observations. However. the reconnection electric field may depend on the initial fluctuation level, and thus a comparison with the other models would be difficult. The spatial characteristics and the dynamics of beam-like distributions at the plasma sheet boundary serve as a sensitive tracer of the topology of the tail configuration. In particular, during active times the boundary flows show similar directional characteristics to the energetic ions. These signatures have been used as an argument for the boundary layer model of substorms and as a contradiction to the neutral line model, since the latter model scemcd to predict large flows in the neutral sheet and not at

1173

Energetic particles in the magnetosphere

the boundary. SCHINDLER and BIRN (1987) however, have shown in an analytical treatment that the intense boundary flows are indeed a natural consequence of the reconnection geometry : the plasma flow is mainly forced along the magnetic field lines, since the flux tubes are compressed in addition to their convection. This view is confirmed in MHD simulations by SCHOLER (1987) who found strong earthward flows inside the plasma sheet boundary and weaker tailward flows outside the boundary. Strong earthward flows in the neutral sheet occurred after the plasmoid was pinched off into the tailward direction. MARTIN and SPEISER (1988) come to similar results in their study of the evolution of ion beams in the plasma sheet boundary with distance from a neutral line. The calculated pitch-angle distributions are in good accordance with earlier experimental results. It should be noted that DELC~URT et al. (1989) also find an earthward streaming boundary layer (on field lines at ~67. latitude) as a consequence of neutral sheet acceleration of ions flowing out of the polar ionosphere. There seems to be a general agreement that particles accelerated in the tail magnetic field configuration emerge as streaming boundary layer particle populations. Recent studies in the distant tail also concentrated on the boundary layer. Data from ISEEshowed that the plasmoid which is ejected in the course of a substorm is embedded in layers of energetic ions and electrons streaming in beam-like distributions downtail (RICHARDSON et al., 1987; SCHOLERet al., 1987). The signatures of these beam distributions as well as the temporal and spatial ordering with energy could be reproduced in detail in a trajectory calculation by SCHOLERand JAMITZKY(1989), who have used a magnctic field geometry from MHD simulations of reconnection in the near-Earth magnetotail (Fig. 3). NISHIDA et al. (1988) have utilized energetic particle measurements during a period when ISEEwas at distances around 80 I?,: from the Earth, that is, in a regime clearly earthward of the distant neutral line of to the magnetosphere (at z 120 R,), particularly diffcrcntiate between signatures consistent with the boundary layer model and the neutral line model of substorms. The most striking evidence for the occurrence of a near-Earth neutral line, with the onset of a substorm, seems to be the high correlation of an antisunward anisotropy of energetic ions with the exit from the plasma sheet and with the substorm onset, while in the near-Earth region (<23 R,, the apogee of ISEE-I and -2). a predominant sunward anisotropy is observed in the plasma sheet boundary. This combination of observations may be taken as indicative of a near-Earth neutral line rather than enhanced activity of a boundary layer up to the distant neutral

line, although not all of the clear substorm onset cases were accompanied with a southward magnetic field direction. The results reviewed above lead to the impression that significant progress has been made during the last few years in the understanding of the structure of the magnetotail, the topological changes in the course of substorms and the acceleration of ions. The major observations seem to fit into a framework with a nearEarth neutral line emerging at substorm onset and a current sheet acceleration in the corresponding field geometry. Yet the observed angular distributions of the energetic ions do not seem to fit into the current models, which still do not predict an isotropization of the accelerated particle distributions. Therefore, the tail studies should be extended to correlative data analysis making use of the mature data sets available from the ISEE and AMPTE satellites to separate temporal from spatial convection effects. They should address the question of whether the central plasma sheet can be filled by plasma injected into the boundary layer, and then convected into the centre, or whether energization occurs at the different locations simultaneously. In addition, the question of turbulent reconnection and possible scattering mechanisms, as well as contributions from the three-dimensionality of the problem, need to be addressed in future simulations. To complement the MHD models of the magnetotail, collisionless reconnection should be addressed by the use of a kinetic theory. In such models stronger electric fields may emerge than in MHD models in which the electric fields are directly tied to the resistivity in the reconnection region.

4. CHARGE STATE AND PITCH-ANGLE DISTRIBL!TIONS

IN THE EQUATORIAL

MAGNETOSPHERE

The charge state distributions in the ring current region have been extensively studied over the last 2 years using the instrumentation on the AMPTEjCCE satellite. In a series of papers KREMSERet al. (1987a,b ; 1988) have used the charge states of 0 and C ions to assess the sources of the ring current population as well as the transport and loss of ions. The average distribution of the 0 and C ions with L-shell is compatible with two different sources, the ionosphere providing Of ions and the solar wind providing Ob+ and C? ions (Fig. 4). The distribution of intermediate charge states can be understood in terms of diffusion and charge exchange according to the model by SPJELDVIKand FRITZ (1978). The two sources mix for 03+ and 04+ Solar wind ions are preferentially found

E. M~BKJS 12. FEE

ISEE-

1993

10-x

PROTONS SO-36

I

keV

102

10' ELECTRONS 7S-115

keV

I

100

Fig. 3. Left: temporal variation of the magnetic field and energetic particle fluxes on entry into and exit from the plasmoid, indicating the ordering of the layers of energetic particle beam distributions in the boundary (fromSCHOLEReraf., 1987). Right : variation of the Buxes of energetic ions with the distance from the neutral sheet (z) from test particle calculations in a time dependent two-dimensional resistive MHD model of tail reconnection (from SCHOLERand JAMITZKY. 1989).

in the outer L-shells, while the ionosphere dominates in the inner L-shells with a strong dependence on K,. In a study of the substorm injection of ions with different charge states, SIBECK et a!. (1988a) have found that the high charge states, particularly of oxygen, dominate at high energies (> 300 keV total), while singly charged ions prevail at low energies. They suggest two different explanations. First this result could be understood with injections from two different sources, the ionosphere and the solar wind, and an acceleration by electric fields which leads to energies proportiona to the charge states. A second possibility is that the pre-existing quiet time ring current population, which already shows this feature, may gain an incremental energy increase during substorms. This may in principle explain the energy distribution of the charge states ; however, at least an additional injection of ionospheric ions will be needed to account for the rC, variations of the Of ions in the magnetosphere, as

mentioned already above. Further systematic studies of the abundance variations during quiet times and after substorm injections may be needed to assess the additional injection of solar wind ions, which has been identified already for a Iarge storm by HA,MILTONet al. (1988). While charge state distributions contain information about the ion sources and the energization, the pitch-angle distributions highlight the transport and loss processes involved. A statistical study by SIBECKet al. (1987b) showed that the general pattern of the ion pitch-angle distributions in the ring current is a mirror image of the electron distribution. Deviations from the usual trapped distribution in the outer L-shells, with losses at pitch-angles near 90’, can be explained in terms of drift-shell splitting and magnetopause shadowing in the direction of the drift paths. This result is also consistent with the diurnal variations of the abundance of different charge states

Energetic particles in the magnetosphere

0

3s

L. 0

05'

Ob.

,.--. Calculated intensity, source-O+,energy: 100 keV,after Spjeldvtkand Fritz 11978) ,Q f VT Measured relative flux:

.

far Kp: O-1;

n for Kp:?-2;

0 for Kb: 2-3

/

t; .:

IVII""'

DRIFT SHELL PARAMETER

L

lOOkeV,afterSpjeldvik and Fritz Measured relahve flux:

DRIFT SHELL PARAMETER

L

Fig.4. Comparison of the measured relative fluxes of oxygen ions with charge states 1-6 as a function of L,, with the calculated fluxes of these ions based on the model by SPJELDVIEC and FRITZ (1978) for an O6 + source (upper panel) and an O+ source (lower panel) at L = 6.6 (from KREMSERef ul., 1988).

1176

E. Miie~us

reported by KREMSERet al. (19&7a), which they attribute to drift-shell splitting on the outer L-shells. An enhanced drift-shell splitting is usually observed during substorms and storms (SIBECKet nl., 1987~). The development of magnetospherjc substorms and storms was studied in detail for several examples using the instrumentation of the AMPTEjCCE sateflite. Because of the elliptical orbit close to the geomagnetic equatorial plane, the radial and temporal evolution of storms could be attacked. By measuring the total ion flux between 25 and 1000 keV, where 75-800/o of the ring current is concentrated, Lur EI ui. (1987) showed that the particle pressure increases start in the outer regions-often with multiple injections, which could bc interpreted as multiple substorms-and in the later phase propagate to the innermost regions of the ring current. In some regions the pressure increase leads to plasma beta of the order of, or even exceeding. i. However, the maximuln of the current density may be several R, away. due to the fact that the current is mainly driven by pressure gradients rather than curvature drifts. A comparison of &helocal currents with the actual Dst values shows that the Dessfer-ParkerSckopke relation basically describes the relationship between the measured particle energy densities and the magnetic field disturbance. A similar conclusion- with an absolute agreement within a factor of 2-is quoted by HAMILTONet al. (1988) who examined the great storm in February 1986, including the detailed ion composition. During the maximum of this storm the energy density of Of exceeded that of H+ with O+ and N’ together constituting about 60% of the total energy density. The ionospheric source was estimated to contribute 70-800/b of the total ring current at this time, yet a large increase of the He”+ flux showed that the solar wind was also a potent plasma source. The rapid initial decay of the ring current after the storm (see Fig. 5) was identified from the composition measurements as the rapid loss of O+ and N’+, while the energetic protons (> 100 keV) were stored for several weeks, because of a significant difference in the charge exchange cross-sections. This typical two-phase recovery behaviour of large storms, which was already reported by AKASOFU et al. (1963, suggests that the enhancement of O+ and N’ tnay be a general feature of large magnetospheric storms. Figure 5 also shows that, white the ring currcnt energy density generally tracks the Dst value very well, significant deviations are seen during the main phase of the storm. These irregularities may be attributed to asymmetries often observed during this stage of magnetospheric storms. Dawn-dusk asymmetries which were sustained for several hours have been reemphasized by STI~DEMANN c’t al. (1987) in a study of

a storm with VIKING data. A global image of the ring current region using energetic neutral imaging of particles escaping the ring current region, after charge exchange with exospheric atoms, has reveatcd a very strong (220: 1) flux asymmetry between midnight and noon at the peak energy (ROELOF,1987). A first quantitative comparison of individual ion energy spectra at various locations in the ring current with a dynamical model of the ion transport during magnetospheric storms has been presented by KISTLERCt (11. (1989). Figure 6 shows a comparison of the H *. 0’. He+ and He’+ phase space densities in the post-noon region with the model predictions, assuming a Volland-Stern electric held with a radial dependence of R’ for K,, = 6. an offset of the symmetry axis of the ion drift paths of 2 and 3 h eastward, and charge exchange as the only loss process. The energy spectra of all ions are described remarkably well with a prcfercnce for a 2-h offset. Inclusion of strong pitch-angle scattering. at least over wide spatial regions, leads to significantly stronger losses than actually observed. This seems to be in contradiction with other observations. for example, ST~~UEMA~Net al. (1987) who attribute isotropic pitch-angle distributions during a storm to the presence of pitchangle scattering. However, such processes may be localized and time dependent. To cstabhsh this possibility and to disentangle temporal from spatial variations, a correlated data analysis with simultaneous data from several spacecraft should be applied. Such data sets may be available from the combination of AMPTEiCCE and IRM, VIKING and possibly some geostationary satellites.

5.

ENERGETIC PARTICLES

FROM ORBITING

NU<‘LEAR REACTORS IN THE MAGNETOSPHERE

In a series of five articles (HONESand HEBE, 1989; O’NEILL t’t al., 1989; PRIMAC’K,1989: RFLXR et al., 1989; SHAREet ni.. 1989) the features and consequences of transient y-ray events originating from nuclear-powered satellites, which were observed by the SMM satellite in the equatorial magnetosphere, are described in a comprehensive way. The detection of these events and the first interpretation dates back several years ; however, the disclosure of this analysis was declassified only very recently. The observed y-ray transients can be organized in a scheme of three different event types (RIE:GERei al., 1989). which are different in both their temporal and spectral characteristics : (1) a sharp ;.l-line at 0.511 MeV, with a steep increase and decrease in intensity:

100

II77

Dst and Rinq Current Energy Density ,"'I',' IV t - II I’,“’ I”, I -I’,‘~‘,~“,“’ Pass 1

2 -----__

0

3

4

6

5

7

8

+ +

W SC - -100 b-

c 42

:: -200

-300

Fig. 5. Comparison

of the measured total ring current energy density with Dst values in the course of the February 1986 geomagnetic storm (from HAMILTON ef ~1.. 1988).

104 103 102

lO_‘L

L

100

IO1

Energy/Charge

102 IkeV/e)

103

IO0

101

Energy/Charge

102

103

ikeV/e)

Fig. 6. Comparison of the observed phase space density vs energy~char~e for H+, 0 I, He+, and He’+ in the post-noon sector of the ring current during a ma~etospheric storm on day 263 of 1984, with model predictions assuming adiabatic drift and charge exchange losses in a Volland-Stern electric field continuration for K, = 6 t’from KISTLEK cf al.. 1989).

1178

E. Mdalus

600

200

I

t

t

/

760

770

780

790

I

800

!

810

1

820

830

MINUTES

by the nuclear-powered satellite. A second maximum is seen when both source and observer cross the same L-shell simultaneously. These characteristics indicate that the positrons and electrons are trapped very efficiently in the L-shell of injection. The spatial and temporal distribution of these particles around the orbiting reactors was modclled quantitatively in terms of drift and diffusion on the original L-shell by HONES and HIGBK (I 989). These observations on the one hand could serve as a surveillance method for orbiting nuclear reactors which are being used for various military purposes. On the other hand the injected electrons and positrons from such satellites are of great concern to y-astronomers, since they constitute a source for background which has to be taken very seriously for the very scnsitivc future instrumentation.

6. THE ROLE OF ESCAPE AND PARTICLE ACCELERATION DURING UPSTREAM PARTICLE EVENTS

POSITRONS.

,,

. . . . ../

ELECTRoNS

Fig. 7. Upper panel : variation of the 511 keV positron annihilation line on board SMM with time correlated with the relative position of SMM and Cosmos 1818 in terms of L. Lower panel : illustration of the transport of electrons and positrons from the orbiting reactors (Cosmos satellite) on the geomagnetic field lines (from SHARE rtal.,1989).

(2) a broad energy spectrum up to ~3 MeV, with a temporal structure similar to type (1) ; (3) a broad energy spectrum, with slow intensity increase and decrease. All events could be correlated by comparing the SMM orbit with the orbits of Soviet nuclear-powered satellites. While event type (3) was identified as direct yrays from the nuclear reactor during very close encounters with SMM, the other events could be organized by the relative position of the two satellites with respect to the Mcllwain L-parameter. Event type (I) is attributed to positrons producing a 0.51 I MeV annihilation line in the structure and shielding of SMM, and event type (2) is electron bremsstrahlung. Both positrons and electrons are injected into the Earth’s magnetic field and are transported away from their source by means of drift and diffusion within the original L-shell (Fig. 7). An intensity maximum with a sharp cut-off is found at the lowest L-shell reached

Energetic particle distributions are frequently observed upstream of the Earth’s bow shock. It has been the subject of an ongoing dispute, over the last years, as to what extent these energetic ions are produced by in-situ Fermi and shock-drift acceleration or whether their origin is solely in the magnetosphere. A major problem is that some of the arguments under discussion do not allow a decision between both possibilities. The magnetic connection of the spacecraft location to the bow shock, for example. is known as a necessary condition for both processes. Other conditions under which upstream particle events are observed, for example, enhanced magnetic activity, nzu,r constitute a trigger for both possibilities : during periods of high magnetic activity the magnetosphere, of course, is filled with a significantly enhanced population of energetic particles, which may leak out into the solar wind, but the enhanced interaction of the solar wind with the bow shock may also be a reason for an increased acceleration efficiency. Thus, without a comparison between the data and a quantitative modeling of boflz physical processes, leakage and in.situ acceleration, or without a decisive ‘experimentum crucis’. no satisfying conclusion may be reached in this controversy. The work by ANAGNOSTOPOULOS et ul. (1986) about the upstream event of 31 October 1978, which was studied earlier by, for example, IPAVICH et al. (1981), prompted a dispute with a Comment and a Reply. In contrast to the earlier studies in which the data were related to Fermi acceleration at the bow shock. ANAG-

Energetic particles in the magnetosphere NOSTOPOULOS et al.(1986), using correlative studies of IMP 7 and 8 upstream and in the tail, argued that the upstream events should be identified as of magnetospheric origin. Their main arguments are based on :

1179 overlapping

range

22LT 0700-0710 UT

l a correlation of upstream ions with IMF variations; l a correlation of ion flux increases upstream and in the tail ; l a power law energy spectrum extending to energies > 1 MeV at both locations.

SARRISetal.(1987) follow a similar approach for two other events in 1977. In his Comment on the paper by ANAGNOSTOPOULOS et ui. (1986), ELLISON (1987) compared the combined energy spectra from the ISEE and IMP spacecraft with a model calculation. He showed that the observed spectra could also be explained in terms of Fermi acceleration of the solar wind and a pre-existing solar energetic particle population, which was indeed present during the event. ANAGNOSTOPOULOS et al. (1987) responded to this criticism by arguing that the two spectra from satellites which arc separated by several R, in distance would need an adjustment in flux before they can be combined and that, therefore, the comparison with the Fermi model is irrelevant. It is obvious from this discussion that in order to decide on the source of the upstream particles it is very important to use multi-spacecraft measurements wherever possible. However, great care is necessary when comparing energy spectra at different locations. Both the variation of the seed population (e.g. presence of solar energetic particles) and propagation and acceleration effects between different locations (in the solar wind and in the magnetosphere) have to be taken into account in both approaches. Apart from the points discussed above a particular problem arises for comparisons with energetic particle fluxes in the tail : flux variations here can indicate either temporal variations of the activity or a crossing of the plasma sheet boundary by the satellite, as has been shown extensively in the past. The latter problem usually does not apply to particle populations in the ring current, which are well defined in their geometry. Thus BAKER et al. (1988) and SIBBCKct d. (I 988b) have studied a strong compression of the magnetosphere on 1 November 1984, using data from satellites in the ring current and upstream of the bow shock. Besides demonstrating that a number of necessary conditions for an efficient leakage of magnetospheric particles were fulfilled during this very active day. the authors present energy spectra

ENERGY

(kd)

Fig. 8. Simultaneous measurements of differential flux of energetic ions vs total energy upstream of the bow shock and in the magnetosphere at geostationary orbit at two different local times during an upstream energetic ion event on 1 November 1984 (adapted from BAKER e/al., 1988).

of the ions upstream of the bow shock, in the boundary layer at the magnetopause, and in the magnetosphere (Fig. 8). SIBECK et al. (1988b) claim that after the strong compression the flux levels of all three species (H, He, 0) are large enough in the boundary layer to account for the fluxes seen upstream of the bow shock and that the spectra are similar. They conclude that the magnetosphere constitutes a sufficient source of upstream particles in this case. BAKER rt al. (1988) present spectra which were obtained simultaneously in the magnetosphere and upstream for a combination of all ion species (Fig. 9). They argue that the spectra-upstream and at the location from which, according to the ion drift paths, leakage is to be expected-follow a power law with the same spectral slope of z -5.5. Interestingly enough the spectral slope is significantly harder in the magnetosphere in that energy range (< 200 keV) which overlaps with the presented upstream spectrum. as can be clearly seen in Fig. 9. The same trend seems to be true for the various ion spectra shown in Fig. 8 : for a factor of 4 increase in energy the upstream spec-

E. M&KJS MEPA

ion flux versus energy

November 1, 1984

“~~

~~

~~

0 Upstream 0717-0723 m Boundary layer 1036-1048

-4 i.lltrrlllj 10

100 Energy

1000 (keV)

10,000

0 Upstream 071 T-0723 n Boundary layer 1036-1048

t TO

0 Upstream 0717-0723 9 Boundary layer 1036-l 048

1 L lStS*Ii 100 Energy

1000 IkeV)

10,000

10

100 Energy

1000

1, 1Luill 10,000

(keVI

Fig. 9. Comparison of the energy spectra of hydrogen, helium and oxygen ions (with no charge state differentiation) upstream of the bow shock, in the magnetosphere and the boundary layer on 1 November 1984 (from SIHECK cf cd.. 19Wb).

tra fall off by more than one order of magnitude more than the magnetospheric spectra. Therefore, this argument. which is used as one of the key elements in the discussion, does not seem to be as strong as has been claimed by the authors. Their interpretation would imply that the escape of ions from the magnetosphere with higher rigidity is suppressed relative to ions with lower rigidity. Observations of magnetospheric energetic ions in the magnetosheath, reported by WILLIAMS et al. (1988b), suggest that. at least for those cases with a clear identification of plasma sheet ions, their spectra remain unchanged after leakage into the sheath. Thus a change of the spectral slope would have to occur on the way through

the magnetosheath and across the bow shock. Another proposed test for the source of upstream ions would be the simultaneous measurement of the ion composition. SIBXXK ~2 ~1. (1988b) present an example for similar composition of He and 0 ions upstream and in the magnetosphere during the observation period. However, the measurement did not distinguish between O+ and highly charged oxygen ions found in the solar wind and in the outer L-shells of the ring current. Therefore, without additional information this test does not seem to be selective between the two models. The geometry of the flux tubes along which particles may find their way upstream and the physical pro-

servdtlons of an upstream rvent at the magrrclopause. at lhc bow shock and upstream It WRS shown that : o

the energy

in the

spectra a~-e difTzrzn[ upstream

magnetosheath

and

on the ens hand, and m the

E. Miierus

ilX2

situ acceleration prevails. These questions are intimately tied to the lack of an appropriate quantitative model for both processes which can be compared with the observations. It is obvious that the models of insitu acceleration are much more advanced than any treatments of the escape from the magnetosphere and the subsequent transport through the magnetosheath. As far as the acceleration models are concerned, the production of seed particles needs to be addressed, possibly by more detailed simulations of the shock interaction with the local wave populatjon. A specific effort in this field, including a detailed correlative data analysis in the appropriate regions, may lead to very fruitful results in the coming years. 7. CONCLUSIONS AND

EXPIRATIONS

Major progress in the understanding of the morphology, transport and acceleration of energetic particles inside the magnetosphere and across its boundaries has become possible due to comprehensive data sets from various satellites with improved instrumentation for the study of the ion composition and the development of more sophisticated simulation tools. In particular, the studies of particles fed into the magnetosphere from the polar ionospheric region, of the acceleration of ions in the tail and of the behaviour of ions in the ring current have profited from missions dedicated to these questions. The recent

development of global imaging of the magnetosphere begins to complement the in-situ measurements by an instantaneous view of the spatial distribution of ions during storm and substorm events in the ring current. An additional interesting discovery of the last two years is the observation of electrons and positrons injected from orbiting nuclear reactors which can serve as a diagnostic tool for the surveillance of nuclearpowered satellites and as an additional data base for the study of particle transport in the radiation belt. The use of multi-spacecraft measurements in order to trace particles on their way through and out of the magnetosphere has enabled steady progress in the understanding of particle escape from the magnetosphere and the acceleration at magnetospheric boundaries. Within the forseeable future the now mature data sets from the particle experiments on satellites like ,AMPTE/CCE and IRM, DE and VIKING will be used for further correlative data analysis in combination with advanced computer simulations of the major dynamic effects. These will http to reveal the complex interactions between the various regions in the magnetosphere during magnetospheric substorms and storms. Azknoll,/ruTgementThe author is grateful to M. Scholer, B. Klecker and D. Hovestadt. for helpful discussions during the preparation of the review. The assistance of D. Sibeck and 8. Tucker in compiling the extensive collection of literature is gratefully acknowledged.

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