Pages 851-856

ENERGETIC PARTICLES IN THE VICINITY OF THE DAWN MAGNETOPAUSE

Z. Nemecek1, J. ›Safrankova1, O. Santolik1, K. Kudela2 , E. T. Sarris3

1 Faculty of Mathematics and Physics, Charles University, E-mail:   nemecek@kevf05.troja.mff.cuni.cz
2 V Holesovickach 2, 18000 Prague 8, Czech Republic
3 Institute of Experimental Physics, Kosice, Slovakia
DUT, Xanthi, Greece

ABSTRACT

This paper presents first results of two-point measurements carried out onboard two satellites of the INTERBALL project which was launched into a highly elliptical orbit in August, 1995. Both satellites are provided with instrumentation for charged particle measurements in a wide energy range (from 0.1 keV to 1 MeV). The distance between the satellites was about 1000 km for the presented events. During the crossings of the magnetopause region, the flux and angular distribution of electrons just inside the low-latitude magnetosphere are compared with those just outside the boundary layer and the electron leakage from the magnetosphere is examined. The peculiarities of the distributions observed by different satellites are discussed in terms of boundary position shifting. The results of the study of the low-latitude magnetopause are compared with the measurements carried out in the plasma mantle region.

INTRODUCTION

The presence of accelerated particles of different origin is a common feature of the interplanetary medium. One of the dominant sources of these particles in near-Earth space is the Earth's magnetosphere. Since the magnetospheric particles should cross the magnetopause to be observed in the outer space the observation of the energetic particles at the magnetopause can provide important information on its structure. Two mechanisms which can result in particle entering the magnetosheath have been suggested. Sibeck et al. (1987 a,b) argues that the magnetospheric particles reach the tangential discontinuity magnetopause, scatter, and enter the magnetosheath. These particles can be observed over a broad range of local times but the ions are expected to primarily enter the dusk magnetosheath and energetic electrons to enter the dawn magnetosheath (Kudela et al., 1992). The magnetic merging model suggests that the particles can escape along interconnected magnetosheath-magnetospheric field lines (Scholer et al., 1981; Daly et al., 1984), i.e. across a rotational discontinuity magnetopause. Since merging is expected to have a limited spatial extent and temporal duration the observation of the energetic particles would exhibit bursty features. The bursts of energetic particles would be regularly observed during periods of southward interplanetary magnetic field (IMF) which is believed to favor magnetic merging and their occurrence during northward IMF would be sporadic. On the other hand, other possible sources such as Fermi or shock drift acceleration at the bow shock can contribute to the flow of energetic particles in the vicinity of the magnetopause (Galeev et al., 1986) and thus the interpretation of the observed flows can be difficult.

The characteristic of all possible sources of the energetic particles in the magnetosheath and some tests by which they can be distinguished were outlined by Sibeck and McEntire (1988). Kudela et al. (1992) noted that most of the energetic particles have near -90° pitch angles and they are trapped near the geomagnetic equator. Such particles can escape from the magnetosphere and enter the magnetosheath only at low latitudes. Based on this fact, they expect that the flux of energetic particles would be largest just inside the low latitude magnetosphere, less in low latitude magnetosheath, still less at higher magnetosheath latitudes, and smallest inside the middle latitude magnetosphere. The statistical study given in the paper shows that the energetic particles in the magnetosheath exhibit no dependence upon the orientation of the magnetic field. It indicates that the dominant source of these particles is leakage from the magnetosphere and the escape of particles along merged magnetic field lines can be considered as a supplemental source.

Another statistical study (Kudela et al., 1994) shows that the probability of the observations of the energetic particles just outside of the low latitude magnetopause is even higher during periods of strongly northward IMF ( BZ  2 nT). This observation confirms the relative importance of the particle leakage across the tangential discontinuity magnetopause.

The present paper describes a comparative study of the flows of the accelerated electrons in the vicinity of the dawn magnetopause at two different latitudes. The study is based on the data of two INTERBALL satellites and compares the data from different instruments. The study confirms the leakage of the particles through the low latitude magnetopause as a principal source of the observed flows.

THE DATA SET

The data used in the present study have been obtained during August - September, 1995 by the INTERBALL 1 and MAGION-4 satellites. The satellites have been launched into a common highly elliptical orbit with initial apogee ~ 195,000 , km, perigee of ~ 800 km, and inclination 63° August 3, 1995. The distance of the satellites varies along the orbit and evolves in time but the distance of about 1 000 km can be considered during the events which will be presented.

Both satellites are equipped with a set of instruments for plasma investigation. The omnidirectional plasma sensor VDP placed on the main satellite is designed to determine an integral flux vector or an integral energetic spectrum of ions and electrons. For simultaneous measurements in all directions the VDP device contains six independent wide-angle Faraday's cups (FC) (Safrankova et al., 1996). The MAGION-4 satellite is equipped with a similar system, the VDP-S device consists of four independent FCs which are placed symmetrically on the subsatellite with axes which are declined from the main subsatellite's axis by ~ 45°.

The ion and electron energy spectra are measured by the MPS/SPS spectrometer onboard the MAGION-4 satellite in the energy range from 40 eV to 5 keV in several directions (Nemecek et al., 1996). This set of devices allows us to determine all basic plasma parameters - the density, velocity, and temperature. These parameters are, together with magnetic field measurements, used for a determination of magnetopause layers.

The flow of energetic electrons is measured by the DOK-S instruments onboard the MAGION-4 satellite. A full description is in Kudela et al. (1996). Here we are using the integral flux of electrons with energies E > 23 keV measured by the detector with the axis oriented in the antisunward direction for the ideal subsatellite orientation. The silicon detector with a thickness of 300m is used and ions up to 600 keV are stopped in the foil in front of the detector. The maximum viewing cone is 56°, and geometrical factor is 0.117 cm2..sr.

EXPERIMENTAL RESULTS

Orbits of the INTERBALL satellites cover a wide range of local times and latitudes of the magnetopause region. Inbound crossings are located near the equatorial plane, the outbound ones are scanning the range of of ~ 30° to ~ 60° of latitude and, due to the Earth's orbit, consecutive crossings differ by  min of the local time. The preliminary analysis of the data showed that the structure of the magnetopause and, consequently, the conditions for the leakage of the magnetospheric particles, vary with local time and thus the collected data set is not sufficient for a statistical study at a given local time. For this reason we have chosen a comparative study of crossings in different latitudes and limited it to the morning flank of the magnetopause. The INTERBALL satellites registered a few inbound and outbound crossings in this region; the presented results are typical for each of these two groups.

 Figure 1: Magnetic field and plasma parameters from INTERBALL 1 on a pass from the magnetosheath into the magnetosphere at a time of southward IMF ( ne, Te- electron density and temperature, Fe - high energy electron flux).

Figure 1 shows the magnetopause crossing as registered by the INTERBALL 1 satellite on August, 29th, 1995. The GSE coordinates of the spacecraft were GSE (X,Y,Z)=(1.6,-11.1,-1.7) RE at 09:00 UT. The crossing of the magnetopause layers starts at 08:59 UT with the decrease of the plasma density and the increase of the magnetic field strength. The first encounter with the low latitude boundary layer (LLBL) is indicated by the sharp rise of the electron temperature. The transition low latitude boundary layer - inner magnetosphere - can be identified by the increase of the flux of the high energy electrons (bottom panel) at 09:58 UT. It can be noted that according to the criterion given by Phan and Paschmann (1996) for the inner edge of the low latitude boundary layer, the falling of the plasma density to 5% of the magnetosheath value, the crossing occurred half an hour later, but we think that the flow of the energetic particles is a better indicator of the magnetosphere.

 Figure 2: Detail plot of the magnetopause crossing from Figure 1 as measured by the MAGION-4 satellite.

We will concentrate our attention on the first crossing of the magnetopause. The fluctuation of the magnetic field in the interval from 09:00 to 09:15 UT indicates that the crossing is a multiple one. The plasma data in Figure 1 are two-minute averages of the key parameters and thus they cannot reflect the fine structure of the crossing. Figure 2 shows this interval with a better temporal resolution. The data have been collected by the subsatellite MAGION-4 which was located only 1000  km away. This distance is too small for the study of the high energy particle peculiarities because it represents only a few ion gyroradii but it is sufficient for the study of the boundary structure, Nemecek et al. (1996); Safrankova et al. (1996). From these studies it follows that the regions of high plasma density can be attributed to the magnetosheath. During the intervals characterized by low plasma density and low temperature the satellite moves in the outer part of the LLBL and the increased electron temperature indicates that this is the inner part of the LLBL. It means that the satellite crossed the magnetopause 11 times during the depicted time interval. The flux of the high energy electrons is nearly constant through all crossings of the magnetopause and it increases only after transition to the inner part of the LLBL. The detector is scanning a broad range of pitch angles (from ~ 20° to ~ 110° ) during the depicted time interval due to the satellite's rotation and due to the changes of the magnetic field direction. Since neither the modulation of the count rate by the satellite spin period nor the changes of the magnetic field direction can be seen it can be concluded that the pitch angle distribution of the particles is nearly isotropic on both sides of the magnetopause, as well as in the inner part of the LLBL. As the density in LLBL region is substantially higher this region can be considered as a source of the observed particles. The high energy electrons are trapped in this region because it is bounded by the higher magnetic field strength and they leave the region by the diffusion caused by the small scale instabilities. The interconnection of the magnetic field lines can be ruled out because it would result in streaming flows with a clear maximum in pitch angle distribution. A possible explanation is the particle leakage due to the locally increased curvature of the magnetopause caused by the surface waves on the magnetopause layers. This is a similar mechanism to that suggested by Book and Sibeck (1995) for the particle transport from the magnetosheath to the magnetosphere. The orientation of the interplanetary magnetic field was southward (IMF  Bz ~ -2.5 nT) well before and during the event and this condition is supposed to be favorable for the reconnection processes. On the other hand, the magnetopause crossing exhibits the features which are typical for the low shear magnetopause (Phan and Paschmann, 1995). A small change of the magnetic field direction across the magnetopause is caused probably by the draping of the magnetic field lines just inside the magnetopause, which leads to its orientation nearly along the XGSE axis (compare the BTOT and Bx values in Figure 1).

Figure 3: Magnetic field and plasma parameters from MAGION-4 measured on a high latitude pass from the magnetosphere into the magnetosheath.

A completely different situation occurs at higher latitudes. Figure 3 shows the outbound magnetopause crossing observed by the MAGION-4 satellite on September 2, 1995. In plasma and magnetic field data two characteristic boundaries can be identified. The first one, at 18:44 UT, characterized by the sharp increase of the plasma density without a substantial change of the magnetic field strength, is probably the inner edge of the plasma mantle. The second boundary, at 18:57 UT, which is distinctly marked by the decrease of the magnetic field, is the magnetopause. The flux of the high energy electrons (bottom panel in Figure 3) is nearly zero through the whole magnetopause layer. The first enhancement of the electron flux is registered at 19:20 UT and lasts for 7 minutes. The strong modulation of this flow by the satellite's rotation indicates the narrow pitch angle distribution; further examination shows that it peaks at ~ 90° . The origin of these particles is unknown because the direction of the magnetic field is highly fluctuating. Its strength is smaller during this event and the particles are probably trapped inside the region of the depressed magnetic field.

CONCLUSION

The observations provided by the pair of the INTERBALL satellites are consistent with the conclusion of Kudela et al. (1992) that the main source of the energetic electrons in the vicinity of the magnetopause is the low latitude magnetosphere. The principal boundaries for the leakage of the high energy electrons are probably the inner edge of the LLBL which divides the dense magnetospheric population of these electrons from the less dense population inside the LLBL. On the contrary, the magnetopause itself does not represent any obstacle for the leakage of the magnetospheric particles to the magnetosheath. The most probable mechanism of the leakage is the diffusion due to the small scale instabilities and/or due to the high local curvature of the boundary surface.

The escaping of the magnetospheric particles along reconnected field lines or the streams of particles accelerated in the bow shock region can represent additional sources.

At higher latitudes (in the plasma mantle region) the density of energetic particles is low. The particles can freely escape this region along magnetic field lines which are open in the distant magnetotail and the reconnection is probably too sporadic to represent an effective source. Energetic particles observed in the vicinity of the high latitude magnetopause are predominantly generated at the bow shock and/or at the low latitude magnetopause.

ACKNOWLEDGEMENTS. The present work was supported by the Czech Grant Agency under Contracts No 205/96/1575 and 202/94/0467 and by the Slovak Agency, Contract No 1353.

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