Pages 983-991

INTERBALL OBSERVATIONS OF THE PLASMA SHEET

Yuri I. Yermolaev

Space Research Institute, Profsoyuznaya 84/32, 117810 Moscow, Russia
E-mail: yermol@afed.iki.rssi.ru

ABSTRACT

One of the important aims of the INTERBALL project is the study of the structure and dynamics of the geomagnetic tail including the plasma sheet and neutral sheet as well as plasmoids. In accordance with the orbit evolution during a period from the middle of September, 1995 to the middle of March, 1996, the satellite passes the plasma sheet at a regular interval of about four days in the geomagnetic time regions of 06-00 and then 24-18 LT and at geocentric distances from 3 to 28 Earth radii. On the basis of the plasma and magnetic field observations, the positions of the plasma sheet and its dependence in a statistical sense on the interplanetary magnetic field By and Bz components are studied and compared with previous results and models.

INTRODUCTION

The geomagnetic tail, which consists of relatively empty lobes and plasma sheet, is one of the most important elements of the global plasma transport within the geomagnetosphere as well as of the intercoupling between the solar wind and the magnetosphere. It is the region where energy from the solar wind is stored as magnetic energy and then converted into particle energy (see reviews Akasofu, 1981; Nishida, 1983; Frank, 1985; Ness, 1987; Fairfield, 1987; Baumjohann, 1994 and references therein).

A number of spacecraft have studied in detail the near-Earth (the Explorer series, Vela, IMP, Prognoz, ISEE, AMPTE and others) and distant (ISEE 3 and GEOTAIL) magnetic tail (see papers Hones et al., 1971; Fairfield, 1980, 1987; Frank, 1985; Ness, 1987 and Baumjohann, 1994 as well as GEOTAIL Special Issue of GRL, December 1994 and references therein). These space observations showed that the position and the configuration of the magnetic tail depend on the solar wind magnetic field and plasma parameters (Fairfield, 1979, 1980, 1987, 1991, 1993; Akasofu, 1981; Cowley, 1981; Nishida, 1983; Frank, 1985; Baumjohann et al., 1986; Huang and Frank, 1994). The magnetic flux in the tail increases with the southward turning of the interplanetary magnetic field (IMF), because reconnection is most efficient when the IMF Bz is negative. The IMF By penetrates the tail (lobes and plasma sheet) in an asymmetric manner (it penetrates more effectively in the Z > 0, Y < 0 and Z < 0, Y > 0 quadrants when By > 0 and in the Z > 0, Y > 0 and Z < 0, Y < 0 quadrants when By < 0) and may result in a twist of the tail (Cowley, 1981; Tsurutani et al., 1984; Sibeck et al., 1985).

The main scientific objective of the INTERBALL project is the study of Earth's magnetosphere and its interaction with the solar wind plasma and the interplanetary magnetic field (Galeev et al., 1995; Zelenyi et al., 1997). Initial results of magnetosphere boundary layer and magnetotail observations were described by Zastenker et al. (1996), Sandahl et al. (1997), Savin et al. (1997), Santolik et al. (1997). Comparison of INTERBALL/Tail Probe measurements with GEOTAIL data and models were presented by Mukai et al., (1997). In this paper the INTERBALL/Tail Probe data are used to study the shape and position of the near-Earth (0 > X > -30 RE) plasma sheet and its dependence on IMF Bz and By components measured on the WIND spacecraft.

INSTRUMENTS AND DATA ANALYSIS

The INTERBALL/Tail Probe was launched on August 3, 1995 in an orbit with parameters: Ha = 184000 km, Hp = 900 km, inclination 65 degrees, and T = 91 hours. This orbit allows measurements of the outer boundaries of the magnetosphere (bow shock and magnetopause) as well as the magnetotail at high and low latitudes (Prokhorenko, 1995).

The main aims and instrument payload of the project were described in a paper by Galeev et al. (1995). In our study we mainly used the data of the 3-D ion energy (E/q = 0.03 - 24.2 keV) spectrometer CORALL (Yermolaev et al., 1997) and magnetometers FM-3 and MIF-M (Nozdrachev et al., 1995; Klimov et al., 1995). In complicated cases the data of the electron spectrometer ELECTRON (Sauvaud et al., 1995) and ion spectrometer PROMICS-3 (Sandahl et al., 1995) were also used for determination of space regions.

The selection criteria used to identify the plasma sheet were the same as in previous experiments: measurements of approximately isotropic high-energy ions and electrons on the energy-time spectrograms (CORALL, ELECTRON and PROMICS-3 INTERBALL plasma data are very similar to those of LEPEDEA on ISEE 1 [see Plate III-3A in paper by Huang et al., 1987], LEP and CPI-HOT on GEOTAIL [see Figure 2 in paper by Fujimoto et al., 1996 and Figure 1 in paper by Frank et al., 1996]). The numerical values of the parameters in the plasma sheet were as follows: proton density np = 0.1-1 cm-3, magnitude of magnetic field B = 0-20 nT and ion and electron temperatures Tp = (1-5)x107 K and Te = (0.5-1)x107K which are in good agreement with previous observations (Lui, 1987).

This approach allowed us to select the plasma sheet and neutral (current) sheet as well as boundaries: plasma sheet boundary layer (PSBL) and low-latitude boundary layer (LLBL). In this preliminary analysis we excluded LLBL intervals because of the vicinity of the magnetopause that is discussed in other papers. Because we used INTERBALL data with time resolution 2 min (satellite spin period), the 2-min averaged IMF Bz and By components (in the GSE coordinate system) observed one hour prior to the INTERBALL observations as measured by the MFI magnetometer on the WIND spacecraft (Lepping et al., 1995) were included. The final data set for the time interval of October, 1995 - January, 1996 consists of about 570 hours of plasma sheet observations.

RESULTS

The positions of INTERBALL/Tail Probe when the satellite was located in the plasma sheet are presented in Figures 1-4 for different IMF conditions: in Figure 1 for Bz > 0 and By > 0; in Figure 2 for Bz > 0 and By < 0; in Figure 3 for B z< 0 and By > 0; and in Figure 4 for Bz < 0 and By < 0. The figures show three projections in the GSM coordinate system: A - projection in ZY (dawn-dusk) plane; B - in ZX (noon-midnight) plane; and C - in YX (ecliptic) plane.

Fig. 1. The Interball/Tail Probe plasma sheet crossings for the IMF Bz > 0 and By > 0:
A - GSM projection in ZY (dawn-dusk) plane;
B - in ZX (noon-midnight) plane and
C - in YX (ecliptic) plane.

Fig. 2. The Interball/Tail Probe plasma sheet crossings for the IMF Bz > 0 and By < 0:
A - GSM projection in ZY (dawn-dusk) plane;
B - in ZX (noon-midnight) plane and
C - in YX (ecliptic) plane.

Fig. 3. The Interball/Tail Probe plasma sheet crossings for the IMF Bz < 0 and By > 0:
A - GSM projection in ZY (dawn-dusk) plane;
B - in ZX (noon-midnight) plane and
C - in YX (ecliptic) plane.

Fig. 4. The Interball/Tail Probe plasma sheet crossings for the IMF Bz < 0 and By < 0:
A - GSM projection in ZY (dawn-dusk) plane;
B - in ZX (noon-midnight) plane and
C - in YX (ecliptic) plane.

In accordance with the satellite orbit, the measurements were made for the most part in the northern hemisphere (Z ranges from -3 to +23 RE) for X from -3 to -30 RE. The range of Y (from -17 to 20 RE) was limited by the period of observations (October 2, 1995 - January 31, 1996) and by the omission of the low-latitude boundary layer crossings from the data set.

The width of the plasma sheet is not constant as Y changes. The half-width has a minimum value of Z ~ 5 RE for Y = 0; it increases up to 10-12 RE (PSBL up to 16 RE) with increasing Y from 0 to 5 RE and is approximately constant for larger Y.

Data selection according to the IMF Bz and By components allows for a study of the asymmetry of the plasma sheet shape as a function of the solar wind magnetic field orientation. Figures 1-4 show that the duration of plasma sheet observations for Bz < 0 is significantly less (about 23%) than that for Bz > 0. Because of the different statistics for these cases, it is difficult to evaluate differences between the figures with Bz < 0 and Bz > 0. Nevertheless, the figures show that the plasma sheet crossings with Y > 15 RE and Z > 5 RE are observed mostly when IMF Bz > 0.

The difference between the figures with different IMF By is more clear. The change in the sign of By results in a change of the plasma sheet position in the Y direction: when By > 0 the plasma sheet shifts in the -Y direction (that is, there are more observations in the region with Y < -10 RE and less observations with Y > 10 RE) and when By < 0, it shifts in +Y direction.

DISCUSSION AND CONCLUSION

On the whole, the presented shape of the plasma sheet is in good agreement with previous investigations of the near-Earth plasma sheet (see review by Fairfield, 1987 and references therein): the width of the plasma sheet is about 10 RE at Y = 0 and it increases up to 20-30 near the magnetopause.

The preferential observations of the plasma sheet with Bz > 0 is unlikely to be connected only with the increase in the plasma sheet width under northward IMF because this fact would be observed only for high Z but we see it for low Z too. We propose that it may be the result of processes on the sun, for instance, the IMF variations of the solar cycle. During previous solar cycles, in 1973-74 and 1984-85, a similar north-south asymmetry of the IMF was observed with different spacecraft (Smith and Bieber, 1992). Unfortunately this asymmetry resulted in small number of plasma sheet observations with Bz < 0 and consequently a reliable comparison of the plasma sheet structure with Bz > 0 and Bz < 0 cannot be made. Nevertheless, it is possible to suggest that the plasma sheet crossings with Y > 15 RE and Z > 5 RE are associated with IMF Bz > 0 but this result requires further investigation.

The asymmetry of the plasma sheet which was observed with the IMF By > 0 and By < 0 cannot be explained by the twist of the tail predicted by Cowley (1981) because the twist would result in the reverse dependency on IMF By. The asymmetric penetration of the IMF By in the tail (Cowley, 1981; Tsurutani et al., 1984; Sibeck et al., 1985) may be described in another words: when IMF By > 0 the IMF By is parallel to the magnetospheric By component in Z > 0, Y < 0 and Z < 0, Y > 0 quadrants (i.e. the total magnetic field component increases) and antiparallel in Z > 0, Y > 0 and Z < 0, Y < 0 quadrants (i.e. the component decreases) and when IMF By < 0 the reverse dependency must prevail. The INTERBALL data show that the plasma sheet shifts in the direction where the IMF By and the magnetospheric By compo-nents are parallel. It is possible to suggest that when the IMF By and the magnetospheric By components are parallel that the total plasma sheet magnetic field is more close to the Y-axis (i.e. has a smaller perpendicular component to the Y-axis) than when they are antiparallel and the plasma of the plasma sheet may move farther from the center of the plasma sheet along the Y-axis.

It should be noted that a very important problem is the choice of the coordinate system. In several papers the IMF By and Bz components in the GSM coordinate system are used for similar investigation. Such a study will be the subject of further publications. We also plan to investigate the distribution of plasma bulk parameters and the magnetic field in the plasma sheet as functions of solar wind parameters and the IMF.

ACKNOWLEDGMENT

This work was supported in part by INTAS Grant 93-2031. The key parameter data of the WIND magnetic field (PI is R. P. Lepping) were provided by NASA Goddard Space Flight Center. The author thanks V .I. Prokhorenko for INTERBALL/Tail Probe orbit calculation and help in figure preparation,

L. M. Zelenyi and S. P. Savin for useful comments regarding the manuscript, and T. Mukai and A. Nishida for their hospitality and support while visiting ISAS.

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