J. ›Safrankova1, Z. Nemecek1, L. Prech1, G. Zastenker2, A. Fedorov2, S. Somanov2, D. Sibeck3, J. ›Simunek4
1 Faculty of Mathematics and Physics,
Charles University, V Holesovickach 2, 18000 Prague 8, Czech Republic,
2 Space Research Institute, Moscow, Russia
3 APL, Johns Hopkins University, Laurel, USA
4Institute of Atmospheric Physics, AV CR, Prague, Czech Republic
On August 3, 1995, two satellites of the INTERBALL mission were launched into a highly elliptical dawnside orbit with an apogee near 200 000 km and a perigee of 800 km. The distance separating the satellites varies from one to several thousand km. Both satellites carry sets of Faraday cups which cover a spatial angle approaching nearly 4. The cups provide high resolution data (up to 16 Hz for the main satellite and 10 Hz for the subsatellite) which we supplement with magnetic field measurements and observations of the electron/ion energy and angular distributions. The configuration of both satellites allows us to determine small-scale variations of the magnetopause shape and position and to estimate the velocity of the magnetopause motion. A preliminary analysis of the data suggests that the nature of magnetopause motion depends significantly on the geomagnetic latitude. Wavy motion of the magnetopause boundaries seems to be preferred at the low-latitude boundary layer near the equatorial plane whereas a radial expansion/contraction seems to be most common at the high latitude plasma mantle.
The state of the magnetosphere is predominantly controlled by the state of the interplanetary medium. The solar wind and interplanetary magnetic field (IMF) parameters control the magnetospheric dynamics and the shape of the magnetopause. Early works concentrated on the solar wind dynamic pressure and demonstrated that the response time was no more than that required for pressure discontinuities to sweep over the dayside magnetosphere ( 15 min). Later work identified the comparable importance of the IMF BZ component and revealed a prompt response (as with the pressure) to southward turns of the IMF. Early statistical studies confirmed that the mean position of the magnetopause lies near that predicted by theory but also revealed that the magnetopause is often encountered several radii from its average position (Howe and Binsack, 1972). To produce the observed range of crossings, the magnetopause must exhibit substantial motion, and it is indeed observed to do so over time scales ranging from minutes to hours (Aubry et al., 1970). Several processes have been proposed to account for this motion. Fairfield (1971) showed that variations in the solar wind dynamic pressure can satisfactorily account for most of the motion on time scales of several hours or longer. When the dynamic pressure increases, the magnetopause moves inward, and when it decreases, the magnetopause relaxes outward.
Early studies also illustrated the role of the IMF - the subsolar magnetopause moves inward during periods of southward IMF BZ(Fairfield, 1971). Holzer and Slavin (1978) demonstrated that the subsolar magnetopause moves inward during periods of southward IMF and showed that the dependence of the subsolar magnetopause position upon solar wind pressure was nearly exactly that predicted by theory. Formisano et al. (1979) argued that the subsolar magnetopause moves inward slightly and that the magnetotail magnetopause moves outward substantially when the solar wind density increases and showed the same magnetopause motion with southward IMF. Petrinec et al. (1991) noted a better correlation of the magnetopause position with the solar wind dynamic pressure during periods of northward IMF than during periods of southward IMF. Using the two average shapes of the magnetopause from that study (one for all northward IMF and one for all southward IMF, both as a function of solar wind pressure), Petrinec and Russell (1993) mapped magnetopause crossings for BZ strongly northward or southward. The authors reported that their mapped subsolar distance decreases with more southward IMF BZbut remains approximately independent of BZ for northward IMF. Roelof and Sibeck (1993) determined that during periods of southward BZ the subsolar magnetopause moves inward, while at XGSE = 0 the flank magnetopause moves outward and the flaring angle increases. These changes are most pronounced during periods of low pressure wherein all have a dependence on BZ that is stronger and functionally different for BZ southward as compared to BZnorthward. In contrast, all these changes are much less sensitive to IMF BZ at the highest pressures.
The magnetopause is in motion on almost all time scales. Short transient events which are common in the outer dayside magnetosphere provide clear evidence for unsteady solar wind - magnetosphere interaction. Surface waves have long been observed at the magnetopause as multiple boundary crossings with oscillatory variations in the boundary normal which are characteristic of a tailward moving surface perturbation (e.g., Fairfield et al., 1990). A burst of magnetopause merging produces a bulge or flux rope of interconnected magnetospheric and magnetosheath magnetic field lines. However, several other mechanisms have been forwarded, including the Kelvin-Helmholtz instability, impulsive penetration and pressure pulse driven ripplets on the magnetopause.
The Kelvin-Helmholtz instability has often been suggested as the cause of short-period (1/10 min) magnetopause motion (Aubry et al., 1971), although recent simultaneous high time resolution solar wind and magnetopause observations have demonstrated that the short-period magnetopause motion often directly corresponds to equally brief variations in the solar wind dynamic pressure (Sibeck et al., 1989).
The K-H instability is a candidate for generating the waves because, at low latitudes and away from the subsolar region, the plasma bulk flow in both the magnetosheath and boundary layer are tailward and form a shear layer capable of driving the instability (Fitzenreiter and Ogilvie, 1995). Magnetopause oscillations can be also driven by solar wind dynamic pressure variations more tailward (Sibeck et al., 1990) or they can be attributed to the magnetic flux tubes associated with flux transfer events (FTE's) moving across the magnetopause (Russell and Elphic, 1978).
THE DATA SET
The data analyzed in this paper have been obtained by two INTERBALL satellites launched into orbit on August 3, 1995 (apogee ~ 195,000 km, perigee ~800 km, inclination 63°). The satellite pair consisted of the main satellite (INTERBALL 1) and the small subsatellite (MAGION-4). The high inclination of the orbit is favourable for the study of the latitudinal variations because consecutive crossings of the magnetopause (inbound and outbound ones) occur nearly for the same local time. On the other hand, the study of the dawn - dusk variations can be affected by the seasonal changes because one should compare the data measured with a lapse of about half a year. The distance between INTERBALL satellites was ~ 1000 km and the subsatellite moved along the same orbit ahead of the main satellite in all cases reported in the present study. The main satellite rotates with the period of about 120 s and the period of a rotation of the subsatellite was ~ 60 s. The rotational axes were parallel to the Sun - Earth line.
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). Their axes form a three-dimensional orthogonal system. In the basic mode which was used in the presented cases - ion flux measurements - the first FC (FC 0), looking to the Sun, is provided with a negative potential (-170 V) applied on the suppressor grid and the result of FC measurements is the sum of the ion flux and the flux of electrons with energy greater than 170 eV (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 the axes which are declined from the main subsatellite's axis at ~ 45°. As a consequence of an unexpected orientation of the subsatellite, the axis of the first FC (FC 0 - S) is directed nearly toward the Sun. Due to the negative voltage (-170 V) which is supplied permanently to the suppressor grid, the VDP-S registers the sum of all ions and electrons with energy greater than 170 eV.
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., 1996a). Unfortunately, due to an unexpected satellite orientation, a zone of about of ± 40° angular width centered sunward is not covered by the spectrometer measurements.
To study the magnetopause structure we have combined our measurements with the magnetic field data from the MIF-M magnetometer onboard the INTERBALL 1 satellite.
The paper presents the data which are in a preliminary stage of the processing and thus not all parameters were available.
Fig. 1: A comparison of magnetic field and plasma data from INTERBALL 1 and MAGION-4 on their pass from the magnetosheath into the magnetosphere. The magnetic field data from INTERBALL 1 are plotted in the boundary normal coordinates, BSGR is a modulus of magnetic field measured by MAGION-4.
The INTERBALL satellites carry out the measurements of the magnetopause region in a wide range of local times and in two intervals of latitudes. Inbound crossings are located near the equatorial plane, outbound ones are scanning a range of 30°/60° of latitude. A preliminary analysis of the obtained data shows that the structure of the magnetopause varies significantly with the latitude and local time; the influence of the solar wind and IMF conditions on the magnetopause position and structure has been mentioned many times. We have chosen the dawn flank for our study of the latitudinal dependence of the magnetopause motion. Among crossings registered during August - September, 1995 we have selected those for which the solar wind dynamic pressure and IMF were constant well before and during the event. After this selection, there remained 5 inbound and 4 outbound crossings. It should be noted that the behaviour of the plasma parameters and magnetic field were similar for each group.
The position of the observed magnetopause crossings is consistent with the model of Sibeck et al. (1991) within ±0.5 RE of accuracy, if the corresponding solar wind data from the WIND spacecraft are used. The only exception is the crossing which occurred on August 29, 1995 at 09:00 UT, and which will be discussed later where a difference between the expected and observed crossings is ~ 2 RE. This shift cannot be explained by the introduction of the models of Petrinec and Russell (1993); Roelof and Sibeck (1993) because under the given solar wind conditions (pressure ~ 2.5 nPa, BZ ~ -2.5 nT) all three models lead nearly to the same position.
The spacecraft spends usually more than one hour inside the magnetopause layers and the boundaries are met many times under the conditions mentioned above. Figure 1 shows the first encounter with the magnetopause which occurred on August 29, 1995. The INTERBALL 1 satellite was located at GSE (X, Y, Z) = (1.6, -11.1, -1.7) RE at 09:00 UT and moved with the velocity of about 2.6 km/s toward the Earth. The MAGION-4 satellite was located 1020 km ahead of INTERBALL 1.
The sharp decrease of the ion density (top panel of Figure 1) at 08:58:40 UT which corresponds to the increase of the magnetic field value can be interpreted as a magnetopause crossing. The preceeding gradual decrease of the ion density which starts at 08:56:20 UT by a turn of the magnetic field (MF) direction (BM component of the MF decreases and BL component becomes principal) can be interpreted as a plasma depletion layer which is often observed just outside of the low-latitude magnetopause (Russell, 1995). After the magnetopause crossing one can identify five density enhancements which are numbered at the top panel. These enhancements correspond to the decrease of the magnetic field nearly to the magnetosheath value. Our interpretation of these enhancements as consecutive encounters with the magnetopause is supported by the rotation of the MF on the edges of these enhancements.
Similar behaviour of the plasma density can be identified in the measurements of the MAGION-4 satellite. Unfortunately, the density is out of range of our device in the magnetosheath but the enhancements which follow after the first magnetopause crossing can be clearly distinguished. According to the measurements of the WIND at a distance of about 100 RE upstream (Figure 2), the pressure of the solar wind had a nearly constant value of 2.4 nPa and the IMF was directed southward (BZ oscillated between -2.5 nT and -3.5 nT) during the time interval from 08:00 UT to 09:00 UT. Nevertheless, it should be taken into account that the measurements in far upstream do not always reflect the actual state of the shocked solar wind in front of the magnetopause (Nemecek, et al., 1996b).
Fig. 2: Solar wind plasma and magnetic field data from the WIND during the INTERBALL pass from the magnetosheath into the magnetosphere (August 29, 1995).
Our understanding of the observed features is illustrated in Figure 3. The dashed lines show the position of the spacecraft along the -YGSE axis (other two components are negligible) and the heavy bars on these lines indicate the intervals of observation of the magnetosheath-like plasma. The numbers in the vicinity of the bars relate the events to Figure 1. The full line which connects the ends of the bars shows the expected position of the magnetopause if only radial expansion of the magnetopause is taken into account. The timing of the remaining two events connected by dotted line (at and UT) excludes this interpretation. We think that all observed events are caused by the wavy motion of the magnetopause (Sibeck et al., 1989). If this is this case, the surface waves should move tailward with the magnetosheath velocity (213 km/s in this case according to Nemecek et al., 1996a). It allows us to convert the temporal scale to the spatial scale which is shown in the top corner of the Figure and to estimate the dimensions of the waves. The typical wavelength along the XGSE axis is ~7500 km. The amplitude of the waves is probably a few times larger than the distance between the spacecraft because most of the crossings are observed by both spacecraft.
|Fig. 3: Schematic drawing of the magnetopause motion (September 2, 1995).|
Figure 4 shows a similar drawing of the magnetopause crossing which occurred on September, 2nd, 1995 between 01:35 and 02:10 UT. The location of the satellites along the YGSE axis is shown in Figure 4, the XGSE and ZGSE components were 0.47 RE and -0.94 RE, respectively. The solar wind pressure and orientation of the IMF were nearly the same as during the previous event. There are two boundaries depicted in Figure 4, the upper boundary is constructed in the same way as the boundary in Figure 3, the lower boundary shows the position of the outer edge of the low-latitude boundary layer (LLBL) or the sheath transition layer (according to Russell (1995)) which is characterized by the sharp rise of the electron temperature (see for example a similar temperature behaviour in Figure 1). It should be noted that the computation of the velocity of the boundary motion from the time delay between the satellites has no sense in these cases because one can obtain an arbitrary value ranging from 30 to 300 km/s.
Fig. 4 : Schematic drawing of the magnetopause motion (August 29, 1995).
An example of a high-latitude crossing is depicted in Figure 5. The behaviour of the plasma and the magnetic field are completely different from the low-latitude crossing depicted in Figure 1. The enhancements of plasma density and the turning the MF (see the first and the second panels in Figure 5) which occurred at 18:45:50 UT do not change the value of the MF. We connect this observation with the encounter of the plasma mantle and the region with low plasma density as the magnetospheric lobes. The second boundary, which is more distinct in the magnetic field data, is crossed at 18:57:20 UT. We interpret this boundary as the magnetopause itself.
Fig. 5: A comparison of plasma and magnetic field data from INTERBALL 1 and MAGION-4 (upper 3 and lower 3 panels, respectively) during a high latitude magnetopause crossing. The dashed lines indicate the inner (left) and outer (right) edges of the magnetopause layer.
The same boundaries can be distinguished in the MAGION-4 data (bottom panels). The magnetic field is highly fluctuating after the crossing of the second boundary but there is no indication of the multiple crossing. On the other hand, the jump in MAGION-4 plasma density on the first boundary can indicate that this boundary was crossed two times as well as the MF enhancement of the INTERBALL 1 magnetic field at 18:58:30 UT can be interpreted as a consecutive crossing of the second boundary. However, the possible crossings are seen only by one satellite in both cases. One can conclude that if there exist surface waves in the mantle region, their amplitudes are lower than the distance between the satellites (930 km). The velocities of the motion computed from the time delays between the observations of both satellites are 12.4 km/s for the inner and 17.8 km/s for the outer boundary. We have used only these magnetopause crossings which have been observed under quiet solar wind and IMF conditions. It results in relatively small set of data and thus we cannot exclude the possibility that the changes of the magnetopause position observed in low-latitudes can be connected with some external influence as variations of the magnetosheath flow generated at the bow shock.
The comparison of the magnetopause crossings in different latitudes leads to the conclusion that the low-latitude magnetopause is much less stable than the high-latitude magnetopause. Since there is no difference between these two latitudes in the magnetosheath the unstableness would be the property of the boundary. From the mechanisms which have been suggested for explanation of the similar observations - flux transfer events, pressure pulses, Kelvin-Helmholtz instability - only the last one is related to the boundary itself.
The present work was supported by the Czech Grant Agency under Contracts No. 205/96/1575 and 202/94/0467. Authors from Russia were partly supported by the RFFI under Contract No. 95-02-03998. Authors are grateful to A. Skalsky and N. Ryb'eva for the magnetic field data.
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