O. Santolík1, J. Safránková1, Z. Nemecek1, J.-A. Sauvaud2, A. Fedorov3, G. Zastenker3
1Faculty of Mathematics and Physics,
Charles University, V Holesovickách 2, CZ-18000 Praha 8, Czech
Republic, E-mail: firstname.lastname@example.org
2CESR, CNRS/University of Toulouse, France
3Space Research Institute, Moscow, Russia
The magnetotail lobes contain mainly a plasma of low density and energy. However, structures consisting of energetic plasma particles have been also previously reported in this region. The present contribution is devoted to a two-point analysis of such structures. We use the data collected by the recently launched INTERBALL project. Energy analyzers and Faraday's cups onboard the main satellite and the sub-satellite detect localized short duration fluxes of electrons at higher energies, surrounded by a regular lobe plasma. The size of these plasma structures is about 0.5 RE (Earth's radii) and their velocity ranges from several km/s to several tens of km/s. We give examples of (1) plasma structures which change their position while the spatial profile of plasma characteristics remains approximately constant, (2) expanding plasma structures, and (3) structures which seem to flap over the satellite pair.
The northern and southern lobes of the magnetospheric tail occupy vast regions between the plasma sheet and the plasma mantle. The lobes are magnetically mapped to the polar caps. The position of the lobe boundaries is quite variable and, under some circumstances (usually active geomagnetic conditions), the lobe plasma can be observed even on the geosynchronous orbit (Moldwin et al., 1995). Inside the lobe region, the plasma density and temperature are extremely low (n ~ 10-2cm-3, T ~ 20 eV) and a systematic tailward component of the plasma bulk velocity has been reported (Sharp et al., 1981). The GEOTAIL spacecraft frequently observes cold dense ion flows streaming tailward in the magnetotail lobe region (XGSM ~ -10 to -200 RE ,GSM means Geocentric solar-Magnetospheric Coordinates, and 1 RE is one Earth's radius). The density is comparable to, or larger than, that in the plasma sheet and the tailward speed ranges from 50 to 500 km/s (Hirahara et al., 1996). These flows are often associated with the ring shaped distribution of the ions, which is probably generated by the thermoelectric field that exists at the plasma sheet - lobe boundary (Saito et al., 1994). The ion composition of the ion flows suggests their ionospheric origin. On the other hand, there are also observations of discrete structures which can be attributed to the precipitation of high energy particles into the cap region. Based on the ISEE observations, Huang et al. (1987) have brought an evidence of the filamentary plasma structures which are connected with the plasma sheet (plasma sheet boundary layer) and which extend to the lobe region. The authors interpret these filaments as a source of the polar cap auroral arcs. The scale size of the filaments contributing to the arcs is of the order of 0.3 RE(Huang et al., 1989).
The present contribution deals with plasma density enhancements observed in the northern magnetotail lobe by two INTERBALL satellites. The study is based on the electron signature of these events. A data set of several tens of observations has been collected during the INTERBALL passages through the northern lobe. Several cases from this data set are analyzed and two-point observations are used to estimate the shape and the motion of the plasma structures.
THE DATA SET
The data analyzed in this paper have been obtained by two INTERBALL spacecraft (the main satellite INTERBALL-1 and the sub-satellite MAGION-4). In all cases reported in the present study, the distance between them ranges from 1000 to 13000 km, the sub-satellite moving along the same orbit ahead of the main satellite.
Both satellites are equipped with a set of instruments for plasma investigation. We use simultaneous measurements from the MPS/SPS spectrometer onboard the MAGION-4 sub-satellite (ions and electrons from 40 eV to 5 keV in several directions, for more details see Nemecek et al., this issue) and the ELECTRON spectrometer on the main satellite (electrons between 10 eV and 22 keV with full solid angle coverage, see Sauvaud, this issue). Independent information on the electron flux at energies above 170 eV is provided by the system of Faraday cups - the VDP device onboard the main satellite and the VDP-S sensor on the MAGION-4 (Nemecek et al., this issue).
Our data set consists of 60 intervals where the INTERBALL satellites encountered isolated fluxes of hot electrons in the northern lobe between August 25, 1995 and February 12, 1996. The intervals have been selected using the following criteria: (1) Energy of observed electrons is greater than 170 eV; (2) Temporal duration lasts from several minutes up to about one hour. The events are separated from continuous intervals of observation of hot electrons longer than 1 hour (e.g. plasma sheet or plasma mantle) by at least several minutes of the absence of hot electrons.
The data set is obtained under relatively quiet geomagnetic conditions, Kp < 2+ for about 75% of cases and Kp < 4+ for nearly all cases (98%).
Figure 1 shows the distribution of the positions of these events in the GSM coordinates. The magnetic field model of Tsyganenko (1989) has been used to trace the magnetic field lines from each point of observation to the plane XGSM = -10 RE. Blue points have been traced in the direction of the magnetic field, whereas red points result from the tracing in the direction anti-parallel to the field. As all the points lie northward of the neutral sheet, red colour indicates XGSM > -10 RE and the blue points have been traced from XGSM < -10 RE.
Fig. 1: Magnetic mapping of the GSM coordinates of the hot electron encounters to the plane XGSM = -10 RE. The dashed line shows the average magnetopause position.
The points correspond to the beginning of each event and the short lines mark the end-positions of the events. The dashed curve indicates the position of the model magnetopause assuming an average solar wind dynamic pressure of 2.04 nPa.
The Figure indicates that our data set is distributed almost uniformly across the northern lobe. The shift of the gray points towards higher ZGSM values is probably due to the limitations of the satellite orbit which never extends to lower ZGSM when the spacecraft is in the near-Earth northern lobe. The plasma sheet passages are excluded from our data set, so that there is a lack of points in the bottom corners of the presented portion of the YGSM - ZGSM plane. This reflects increasing width of the plasma sheet with growing absolute values of YGSM.
The events registered in the vicinity of the lobe boundaries can probably be attributed to short excursions into the plasma sheet or plasma mantle. However, the flux enhancements located deep inside the lobe could be caused by spatial plasma structures similar to the filamentary objects proposed by Huang et al. (1987).
To demonstrate the variability of the hot plasma encounters, several events from the above data set will be presented in some detail. First we will analyze a set of subsequent encounters with electron fluxes at XGSM < -10 RE, between 17:00 UT on October 31 and 8:00 UT on November 1, 1995.
Figure 2 shows two orthogonal projections of the corresponding part of the INTERBALL orbit in the GSM coordinates. Small points on the orbit trajectory represent one-hour ticks. The distance between the sub-satellite and the main satellite increases from about 1000 km at the beginning of the interval to 2000 km at its end. The distance from a model neutral sheet (Fairfield, 1980) is between 8.8 and 11 RE during the interval. The electron flux enhancements are marked with thick lines along the orbit and by the letters A-F. The events D and F are characterized by electron energies above 1 keV while the sub-satellite ion spectrometer measures streaming ions of several keV. The plasma parameters for these events resemble those of the plasma sheet passages. Oppositely, during the intervals A, B, C, and E the electron energy is lower (several hundreds of eV). These events are also accompanied by ion streams but the plasma resembles that of the plasma sheet boundary layer.
Fig. 2: Part of the INTERBALL orbit in GSM coordinates on 31-Oct/ 1-Nov-1995. The hot electron encounters are marked with letters A - F.
Registered by a single spacecraft, this sequence of events would allow a multiplicity of different hypotheses on its origin. A measurement by two separate satellites provides information about the one-dimensional dynamics of the the plasma structures. This can help us to understand the relation of these events to the plasma sheet or the plasma sheet boundary layer.
As an example of a two-point analysis, the event C is presented in Figure 3. The top part contains the electron energy spectrograms measured during the electron flux encounter onboard the main satellite (top) and on the sub-satellite (middle). The overall shape of the spectrograms is the same in both figures: starting at 19:45 a gradual growth of the number of electrons is detected at energies of several hundreds eV, followed by an abrupt decrease of the electron counts at 19:56. The distinct differences for energies below 70 eV are caused probably by a higher photoelectron or secondary electron flux from the body of the sub-satellite. The differences for higher energies reflect, however, the spatial and/or temporal structure of the natural electron flux: the peak around 19:55 is wider in the sub-satellite observations than the corresponding peak recorded by the main satellite.
Fig. 3: Two-point measurement of the event C of the electron flux enhancement. Top: energy spectrogram measured by the ELECTRON spectrometer onboard the main satellite. Middle: spectrogram obtained by the SPS device (sub-satellite). Bottom: flux of electrons with energy exceeding 170 eV as measured by the VDP-S Faraday's cup on the sub-satellite (blue) and by the analogous VDP device onboard the main satellite (yellow).
In the bottom part of Figure 3, an independent measurement of the electron fluxes is presented. The integral flux of ions and electrons above 170 eV registered by the sub-satellite is given by the solid line and the analogous quantity provided by the main satellite is represented by the dashed line. The values are negative and the two curves follow the same pattern in accord with the shape of the electron spectrograms. This suggests that the main component of the flux are hot electrons. Note that the peak at 19:55 is again wider in the sub-satellite data.
During the event presented in Figure 3, the sub-satellite moves ahead of the main satellite at a distance of about 1100 km. The temporal resolution of our simultaneous two-point measurement is 10 s. If a planar plasma structure moved with a velocity v deviated by an angle a from the direction of the satellite - sub-satellite vector, the values v/cosa < 110 km/s should lead to observable time shifts between the data of the two satellites. In the present case, the trailing edge of the structure shows a small time shift corresponding to v/cosa of about 60 km/s. However, there is no distinct time shift of the overall shape; it seems rather that the main satellite always detects a lower electron flux than the sub-satellite. This could be explained either by a spatial structure transverse to the direction of the satellite motion, or by a simple flapping of the plasma structure over the INTERBALL position. In the latter case, the sub-satellite first encounters a peak value of the electron flux, and then the flux also begins to grow at the main-satellite position. Suddenly, the structure contracts back, allowing the main satellite to detect only a small peak, and due to this movement, the flux at the sub-satellite position also vanishes abruptly. This interpretation seems to be supported by other flux encounters presented in Figure 2. The occurrence of this sequence of events can be therefore attributed to a multiple flapping of the plasma sheet over the INTERBALL satellites.
However, this is not a general feature of the plasma structures observed by the INTERBALL in the lobe region. As examples, two electron flux enhancements observed at XGSM > -10 RE are plotted in Figure 4.
Fig. 4: Two-point measurement of two events at XGSM > -10 RE. The data of the VDP-S Faraday's cup onboard the sub-satellite (blue) and the analogous measurement of the VDP cup on the main satellite (yellow) are simultaneously presented.
The presented cases of increased negative current flowing to Faraday's cups are always accompanied by electrons with energies of several hundreds eV registered by the sub-satellite energy spectrometer (not shown). The common features of these isolated events are (1) short duration, and (2) observable time shifts between the data of both satellites. The left panel shows a plasma structure encountered at XGSM = -2.2 RE, YGSM = 0.7 RE and ZGSM = 6.0 RE. The separation of the satellite pair is about 2700 km, and they move into the northern lobe with a velocity of 4 km/s. The form of the electron flux profiles measured by the two satellites is nearly the same. The observed time shift suggests that the plasma structure moves in the direction opposite to the satellite motion, with v/cosa of about 13 km/s, and its spatial form is conserved. The dimension of the encountered region measured along the satellite orbit is approximately 0.25 - 0.5 RE depending on the level of electron flux that defines the boundary of the structure.
The second example in the right panel of Figure 4 is more complicated. The electron flux enhancement was observed at a similar position, XGSM = -3.1 RE, YGSM = -1.8 RE and ZGSM= 7.1 RE, and again the satellites (with a separation of 2900 km) move into the lobe with a velocity of 3.5 km/s. The profile of the electron flux is more compressed in the sub-satellite data although the detailed shape of the trailing edge is conserved. A simple interpretation is possible: the sub-satellite first encounters the leading edge of the structure, which moves opposite to the satellite motion with v/cosa = 3 km/s. At the moment when the leading edge arrives at the main satellite, the sub-satellite leaves the plasma structure due to its own orbital motion, and after some time does the same the main satellite. This means that the trailing edge is standing (compared with the orbital velocity) while the leading edge of the structure slowly expands. When the sub-satellite exits the structure (and the main satellite enters it), the transverse dimension of the electron flux region is approximately equal to the relative distance of the satellite pair, i.e. nearly 0.5 RE.
The paper brings first information on structures observed in the magnetotail lobes by the INTERBALL satellite pair. The observed electron flux enhancements can be divided into two groups: events which could be attributed to flapping of the plasma sheet and its boundary layer, and events which can be interpreted as real structures having a transversal dimension of about 0.5 RE and moving inside the lobe region.
A clear distinction between these two groups requires a further data treatment. This treatment should include a detailed analysis of the velocity directions of the plasma particles (although the majority of the analyzed electron fluxes seems to be nearly isotropic) and should be completed with a high resolution magnetic field data. Furthermore, comparisons of our data with the measurement of other satellites and remote polar cap observations would be helpful.
ACKNOWLEDGEMENTS. The present work was supported by the Czech Grant Agency under Contracts No 205/96/1575 and 202/94/0467. The Russian coauthors were partly supported by the RFFI grant No 95-02-03998.
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