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Observations

We analyze the data obtained after Interball Tail crossed the magnetopause on an inbound trajectory at ~ 22:50 UT at geomagnetic latitude 27.5o N and at 18.92 Local Geomagnetic Time on February 15, 1996. The angle between the magnetosheath magnetic field (as measured prior to the disturbed region and adjacent to the magnetopause) and the magnetospheric field close to the magnetopause was about 158o. We use the data from the ion spectrometer SCA-1 [Vaisberg et al., 1995], which provides 3-D distributions in the energy range 0.05-5.0 keV/Q within ~ 10s, and from the three-axis fluxgate magnetometer MIF from the ASPI complex [Klimov et al., 1995], with sampling frequency of 1 Hz.

Figure 1 shows the ion counting rate spectra, ion bulk parameters, and magnetic field characteristics observed at the magnetopause crossing on February 15, 1996. There are four distinct plasma and magnetic field transients after the magnetopause crossing at ~ 22:50 UT: (1) around 23:00 UT, (2) at ~ 23:35 UT, (3) at ~ 23:55 UT, and (4) at ~ 00:08 UT on February 16, 1996. The number density, the ion temperature and the velocity within these transients change systematically, varying from values closer to magnetosheath levels to those closer to magnetospheric values. All these transients show a double structure in plasma parameters, seen in the number density and temperature profiles. Transients (2) and (3) have a classic reversed FTE signature with a negative excursion of Bn followed by a positive excursion, indicating the reconnection site is to the north of the satellite's location. An important feature for all of these transients is the depletion of magnetospheric ions (seen in the higher energy range) that is strongest in the first transient and diminishes progressively towards the last one. Sporadic plasma jetting and magnetospheric ion leakage to the magnetosheath indicate ongoing reconnection, which is possibly not far from the observation site.

WIND and IMP-8 spacecraft show stable solar conditions with negative Bz ~ 2.5 nT and with small variations of magnetic field magnitude and its components. This stability suggests that the phenomena observed with Interball Tail are not associated with solar wind variations.

Figure 2 shows the structure of transient (3) observed at ~ 23:55 UT. As in the case of the FTE studied by Rijnbeek et al. [1987], Farrugia et al. [1988], Smith and Owen [1992], and Lockwood and Hapgood [1997] (AMPTE-UKS FTE), we can separate the structure of the transient into several regions (indicated by the letters between the vertical lines). The outer regions of transient (3), R1 and R1', contain disturbed magnetospheric plasma and have a slightly increased magnetic field. Within regions R2 and R2' magnetospheric-like plasma and magnetosheath-like plasma mixing occurs. The leading region L has systematically higher number density, and lower ion temperature, than the trailing region T, but the number density and temperature do not reach respective magnetosheath values. The leading region L can be separated in two parts: the boundary layer L1 with the strong velocity shear, and the main body L2, where the velocity is most stable and Vn is directed inside the magnetosphere. The central region of the transient, S, is defined as the location where the normal component of the magnetic field changes its sign. There are strong variations in the velocity and its components in the trailing region T, which can also be separated in two parts by the differences of plasma and magnetic parameters. Depletion of magnetospheric ions is easily seen within the transient.

Transient (3) is similar to the AMPTE-UKS FTE in having disturbed magnetospheric regions R1 and R1' and mixing regions R2 and R2'. There is a slight similarity between the two events in that b i reaches a local maximum near the edges of each event. However, unlike the AMPTE-UKS FTE, transient (3) has a very asymmetric plasma signature and does not have a high b i region in the central region of the event. The flow direction variations are smallest in the main part of the leading region (L) and highest in the trailing region (T) of the transient (Fig. 2). This demonstrates that there is a pronounced difference between the steady plasma flow in the leading part of the transient and the turbulent flow in the trailing part of the transient, which is indicative of dissipation of the transient within the magnetosphere.

Figure 3 is a scatter plot of the ion density and temperature (Ni-Ti) for the entire time interval shown in Fig. 1 (black dots). The magnetosheath state (upper left corner) and the magnetospheric state (lower right end of the scatter plot) are connected by a band of values representative of the boundary layer and transients. Partial scatter plots for three successive transients are also shown in the figure in different colors. The abscissa values for transients have been progressively displaced by a factor of 2.5 in order to avoid overlapping (see caption for explanation). The figure clearly indicates three important points: 1) the leading portion of all three events has plasma characteristics that are more magnetosheath-like than the trailing portion of the transient, 2) none of the observed events have plasma parameters at the core that match those of the local magnetosheath (i.e. they are less magnetosheath-like), and 3) the events further from the magnetopause boundary (i.e. 23:55) are more magnetospheric-like at the core than the events that are close to the magnetopause (i.e. 23:35). This figure best represents the difference between these transients and the earlier AMPTE-UKS FTE.



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Next: Discussion Up: Observation of Isolated Structures of the LLBL Previous: Introduction