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II. Data sets and observations

The data for this study were obtained as the Polar satellite traveled through the cusp and polar cap at radial distances of ~5 - 9 Re, and traversed the plasma sheet boundary at ~4-7 Re. The spacecraft potential, indicative of thermal electron density [Pedersen, 1995], was used to provide the initial identification of the plasma sheet boundary and regions of cusp particle injections. Ion composition measurements, made by the TIMAS instrument [Shelley et al., 1995], were examined to determine the probable source region and plasma characteristics. Hydra instrument observations [Scudder et al., 1995] provided additional information on the particle distributions and moments in these regions. The electric field and spacecraft potential were measured by the double probe electric field instrument on the Polar satellite [Harvey et al., 1995] which acquired bursts of high time resolution data at rates up to 8000 samples per second. Electric field data were rotated into a magnetic field-aligned coordinate system, and the parallel component was examined for candidate solitary waves. Timing analysis was limited to events where one of the two spin plane boom pairs is nearly aligned along the magnetic field. In these cases, the signal from each single probe was splined to provide an identical time basis for all signals and spacecraft potential variations were removed using the data from the perpendicular boom pair. As shown schematically in Figure 1, the time delay between the solitary wave signal measured at opposing probes was determined using a cross correlation analysis. Coupled with the projected probe separation along the magnetic field, the time delay was used to estimate the propagation velocity of the solitary waves. Details of this procedure will be described elsewhere [Dombeck et al., manuscript in preparation]. The AC magnetic field from the search coils [Gurnett et al., 1995] were sampled in the burst memory at the same rate as the electric field. DC magnetic field data, obtained from the fluxgate magnetometers [Russell et al., 1995], were utilized to determine the association of the solitary waves with field-aligned currents.

An overview of a plasma sheet boundary crossing at ~6.3 Re and at ~01:30 MLT on 3/28/97 is shown in Figure 2. The transition from lobe to the plasma sheet is indicated by the increase in the negative of the spacecraft potential (panel a), corresponding to the increase in electron flux (panel b). The plasma sheet boundary, indicated by the variable values of the spacecraft potential and the electron flux (intermediate between lobe and plasma sheet values), contained several field-aligned current sheets indicated by changes in the eastward component of the magnetic field (panel c). The waveform burst occurred at a transition between downward and upward current, in a region containing low energy oxygen and hydrogen conics, and low energy electrons peaked in the upflowing direction. A 50ms sample of the electric field parallel to the geomagnetic field (panel d) indicates that the solitary wave field is first negative (upward, away from the earth) and then positive (downward, towards the earth) with amplitudes up to >50 mV/m. Since the measured time delays correspond to upward velocities (of ~1000 — 2500 km/s), the solitary waves are positive potential structures (i.e. ion enhancements or electron holes) with parallel scale sizes of ~2-8 km. In addition, there are often unipolar perpendicular fields, indicating that the solitary waves are not one-dimensional. There were no signatures of the solitary waves observable in the AC magnetic field. Using Hydra estimates of density and temperature, the Debye length, l D, is ~0.25 km, and the ion sound speed, cs, is ~100 km/s. There is, however, evidence of an energetic ion population, suggesting that cs is probably closer to 500 km/s.

In contrast to the 3/28 event, the solitary waves observed during the burst event on 3/10/87 (Figure 3) occurred in a region of downward current (panel c), well inside the plasma sheet. At this time, there was a few keV upflowing ion beam (see panel bb which shows the ions with pitch angles between 150° and 180° ). The observed solitary wave electric field signatures (see examples shown in panel d) had the opposite polarity (first downward, then upward) from the previous example. However, since the measured time delays correspond to a downward velocity, the structures were again positive potential pulses. In this case, the Hydra moments correspond to cs ~400 km/s. The observed velocities in both plasma sheet cases were ~2-5 times the ion sound speed. The observed scale sizes were ~2-30 l D. For some structures, no delay was observed with propagation speeds of >2500 km/s).

An example of a cusp injection on 4/24/97 at ~6 Re and ~11:30 MLT is presented in Figure 4. The injection which occurred at ~17:12 UT can be seen in the spacecraft potential (panel a). This identification was confirmed by examination of the TIMAS data which showed an intense velocity dispersed injection of H+ (panel b) and He++ (panel c). The perturbation in the eastward component of the DC magnetic field (not shown) was indicative of a current into the ionosphere. An ~6s burst began at 17:40:20 UT, and the component of the burst electric field which is parallel to the magnetic field is shown in panel d. Packets in the parallel component with magnitudes of 10-20 mV/m are groups of solitary waves. The quasi-periodic nature of the solitary wave bursts and other aspects of the waves are discussed elsewhere [Cattell et al., 1998b]. The measured time delays correspond to propagation towards the earth (i.e in the direction of the injected ions) at speeds of ~1000-2000 km/s. Individual solitary waves initially have a positive (downward) electric field, followed by a negative (upward) field. For the observed downward propagation, this is consistent with a positive potential solitary wave, as was also observed in the plasma sheet boundary events. Although plasma measurements from Hydra were not available at the time of the burst, electron density and temperature for this interval, estimated from the spacecraft potential and Hydra observations in similar cusp injections, resulted in l D ~0.01- 0.03 km, and cs ~40 km/s. The parallel scale sizes are ~0.5-1 km, or ~15-100 l D. Several other cases of solitary waves during cusp injections have been examined and the solitary waves have comparable structure and velocities. In addition, the characteristics of solitary waves during several high altitude cusp/polar cap crossings which were not associated with injections have been determined. In contrast to the injection events, the wave velocity in these cases was usually upward. Statistical studies are underway to verify this correlation.

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Next: IV. Discussion and conclusions Up: Comparisons of Polar satellite observations Previous: I. Introduction