Comparison of solitary waves and wave packets observed at plasma sheet boundary to results from the auroral zone

The plasma sheet boundary, at distances intermediate between the auroral ac celeration region and the regions where energy conversion associated with s ubstorms occurs, is very dynamic with electron and ion beams, field-aligned currents and many types of waves and non-linear structures. We discuss ele ctric and magnetic fields observations of waves occurring at two very diffe rent timescales. At the longer scales (10's of seconds), Wygant et al. (200 0) have shown that the observed fields are associated with Alfvenic fluctua tions which have their largest electric field normal to the average plane o f the plasma sheet (deltaE(N)). The simultaneously observed magnetic field perturbations are azimuthal(deltaB(T)), resulting in a Poynting flux along the geomagnetic field. The observations are consistent with an incompressib le, transverse electromagnetic surface shear Alfven mode at the surface of the plasma sheet boundary. The local deltaE(N)/deltaB(T) is consistent with V-A. The waves provide an intense earthward Poynting flux sufficient to pr ovide the energy necessary for the energization of auroral electron beams. In addition, the large amplitude surface waves are magnetically conjugate ( to within 1 degree) to intense auroral emission as determined from the UVI imager, whereas weak aurora are correlated with small amplitude electric fi elds. Particle detectors simultaneously observe ions flowing up the field l ine away from the earth, providing further evidence that low altitude accel eration is occurring on conjugate magnetic field lines. At small scales, la rge amplitude solitary waves are frequently observed, and ion acoustic, low er hybrid, and Langmuir wave packets are sometimes seen. There are clear di fferences between the solitary wave observations at the plasma sheet bounda ry and in the low altitude auroral zone. At high altitudes, only electron m ode solitary waves have been identified and they occur both in regions of u pward and downward field-aligned current, in contrast to the auroral zone w here ion solitary waves occur in upward currents ana electron solitary wave s occur primarily in downward currents. This difference may because the gro wth of ion acoustic solitons requires that the plasma be strongly magnetize d (f(ce)/ f(pe) >>1) which is not the case for the observed high altitude p lasma sheet boundary crossings. The high altitude events are associated wit h a wide variety of electron distribution types, whereas the low altitude e vents occur in regions of flat-top electron beam distributions. Preliminary evidence suggests that the high altitude events may be BGK electron holes, as has been shown for the low altitude events. For the parameter regime us ually observed at high altitudes, electron holes would be stable. In additi on, initial work on electron acoustic solitons suggests that these compress ive waves would occur only for a limited range of parameters, so they are u nlikely to explain the high altitude solitary waves, (C) 2001 Elsevier Scie nce Ltd. All rights reserved.