Comparison of Solitary Waves and Wave Packets Observed at Plasma Sheet Boundary to Results from the Auroral Zone

C. Cattell 1, J. Dombeck ~, A. Keiling ~, J. WyganP, R. Bergmann 2, M. K. Hudson ~, C. Kletzing 4, F. S. Mozer 5, M. Temerin s, I. Roth s and G. Parks 6 ~School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA 'Eastern Illinois University 3Dept. of Physics and Astronomy, Dartmouth College, Hanover, NH 4Dept. of Physics and Astronomy, University of Iowa, Iowa City, IA, USA 5Space Sciences Laboratory, University of California, Berkeley, CA, USA 6Geophysics Program, University of Washington, Seattle, WA, USA Received 22 November 1999; revised 19 may 2000; accepted 30 June 2000 Abstract.The plasma sheet boundary, at distances intermediate between the auroral acceleration region and the regions where energy conversion associated with substorms occurs, is very dynamic with electron and ion beams, field- aligned currents and many types of waves and non-linear structures. We discuss electric and magnetic fields observations of waves occurring at two very different time- scales. At the longer scales (10's of seconds), Wygant et al. (2000) have shown that the observed fields are associated with Alfvenic fluctuations which have their largest electric field normal to the average plane of the plasma sheet (~iEN). The simultaneously observed magnetic field perturbations are azimuthal(SBT), resulting in a Poynting flux along the geomagnetic field. The observations are consistent with an incompressible, transverse electromagnetic surface shear Alfven mode at the surface of the plasma sheet boundary. The local 8EN/SBv is consistent with VA. The waves provide an intense earthward Poynting flux sufficient to provide 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 fields. Particle detectors simultaneously observe ions flowing up the field line away from the earth, providing further evidence that low altitude acceleration is occurring on conjugate magnetic field lines. At small scales, large amplitude solitary waves are frequently observed, and ion acoustic, lower hybrid, and Langmuir wave packets are sometimes seen. There are clear differences between the solitary wave observations at the plasma sheet boundary and Correspondence to: Cynthia Cattell in the low altitude auroral zone. At high altitudes, only electron mode solitary waves have been identified and they occur both in regions of upward and downward field-aligned current, in contrast to the auroral zone where ion solitary waves occur in upward currents and electron solitary waves occur primarily in downward currents. This difference may because the growth of ion acoustic solitons requires that the plasma be strongly magnetized (fee/ fpe >>l) which is not the case for the observed high altitude plasma sheet boundary crossings. The high altitude events are associated with a wide variety of electron distribution types, whereas the low altitude events 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 usually observed at high altitudes, electron holes would be stable. In addition, initial work on electron acoustic solitons suggests that these compressive waves would occur only for a limited range of parameters, so they are unlikely to explain the high altitude solitary waves. © 2001 Elsevier Science Ltd. All fights reserved 1 Introduction 97 The plasma sheet boundary is a very dynamic region which often contains ion and electron beams, field-aligned currents, spiky electric fields and strong waves. It is generally thought to be on magnetic field lines that map to the poleward boundary of the night-side auroral zone, as well as to the more distant portions of the geomagnetic tail where energy conversion processes associated with reconnection occur.