Intense Alfven Waves With Compressional Mode Waves Well Within the Plasma Sheet During the Main Phase of the 21 October 1999 Major Storm and Comparison of Major Storm to Non-Storm Substorm PSBL Alfven Wave Events

We present the first significant compressional mode MHD waves observed concurrent with very intense earthward Alfvenic Poynting flux (>1000 ergs cm-2s-1 peaks in the time series data, filtered from 5 mHz to 4 Hz, when mapped to ionospheric altitudes) in the plasma sheet during a major geomagnetic storm. These waves were observed by Polar at ~5.5 Re geocentric and ~23 MLT during the main phase of the 21 October 1999 major storm in relation to a tail reconfiguration. A lobe excursion and a superdense plasma sheet population were also observed in relation to the event by two LANL geosynchronous satellites in the dawn sector. Although the event is unique in Polar observations reported to date, similar geosynchronous events associated with large storms or other significant geomagnetic activity have been observed ~30 times over the course of about one year. A sequence of FAST passes during the same storm provides additional evidence for Alfvenic Poynting flux powering auroral acceleration at low latitude during the storm. Further, the first study of the properties of a significant number (~10) of Alfven wave events with large earthward Poynting flux (tens to hundreds of ergs cm-2s-1 mapped) on the plasma sheet boundary layer (PSBL) at ~4-9 Re during major storms is summarized. These properties are found to be similar to PSBL events observed during non-storm substorms. A PSBL excursion during the 21 October 1999 event also shows similar properties indicating that the proximity to the boundary controls the wave properties to a large degree, and also indicate that the central plasma sheet waves during major storms, at least in this case, have significantly different properties. These waves are capable of bringing significant amounts of energy into the inner magnetosphere during major storms.