Properties of Alfven waves in the magnetotail below 9 R(E) and their relation to auroral acceleration and major geomagnetic storms.

摘要

The presented studies investigate the characteristics of Alfven wave events in the geomagnetic tail on the plasma sheet boundary layer (PSBL) and possibly well within the plasma sheet during substorms and major geomagnetic storms (-200 Dst). Such storms are rare but dramatically affect the state of the magnetosphere in ways that we have only recently been capable of investigating with sufficient in situ instrumentation. The first comparative study of major storm PSBL Alfven waves events is presented. Properties of eight substorm and ten major storm events are compared using a new method, providing new insights into the phenomena, their interactions in the auroral acceleration region (AAR), and their generation. Direct comparison between Polar and FAST indicating a decrease (increase) in low-(high-)frequency shear (kinetic) earthward Alfvenic Poynting flux and an increase in earthward electron energy flux strongly suggests transfer of shear Alfven wave Poynting flux to kinetic Alfven waves which then accelerate auroral electrons. Polar observations also suggest a broadband source and indicate that small-scale, temporally/spatially variable factors, likely including density cavities and ionospheric conductivity structure, strongly affect the reflectivity/dissipation properties, as the waves in each frequency band contain a mixture of earthward, tailward, reflecting and incoherent wave intervals. Averages of these properties are consistent with theory, but the detailed structure has not been predicted. Tailward intervals also suggest ionospheric field line shear. Most major storm events were found have similar properties to substorm events with a few notable differences, consistent with effects related to the extended duration of storms. Low-latitude broadband auroral electrons and high-frequency Alfven waves along with properties of a unique Alfven wave event, related to a major storm tail reconfiguration, with very intense Poynting flux and the first reported concurrent compressional mode waves, strongly suggest Alfven wave powered auroral acceleration on field lines mapping well within the plasma sheet during major storms. Geosynchronous satellite observations during this event also indicate that such events may occur ∼30 times within a year. Ion solitary waves in the AAR are also explored, and are found to likely be produced by a hydrogen and oxygen beam two-stream instability.