Free energy to drive equatorial magnetosonic wave instability at geosynchronous orbit

Abstract

The magnetosonic (or ion Bernstein) instability is driven by a positive slope in the ion distribution function perpendicular to the magnetic field at energies above about 1 keV. Fifteen years of multisatellite geosynchronous observations are used to determine the statistical occurrence of ion distributions with positive slopes as a function of energy, local time, geomagnetic activity, and phase of the solar cycle. There is no discernable dependence on phase of the solar cycle, but there are clear dependences on the other parameters. Positive slopes are seen primarily in the energy range between similar to 3 and similar to 24 keV. The peak occurrence of positive slopes is between midmorning and dusk and moves progressively toward earlier local times for higher energies. The occurrence is significantly greater and extends over a broader local time range for low levels of geomagnetic activity than for high activity, for all energies. At high activity levels, the occurrence tends to be more closely confined near noon. Peak occurrence rates are similar to 30% at energies just below 10 keV. A superposed epoch analysis of 77 coronal mass ejection (CME)-driven storms and 93 high-speed solar wind (HSS)-driven storms shows a relative suppression of the occurrence frequency of positive slopes during the recovery phase. The suppression is particularly long-lived for HSS-driven streams.