ULF waves excited by negative/positive solar wind dynamic pressure impulses and interplanetary shocks at geosynchronous orbit

When a solar wind dynamic pressure impulse impinges on the magnetophere, ultralow frequency (ULF) waves can be excited in the magnetosphere and the solar wind energy can be transported from interplanetary space into the inner magnetosphere. In this paper, we have systematically studied ULF waves excited at geosynchronous orbit by both positive and negative solar wind dynamic pressure pulses. We have identified 270 ULFevents excited by positive solar wind dynamic pressure pulses and 254 ULF events excited by negative pulses from 1 January 2001 to 31 March 2009. We have found that the poloidal and toroidal waves excited by positive and negative pressure pulses oscillate in a similar manner of phase near 06:00 local time (LT) and 18:00 LT, but in antiphase near 12:00 LT and 0:00 LT. Furthermore, it is shown that excited ULF oscillations are in general stronger around local noon than those in the dawn and dusk flanks. It is demonstrated that disturbances induced by negative impulses are weaker than those by positive ones, and the poloidal wave amplitudes are stronger than the toroidal wave amplitudes both in positive and negative events. The potential impact of these excited waves on energetic electrons at geosynchronous orbit has also been discussed.