Non-stationary effects in wave-particle interactions during storm sudden commencements (SSC)

Simultaneous observations of energetic electrons (16-300 keV) and ELF/VLF waves (200 Hz-3 kHZ) are analysed, that are obtained onboard GEOS 2 in the dayside magnetosphere during Storm Sudden Commencements (SSC's). The electron distribution exhibits a large temperature anisotropy, leading to the amplification of whistler waves. Wave particle interactions (WPI's) are studied by calculating the temperature anisotropy and the wave growth rate from one minute averages of the particle data. Following the SSC, an increase in the electron distribution anisotropy results in an increase of the wave spectral intensity B_f². Oscillations with a period of 2-4 min are observed in the maximum intensity B_fmax² that are in antiphase both with the corresponding growth rate and the anisotropy. This last effect is interpreted in terms of the quasilinear pitch-angle diffusion of the resonant electrons.