Disappearance, recovery and patchiness of plasmaspheric hiss following
two consecutive interplanetary shocks: First results
Abstract
We present, for the first time, a plasmaspheric hiss event observed by
the Van Allen probes in response to two successive interplanetary shocks
occurring within an interval of ~2 hours on December 19,
2015. The first shock arrived at 16:16 UT and caused disappearance of
hiss for ~30 minutes. Significant Landau damping by
suprathermal electrons followed by their gradual removal by
magnetospheric compression opposed the generation of hiss causing the
disappearance. Calculation of electron phase space density and linear
wave growth rates showed that the shock did not change the growth rate
of whistler mode waves within the core frequency range of plasmaspheric
hiss (0.1 - 0.5 kHz) during this interval making conditions unfavorable
for the generation of the waves. The recovery began at
~16:45 UT which is attributed to an enhancement in local
plasma instability initiated by the first shock-induced substorm and
additional possible contribution from chorus waves. This time, the wave
growth rate peaked within the core frequency range (~350
Hz). The second shock arrived at 18:02 UT and generated patchy hiss
persisting up to ~19:00 UT. It is shown that an enhanced
growth rate and additional contribution from shock-induced poloidal Pc5
mode (periodicity ∼240 sec) ULF waves resulted in the excitation of hiss
waves during this period. The hiss wave amplitudes were found to be
additionally modulated by background plasma density and fluctuating
plasmapause location. The investigation highlights the important roles
of interplanetary shocks, substorms, ULF waves and background plasma
density in the variability of plasmaspheric hiss.