Van Allen Probe Observations Of Disappearance, Recovery And Patchiness Of Plasmaspheric Hiss Following Two Consecutive Interplanetary Shocks: First Results

We present, for the first time, a plasmaspheric hiss event observed by the Van Allen probes in response to two successive interplanetary (IP) shocks occurring within an interval of similar to 2 h on December 19, 2015. The first shock arrived at 16:16 UT and caused disappearance of hiss for similar to 30 min. Combined effect of plasmapause crossing, significant Landau damping by suprathermal electrons and their gradual removal by magnetospheric compression led to the disappearance of hiss. Calculation of electron phase space density and linear wave growth rates showed that the shock did not change the growth rate of whistler waves within the core frequency range of plasmaspheric hiss (0.1-0.5 kHz) during this interval making conditions unfavorable for the generation of hiss. The recovery began at similar to 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 (similar to 350 Hz). The second shock arrived at 18:02 UT and generated patchy hiss persisting up to similar to 19:00 UT. It is shown that an enhanced growth rate and additional contribution from shock-induced poloidal Pc5 mode (periodicity similar to 240 s) ultralow frequency (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 IP shocks, substorms, ULF waves, and background plasma density in the variability of plasmaspheric hiss.