Response of relativistic electron microbursts to the arrival of high speed solar wind streams

Relativistic electron microbursts are frequently observed during high-speed solar wind stream (HSS) events [e.g., Blum et al., 2015], while important solar wind parameters for the frequent microburst precipitations have not been well understood. We perform a superposed epoch analysis of the microburst occurrence during HSS events, considering the polarity of interplanetary magnetic field (IMF) and solar wind speed according to the method used by Miyoshi and Kataoka (2008). We find the most frequent microburst precipitations during the higher-speed solar wind streams with a southward offset of IMF (SBZ-fast HSS events), indicating that both the southward IMF and fast solar wind are important for frequent microburst precipitations. It reveals that precipitation loss through relativistic electron microbursts is more important during the SBZ-fast HSS events than during the other HSS events. Since fluxes of radiation belt electrons largely increase during SBZ-fast HSS events [Miyoshi and Kataoka, 2008], the result is observational evidence that relativistic electron microbursts can be a proxy of acceleration of MeV electrons by chorus.