Properties of energetic particle bursts at dawnside magnetosheath: Cassini observations during the 1999 Earth swing-by

Abstract

[1] During Cassini's Earth swing-by in 1999, the Low-Energy Magnetospheric Measurement System (LEMMS) detected energetic particles at the distant dawn magnetosheath. LEMMS identified 9 electron bursts and continuous proton flux surges at least up to 100 keV at X = −72 Re to −102 Re during the southward turning of the interplanetary magnetic field. All bursts were anti-field-aligned and correlated with the Bz directions streaming away from the magnetosphere, indicating that they are originated in the magnetosphere. The different source locations of protons and electron bursts were suggested from the magnetic field direction during the flux enhancement owing to the high time resolution mode of LEMMS: electrons were from flank low-latitude boundary layer and protons were from wide and dispersed region in the magnetopause. Although the energy spectra of the particles in the magnetosheath were similar to that in the plasma sheet, the proton spectra showed a bend around 5 keV possibly due to scatterings and accelerations by large amplitude waves. Most of these proton spectra were well described by the kappa distributions. The fact that the electron bursts were identified only at limited locations suggested that the reconnection on the magnetopause was required at the source, while protons could leak out both from the magnetic reconnection and the continuous gradient drift with wave scatterings at the magnetopause. Within the short time snapshot of the magnetosheath, extremely active feature of the particle leakage throughout the magnetopause was revealed.