Multi-Instrument Observations of Soft Electron Precipitation and Its Association with Magnetospheric Flows

Abstract

We present a multi-instrument study on the variations of optical auroras and ionospheric electron densities during an interval of a series of fast earthward flows in the magnetotail on 3 March 2009. The flow-related auroral signatures include intermittent higher-latitude (>68° magnetic latitude) intensifications manifested in green and blue line auroras and more latitudinally extended red line auroral intensifications and expansions. During the same interval the Poker Flat incoherent scatter radar (PFISR) detected F region ionospheric electron density enhancements which, together with the red line auroral intensifications, give evidence for soft electron (<1 keV) precipitation associated with fast magnetospheric flow activity. We demonstrated the southward motion of ionospheric electron density patches in correspondence to individual earthward flow bursts and auroral activations. By virtue of the multibeam technique of PFISR we construct the altitudinal profile of the density patches and estimate that the characteristic energies of the precipitating electrons were on order of a few hundred eV, comparable to the observed electron temperature in the near-Earth central plasma sheet (CPS). We propose that the fast flows give rise to enhanced ELF wave activity, which causes strong pitch angle diffusion of the soft electron population in the CPS via wave-particle interactions. The precipitation may be further aided with a moderate field-aligned potential drop comparable to or smaller than the CPS electron temperature. When the flows penetrate into the inner plasma sheet, the adiabatic drift motion of soft electrons may lead to a decreasing trend of electron energy with decreasing radial distance, which is manifested in PFISR observations as an ascending trend of the altitude range of the density patches toward the equatorward auroral border.

Key Points:

• Magnetospheric fast flows produce soft electron precipitation in the ionosphere
• Soft electron precipitation is mainly caused by enhanced scattering by wave
• The auroral morphology is consistent with the adiabatic drift of electrons