Revisiting the Origin of Impulsive Electron Events: Coronal Magnetic Restructuring

Abstract

We revisit the impulsive beamlike particle events detected in situ from 1997 to 2000 by the Electron, Proton, and Alpha Monitor (EPAM) experiment on the Advanced Composition Explorer spacecraft. We study in detail a subset of events for which there are radio coronal observations from the Nancay Radioheliograph. EPAM measures electrons in the energy range from 40 to 300 keV over a wide range of look directions and with better than 1 minute time resolution, while the Nancay radioheliograph provides images of the solar corona at five different frequencies with time cadence of eight images per second and per frequency. The radio images are complemented with spectral information from a series of radiospectrographs over a wide frequency range (from dm to km wavelengths), white-light coronagraphic images from the Large Angle Spectroscopic Coronagraph (LASCO) on the the Solar and Heliospheric Observatory (SOHO) spacecraft, and EUV images from the Extreme Ultraviolet Imaging Telescope (EIT), also on SOHO. We separate the particle events according to their associated radio emissions in the meter to decameter wavelengths, in radio-simple (only type III bursts) and radio-complex (also type II bursts and/or continua). The electron events in the radio-simple category have rather short durations, are very weak, and show essentially no delay between the onset of type III emission and the inferred release time for the energetic electrons. The electron events in the radio-complex category present variable delays between the onset of type III emission and the inferred release time for the energetic electrons. The inferred release time for the particles in the radio-complex category always coincides with the onset or major changes in the complex radio emissions; this good association suggests that the coronal processes involved in the radio emissions are at the origin of the electron acceleration. The timing and spectral characteristics of the radio emissions, when compared with the properties of the particles seen at EPAM, and the white-light information from LASCO, strongly support an acceleration process in the corona, at variable heights and below the leading edge of the associated coronal mass ejection. The coronal restructuring put in evidence by the radio signatures is the simplest explanation for the origin of those energetic particles.