The Coronal Mass Ejection of 1998 April 20: Direct Imaging at Radio Wavelengths

We observed the fast coronal mass ejection (CME) of 1998 April 20 with the radioheliograph at Nançay, France, between 164 and 432 MHz. Spectroscopic data were obtained between 40 and 800 MHz by the spectrometer at Tremsdorf, Germany, and between 20 kHz and 14 MHz with the WAVES instrument on board the Wind spacecraft. Energetic particle data were obtained from the Wind 3D Plasma and Energetic Particle experiment. The CME was observed in white light by the Large-Angle Spectrometric COronagraph experiment on board the Solar and Heliospheric Observatory spacecraft. For the first time, the expanding CME loops are imaged directly at radio wavelengths. We show that the radio-emitting CME loops are the result of nonthermal synchrotron emission from electrons with energies of ~0.5-5 MeV interacting with magnetic fields of ~0.1 to a few gauss. They appear nearly simultaneously with the onset of an associated type II radio burst, shock-accelerated type III radio bursts, and the initiation of a solar energetic particle event. We suggest possible sources of the energetic electrons responsible for this "radio CME" and point out diagnostic uses for synchrotron emission from CME loops.