The Acceleration and Release of Near‐relativistic Electrons by Coronal Mass Ejections

Abstract

We have examined transient coronal activity observed by the Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph Experiment (LASCO) around the release time of beams of near-relativistic electrons (?40–300 keV) observed by the Advanced Composition Explorer (ACE)/ Electron, Proton, and Alpha Monitor (EPAM) at 1 AU. The electron beams are strongly associated with coronal mass ejections (CMEs) emitted from near the Sun’s western limb, although the converse is not true; many more west hemisphere CMEs are seen by LASCO than are relativistic electron beams seen by ACE/ EPAM. However, when they are present, strong beams of electrons may be used to infer reliably the actual solar release time as they propagate scatter free. Fifty-two electron beams were observed from 1997 Septem- ber to 2000 September at times ofLASCOobservations. Of these, there were 47 observations of an associated coronal transient, of which 33 were ‘‘ classical ’’ CMEs, as distinct from blobs and jets. We were able to extrapolate the height-time plot back to a nominal 1 R?, to estimate a CME launch time. All but two of the CMEs were seen in projection off the west solar limb. For 37 events there was an associated soft X-ray event, the majority of which were from western active regions.All electron events were accompanied by decametric type III emission consistent with electron beams having exciter energies of around a few keV. The near-rela- tivistic electron injection time was typically delayed by ?20 minutes from the CME launch time and greater than ?10 minutes after the onset of the electromagnetic radio and X-ray signatures of the flare (when present). Therefore, the near-relativistic electrons that must be present to produce the chromospheric electro- magnetic emission do not escape promptly (at least at detectable intensities). The radial distance of most CMEs at the electron release time was between 1.5 and 3.5 R?. Both the peak electron intensity and the spec- tral hardness of the electrons were positively correlated with the CME speed, a signature of shock accelera- tion. We therefore suggest that most of the near-relativistic electrons seen by ACE/EPAM are accelerated by the shock driven by the coronal transient and are released at a radial distance around 2–3R?.