Energetic electrons accelerated in solar particle events

Abstract

New measurements of energetic solar electrons from the WIND spacecraft are reviewed, and the implications for particle acceleration mechanisms discussed. In non-relativistic electron/3He-rich (so-called impulsive) events the electron energy spectrum is often found to extend below ∼1 keV, indicating that acceleration occurs high in the corona. Comparison of the escaping electrons with the electrons producing the associated hard X-ray burst suggests that acceleration is occurring over a wide range of altitudes. For Large Solar Energetic Particle (LSEP, or so-called gradual) events, WIND observations show the low energy ∼1–10 keV electron component is sometimes missing. In many LSEP events the electrons are released from the Sun up to ∼0.5 hour later than the onset of the solar type III radio burst, and coronal transient waves are detected traveling across the Sun by the SOHO EIT instrument. Onset timing analyses show two types of LSEPs; in some events the first arriving ∼0.1–6 MeV protons are released ∼0.5–2 hours after the electrons and travel a path length of ∼1.2 AU (essentially scatter-free), while in other events the protons are released at the same time as the electrons but appear to travel ∼2 AU. If we assume the observed energetic particles are accelerated by a shock in front of an outward propagating fast CME, the electrons are accelerated earlier and lower in the corona (∼0.5 RSun) and the protons later and higher, ∼4 RSun for the first type of event, and from ∼4 to >∼10 RSun, with the more energetic protons accelerated lower for the second type. In mid-2000 the High Energy Solar Spectroscopic Imager (HESSI) mission will be launched to provide detailed X-ray and gamma-ray imaging and spectroscopy observations to study particle acceleration and energy release processes at the Sun. Comparisons between HESSI and ACE/WIND should provide new insights into the origins of energetic solar particles.