High-Energy 3He-Rich Solar Particle Events

Abstract

Energetic particle observations of the ERNE instrument on board SOHO enable measurements of 3He and 4He fluxes beyond 15 MeV nucleon−1 with good statistical resolution. We report results of a survey of the ERNE observations covering the period from 8 February 1999 to 6 December 2000. We find 10 and 5 days during which the 3He-to-4He ratio exceeds the levels of 20% and 50%, respectively. Those periods include, in particular, four 3He-rich events that are sufficiently strong for a reasonably accurate estimate of the time-intensity profiles. We analyze the history of solar and interplanetary phenomena associated with these high-energy 3He-rich events. Basic properties of such events and significant solar and interplanetary factors are formulated. The significant factors comprise, in particular, a strong, impulsive flare, typically observed about \(\frac{2}{3}\) day before the 3He onset, and an interplanetary shock wave or magnetic field enhancement arriving at 1 AU about frac43 day after the 3He onset. The high-energy 3He-rich events make up a new kind of hybrid events, possessing the impulsive-type composition and the gradual-type time-profiles. We emphasize a dependence of the resultant particle event on the history of the particular solar eruption comprising coronal mass ejection (CME) and the flare associated with the CME.