The acceleration and release of solar energetic particles in events from April 20 – May 9, 1998

Abstract

The most active period in X-rays of the current solar cycle to date (August 1998) occurred from April 23 – May 9, 1998, when there were four X-class flares and two more which exceeded M6. All were likely to have been from AR 8210 (two were occulted and thus didn't have positive identification, although the coronal Yohkoh images were consistent with this hypothesis). They were associated with fast, extensive coronal mass ejections as seen by the LASCO coronagraphs on SOHO. This time interval also saw the largest energetic particle activity of the cycle so far, and it is reasonable to hypothesise that, the activity that produced the X-ray events also was responsible for accelerating the particles. In fact, the largest energetic particle event in this period was associated with an occulted flare on April 20, which Yohkoh observed in X-rays off the southwest limb; this flare plus the other six comprise the seven events studied in this paper. The largest X-ray event of the series, an X2.7 flare from S11W65, which is normally magnetically well-connected to the Earth, produced a peak >10 MeV proton intensity at the Earth an order-of-magnitude less than that produced by the April 20 event. Relativistic electrons from the April 20 event were detected promptly at the ACE spacecraft without velocity dispersion, indicating that ACE was moving into a magnetic structure already populated with electrons. The electrons were seen at the Ulysses spacecraft around a day later as an isotropic increase, indicating that the electrons had reached Ulysses, at around 5.4 AU, via a wide range of magnetic field lines. We compare the solar X-ray and radio signatures for the seven events, together with the characteristics of the coronal mass ejections seen with LASCO. We conclude that the energetic particles accelerated in these events were unlikely to have been accelerated in the interplanetary medium by the CMEs, but were more likely to have been accelerated in the closed magnetic field region of the corona, whence they either propagated within the closed corona until they escaped onto open field lines, or they lost their energy.