First Energetic Particle Events Observed by the ERNE Instrument

Abstract

The energetic particle instrument ERNE on-board SOHO started its observations on December 15, 1995. The low-energy sensor of ERNE, LED, is capable of measuring particles in the energy range from 1 to 10 MeV nucl^-1. From the beginning of the year 1996 until May 22, 1996, LED-observations included four energetic particle events above threshold intensities. An energetic particle event caused by a corotating interaction region that accelerated protons upto ∽ 10 MeV, was observed during January 20-25. Another similar particle event occured on May 6-12. The events were separated by four solar rotation periods. They had similar time profiles, but the one in May had a harder spectrum and a lower intensity level. The ⁴He-to-proton ratios were in accordance with the solar wind value. Energetic particles observed during April 22-23 and May 14-17 were accelerated at the Sun. The first one was apparently an outcome from an active region observed on the west limb by telescopes on-board SOHO. Protons were detected at energies from 1 to ∽10 MeV. For this event, the⁴He-to-proton ratio in the range 1.5-5 MeV nucl^-1 was ∽ 3%. No ³He ions were detected. The period of May 14-15 was, in contrast, extremely ³He-rich: it had a³He-to-proton ratio of 1.5 ± 0.6 and a ³He-to- ^4He ratio as high as ∽ 8. The period of May 14-17 comprised at least three individual, one-day-long events. The first two events were ³He-rich, while the last one seemed to have a normal composition.