First Energetic Particle Events Observed by the ERNE Instrument
J. Torsti, E. Valtonen, L. G. Kocharov, R. Vainio, E. Riihonen, A. Anttila, T. Laitinen, M. Teittinen, J. Kuusela
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 \ensuremath\backsim 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 $^4$He-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 \ensuremath\backsim10 MeV. For this event, the$^4$He-to-proton ratio in the range 1.5-5 MeV nucl$^-1$ was \ensuremath\backsim 3\%. No $^3$He ions were detected. The period of May 14-15 was, in contrast, extremely $^3$He-rich: it had a$^3$He-to-proton ratio of 1.5 \ensuremath\pm 0.6 and a $^3$He-to- $^4He$ ratio as high as \ensuremath\backsim 8. The period of May 14-17 comprised at least three individual, one-day-long events. The first two events were $^3$He-rich, while the last one seemed to have a normal composition.
Sol. Phys., 170, 179-191, 1997