MeV He3/He4 isotope abundances in solar energetic particle events: SOHO/COSTEP observations
Abstract
We present first results based on a systematic survey of 4-41 MeV/N 3He/4He isotope abundances with ratios >0.01 detected by the COmprehensive SupraThermal and Energetic Particle analyzer (COSTEP) onboard the SOHO (SOlar and Heliospheric Observatory) spacecraft. More than about 25% of the identified events showed 3He/4He ratios in the range 0.1-1. For events with sufficiently high detector count rates the atomic mass plots can be resolved up to a time resolution of about 1 hour. These events are most suitable for comparisons with in situ solar wind plasma and magnetic field measurements and SOHO's optical white-light and extreme ultraviolet (EUV) observations of the Sun. The correlations show an association with passages of shock associated coronal mass ejections (CMEs) in the solar wind that inhibit high He/H plasma overabundances. It is likely that the CMEs have been released in strong magnetic reconfiguration processes at the solar source sites. Here we present a brief overview of such an event detected on October 30, 2000. 1. Introduction The SOHO/COSTEP instrument measures solar energetic particles (SEPs) at MeV energies in the interplanetary medium. The solid state detectors are capable to detect 3 He/4 He-enrichments at these energies (Müller-Mellin et al., 1995). Usually, the 3 He/4 He-ratio in the solar wind is at the order of 10-4 , but occasionally ratios up to about values of ~1 or even above have been observed in SEP events (e.g., Mason et al., 1999). The origin of these isotope abundances has commonly been attributed to impulsive solar flares and wave-particle interaction mechanisms (Temerin and Roth, 1992). However, fully satisfying physical explanations are still lacking. Here we present first results of a systematic survey of the He-measurements taken by COSTEP since launch in 1995 until the end of the year 2000. 2. Data For this study we have analyzed SOHO/COSTEP measurements of 4.3-40.9 MeV/N helium particles as well as COSTEP data covering proton and electron measurements at energies below 10 MeV. These data were compared with magnetic field and plasma data from the Advanced Coposition Explorer (ACE), SOHO/LASCO (Large Angle Spectroscopic COronagraph) observations of CMEs and X-ray flare measurements taken by the GOES satellite. 3. Identification of 3 He/4 He-rich events Figure 1 shows an example of a 3 He/4 He-rich event as identified from the COSTEP mass separation plot for Oct. 30, 2000. The vertical axis in Figure 1 provides information about the detector count rates, the horizontal axis is labelled such that 4 He corresponds to a value of 0. The two largest peaks in the count rates at -0.6 and 0 correspond to proton (-0.6) and 4 He particles. The presence of a major contribution of 3 He isotopes at about -0.05 is very distinguished. The ratio of 3 He/4 He was 0.7 (see Table 1, doy 304 in 2000). All identified ratios >0.01 are listed in Table 1. Figure 1. COSTEP mass plot for 30 October 2000. The peaks at -0.6, -0.05 and 0 correspond to p, 3 He and 4 He. p 3 He 4 He
- Publication:
-
International Cosmic Ray Conference
- Pub Date:
- August 2001
- Bibcode:
- 2001ICRC....8.3095B