Light isotope abundances in SEPs measured by NINA

Observations of 9 Solar Energetic Particle events detected by the instrument NINA from November 1998 to April 1999 will be presented. NINA is a silicon-based space detector in orbit since July 1998 on board the Russian satellite Resurs-01-N4, which flies at low altitude (about 800 km) in polar inclination. For every SEP event we reconstructed the power-law He spectrum in the energy interval 10–50 MeV/n, extracting spectral indexes from 1.8 to 6.8. Data of He and He were then employed to determine the He/He ratio, that turned out to be high for some SEP events showing the enrichment in He. For the 7 November 1998 event this ratio reached the maximum value of 0.33 ± 0.06, with spectral indexes 2.5 ± 0.6 and 3.7 ± 0.3 for He and He, respectively. The He/He ratio averaged over the remaining events was 0.011 ± 0.004. For all events we determined the deuterium-to-proton ratio. The average value of the H/H ratio, over all events, was (3.9±1.4)× 10−5 in the energy interval 9–12 MeV/n. During 24 November 1998 event, however, this ratio resulted about 10 times higher than normal coronal values.