Solar Energetic Particles

SEP events were identified by an unpredictable increase in the trigger rate of the instrument, at least one order of magnitude with respect to the averaged solar quiet values. Figure 1 shows typical NINA counting rates, plotted together with data from GOES-8 and BATSE; sudden increases of particles are visible. Nine such bursts in the period October 1998--April 1999 have been detected and identified as Solar Energetic Particle events.

Figure 1

For the 7 November 1998 event, one of the most interesting that NINA detector, we report in Figure 2 the reconstructed helium isotope masses (left) and the 3He and 4He differential energy spectrum in the full energy interval (right). The energy spectrum S(E) during SEPs was fitted by a power-law component plus the galactic background B(E) measured by our own instrument in the same period:
S(E) = A E- + B(E) .
The 3He spectrum is slightly harder ( = 2.5 ± 0.6) than 4He ( = 3.7 ± 0.3). This implies that the 3He/4He ratio increases with energy in this SEP event.

Figure 2

The strongest solar event that we detected was the one of 14 November 1998. In this event we recorded several high Z particles, as shown in Figure 3.

Figure 3

For all events we determined the deuterium-to-proton ratio. During the 24 November 1998 event the ratio resulted about 10 times higher than normal coronal values. Figure 4 shows the mass resolution of hydrogen isotopes for the 24 November 1998 (right) together with the distribution of the 6 November 1998 (left). These two SEP events correspond to the maximum and the minimum of the deuterium component that NINA detected.

Figure 4

For more details about the analysis of SEPs performed by NINA please sea the NINA