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Helium-3-Rich Solar Particle Events Observed by SEPICA

Uncalibrated mass histograms for helium as obtained during the Sept. 18-20, 1997, solar energetic particle events

The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) was first turned on Thursday, August 28, 1997, and has been taking data since August 29. SEPICA is designed to measure the elemental and ionic charge state composition of energetic particle fluxes from H to Ni (1 <= Z <= 28), as well as the isotopic composition of He. For the first four weeks the deflection high voltage that allows discrimination between charge states was set to 3 kV (10% of its maximum setting) to allow for outgassing. Subsequently, the deflection voltage has been slowly raised to 24 kV. SEPICA was therefore only partially operational during the first month.

Identification of He isotopes by SEPICA is demonstrated in the Figures above, which include energetic particle events that occurred during Sept. 18 through 20 (featured in the 10/27/97 ACE News item from EPAM). Shown are helium mass histograms (not yet fully calibrated) for two energy ranges from these first He-rich solar events recorded by SEPICA. The two histograms suggest a strong variation of the 3He/4He ratio with energy. Data from the SOHO/HSTOF instrument indicate that this trend extends to lower energies, which may open the way for a multi-mission collaborative study.

These solar events also show a substantial enrichment in Fe (as already indicated by EPAM). SEPICA has collected more Fe ions during these three days than were integrated over all 3He-rich events during the entire ISEE-3 mission with SEPICA's predecessor. Although the deflection voltage was only at 3 kV, preliminary analysis shows that low energy Fe ions (50 - 200 keV/nucleon) have charge states of Q = 15 - 25, compatible with results for the entire ISEE-3 sample of 3He-rich events. SEPICA will be able to carry out this charge state analysis for individual flares.

SEPICA was developed by the University of New Hampshire and the Max-Planck-Institut for Extraterrestrial Physics, Germany.

Contributed by Dr. Eberhard Moebius, UNH.