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ACE News #77 - January 2, 2004

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Upper Limit on 3He Fluence in Solar Energetic Particle Events

The remarkable enrichment of the isotope 3He in solar energetic particle (SEP) events is usually described in terms of the abundance ratio 3He/4He. This isotope abundance ratio is estimated for SEPs by taking the ratio of the fluences (event-integrated omni-directional fluxes) of the two isotopes. Our continuing mission-long analysis of 3He-rich SEP events using 0.2 - 2.0 MeV/nucleon helium measurements with ACE/ULEIS [Ho et al., Ap. J., 552, 863, 2001; Mason et al., Ap. J., 525, L133, 1999] has revealed a striking pattern in the fluences themselves. The figure above shows 96 3He-rich SEP events (events with 3He/4He above the solar wind value of 4 x 10-4 ). The events in red were selected based on the criteria in Ho et al. [2001], where solid circles denote launch to 1999 data and open circles denote 2000-2002 data. Both "impulsive" (flare-related) and "gradual" (CME-related) events are included. The blue squares are the large SEP events from the list compiled by Mason et al. [1999]. The Figure shows that 4He fluences range over a factor of 10,000 while the 3He fluences in the same SEP events range over only a factor of 100. Our measurement range is limited by the instrument sensitivity threshold for events lasting ~1 day (cross-hatched boundaries along each axis), while in very large events (which can last more than 5 days), higher background and the large number of 4He counts may spill over and obscure the 3He (dark triangle, lower right). However, both 3He and 4He fluences can be registered anywhere else within the remaining area of the diagram. Nonetheless, ULEIS measured no 3He fluences above 105/cm2-sr-MeV/nuc during 1997-2002, so the upper half of the diagram (gray) is devoid of events. The diagonal dotted lines indicating constant 3 He/4 He ratios in no way order the data.

Consequently, not only can the fluence of 4He vary by 4 orders of magnitude for a given fluence of 3He, but there appears to be an upper limit to the 3He fluence. An interpretation that immediately suggests itself is that only a limited number of energetic 3He ions can be released from the Sun in a SEP event since the 3He probably originates in compact regions (e.g., see Reames et al., Ap. J., 327, 998, 1988). If no other disturbance takes place, then very high 3He/4He ratios can result; but if this compact volume of 3He is commingled with a larger volume of coronal material in a large SEP event, it will be diluted, resulting in a lower 3He/4He ratio. Thus, the upper limit on the 3He fluence that we observe may be giving us information on the maximum size of the 3He acceleration region. Although a proper analysis of the interplanetary propagation of the helium ions is required before we can relate the net flux of 3He ions to the measured fluence, we consider this to be a viable explanation of the observed ULEIS upper limit on the fluence of 0.2-2.0 MeV/nucleon 3He ions.

Submitted by George C. Ho and Edmond C. Roelof of JHU/APL and Glenn Mason of the University of Maryland.

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