The Ultra-Low Energy Isotope Spectrometer (ULEIS) on ACE is making new observations of heavy ions from impulsive solar flares that will help determine not only how the ions are accelerated but also how they propagate from the Sun to Earth. The energy source for an impulsive flare is probably the solar magnetic field, although the details of how the energy gets transferred from the magnetic field into particle and photon emissions are still unknown. We do know that these impulsive events have characteristic abundances that distinguish them from any other sample of accelerated matter in the heliosphere: the rare isotope ³He is enhanced by factors of ~100 to 1000 over its abundance in the Sun, and heavy ions are enhanced by factors of ~3 to 10 over their solar abundances. Resonant absorption of waves produced by beams of energetic electrons may be the cause of the peculiar composition enrichments.

The particles from impulsive flares propagate outward from the flare along the spiral interplanetary magnetic field; in order to observe them at ACE, the magnetic field line through ACE must connect to a flare site at western solar longitudes. Above we show several events enriched in ³He and heavy ions observed with ULEIS during 15-19 August 1998; active regions 8297 & 8299 passed through western solar longitudes during this time and flared multiple times. The figure shows particle velocity in (MeV/nucleon)^1/2 versus arrival time at ACE for ions of carbon through iron. The distinct curves are the result of velocity dispersion: faster particles arrived from the flares before the slower particles, forming curves of velocity = distance/time. With its high sensitivity, ULEIS can distinguish the individual ion injections labeled 1-11 in the figure; some of these low intensity events could not be resolved with previous instruments. Also evident are differences in the event profiles which suggest varying amounts of particle scattering in the interplanetary medium (e.g. smaller width in velocity of event 2 versus event 6) as well as dramatic effects of the meandering interplanetary magnetic field (e.g. cutoffs of events 4 and 5 while event 6 is unaffected). These high time resolution measurements of ions from impulsive solar flares will not only single out individual events for further study with ACE, but may also yield further insight into the magnetic connection between ACE and the flare site.

Contributed by Joe Mazur of The Aerospace Corporation and Glenn Mason & Joe Dwyer of the University of Maryland.

See The ULEIS Home Page for more information about the ULEIS instrument.

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Last modified 11 November 1998, Andrew Davis
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