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ACE News #90 - Jul 8, 2005
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Impulsive Solar Energetic Particle (SEP) events are known for huge enhancements of heavy ions and the isotope 3He. Mass per charge (M/Q) dependent acceleration mechanisms have been proposed as an explanation for these enhancements. However, charge state observations have indicated that ions from C to Mg are fully stripped in these events giving them identical M/Q. Since there is no known preferential acceleration mechanism for ions with identical M/Q, these charge state observations create a problem for the models. As a result, it has been suggested that ions are first accelerated and then stripped due to collisions low in the solar corona. This would lead to energy-dependent charge states. Recent observations show indeed that the charge states increase with energy (see ACE News #80).
To see if there is a connection between charge states in any of the energy ranges and the ion enhancements that would reflect a M/Q dependent fractionation process, ACE SEPICA charge states were combined with abundance ratios from ULEIS. The figure shows Fe charge states in four different energy ranges as a function of the Fe/O ratio. While the Fe charge states at low energies appear to increase with the Fe/O ratio, the highest energy range shows no such correlation. A similar behavior is also observed for the Mg/O, Ne/O, and 3He/4He ratios.
These results support the idea that ions are first accelerated and then stripped low in the solar corona. At high energies, which are only reached after substantial acceleration, stripping leads to an equilibrium charge state that is solely a function of the particles' energy. As a result they will not correlate with the abundance ratios. The charge states of the low energy ions tend to reflect the plasma conditions at the acceleration site. The correlation at these low energies suggests a mass per charge dependent acceleration mechanism, although the details of the process are still not understood.
Submitted by Robert DiFabio (Honors Thesis) and Eberhard Moebius of the University of New Hampshire and Glenn M. Mason of the John Hopkins University Applied Physics Lab. Address comments or questions to firstname.lastname@example.org.
Last modified 8 Jul 2005, by