Relative distributions of fluences of 3He and 4He in solar energetic particles

Abstract

Solar energetic particles show a rich variety of spectra and relative abundances of many ionic species and their isotopes. A long-standing puzzle has been the extreme enrichments of 3He ions. The most extreme enrichments are observed in low-fluence, the so-called impulsive, events which are believed to be produced at the flare site in the solar corona with little scattering and acceleration during transport to the Earth. In such events, 3He ions show a characteristic concave curved spectra in a log-log plot. In two earlier papers of Liu etal., we showed how such extreme enrichments and spectra can result in the model developed by Petrosian and Liu, where ions are accelerated stochastically by plasma waves or turbulence. In this paper, we address the relative distributions of the fluences of 3He and 4He ions presented by Ho etal., which show that while the distribution of 4He fluence (which we believe is a good measure of the flare strength) like many other extensive characteristics of solar flare is fairly broad, the 3He fluence is limited to a narrow range. These characteristics introduce a strong anticorrelation between the ratio of the fluences and the 4He fluence. One of the predictions of our model presented in the 2006 paper was the presence of steep variation of the fluence ratio with the level of turbulence or the rate of acceleration. We show here that this feature of the model can reproduce the observed distribution of the fluences with very few free parameters. The primary reason for the success of the model in both fronts is because fully ionized 3He ion, with its unique charge-to-mass ratio, can resonantly interact with plasma modes not accessible to 4He and be accelerated more readily than 4He. Essentially in most flares, all background 3He ions are accelerated to few MeV/nucleon range, while this happens for 4He ions only in very strong events. A much smaller fraction of 4He ions reach such energies in weaker events. © 2009. The American Astronomical Society. All rights reserved.