Relative Spectra and Distributions of Fluences of 3He and 4He in Solar Energetic Particles

TL;DR

This paper models the distributions of 3He and 4He fluences in solar energetic particles, explaining observed ratios and their correlation with turbulence levels through stochastic acceleration mechanisms.

Contribution

It demonstrates that a turbulence-based stochastic acceleration model can reproduce the observed fluence distributions and ratios of 3He and 4He in solar energetic particle events with minimal free parameters.

Findings

The model reproduces the narrow 3He fluence distribution.

The fluence ratio varies steeply with turbulence levels.

3He ions are more readily accelerated due to their charge-to-mass ratio.

Abstract

Solar Energetic Particles (SEPs) 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 two earlier papers (Liu et al. 2004 and 2006) we showed how such extreme enrichments can result in the model developed by Petrosian and Liu (2004), 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 et al. (2005) which show that while the distribution of 4He fluence like many other extensive characteristics of solar flare, is fairly broad, the 3He fluence is limited to a narrow range. Moreover, the ratio of the fluences shows a strong correlation with the 4He fluence. One of the predictions of our model was 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 more plasma modes and accelerate 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.