Ionic charge states of solar energetic particles

Abstract

A novel technique to infer average ionic charge states of high energy (>= 10 MeV/nuc) solar energetic particles (SEPs) in large solar events is presented. In some large SEP events, it is observed that higher energy SEPs decay in intensity more rapidly than at lower energies. Furthermore, this energy dependence varies with particle species, as would be expected if the decay timescale depended on a rigidity-dependent diffusive mean free path. Observations are done with the Solar Isotope Spectrometer (SIS) on board the Advanced Composition Explorer spacecraft. By comparing the decay timescales of nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron to a reference element, such as carbon, charge states are inferred for these elements in three SEP events between 1997 and 2002. In a fourth event, upper limits of charge states are inferred. For the solar event of November 6, 1997, charge states are also inferred for sodium, calcium, and nickel. These charge states are compared with other measurements at similar energies, and with measurements at lower energies. Two interpretations of the data are discussed. First, if there is no stripping in the shock acceleration process, the charge states inferred for the events might be indicative of source plasma temperatures (Arnaud and Rothenflug, 1985; Arnaud and Raymond, 1992). It is found that two of the events examined have temperatures consistent with the acceleration of particles from the corona. The other two events have best fit temperatures that might indicate a mixture of sources, including the corona and a hot flare region. Second, iron charge states from this work and from other work (Mazur et al., 1999; Mobius et al., 1999; Cohen et al., 1999) at various energies for the November 6, 1997, event are compared to the models of Barghouty and Mewaldt (2000) and Kovaltsov et al. (2001) for shock acceleration in a dense plasma, which include the effects of stripping and recombination due to interactions with protons and electrons. These models can describe the observed charge state spectra with acceleration from the corona without invoking mixture with a hot source.