Suprathermal particles in magnetic clouds

Abstract

Measurements with the Solar Wind Ion Composition Spectrometer (SWICS) have shown ubiquitous and prominent suprathermal tails with a power-law dependence on velocity, ∼ ν−5 [1, 2]. Those and similar studies have mostly concentrated on long-term averages. Several explanations of these power-law velocity distribution functions (VDFs) have been proposed. Many of them rely on statistical arguments, e.g., that these VDFs result from superpositions of many VDFs, from multiple acceleration sites or processes, from successions of compressions and rarefactions, or from jumps from slow to fast wind and vice versa. To limit such proposed explanations we have investigated short-term spectra of such suprathermal particles (STPs) in magnetic clouds (MCs). Because MCs are generally globally expanding and show very low turbulence and only very limited variability in the overall plasma parameters, we do not expect a similar power law VDF to be observed in MCs. However, we found that the VDFs within MCs are similar to the average VDFs in the solar wind, that they often exhibit power laws and occasionally also the quiet-time ∼ ν−5 behaviour.