On the Generation Mechanism of Electromagnetic Cyclotron Waves in the Solar Wind: Statistical Results from Wind Observations
Abstract
Electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency are frequently observed in the solar wind, yet their generation mechanism is still an open question. Based on the Wind data from 2005-2015, this paper carries out a statistical study on the plasma characteristics associated with the occurrence of ECWs.
The probability density distributions (PDDs) of proton temperature anisotropy (T-perpendicular to/T-parallel to) and proton parallel beta (beta(parallel to)) are investigated, where perpendicular to and parallel to refer to perpendicular and parallel to the background magnetic field, respectively. The PDDs depend on solar wind types as well as wave polarizations, and those for ECWs with left-handed (LH) polarization exhibit considerable differences from the PDDs for ambient solar winds.
The distributions of occurrence rates of LH ECWs in (beta(parallel to), T-perpendicular to/T-parallel to) space show a tendency that the occurrence rates increase with proton temperature anisotropies. The beta(parallel to) with the maximum of occurrence rates is near 0.1 when T-perpendicular to/T-parallel to > 1, while it is around 1 when T-perpendicular to/T-parallel to < 1.
The presence of alpha-proton differential flow with large kinetic energy corresponds to a much higher occurrence rate as well as the domination of LH polarization of ECWs. Based on these observations and existing theories, we propose that the proton cyclotron and parallel firehose instabilities with effects of alpha-proton differential flow are likely responsible for the local generation of LH ECWs in the solar wind.
The generation mechanism of right-handed (RH) ECWs seems to be complicated and more discussions are needed in future research.