Time-dependent occurrence rate of electromagnetic cyclotron waves in the solar wind: evidence for effect of alpha particles?

TL;DR

This study analyzes 7 years of STEREO data to examine how the occurrence rates of electromagnetic cyclotron waves in the solar wind depend on polarization and plasma conditions, suggesting alpha particles influence their generation.

Contribution

It provides the first long-term statistical survey of ECWs with polarization-specific occurrence rates and links LH ECWs to plasma conditions and alpha particle effects.

Findings

LH ECWs occur more frequently in hotter, less dense plasma

Right-handed ECWs have a nearly constant occurrence rate

Occurrence rate of LH ECWs varies significantly with plasma properties

Abstract

Previous studies revealed that electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency exist widely in the solar wind, and the majority of ECWs are left-handed (LH) polarized waves. Using the magnetic field data from the STEREO mission, this Letter carries out a survey of ECWs over a long period of 7 years, and calculates the occurrence rates of ECWs with different polarization senses. Results show that the occurrence rate is nearly a constant for the ECWs with right-handed polarization, but it varies significantly for the ECWs with LH polarization. Further investigation of plasma conditions reveals that the LH ECWs take place preferentially in a plasma characterized by higher temperature, lower density, and larger velocity. Some considerable correlations between the occurrence rate of LH ECWs and the properties of ambient plasmas are discussed. The present research may provide evidence for effect of alpha particles on generation of ECWs.