Spatial Distribution of Electromagnetic Waves near the Proton Cyclotron Frequency in ICME Sheath Regions Associated with Quasi-perpendicular Shocks: Wind Observations

Abstract

Electromagnetic waves (EMWs) near the proton cyclotron frequency f cp are transverse left-handed (LH) or right-handed (RH) polarized waves, and are ubiquitous in the solar wind. However, the characteristics of these waves in the sheath regions of interplanetary coronal mass ejections (ICMEs) are poorly understood. Through a comprehensive survey of Wind magnetic field and plasma data using dynamic spectra and repeated filtering analyses, 700 EMW events (7.1% of the analysis time) are identified in the 62 ICME sheath regions associated with quasi-perpendicular shocks involved with a low shock Mach number M f and low upstream β 1 . In the ICME sheath regions, outward (inward)-propagating LH (RH) EMWs have relatively higher counts and longer duration than inward (outward)-propagating LH (RH) EMWs in the plasma frame, consistent with previous STEREO observations. The spatial distributions of the magnetic field, plasma, and frequency parameters of EMWs are also presented in both spacecraft and plasma frames, especially the proton (alpha) temperature anisotropy , α abundance N α / N p , and normalized differential alpha-proton speed V d / V A . After removing the Doppler shift, 81.1% (59%) of all outward (inward)-propagating LH EMWs have a frequency below (above) 0.5 f cp , while 68.3% (64%) of all outward (inward)-propagating RH EMWs have a frequency smaller (greater) than 0.5 f cp . Further investigations of local plasma parameters reveal that different excitation mechanisms for EMWs are in different subregions of the ICME sheath regions. These results are helpful in understanding the important role of EMWs in the solar wind–ICME coupling process with different sheath regions.