Wave-polarization Analysis of the Alfvénic Slow Solar Wind at Kinetic Scales

Abstract

This paper reports the first polarization measurement in the Alfvénic slow solar wind. The normalized magnetic helicity is used as a diagnostic parameter for studying the polarization status of the high-frequency magnetic fluctuations, along with an attempt to identify various wave modes in the solar wind turbulence. Clear evidence for the existence of ion cyclotron waves (ICWs) and kinetic Alfvén waves (KAWs) is also found in the Alfvénic lowspeed plasma, robustly supporting the idea that the Alfvénic content of the solar wind fluctuations at fluid scales is the key parameter driving wave generation at kinetic scales. By separating the contributions to helicity from the two modes, it is possible to address the thermodynamical properties of ICWs and KAWs and provide the first direct estimate of their magnetic compressibility. In particular, while ICWs are mainly associated with higher levels of anisotropy and appear to be bounded by the threshold of proton–cyclotron kinetic instability, KAWs (which end up being more compressive than ICWs) are found at lower anisotropies and seem to be limited by the mirror mode instability threshold, extending as well to near the parallel fire hose unstable region. These result are relevant to theories of turbulence and dissipation in the solar wind.