The slow-mode nature of compressible wave power in solar wind turbulence

TL;DR

This study demonstrates that in solar wind turbulence at 1 AU, the compressible fluctuations are mainly in the kinetic slow mode, influencing the turbulence cascade and density fluctuation spectrum.

Contribution

It provides the first comprehensive statistical evidence that the compressible component in solar wind turbulence is predominantly in the kinetic slow mode, supported by spacecraft data and kinetic plasma theory.

Findings

Compressible fluctuations are mainly in the kinetic slow mode.

Negative correlation between density and magnetic field fluctuations.

Implications for turbulence cascade favoring kinetic Alfvén waves.

Abstract

We use a large, statistical set of measurements from the Wind spacecraft at 1 AU, and supporting synthetic spacecraft data based on kinetic plasma theory, to show that the compressible component of inertial range solar wind turbulence is primarily in the kinetic slow mode. The zero-lag cross correlation C(delta n, delta B_parallel) between proton density fluctuations delta n and the field-aligned (compressible) component of the magnetic field delta B_parallel is negative and close to -1. The typical dependence of C(delta n,delta B_parallel) on the ion plasma beta_i is consistent with a spectrum of compressible wave energy that is almost entirely in the kinetic slow mode. This has important implications for both the nature of the density fluctuation spectrum and for the cascade of kinetic turbulence to short wavelengths, favoring evolution to the kinetic Alfven wave mode rather than the (fast) whistler mode.