Using Synthetic Spacecraft Data to Interpret Compressible Fluctuations in Solar Wind Turbulence

TL;DR

This study uses synthetic spacecraft data based on kinetic plasma theory to analyze compressible fluctuations in solar wind turbulence, revealing that MHD theory is insufficient and that slow waves dominate the compressible component.

Contribution

The paper introduces a method to generate synthetic spacecraft data from kinetic plasma theory and compares it with observations, highlighting the dominance of slow waves in compressible turbulence.

Findings

MHD theory inadequately describes compressible fluctuations.

Observed density-parallel magnetic field correlation aligns with negligible fast wave energy.

Solar wind fluctuations are modeled as a mixture of incompressible Alfvenic and slow wave turbulence.

Abstract

Kinetic plasma theory is used to generate synthetic spacecraft data to analyze and interpret the compressible fluctuations in the inertial range of solar wind turbulence. The kinetic counterparts of the three familiar linear MHD wave modes---the fast, Alfven, and slow waves---are identified and the properties of the density-parallel magnetic field correlation for these kinetic wave modes is presented. The construction of synthetic spacecraft data, based on the quasi-linear premise---that some characteristics of magnetized plasma turbulence can be usefully modeled as a collection of randomly phased, linear wave modes---is described in detail. Theoretical predictions of the density-parallel magnetic field correlation based on MHD and Vlasov-Maxwell linear eigenfunctions are presented and compared to the observational determination of this correlation based on 10 years of Wind spacecraft data. It is demonstrated that MHD theory is inadequate to describe the compressible turbulent fluctuations and that the observed density-parallel magnetic field correlation is consistent with a statistically negligible kinetic fast wave energy contribution for the large sample used in this study. A model of the solar wind inertial range fluctuations is proposed comprised of a mixture of a critically balanced distribution of incompressible Alfvenic fluctuations and a critically balanced or more anisotropic than critical balance distribution of compressible slow wave fluctuations. These results imply that there is little or no transfer of large scale turbulent energy through the inertial range down to whistler waves at small scales.