Statistics of magnetic field fluctuations in a partially ionized space plasma

TL;DR

This paper investigates magnetic field fluctuations in space plasmas, revealing different statistical behaviors in heliosheath and solar wind, and introduces a nonlinear multifluid simulation model to explain the evolution of these fluctuations.

Contribution

It develops a self-consistent 2D multifluid simulation model for partially ionized plasmas, elucidating mode conversion processes affecting magnetic field fluctuation statistics.

Findings

Magnetic fluctuations are Gaussian in heliosheath and lognormal in solar wind.

Mode conversion suppresses vortical modes and amplifies compressive modes.

Simulations show fluctuations tend toward isotropy and Gaussian distribution due to mode dynamics.

Abstract

{\em Voyager 1} and {\em 2} data reveals that magnetic field fluctuations are compressive and exhibit a Gaussian distribution in the compressed heliosheath plasma, whereas they follow a lognormal distribution in a nearly incompressible supersonic solar wind plasma. To describe the evolution of magnetic field, we develop a nonlinear simulation model of a partially ionized plasma based on two dimensional time-dependent multifluid model. Our model self-consistently describes solar wind plasma ions, electrons, neutrals and pickup ions. It is found from our simulations that the magnetic field evolution is governed by mode conversion process that leads to the suppression of vortical modes, whereas the compressive modes are amplified. An implication of the mode conversion process is to quench the Alfv\'enic interactions associated with the vortical motions. Consequently anisotropic cascades are reduced. This is accompanied by the amplification of compressional modes that tend to isotropize the plasma fluctuations and lead to a Gaussian distribution of the magnetic field.