Non-thermal plasma density redistribution in planetary magnetospheres due to ion-cyclotron waves

TL;DR

This study models how non-thermal plasma populations influence density redistribution in planetary magnetospheres caused by electromagnetic ion cyclotron waves, emphasizing the importance of non-thermal effects for accurate modeling.

Contribution

It introduces a generalized force balance model incorporating Kappa-distributed non-thermal plasmas to analyze density responses to EMIC waves in various magnetospheric conditions.

Findings

Non-thermal effects can significantly alter plasma accumulation patterns.

Decreasing Kappa and increasing plasma beta reduce the nonlinear response.

The phase transition between density minima and maxima depends on plasma parameters.

Abstract

Planetary magnetospheres exhibit diverse environments where Ultra-low frequency (ULF) pulsations induce nonlinear ponderomotive effects. Since suprathermal populations modeled by Kappa distributions are ubiquitous in these regions, their significant influence on the ponderomotive force (PF) induced by electromagnetic ion cyclotron (EMIC) waves must be accounted for. We investigate field-aligned plasma density redistribution driven by the PF of traveling EMIC waves across different planetary magnetospheres. We apply a generalized slow-time-scale force balance equation to model stationary density solutions in low-beta plasmas ($\beta \ll 1$) with isotropic Kappa distributions. To enable systematic comparison, wave modulation is described using the WKB approximation in a dipole magnetic field, neglecting first-order curvature effects. The plasma response varies significantly with magnetospheric parameters: decreasing the kappa parameter and increasing plasma beta counteract plasma accumulation towards the equator. In low-beta environments, non-thermal effects substantially reduce the nonlinear response to short-period pulsations, though preserving the qualitative behavior of Maxwellian models. Furthermore, we characterize how the critical parameter governing the phase transition between equatorial density minima and maxima depends on the specific combination of plasma beta, kappa, and L-shell. Our study demonstrates that non-thermal plasma properties are a governing factor in field-aligned density redistribution driven by ULF waves, highlighting the necessity of incorporating them to accurately model ponderomotive phenomena across multifaceted planetary magnetospheres.