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

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 ($β\ll 1$) 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.

Figures (5)

• Figure 1: Normalized density redistribution along the geomagnetic field lines as a function of the latitude for different values of $\Lambda$ depending on the planet for $\nu=0.1$, $\bar{\omega} = 0.1$, $L=2$, $c/c_{A0} = 10^3$, $\beta_{0} = 0.1$ and a Maxwellian distribution.
• Figure 2: Normalized density redistribution along the geomagnetic field lines as function of the colatitude for $\nu=0.1$, $\bar{\omega} = 0.1$, $L=2$, $c/c_{A0} = 10^3$ and for (a) different values of $\beta_{0}$ with a Maxwellian distribution (b) different values of $\kappa$, with $\beta_{0} = 0.03$.
• Figure 3: Shaded isocontours of $d\bar{n}/d\bar{x}_\parallel$ as a function of the colatitude and the normalized density with the nullcline $n_c$ represented with the black curve; with $\nu = 0.1$, $\bar{\omega} = 0.1$, $c/c_{A0} = 10^3$, $\beta_{i0\kappa} = 0.1$, $L = 2$ and the $C_g$ of Earth.
• Figure 4: Critical value $\Lambda_c$ for different values of the kappa parameter as a function of: (a) the plasma beta $\beta_0$, with $\bar{\omega} = 0.1$ and $L=2$; (b) the normalized frequency $\omega/\Omega_{i0}$, with $\beta_{0} = 0.1$ and $L = 2$; and (c) the L-shell parameter $L$, with $\beta_{0} = 0.1$ and $\bar{\omega} = 0.1$. In panel (c), only the Maxwellian case is displayed, as the curves for other $\kappa$ values overlap and are omitted for clarity.
• Figure 5: Panels (a)-(b): Normalized forces that act on the equilibrium Equation \ref{['eq:Force_balance_normalized']}, as a function of the colatitude $\theta$ with $\nu=0.1$, $\bar{\omega} = 0.1$, $c/c_{A0} = 10^3$, $\beta_{i0\kappa} = 0.1$, $L=2$ for (a) $\Lambda = 10 >\Lambda_c$ (b) $\Lambda = 0.3 <\Lambda_c$. Panel (c): Zoom over the gray-shaded region in panel (b).