On parallel electric field generation in transversely inhomogeneous plasmas

TL;DR

This study demonstrates that parallel electric fields in transversely inhomogeneous plasmas are generated by electron-ion flow separation caused by density gradients, with the amplitude inversely proportional to ion mass, using a minimal two-fluid cold plasma model.

Contribution

It shows that a simple two-fluid cold plasma model suffices to reproduce kinetic simulation results of E_{||} generation in inhomogeneous plasmas, highlighting the role of flow separation.

Findings

E_{||} amplitude decreases linearly with increasing ion mass ratio

Electron parallel speed is unaffected by ion mass ratio

Ion velocity decreases linearly with inverse of mass ratio

Abstract

The generation of parallel electric fields by the propagation of ion cyclotron waves in the plasma with a transverse density inhomogeneity was studied. It was proven that the minimal model required to reproduce the previous kinetic simulation results of E_{||} generation [Tsiklauri et al 2005, Genot et al 2004] is the two-fluid, cold plasma approximation in the linear regime. By considering the numerical solutions it was also shown that the cause of E_{||} generation is the electron and ion flow separation induced by the transverse density inhomogeneity. We also investigate how E_{||} generation is affected by the mass ratio and found that amplitude attained by E_{||} decreases linearly as inverse of the mass ratio m_i/m_e. For realistic mass ratio of m_i/m_e=1836, such empirical scaling law, within a time corresponding to 3 periods of the driving ion cyclotron wave, is producing E_{||}=14 Vm^{-1} for solar coronal parameters. Increase in mass ratio does not have any effect on final parallel (magnetic field aligned) speed attained by electrons. However, parallel ion velocity decreases linearly with inverse of the mass ratio m_i/m_e. These results can be interpreted as following: (i) ion dynamics plays no role in the E_{||} generation; (ii) E_{||} \propto 1/m_i scaling is caused by the fact that omega_d = 0.3 omega_{ci} \propto 1/m_i is decreasing with the increase of ion mass, and hence the electron fluid can effectively "short-circuit" (recombine with) the slowly oscillating ions, hence producing smaller E_{||}.