Cut-off wavenumber of Alfven waves in partially ionized plasmas of the solar atmosphere

TL;DR

This paper investigates the origin of the cut-off wavenumber for Alfven waves in the solar atmosphere's partially ionized plasmas, revealing it results from certain approximations rather than physical effects.

Contribution

It demonstrates that neglecting inertial and Hall terms in single-fluid MHD causes the artificial cut-off wavenumber, clarifying its non-physical origin.

Findings

Cut-off wavenumber appears due to specific approximations in MHD.

Neglecting inertial and Hall terms leads to zero real Alfven frequency.

The physical Alfven waves do not have a cut-off wavenumber.

Abstract

Alfven wave dynamics in partially ionized plasmas of the solar atmosphere shows that there is indeed a cut-off wavenumber, i.e. the Alfven waves with wavenumbers higher than the cut-off value are evanescent. The cut-off wavenumber appears in single-fluid magnetohydrodynamic (MHD) approximation but it is absent in a multi-fluid approach. Up to now, an explanation for the existence of the cut-off wavenumber is still missing. The aim of this paper is to point out the reason for the appearance of a cut-off wavenumber in single-fluid MHD. Beginning with three-fluid equations (with electrons, protons and neutral hydrogen atoms), we performed consecutive approximations until we obtained the usual single-fluid description is obtained. We solved the dispersion relation of linear Alfven waves at each step and sought the approximation responsible of the cut-off wavenumber appearance. We have found that neglecting inertial terms significantly reduces the real part of the Alfven frequency although it never becomes zero. Therefore, the cut-off wavenumber does not exist at this stage. However, when the inertial terms together with the Hall term in the induction equation are neglected, the real part of the Alfven frequency becomes zero. The appearance of a cut-off wavenumber, when Alfven waves in partially ionized regions of the solar atmosphere are studied, is the result of neglecting inertial and Hall terms, therefore it has no physical origin.