Overdamped Alfven waves due to ion-neutral collisions in the solar chromosphere

TL;DR

This study investigates how ion-neutral collisions damp Alfven waves in the solar chromosphere, highlighting the impact of different collision cross section models on wave damping and potential observational signatures.

Contribution

It compares classical and quantum-mechanical collision cross sections to determine their effects on Alfven wave damping and identifies critical wavelengths and frequencies relevant for chromospheric heating.

Findings

Critical wavelengths range from 1 km to 50 km (hard-sphere) and 1 m to 1 km (quantum)

Optimal damping frequencies are 1-100 Hz (hard-sphere) and 100-10,000 Hz (quantum)

High-resolution observations like ALMA could test these predictions

Abstract

Alfvenic waves are ubiquitous in the solar atmosphere and their dissipation may play an important role in atmospheric heating. In the partially ionized solar chromosphere, collisions between ions and neutrals are an efficient dissipative mechanism for Alfven waves with frequencies near the ion-neutral collision frequency. The collision frequency is proportional to the ion-neutral collision cross section for momentum transfer. Here, we investigate Alfven wave damping as a function of height in a simplified chromospheric model and compare the results for two sets of collision cross sections, namely those of the classic hard-sphere model and those based on recent quantum-mechanical computations. We find important differences between the results for the two sets of cross sections. There is a critical interval of wavelengths for which impulsively excited Alfven waves are overdamped as a result of the strong ion-neutral dissipation. The critical wavelengths are in the range from 1 km to 50 km for the hard-sphere cross sections, and from 1 m to 1 km for the quantum-mechanical cross sections. Equivalently, for periodically driven Alfven waves there is an optimal frequency for which the damping is most effective. The optimal frequency varies from 1 Hz to 10^2 Hz for the hard-sphere cross sections, and from 10^2 Hz to 10^4 Hz for the quantum-mechanical cross sections. Future observations at sufficiently high spatial or temporal resolution may show the importance of high-frequency Alfven waves for chromospheric heating. For instance, the Atacama Large Millimeter/submillimeter Array (ALMA) may be able to detect the critical wavelengths and optimal frequencies and so to test the effective collision cross section in the chromospheric plasma.