Overdamped Alfvén waves due to ionneutral collisions in the solar chromosphere 

Roberto Soler Submitted: 20141124 07:26
Alfvénic 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 Alfvén waves with frequencies near the ionneutral collision frequency. The collision frequency is proportional to the ionneutral collision cross section for momentum transfer. Here, we investigate Alfvén 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 hardsphere model and those based on recent quantummechanical 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 Alfvén waves are overdamped as a result of the strong ionneutral dissipation. The critical wavelengths are in the range from 1 km to 50 km for the hardsphere cross sections, and from 1 m to 1 km for the quantummechanical cross sections. Equivalently, for periodically driven Alfvén 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 hardsphere cross sections, and from 10^{2} Hz to 10^{4} Hz for the quantummechanical cross sections. Future observations at sufficiently high spatial or temporal resolution may show the importance of highfrequency Alfvén 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.
Authors: R. Soler, J. L. Ballester, T. V. Zaqarashvili
Projects: None

Publication Status: Accepted in A&A
Last Modified: 20141124 07:26


