Dissipation of Magnetohydrodynamic Waves in the Upper Solar Atmosphere

Laing, Gordon Bremner (1996). Dissipation of Magnetohydrodynamic Waves in the Upper Solar Atmosphere. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f5bc

Abstract

The upper solar atmosphere, the corona, is an example of a hot (10₆K), tenuous, structured plasma. This thesis concentrates on two main aspects regarding our understanding of the corona, namely the existence of oscillatory phenomena and their possible connection with explaining the high observed coronal temperatures. The oscillatory phenomena are interpreted in terms of magnetohydrodynamic (mhd) waves and the thesis investigates how the energy of such waves might be converted into heat by the dissipation mechanisms of ion viscosity, electron thermal conduction and radiation in an optically thin atmosphere. An overview of coronal features and the observational evidence for waves are given and coronal heating theories together with dissipative mechanisms are discussed. Detailed calculations of the energy carried by waves in structured media are given showing that the waves can carry sufficient energy to meet the coronal heating requirements provided that the waves are associated with large root-mean-square-velocity amplitudes. The lengths over which ducted waves lose their energy in a weakly dissipative environment for both warm and cold plasmas are calculated. The results show that fast waves with periods 2 - 10 s are likely to dissipate in regions of low magnetic field strength (<15 G), and slow waves that are likely to dissipate have periods in the range 15 - 225 s. Dissipation lengths and rates for waves propagating in slender structures are calculated by two methods. One method considers an isothermal environment; the other considers large Péclet number, and it is found that slow, symmetric waves are likely to dissipate with periods in the range 2 - 80 s and 2 - 38 s, respectively. The final chapter compares the models with each other and with the models in the literature. Period ranges of dissipating and non-dissipating waves are compared with observed waves and it is proposed that waves of 2 - 10 s might contribute to coronal heating, whilst those waves that might survive dissipation have periods of a few and many tens of seconds.