On the non-existence of a cut-off frequency for the propagation of kink-modes on an isothermal slender stratified tube
Abstract
Recently (Andries and Cally, ApJ, 743, 164, 2011) a fairly general analytic theory was established for the dispersion and scattering of magnetohydrodynamic waves by longitudinally stratified flux tubes. The theory provides a common framework for, and synthesis of, many previous studies of flux tube oscillations that were carried out under various simplifying assumptions. In particular we illustrated the unifying theoretical framework underlying both the description of waves scattered by flux tubes and the dispersion of waves carried along flux tubes. In the present contribution we will investigate in more detail the propagation of the kink-mode on a slender flux tube in a stratified atmosphere. Spruit (A&A, 98, 155-160, 1981) has studied this situation before and discussed both the appearance of 'buoyancy' terms in the kink-mode and the reduction to a Klein-Gordon equation in the case of an isothermally stratified medium with the associated cut-off frequency preventing the propagation of kink modes towards the corona for low frequencies. We show in convincing detail why the derivation by Spruit (1983) is erroneous and conclude that there is no cut-off frequency for the vertical propagation of kink-modes along a flux tube. The non-existence of the cut-off frequency implies that, regardless of the frequency, kink-modes may propagate along a flux tube from the photosphere towards the corona. How much of the energy is effectively transmitted into the corona, however, remains subject of further study.
- Publication:
-
40th COSPAR Scientific Assembly
- Pub Date:
- 2014
- Bibcode:
- 2014cosp...40E..99A