Theoretical evidence for cutoff frequencies for Alfvén waves propagating in the solar atmosphere

Abstract

Propagation of linear Alfven waves in the isothermal solar atmosphere is investigated numerically and analytically. It is shown that the two wave variables, the velocity and magnetic field perturbations, behave differently and that there is a range of wave fre- quencies for which the wave behavior changes from propagating to non-propagating. The so-called transition and turning points corresponding to this change are deter- mined analytically, and their locations in the atmosphere are calculated and verified against the numerical results. The transition and turning points are then used to in- troduce cutoff frequencies, which are different for different wave variables. The main result of the paper is that there is no one unique cutoff frequency for Alfven waves but instead a number of cutoff frequencies can be introduced depending upon the method used to define them as well as on the choice of the wave variable used to describe the waves. Relevance of the obtained results to recent observations of Alfven waves in the solar atmosphere is also briefly discussed.