Non-parallel propagation of hydromagnetic surface waves in the presence of steady shear-flows

Abstract

The combined effect of non-parallel propagation and steady shear-flows on the properties of hydromagnetic surface waves is examined for two different orderings of physical parameters that are expected at the edge of a hot and dense coronal loop and an isolated, photospheric magnetic flux-tube. It is found that a finite angle of inclination of the wavevector with respect to the magnetic field generally facilitates the propagation of surface waves by relaxing restrictions imposed on their phase velocities by the shear-flow. Non-parallel propagation, along with a shear-flow, can also give rise to backward propagating surface modes that may be subject to negative energy instabilities. Such a backward surface mode appears when the magnitude of the shear-flow exceeds a certain critical value. Such a critical flow is much larger than the magnitudes of the observed flows in solar coronal loops. Negative energy instabilities of hydromagnetic surface waves may not, therefore, play an important role in the energetics of the hot solar coronal loops. For a photospheric magnetic flux-tube, the critical flow for the appearance of a backward propagating slow surface mode is found to increase with the increase of the angle of propagation of the waves. The surface mode that is most prone to negative energy instabilities, therefore, seems to be the one that propagates parallel to the magnetic field lines.