Photospheric logarithmic velocity spirals as MHD wave generation mechanisms

Abstract

High-resolution observations of the solar photosphere have identified a wide variety of spiralling motions in the solar plasma. These spirals vary in properties, but are observed to be abundant at the solar surface. In this work these spirals are studied for their potential as magnetohydrodynamic (MHD) wave generation mechanisms. The inter-granular lanes, where these spirals are commonly observed, are also regions where the magnetic field strength is higher than average. This combination of magnetic field and spiralling plasma is a recipe for the generation of Alfv\'en waves and other MHD waves. This work employs numerical simulations of a self-similar magnetic flux tube embedded in a realistic, gravitationally stratified, solar atmosphere to study the effects of a single magnetic flux tube perturbed by a logarithmic velocity spiral driver. The expansion factor of the logarithmic spiral driver is varied and multiple simulations are run for a range of values of the expansion factor centred around observational constraints. The simulations are analysed using `flux surfaces' constructed from the magnetic field lines so that the vectors perpendicular, parallel and azimuthal to the local magnetic field vector can be calculated. The results of this analysis show that the Alfv\'en wave is the dominant wave for lower values of the expansion factor, whereas, for the higher values the parallel component is dominant. This transition occurs within the range of the observational constraints, meaning that spiral drivers, as observed in the solar photosphere, have the potential to generate a variety of MHD wave modes.