A wave scattering theory of solar seismic power haloes

Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany e-mail: hanasoge@mps.mpg.de

Received: 7 May 2009
Accepted: 23 June 2009

Abstract

Spatial maps of the high-pass frequency filtered time-averaged root-mean-squared (RMS) Doppler velocities tend to show substantial decrements within regions of strong field and curiously, randomly distributed patches of enhancement in the vicinity. We propose that these haloes or enhancements are a consequence of magnetic-field-induced mode mixing (scattering), resulting in the preferential powering of waves that possess strong surface velocity signatures (i.e. scattering from low to high wavenumbers). Evidently, this process can occur in the reverse, and therefore in order to determine if the haloes are indeed caused by mode mixing, we must answer the question: how are acoustic waves scattered by magnetic fields? Through simulations of the interactions between waves and sunspots and models of plage, we demonstrate that the high to low modal order scattering channels are favoured. With increasing frequency and consequently, decreasing wavelength, a growing number of modes are scattered by the sunspot, thereby rendering the enhancements most visible around the high-frequency parts of the spectrum. The haloes obtained from the simulations are on the same order of magnitude but weaker than those observed. We also present observational evidence to support this theory: observations of active region AR9787 are firstly frequency filtered to isolate the 5–6 mHz signals and secondly, decomposed into three wavenumber bandpasses, . With increasing wavenumber, the extent of the halo effect is seen increase dramatically, in line with theoretical expectation.