Radio Astronomical Constraints on Coronal Heating by High‐Frequency Alfven Waves

Abstract

It has been suggested that the heating of the solar corona and the acceleration of the solar wind to their observed values are due to high-frequency (1 to several hundred Hertz) magnetohydrodynamic (MHD) waves. This suggestion circumvents well-established difficulties with heating and acceleration by observed, low-frequency (milliHertz) MHD waves. There is no widely accepted mechanism for damping the low-frequency waves, and their observed amplitude throughout the inner heliosphere is insufficient to account for observed solar wind properties. By contrast, the alleged high-frequency waves would not be detected by radio propagation observations (specifically Faraday rotation observations) hitherto conducted, so the amplitude of these waves would appear to be a free parameter for the theories. The purpose of this paper is to show that these putative waves might reveal themselves through another observable radio propagation phenomenon, Faraday screen depolarization. Faraday screen depolarization is a reduction of the observed degree of linear polarization of an extended, polarized radio source when viewed through a medium (here referred to as a screen) in which the Faraday rotation varies in a random fashion. Expressions for Faraday screen depolarization are extracted from the literature and calculations appropriate to the solar corona and inner solar wind are made. Depending on the exact properties of the high-frequency turbulence, a detectable (i.e. ~5%-20%) depolarization could result. We present six polarimetric observations at a frequency of 1465 MHz of polarized sources observed close to the Sun with the Very Large Array radio telescope in 1997. These observations show no detectable depolarization and rule out some turbulence models. The existing observations are far from being definitive in ruling out high-frequency turbulence with the required properties. However, additional observations, particularly undertaken at lower radio frequencies, could furnish far more restrictive limits.