Radio Emissions from Solar Active Regions

Abstract

Solar active region coronae are known for strong magnetic fields permeating tenuous plasma, which makes them an ideal astronomical laboratory for magnetohydrodynamics research. It is, however, relatively less known that this physical condition also permits a very efficient radiation mechanism, gyro-resonant emission, produced by hot electrons gyrating in the coronal magnetic field. As a resonant mechanism, gyro-emission produces high enough opacity to fully reveal the coronal temperature, and is concentrated at a few harmonics of the local gyrofrequency to serve as an excellent indicator of the magnetic field. In addition, the polarization of the ubiquitous free-free emission and a phenomenon of depolarization due to mode coupling extend the magnetic field diagnostic to a wide range of coronal heights. The ability to measure the coronal temperature and magnetic field without the complications that arise in other radiative inversion problems is a particular advantage for the active region radio emissions available only at these wavelengths. This article reviews the efforts to understand these radiative processes, and use them as diagnostic tools to address a number of critical issues involved with active regions.

Coupling from the Photosphere to the Chromosphere and the Corona

“…Radio observations are able to determine the magnetic field strength in and around active regions (see review by Lee 2007). Acoustic mapping techniques (Finsterle et al 2004) use the reflection of high-frequency acoustic waves (mHz-range) from the region in the atmosphere where the gas pressure and the magnetic pressure are equal to reveal the structure of the magnetic canopy.…”

Space weather impact on radio device operation

“…Сол-нечный радиошум и радиовсплески (внезапные усиления радиоизлучения), открытые в 40-х гг. прошлого века, достаточно активно исследуются по сей день [Bastian et al, 1998;Solar and Space Weather Radiophysics..., 2004;Lee, 2007;Shibasaki et al, 2011].…”

Radial distributions of magnetic field strength in the solar corona as derived from data on fast halo CMEs

“…There have been no regular measurements of the coronal magnetic field -only individual ones based on the Zeeman effect (in the infrared band) [Lin et al, 2000], Hanle effect [Sahal-Brechot et al, 1986], and Faraday effect [Pätzold et al, 1987;Spangler, 2005], as well as on measurements of solar radio emission [Lee, 2007;Bogod, Yasnov, 2016]. The coronal magnetic field can be reconstructed in different approximations by measuring the photospheric field: in the potential approximation (see [Rudenko, 2001] and references therein), in the force-free approximation [Wiegelmann, 2008;Rudenko, Myshyakov, 2009].…”