Abstract
The structure of the upper solar atmosphere, on all observable scales, is intimately governed by the magnetic field. The same holds for a variety of solar phenomena that constitute solar activity, from tiny transient brightening to huge Coronal Mass Ejections. Due to inherent difficulties in measuring magnetic field effects on atoms (Zeeman and Hanle effects) in the corona, radio methods sensitive to electrons are of primary importance in obtaining quantitative information about its magnetic field. In this review we explore these methods and point out their advantages and limitations. After a brief presentation of the magneto-ionic theory of wave propagation in cold, collisionless plasmas, we discuss how the magnetic field affects the radio emission produced by incoherent emission mechanisms (free-free, gyroresonance, and gyrosynchrotron processes) and give examples of measurements of magnetic filed parameters in the quiet sun, active regions and radio CMEs. We proceed by discussing how the inversion of the sense of circular polarization can be used to measure the field above active regions. Subsequently we pass to coherent emission mechanisms and present results of measurements from fiber bursts, zebra patterns, and type II burst emission. We close this review with a discussion of the variation of the magnetic field, deduced by radio measurements, from the low corona up to ~ 10 solar radii and with some thoughts about future work.
Highlights
The sun is made up of plasma and magnetic field
When the coupling between the xmode and o-mode waves is weak their polarization properties change along the ray path, whereas when the geometrical optics approximation breaks down the waves are strongly coupled and their polarization remains fixed, even if a transverse field region (TFR) is crossed
There are three aspects that one should bear in mind: (a) That the magnetic field is often measured not over a 2-D field of view as in the photosphere but at particular locations, (b) that the vast majority of the measurements refers to active regions or bursts and not to the quiet Sun, and (c) that some methods require additional information for the computation of the magnetic field, such as the density scale and the height of the emission; this has to be provided by other observations, by models, or even by estimates
Summary
The sun is made up of plasma and magnetic field The latter affects practically all solar phenomena, in all layers of the solar atmosphere. In order to understand how the Sun works, and in order to predict the effect of solar phenomena near the Earth in the context of space whether, we need quantitative information on the parameters of both the plasma and the magnetic field, with the highest spatial, spectral, and. The most reliable method for measuring the magnetic field in the corona is through its influence on the radio emission which we will present in this review. We begin by discussing the influence of the magnetic field on the propagation of radio waves and on the free-free emission mechanism. We continue with diagnostics from metric burst emission and finish with a summary and a discussion of prospects
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