Coronal Magnetic Field Measurements through Gyroresonance Emission

This article reviews the use of gyroresonance emission at radio wavelengths to measure coronal magnetic fields. The spiralling motion of electrons in the 200– 2000 G fields in the solar corona produces sufficient opacity to render the corona optically thick, making it easy to recognize such sources in microwave images from their coronal brightness temperatures. Where gyroresonance sources are present they may be used as sensitive probes of the magnetic field strength above active regions, and this unique capability is one of the strengths of radio observations. Typically a gyroresonance radio source shows the temperature on an optically thick surface of constant magnetic field within the corona. Since each radio frequency corresponds to a different magnetic field strength, the coronal structure can be “peeled away” by using different frequencies.