Radio Versus EUV/X{Ray Observations of the Solar Atmosphere

This paper 1 reviews the contrasting properties of radio and EUV/X{ray observations for the study of the solar atmosphere. The emphasis is placed on explaining the nature of radio observations to an EUV/X{ray audience. Radio emission is produced by mechanisms which are well-understood within classical physics. Bremsstrahlung tends to be dominant at low frequencies, while gyroresonance emission from strong magnetic elds produces bright sources at higher frequencies. At most radio frequencies the images of the Sun are dominated almost everywhere by bremsstrahlung opacity, which may be optically thick or thin depending on circumstances. Where gyroresonance sources are present they may be used as sensitive probes of the regions above active regions where magnetic eld strengths exceed several hundred gauss, 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 eld within the corona. Since each radio frequency corresponds to a diierent magnetic eld strength, the coronal structure can be \peeled away" by using diierent frequencies. The peculiarities of radio observing techniques are discussed and contrasted with EUV/X{ ray techniques. Radio observations are strong at determining temperatures and coronal magnetic eld strengths while EUV/X{ray observations better sense densities and reveal coronal magnetic eld lines: in this way the two wavelength domains are nicely complementary.