Line Splitting of Plasma Radiation and Solar Radio Outbursts

The emission of radiation at frequencies near w and 2w from a plasma can become greatly enhanced if a flux of suprathermal electrons coexists with the thermal plasma. The effect of a weak magnetic field in the plasma on this emission is calculated. For some anisotropic distributions of suprathermal electrons, the magnetic field gives rise to a splitting of the plasma lines by an amount /2w for the fundamental and an amount twice this value for the second harmonic. This gives a plausible explanation of the observed line splitting for a Type II solar outburst and also therefore an indirect determination of coronal magnetic fields at the source of emission. The splitting mechanism is similar to that earlier suggested by Sturrock. However, a detailed calculation shows that anisotropic distributions of suprathermal electrons are necessary to produce a line splitting, and that Landau damping controls the energy density of the radiating plasma oscillations in all but very small sources. The intensity profiles of the emission lines would also give information on the type of suprathermal electron distributions from which the emission originates.