SOLAR 2ND-HARMONIC PLASMA EMISSION AND THE HEAD-ON APPROXIMATION

The coalescence of two Langmuir waves, L and L', produces emission at twice the plasma frequency in type II and type III solar radio bursts. The analysis of the coalescence process is usually simplified by assu ming the head-on approximation, where the wavevectors of the coalescin g waves satisfy k(L') approximate to -k(L), corresponding to the two L angmuir waves meeting head on. However, this is not always a valid app roximation, particularly when the peak of the Langmuir spectrum lies a t small wavenumbers, for narrow band spectra, and for spectra with bro ad angular ranges. Realistic Langmuir wave spectra are used to investi gate the effects of relaxing the head-on approximation.