The Mechanism Responsible for 'Shadow' Type III Solar Radio Bursts. I. Absorption due to Langmuir Turbulence

The hypothesis is advanced that for 'shadow' type III solar radio events (absorption features with drift rates and bandwidths typical of type III bursts or U-bursts) the absorption mechanism involves Langmuir turbulence, such absorption being the inverse of either fundamental (I = J;,) or second harmonic (f = 2/p) plasma emission. The theory for both absorption processes is developed and applied to shadow type III events with the following results: (1) the predicted absorption is confined to a very narrow frequency range (fl.I/ I ~ 10- 3); (2) effective absorption requires an energy density in Langmuir turbulence (with phase speeds ~tc) in excess of 10-9 ergcm- 3 for the fundamental and in excess of 3 x 10-6 ergcm- 3 for the second harmonic; (3) the brightness of the background source must exceed 109 and 1016 K for absorption at the fundamental and second harmonic respectively. Comparison of the theory with the properties of an event discussed by Kai (1973) leads to the conclusions: (1) absorption at the second harmonic is unacceptable because of the high brightness temperature required; (2) to explain the observed bandwidth in terms of absorption at the fundamental, the absorbing region and the background source must overlap in height; (3) to explain the observed reduction in brightness temperature requires that the initial brightness temperature exceed 5�5 X 109 K (the observed value was 109 K).