DECIMETRIC TYPE III BURSTS: GENERATION AND PROPAGATION

Simulations are presented for decimetric type III radio bursts at 2fp , where fp is the local electron plasma frequency. The simulations show that 2fp radiation can be observed at Earth in two scenarios for the radiation's generation and propagation. In Scenario A, radiation is produced and propagates in warm plasmas in the lower corona that are caused by previous magnetic reconnection outflows and/or chromospheric evaporation. In Scenario B radiation is generated in normal plasmas, then due to its natural directivity pattern and refraction, radiation partly propagates into nearby regions, which are hot because of previous reconnection/evaporation. The profiles of plasma density ne (r) and electron temperature Te (r) in the lower corona (r - Rsun ∼< 100 Mm) are found to be crucial to whether radiation can be produced and escape at observable levels against the effects of free-free absorption, where r is the heliocentric distance. Significantly, the observed wide ranges of radiation properties (e.g., drift rates) require ne (r) with a large range of scale heights hs , consistent nonetheless for Scenario B with short observed EUV loops. This is relevant to problems with large hs inferred from tall EUV loops. The simulations suggest: (1) ne (r) with small hs , such as ne (r)∝(r - Rsun)-2.38 for flaring regions, are unexpectedly common deep in the corona. This result is consistent with recent work on ne (r) for r ∼ (1.05-2)Rsun extracted from observed metric type IIIs. (2) The dominance of reverse-slope bursts over normal bursts sometimes observed may originate from asymmetric reconnection/acceleration, which favors downgoing beams.