Australian Space Science Conference 2011
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Bo Li

Predictions of Decimetric Type III Solar Radio Bursts

Bo Li
School of Physics, University of Sydney

Iver Cairns
School of Physics, University of Sydney

Yihua Yan
National Astronomical Observatories, Chinese Academy of Sciences

Peter Robinson
School of Physics, University of Sydney

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     Last modified: July 26, 2011

Type III solar radio bursts are produced when energetic electrons accelerated in solar flares propagate as beams along solar magnetic field lines. Simulations are presented for decimetric type III bursts at the second harmonic of the local electron plasma frequency fp. The simulations show that 2fp radiation can be observed remotely at Earth in two scenarios for the radiationís 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 and then partly propagates into nearby regions due to its natural directivity pattern and refraction, where the neighboring regions are hot because of previous reconnection/evaporation. The simulations demonstrate that the profiles of plasma density ne(r) and electron temperature Te(r) in the lower corona (r-RS≤100Mm) are 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 and RS is the solar radius. Significantly, the observed wide ranges of radiation properties (e.g., frequency drift rates) require density profiles with a large range of scale heights. The simulations suggest the following: (1) Density profiles with small scale heights, such as given by offset power-laws like ne(r) ~ (r-RS)-2.38 for flaring regions, are unexpectedly common in the lower corona. This result is consistent with recent work that directly constrained ne(r) profiles for r ≈ (1.05-2)RS from observed metric type IIIs. (2) The dominance of reverse-slope bursts over normal bursts sometimes observed may be caused by asymmetric reconnection or acceleration, which favors downgoing beams.

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