V. Vasanth, S. Umapathy, B. Vršnak, T. Žic, O. Prakash
The Type-II solar radio burst recorded on 13 June 2010 by the radio spectrograph of the Hiraiso Solar Observatory was employed to estimate the magnetic-field strength in the solar corona. The burst was characterized by a well pronounced band-splitting, which we used to estimate the density jump at the shock and Alfven Mach number using the Rankine-Hugoniot relations. The plasma frequency of the Type-II bursts is converted into height [R] in solar radii using the appropriate density model, then we estimated the shock speed [Vs], coronal Alfven velocity [Va], and the magnetic-field strength at different heights. The relative bandwidth of the band-split is found to be in the range 0.2 -- 0.25, corresponding to the density jump of X = 1.44 -- 1.56, and the Alfven Mach number of MA = 1.35 -- 1.45. The inferred mean shock speed was on the order of V ~ 667 km/s. From the dependencies V(R) and MA(R) we found that Alfven speed slightly decreases at R ~ 1.3 -- 1.5. The magnetic-field strength decreases from a value between 2.7 and 1.7 G at R ~ 1.3 -- 1.5 Rs depending on the coronal-density model employed. We find that our results are in good agreement with the empirical scaling by Dulk and McLean (Solar Phys. 57, 279, 1978) and Gopalswamy et al. (Astrophys. J. 744, 72, 2012). Our result shows that Type-II band splitting method is an important tool for inferring the coronal magnetic field, especially when independent measurements were made from white light observations.
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http://arxiv.org/abs/1305.1760
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