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Investigation on the coronal magnetic field using a type II solar radio burst

The type II solar radio burst recorded on June 13th, 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-split, which we used to estimate the density jump at the shock, which was then converted to the Alfven Mach number by applying the Rankine-Hugoniot relations. From the frequency drift of the emission we estimated the shock speed, which provided an estimate of the coronal Alfven velocity and the magnetic field strength. The relative bandwidth of the band-split is found to be in the range 0.2 – 0.3, corresponding to the density jump of X = 1.43 – 1.64, and the Alfven Mach number of MA = 1.34 – 1.52. The inferred mean shock speed was on the order of v ~ 1000 km s-1. From the dependencies v(R) and MA(R) we found that Alfven speed first decreases from ~ 1000 km s-1 to a local minimum of vA ~ 400 – 500 km s-1 at R ~ 1.4 – 1.8, and then increases again. The type II burst appeared in the decreasing-vA environment, and disappears in the increasing-vA environment, as would be expected in the case of freely propagating blast wave. The magnetic field strength decreases from a value between 5 and 7 G at R ~ 1.2 – 1.6, to a value between 1 and 2 G at R ~ 1.4 – 1.8, depending on the coronal density model employed. We find that our results are in quite good agreement with the empirical scaling by Dulk and McLean (1978), and that the scaling appropriate for quiet corona and large radial distances, B = 2R-2, becomes effective only beyond ~1.5 – 2 solar radii.

solar type II radio bursts; solar flares; coronal mass ejections; shock waves

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