Investigation of the Coronal Magnetic Field Using a Type II Solar Radio Burst

Vasanth, V.; Umapathy, S.; Vršnak, Bojan; Žic, Tomislav; Prakash, O.
January 2014
Solar Physics;Jan2014, Vol. 289 Issue 1, p251
Academic Journal
The type II solar radio burst recorded on 13 June 2010 by the Hiraiso Solar Observatory Radio Spectrograph 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 Alfvén Mach number using the Rankine–Hugoniot relation. We convert the plasma frequency of the type II burst into height [ R] in solar radii using an appropriate density model, and then we estimated the shock speed [ Vs], coronal Alfvén velocity [ VA], and the magnetic-field strength at different heights. The relative bandwidth of the band splitting was found to be in the range 0.2 – 0.25, corresponding to a density jump of X=1.44 – 1.56, and an Alfvén Mach number of MA=1.35 – 1.45. The inferred mean shock speed was on the order of V≈667 km s −1. From the dependencies V( R) and MA( R) we found that the Alfvén speed slightly decreases at R≈1.3 – 1.5 R ⊙. The magnetic-field strength decreases from a value between 2.7 and 1.7 G at R≈1.3 – 1.5 R ⊙, depending on the coronal-density model employed. 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 results show that the type II band-splitting method is an important tool for inferring the coronal magnetic field, especially when independent measurements are made from white-light observations.


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