Properties of Type-II Radio Bursts in Relation to Magnetic Complexity of the Solar Active Regions

Abstract

Type-II radio bursts are believed to occur as a result of the shock driven by flares or coronal mass ejections (CMEs). While the shock waves are important for the acceleration of electrons necessary for the generation of the radio emission, the exact nature of the shock and coronal conditions necessary to produce type-II radio emission is still under debate. In this investigation, we probe the relationship of kinematic characteristics of the type-II radio bursts with the magnetic-field complexity (M_j) of the active regions visible on the photosphere. Our investigation of 64 type-II solar radio bursts, which are associated with flares and CMEs, reveals that M_j is linearly correlated in the logarithmic scale with the starting frequency (f_s) and drift-rate (\({\Delta f/\Delta t}\)) of type-II radio burst. Further, M_j exhibits a linear correlation with the shock height (r) and electron density (\(n_{\rm e}\)) in logarithmic scale. This indicates that high frequency (f_s \(\geq 100\) \({\rm MH_{z}}\)) bursts, which occur at the reconnection site near the solar surface, are produced from a strong magnetically complex region. Further, strong and complex magnetic-field regions produce shocks of higher speeds. Based on the derived plasma parameters of the radio bursts and their relationship with f_s as well as with M_j, we propose that the high-frequency type-II bursts were generated in a special situation when the shock is produced due to magnetic reconnection occurring in the low-lying coronal loops. We conclude that type-II radio bursts can occur even in the inner corona as well as in the outer corona; however, it depends on the magnetic complexity of the active region in which the event occurs.