Observations of Near-Simultaneous Split-Band Solar Type-II Radio Bursts at Low Frequencies.

We report ground-based radio spectral and polarimeter observations of two successive split-band Type-II bursts that occurred on 20 February 2014 at low frequencies ( ${<}\,100~\mbox{MHz}$) in association with a solar coronal mass ejection (CME). The temporal interval between the onset of the two bursts was very small, ≈ one minute. Both of the bursts exhibited fundamental-harmonic structure. The coronal magnetic-field strength [ $B$] in the upstream region of the associated magnetohydrodynamic (MHD) shock, estimated from the split-band observations, is in the range $B \approx1.3\,\mbox{--}\,1.1~\mbox{G}$ over the radial distance [ $r$] interval $r \approx1.49\,\mbox{--}\,1.58~\mbox{R}_{\odot}$ for the first Type-II burst, and $B \approx1.3\,\mbox{--}\,1.0~\mbox{G}$ over $r \approx1.49\,\mbox{--}\,1.64~\mbox{R}_{\odot}$ for the second Type-II burst. Based on the results obtained, we show that the first and the second Type-II bursts in the present case were likely due to MHD shocks generated by the near-simultaneous interaction of two different regions of the aforementioned CME with a preceding CME and a pre-existing coronal streamer. [ABSTRACT FROM AUTHOR]