Observations of near-simultaneous split-band solar type-II radio bursts at low frequencies

Abstract

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 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≈1.3--1.1 G over the radial distance [r] interval r≈1.49--1.58 R⊙ for the first Type-II burst, and B≈1.3--1.0 G over r≈1.49--1.64 R⊙ 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.