A Wide-Band High-Frequency Type-II Solar Radio Burst

Illustration 1. Dynamic spectrum of the high-frequency wide-band type-II burst recorded by the ORFEES radio spectrograph (670-144 MHz). The maximum brightness temperatures (TB_max) at different NRH frequencies are plotted with white lines (source: The Authors).

Here we report an unusual wide-band high frequency type-II solar burst on 19 May 2022 with starting frequency as high as 670 MHz and an instantaneous bandwidth as wide as 300 MHz, as recorded by the ORFEES spectrograph (Illustration 1). Such bandwidth is much wider than usual events. In addition, at any specific frequency the burst lasts for 2 minutes. This is also much longer than usual bursts. Simultaneous observations of extreme-ultraviolet (EUV) and radio imaging from the Nançay Radio Heliograph (NRH) are available for this event. This allows us to measure the location of radio sources over the EUV shock structure and to explore the cause of the high-frequency wide-band feature.

By over-plotting the 90 and 95% contours of TB_max onto the closest-in-time AIA 211 Å running difference images (see Illustration 2), we made four observations: (1) the type-II sources are co-spatial with the nose front of the EUV shock wave structure (being slightly separated, by 0.01 R_ʘ; (2) the sources basically overlap with each other before 12:04 UT (without spatial separation 0.001 R_ʘ; later they become spatially dispersed, with lower-frequency sources being farther away from the disk; (3) as already mentioned, the sources at several frequencies appear simultaneously, and they distribute within a broad region of 150 ̶ 200 arcseconds; and (4) during most time of the burst, the sources are spatially dispersed yet still centered around the dip of the shock front that corresponds to the CME-shock transit across the bright dense loop tops.

The dip is likely due to the strong compression caused by the shock propagation into bright and dense loops where the Alfvénic speed (as well as the shock speed) is relatively low. According to the EUV and white light data, bright and dense loops that connect the two spots are spatially correlated with the shock dip, indicating the shock has transited across them with a strong interaction. In this case, electron acceleration is quite efficient according to numerical simulations by Kong et al. (2015, 2016). We suggest that the type-II burst originates from the shock transit across these magnetically closed-loop structures that forms the shock dip (see Illustration3).