Abstract
From 2018 Oct 12 to 13, three successive solar eruptions (E1--E3) with B-class flares and poor white light coronal mass ejections (CMEs) occurred from the same active region NOAA AR 12724. Interestingly, the first two eruptions are associated with Type II radio bursts but the third is not. Using the soft X-ray flux data, radio dynamic spectra and dual perspective EUV intensity images, we comparatively investigate the three events. Our results show that their relevant flares are weak (B2.1, B7.9 and B2.3) and short-lived (13, 9 and 14 minutes). The main eruption directions of E1 and E2 are along $\sim$45$^\circ$ north of their radial directions, while E3 primarily propagated along the radial direction. In the EUV channels, the early speeds of the first two CMEs have apparent speeds of $\sim$320 km s$^{-1}$ and $\sim$380 km s$^{-1}$, which could exceed their respective local Alfv$\acute{e}$n speeds of $\sim$300 km s$^{-1}$ and $\sim$350 km s$^{-1}$. However, the CME in the third eruption possesses a much lower speed of $\sim$160 km s$^{-1}$. These results suggest that the observed Type II radio bursts in the eruptions E1 and E2 are likely triggered by their associated CMEs and the direction of eruption and the ambient plasma and magnetic environments may take an important place in producing Type II radio burst or shock as well.
Highlights
Solar Type II radio bursts were first reported by Payne-Scott et al (1947) and named by Wild and McCready (1950)
The blast wave initiated by flares and piston-driven mechanism associated with coronal mass ejections (CMEs) have become the two main competitors of the triggering mechanism of the Type II bursts as reviewed by the papers (e.g., Cliver et al, 1999; Vršnak and Cliver, 2008; Nindos et al, 2011) and suggested by some recent studies (e.g., Zheng et al, 2018; Eselevich et al, 2019; Frassati et al, 2019)
All the three events originated from the same active region (AR) NOAA 12724 and each of them involved a filament eruption, a B-class flare and a slow poor white light (WL) CME
Summary
Solar Type II radio bursts were first reported by Payne-Scott et al (1947) and named by Wild and McCready (1950). The blast wave initiated by flares and piston-driven mechanism associated with coronal mass ejections (CMEs) have become the two main competitors of the triggering mechanism of the Type II bursts as reviewed by the papers (e.g., Cliver et al, 1999; Vršnak and Cliver, 2008; Nindos et al, 2011) and suggested by some recent studies (e.g., Zheng et al, 2018; Eselevich et al, 2019; Frassati et al, 2019). We present a case study about two successive Type II bursts associated with B-class flares and CMEs with slow speeds below ∼400 km s−1 in the period of solar activity minimum. Our comparative investigations suggest that these Type II radio bursts are likely triggered by their associated CMEs and we discuss the influence from the ambient coronal magnetic structures on the eruptions
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