Abstract
Abstract One of the most important products of solar flares is nonthermal energetic particles, which may carry up to 50% of the energy released in the flaring processes. In radio observations, nonthermal particles generally manifest as spectral fine structures with fast frequency-drifting rates, named as solar fast-drifting radio bursts (FDRBs). This work demonstrated three types of FDRBs, including type III pair bursts, narrowband stochastic spike bursts following the type III bursts, and spike-like bursts superimposed on a type II burst in an X1.3 flare on 2014 April 25. We find that although all of them have fast frequency-drifting rates, they are intrinsically different from each other in frequency bandwidth, drifting rate, and statistical distribution. We suggest that they are possibly generated from different accelerating mechanisms. The type III pair bursts may be triggered by high-energy electron beams accelerated by the flaring magnetic reconnection, spike bursts are produced by the energetic electrons accelerated by a termination shock wave triggered by the fast reconnecting plasma outflows impacting the flaring loop top, and spike-like bursts are possibly generated by nonthermal electrons accelerated by moving magnetic reconnection triggered by interaction between coronal mass ejection and the background magnetized plasma. These results may help us to understand the generation mechanism of nonthermal particles and energy release in solar flares.
Highlights
In physics of solar flares and coronal mass ejections (CMEs), radio observations play a key role to understand the primary energy release, triggering mechanism of eruptions, and the related plasma instabilities
The related flare occurred at the solar west limb on 2014 April 25 which was fully observed by several telescopes, including soft X-ray (SXR) at 0.5 - 4 ̊A and 1 - 8 ̊A observed by GOES, extreme ultraviolet (EUV) images observed by SDO/AIA (Lemen et al 2012), hard X-ray (HXR) by RHESSI (Lin et al 2002), radio dynamic spectrogram at frequency of 100 - 500 MHz by Iitate Planetary radio telescope (IPRT/AMATERAS, Iwai et al 2012) and microwave images at frequency of 17 GHz observed by Nobeyama Radio Heliograph (NoRH), etc
Considering the distribution of the negative drifting (ND) and positive drifting (PD) spike-like bursts is very similar to the type III pair bursts which produced from magnetic reconnection, we proposed that the interaction between CME and the background magnetized coronal plasma may generate moving magnetic reconnection (showing (3) in Fig. 7), and accelerate electrons to produce nonthermal energetic electrons which triggered the formation of type II radio burst and the spike-like bursts
Summary
In physics of solar flares and coronal mass ejections (CMEs), radio observations play a key role to understand the primary energy release, triggering mechanism of eruptions, and the related plasma instabilities. It can provide the most sensitive direct evidences of magnetic reconnections, nonthermal particle accelerations and propagations, and the variations of magnetic field in corona (Dulk 1985, Bastian et al 1998). Its frequency drifting rate is very slow (D < 0.01 s−1 in most cases) and the corresponding moving velocity of the emitting medium is near or slower than the local Alfven speed (vA) They are possibly produced by some plasma flows, jets or the motions of coronal loops.
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