Context. Large-scale fast waves with perturbation of the EUV emission intensity are well resolved in both temporal and spatial scale by SDO/AIA. These waves are prone to propagate along the magnetic field line. Aims. We aim to probe the link between propagating fast wave trains and flaring energy releases. By measuring the wave parameters, we reveal their nature and investigate the potential to diagnose the energy source and waveguide. Methods. The spatial and temporal evolution of the wave amplitude and propagating speed are studied. The correlation of individual wave trains with flare-generated radio bursts is tested. Results. The propagating wave pattern comprises distinct wave trains with varying periods and wavelengths. This characteristic signature is consistent with the patterns formed by waveguide dispersion, when different spectral components propagate at different phase and group speeds. The wave train releases are found to be highly correlated in start time with the radio bursts emitted by the nonthermal electrons that were accelerated in bursty energy releases. The wave amplitude is seen to reach the maximum midway during its course. This can be caused by a combined effect of the waveguide spread in the transverse direction and density stratification. The transverse amplitude distribution perpendicular to the wave vector is found to follow approximately a Gaussian profile. The spatial structure is consistent with the kink mode that is polarised along the line-of-sight. The propagating speed is subject to deceleration from ∼735−845 km s −1 to ∼600 km s −1 . This could be caused by the decrease in the local Alfvén speed and/or the projection effect.

Solar radio type III bursts are believed to be the most sensitive signature of near-relativistic electron beam propagation in the corona. A solar radio type IIIb-III pair burst with fine frequency structures, observed by the Low Frequency Array (LOFAR) with high temporal (∼ 10 ms) and spectral (12.5 kHz) resolutions at 30 -80 MHz, is presented. The observations show that the type III burst consists of many striae, which have a frequency scale of about 0.1 MHz in both the fundamental (plasma) and the harmonic (double plasma) emission. We investigate the effects of background density fluctuations based on the observation of striae structure to estimate the density perturbation in solar corona. It is found that the spectral index of the density fluctuation spectrum is about −1.7, and the characteristic spatial scale of the density perturbation is around 700 km. This spectral index is very close to a Kolmogorov turbulence spectral index of −5/3, consistent with a turbulent cascade. This fact indicates that the coronal turbulence may play the important role of modulating the time structures of solar radio type III bursts, and the fine structure of radio type III bursts could provide a useful and unique tool to diagnose the turbulence in the solar corona.

Quasi-periodic pulsations (QPPs) are frequently observed in solar flares, which may reveal some essential characteristics of both thermal and nonthermal energy releases. This work presents multi-wavelength imaging observations of an M8.7 flare in active region AR 12242 on 2014 December 17. We found that there were three different QPPs: UV QPPs with a period of about 4 minutes at 1600 Å images near the center of the active region lasting from the preflare phase to the impulsive phase; EUV QPPs with a period of about 3 minutes along the circular ribbon during the preflare phase; and radio QPPs with a period of about 2 minutes at frequencies of 1.2–2.0 GHz around the flaring source region during the impulsive phase. The observations include the radio images observed by the Mingantu Spectral Radioheliograph in China at frequencies of 1.2–2.0 GHz for the first time, microwave images by the Nobeyama Radioheliograph, UV and EUV images by AIA/SDO, and a magnetogram by HMI/SDO. We suggest that the 4 minute UV QPPs should be modulated by the sunspot oscillations, and the 3 minute EUV QPPs are closely related to the 2 minute radio QPPs for their source regions connected by a group of coronal loops. We propose that the intermittent magnetic reconnecting downward and upward plasmoids may be the possible trigger of both the preflare 3 minute EUV QPPs and the impulsive 2 minute radio QPPs. The other possible mechanism is LRC oscillation, which is associated with the current-carrying coronal loops. The latter mechanism implies that the existence of preflare QPPs may be a possible precursor to solar flares.

Solar flares are the most powerful explosions occurring in the solar system, which may lead to disastrous space weather events and impact various aspects of our Earth. So far, it is still a big challenge in modern astrophysics to understand the origin of solar flares and predict their onset. Based on the analysis of soft X-ray emission observed by the Geostationary Operational Environmental Satellite (GOES), this work reported a new discovery of very long-periodic pulsations occurred in the preflare phase before the onset of solar flares (preflare-VLPs). These pulsations are typically with period of 8 -30 min and last for about 1 -2 hours. They are possibly generated from LRC oscillations of plasma loops where electric current dominates the physical process during magnetic energy accumulation in the source region. The preflare-VLP provides an essential information for understanding the triggering mechanism and origin of solar flares, and may help us to response to solar explosions and the corresponding disastrous space weather events as a convenient precursory indicator.