The morphology of average solar flare time profiles from observations of the Sun's lower atmosphere

We study the decay phase of solar flares in several spectral bands using a method basedon that successfully applied to white light flares observed on an M4 dwarf. We selectedand processed 102 events detected in the Sun-as-a-star flux obtained with SDO/AIAimages in the 1600 ̊A and 304 ̊A channels and 54 events detected in the 1700 ̊A channel.The main criterion for the selection of time profiles was a slow, continuous flux decaywithout significant new bursts. The obtained averaged time profiles were fitted withanalytical templates, using different time intervals, that consisted of a combination oftwo independent exponents or a broken power law. The average flare profile observedin the 1700 ̊A channel decayed more slowly than the average flare profile observed onthe M4 dwarf. As the 1700 ̊A emission is associated with a similar temperature to thatusually ascribed to M dwarf flares, this implies that the M dwarf flare emission comesfrom a more dense layer than solar flare emission in the 1700 ̊A band. The coolingprocesses in solar flares were best described by the two exponents model, fitted overthe intervals t1=[0, 0.5]t1/2and t2=[3, 10]t1/2wheret1/2is time taken for the profileto decay to half the maximum value. The broken power law model provided a goodfit to the first decay phase, as it was able to account for the impact of chromosphericplasma evaporation, but it did not successfully fit the second decay phase

Editorial : solar radiophysics — recent results on observations and theories

Solar radiophysics is a rapidly developing branch of solar physics and plasma astrophysics. Solar radiophysics has the goal of analyzing observations of radio emissions from the Sun and understanding basic physical processes operating in quiet and active regions of the solar corona. In the near future, the commissioning of a new generation of solar radio observational facilities, which include the Chinese Spectral Radio Heliograph (CSRH) and the upgrade of the Siberian Solar Radio Telescope (SSRT), and the beginning of solar observations with the Atacama Large Millimeter/submillimeter Array (ALMA), is expected to bring us new breakthrough results of a transformative nature. The Marie-Curie International Research Staff Exchange (MC IRSES) “RadioSun” international network aims to create a solid foundation for the successful exploitation of upcoming solar radio observational facilities, as well as intensive use of the existing observational tools, advanced theoretical modeling of relevant physical processes and observables, and training a new generation of solar radio physicists. The RadioSun network links research teams from China, Czech Republic, Poland, Russia and the UK. This mini-volume presents research papers based on invited reviews and contributed talks at the 1st RadioSun workshop in China. These papers cover a broad range of research topics and include recent observational and theoretical advances in solar radiophysics, MHD seismology of the solar corona, physics of solar flares, generation of radio emission, numerical modeling of MHD and plasma physics processes, charged-particle acceleration and novel instrumentation

Common origin of quasi-periodic pulsations in microwave and decimetric solar radio bursts

We analyse quasi-periodic pulsations (QPP) detected in the microwave and decimetre radio emission of the 5 May 2017 7:04 UT (SOL2017-09-05T07:04) solar flare, using simultaneous observations by the Siberian Radioheliograph 48 (SRH-48, 4 – 8 GHz) and Mingantu Spectral Radioheliograph (MUSER-I, 0.4 – 2 GHz). The microwave emission was broadband with a typical gyrosynchrotron spectrum, while a quasi-periodic enhancement of the decimetric emission appeared in a narrow spectral band (500 – 700 MHz), consistent with the coherent-plasma-emission mechanism. The periodicity that we found in microwaves is about 30 seconds, coming from a compact loop-like source with a typical height of about 31 Mm. The decimetric emission exhibited a periodicity of about 6 seconds. We suggest a qualitative scenario linking the QPPs observed in both incoherent and coherent spectral bands and their generation mechanisms. The properties of the QPPs found in the microwave signal are typical for perturbations of the flare loop by the standing sausage mode of a fast magnetohydrodynamic (MHD) wave. Our analysis indicated that this sausage-oscillating flare loop was the primary source of oscillations in the discussed event. The suggested scenario is that a fundamental sausage harmonic is the dominant cause for the observed QPPs in the microwave emission. The initiation of oscillations in the decimetric emission is caused by the third sausage harmonic via periodic and nonlinear triggering of the acceleration processes in the current sheets, formed at the interface between the sausage-oscillating flare loop and the external coronal loop that extended to higher altitudes. Our results demonstrate the possible role of MHD wave processes in the release and transport of energy during solar flares, linking coherent and incoherent radio emission mechanisms