Irregular time evolution of the radio emission generated in a B2-class microflare (SOL2017-01-25T10:15), occurring on 2017 January 25 in active region 12,628, is studied. The microflare was apparently initiated by an appearance of an s-shaped loop, observed in the EUV band. The radio emission is associated with the nonthermal electrons detected with Ramaty High Energy Solar Spectroscopic Imager, and originates simultaneously from two opposite footpoints of a magnetic fan structure beginning at a sunspot. According to the active region geometry, the footpoints are situated in the meridional direction, and hence are observed by RATAN-600 simultaneously. The radio emission intensity signal, as well as the left-hand and right-hand circular polarization signals in the lowfrequency band (3-4GHz) show good correlation with each other, with the average characteristic time of the variation 1.4±0.3s. The polarization signal shows a time variation with the characteristic time of about 0.7±0.2s. The irregular quasi-periodic pulsations of the radio emission are likely to be caused by the superposition of the signals generated at the local electron plasma frequencies by the interaction of nonthermal electrons with the plasma at the footpoints. In this scenario, the precipitation rate of the nonthermal electrons at the opposite footpoints could be modulated by the superposition of fundamental and second harmonic modes of sausage oscillations, resulting in the observed different characteristic times of the intensity and polarization signals. However, other mechanisms, e.g., the oscillatory regime of loop coalescence or magnetic null point oscillation could not be rigorously excluded.

We developed a method to compute the temperature and density structure along the line of sight by inversion of the differential emission measure (DEM), under the assumptions of stratification and hydrostatic equilibrium. We applied this method to the DEM obtained from AIA observations and used the results, together with potential extrapolations of the photosheric magnetic field, to compute the microwave emission of three sunspots, which we compared with observations from the RATAN-600 radio telescope and the Nobeyama Radioheliograph (NoRH). Our DEM based models reproduced very well the observations of the moderate-size spot on October 2011 and within 25% the data of a similar sized spot on March 2016, but predicted too low values for the big spot of April 14, 2016. The latter was better fitted by a constant conductive flux atmospheric model which, however, could not reproduce the peak brightness temperature of 4.7 × 10 6 K and the shape of the source at the NoRH frequency. We propose that these deviations could be due to low intensity non-thermal emission associated to a moving pore and to an opposite polarity light bridge. We also found that the double structure of the big spot at high RATAN-600 frequencies could be interpreted in terms of the variation of the angle between the magnetic field and the line of sight along the sunspot.

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The modulation of the microwave emission intensity from a flaring loop by a standing linear sausage fast magnetoacoustic wave is considered in terms of a straight plasma slab with the perpendicular Epstein profile of the plasma density, penetrated by a magnetic field. The emission is of the gyrosynchrotron (GS) nature, and is caused by mildly relativistic electrons which occupy a layer in the oscillating slab, i.e., the emitting and oscillating volumes do not coincide. It is shown that the microwave response to the linear sausage wave is highly non-linear. The degree of the non-linearity, defined as a ratio of the Fourier power of the second harmonic to the Fourier power of the principal harmonic, is found to depend on the combination of the width of the GS source and the viewing angle, and is different in the optically thick and optically thin parts of the microwave spectrum. This effect could be considered as a potential tool for diagnostics of the transverse scales of the regions filled in by the accelerated electrons.