Victor Fedorovich Melnikov

We report dual-perspective microwave and hard X-ray (HXR) observations of an X-class flare on 2024 May 15, taking advantage of the unique geometry of a front-side view from the Earth and a back-side perspective from the Solar Orbiter. Using spatially resolved imaging spectroscopy from Siberian Radioheliograph together with Chashan Broadband Solar millimeter spectrometer and Spectrometer Telescope for Imaging X-rays data, we identify a set of nonthermal flaring loops in an asymmetric magnetic field, with microwave sources located near the loop top and HXR sources associated with the southern footpoint. Compared with HXR, the microwave emission shows an opposite ascending trend, large time lags, and distinctive “soft–hard–harder” spectral patterns, which we attribute to energy-dependent trapping and precipitation of energetic electrons in an asymmetric magnetic configuration. Flux pulsations and their spectral and polarization signatures are consistent with intermittent particle acceleration rather than magnetohydrodynamic wave modulation. Microwave magnetic diagnostics, corroborated by nonlinear force free field extrapolation, provide key constraints on the three-dimensional magnetic configuration. The dual-perspective flux profile comparison and consistent quasiperiodic pulsation signatures across wavelengths together support a self-consistent picture of energy-dependent electron trapping, precipitation, and transport in these asymmetric loops.

How flares accelerate electrons efficiently remains a major puzzle in solar and stellar astronomy. Crucial information is carried by microwave bursts that are generated as energetic electrons spiraling around magnetic field lines. Here we report the eruption process during the strongest solar microwave burst (>10,000 solar flux units above 20 GHz, associated with an X1.7 flare) in Solar Cycle 25. We found that the interaction between postflare loops of a preceding M2.6 flare and rising southern loops generates abundant superhot plasma ( n _e ∼ 2.0 × 10 ⁹ cm ⁻³ , T _e ∼ 40 MK), which gets involved (by providing heated seed particles) in the subsequent three successive episodes of reconnection processes of the X1.7 flare. Using the gyrosynchrotron mechanism to fit the obtained spectra for the three subsequent microwave peaks, we found that energetic electrons are progressively abundant, with a hardening spectrum with the index reaching up to ∼2. We highlight the importance of preheated plasma and persistent multiepisode acceleration of energetic electrons in generating such extreme microwave bursts.

The radiation mechanism of decimetric wideband and pulsating radio bursts from the Sun (in terms of decimetric type-IV (t-IVdm) burst) and other flaring stars is a long-standing problem. Early investigations were based on the leading-spot hypothesis for the Sun and yielded contradictory results. Here, we analyzed the flare-associated t-IVdm burst on 20110924 with medium-strong levels of polarization and from sources near a sunspot. We found that the emission is intermittent and the maximum T _B exceeds 10¹¹ K, with well-defined upper and lower frequency cutoffs. The radio sources are left-handed polarized, located above the sunspot with a negative polarity. The sources align well with the sites of the second harmonic of the local electron gyrofrequency. These findings provide essential evidence that the burst is induced by the electron cyclotron maser emission (ECME) in the harmonic X mode. We further modeled the transport of downward-streaming energetic electrons along a coronal loop and found that most electrons get mirrored within the specific altitude range of 20–100 Mm. This explains why such bursts tend to have well-defined spectral ranges. We also found the ECME-radiating energetic electrons exhibit a shell-like velocity distribution function instead of the generally presumed loss-cone distribution. The study greatly expands the application of ECME in solar radio astronomy and provides solar samples for similar bursts from other flaring stars.

The spatial dynamics of microwave emission (data from the Nobeyama Radioheliograph) of active regions with a complex magnetic field configuration before powerful flares, as well as the fine structure of active sources of ultraviolet (UV) emission based on AIA/SDO data, are studied. It is shown that there is unsteady behavior of microwave sources, along with the ignition of a system of crossed UV loops (X-structures). A class-M X-ray flare occurs after several hours at the site of ignition of the intersecting UV loops. The similar behavior observed in microwave and ultraviolet emission in the five active regions studied by us likely serves as a sign of a preflare situation in the active region on the Sun.

This paper reports the obtained analysis data on the spatio-temporal evolution of sources of microwave and hard X-ray pulsations for the SOL2011-06-07T06:16 event based on the observation data of the...