Marian Karlicky

Understanding the origin of coherent solar radio bursts requires linking macroscopic coronal structures with the kinetic processes responsible for wave generation. We investigate emissions from slowly positively drifting bursts (SPDBs), a specific type of solar radio emission. SPDBs provide observational constraints for modeling beam–plasma interactions in coronal loops, serving as a basis for a multiscale description. We employ a three-stage numerical framework that combines nonlinear force-free field magnetic extrapolation, guiding-center simulations, and fully kinetic particle-in-cell (PIC) modeling. The background plasma density is described using a hydrostatic model consistent with active-region conditions, producing a plasma–frequency gradient comparable to that inferred from the observed SPDBs spectrum. Energetic electrons injected near the loop top evolve through magnetic mirroring, pitch-angle scattering, turbulence development, and partial precipitation. Evolved velocity distribution functions are sampled after approximately one bounce period and used in PIC simulations to evaluate emission properties. The results show that the evolved beam distribution of energetic electrons predominantly excites beam–Langmuir waves and fundamental plasma emission along the loop, with the emission intensity gradually decreasing from the loop top toward the footpoint. The modest initial beam velocity of energetic electrons explains the inefficient generation of harmonic plasma emission. The temporal evolution of the modeled emission reproduces key SPDB characteristics, including the ∼4 s duration and frequency drift behavior. These results suggest plasma emission explains the mechanisms behind SPDB generation and demonstrate the feasibility of a unified model connecting coronal magnetic topology, particle transport, and radio emission.

In this paper, we present a unique decimetric radio burst that oscillates not only in time but also in frequency. Its duration, exceeding 80 minutes, is significantly longer than any radio oscillations previously reported in the literature. The oscillation period of the burst was about 3 minutes, suggesting a connection with sunspot oscillations observed in EUV and radio wavelengths. We detected the same periodicity in a 3000 MHz radio source located in a loop anchored to sunspots and spanning the entire active region, where flares occurred during the burst. Based on these observations, we propose that the frequency-oscillating burst is generated in the loop, serving as a reservoir of accelerated electrons, by the electron-cyclotron mechanism. Waves originating in the sunspot penetrate into the loop containing the source of the frequency-oscillating burst, and the resonant period of this loop appears to be close to that of the incoming waves. As a result, the radio emission of the burst oscillated for about 80 minutes with a 3-minute period. The magnetic-field strength in the burst source was estimated to be about 250 G.

In this paper, we present the results of our analysis of solar eruptive flares observed by the Ondřejov radiospectrographs over more than three decades. By combining the eruptive flare model with findings from our magnetohydrodynamic and particle-in-cell simulations, we demonstrate the crucial role of decimetric radio bursts in understanding plasma processes during eruptive flares. We describe unusual drifting continua associated with the rise of a magnetic rope at the onset of these flares. Notably, we report very rare slowly positively drifting bursts (SPDBs) linked to the bright helical structure of the ascending rope. Drifting pulsation structures (DPSs) are identified as signatures of plasmoids, while narrowband decimetric spikes are associated with magnetic reconnection outflows. We also examine pairs of decimetric Type III bursts, which indicate electron beams propagating both upward and downward in the solar atmosphere from the acceleration site, as well as a special Type III burst likely traveling around a plasmoid. We introduce a method for computing period maps and identifying a unique wave/shock feature in the radio spectrum. A movie illustrating the plasma processes responsible for generating the drifting pulsation structure is also shown. The interpretations of all presented bursts are based on the standard model of eruptive flares. However, positional data for sources of these radio bursts are often lacking. To emphasize the importance of spatial information, we present an example of a drifting pulsation structure observed simultaneously with observations from the Expanded Owens Valley Solar Array (EOVSA). Finally, we summarize all discussed bursts in a comprehensive scheme that extends our knowledge about a role of decimetric bursts at the onset of eruptive flares.

Context. The slowly positively drifting bursts (SPDBs) are rarely observed in radio emission of solar flares. Aims. To understand how the SPDBs are generated, we studied the radio observations at 600–5000 MHz together with the imaging observations made in ultraviolet (UV) and extreme ultraviolet (EUV) during the SPDB-rich C8.7 flare of 2014 May 10 (SOL2014-05-10T0702). Methods. Because the SPDBs propagate towards locations of higher plasma density, we studied their associations with individual flare kernels, located either within the flare core itself, or distributed at longer distances, but connected to the flaring region by large-scale hot loops. For each kernel we constructed light curves using 1600 Å and 304 Å observations and compared these light curves with the temporal evolution of radio flux at 1190 MHz, representing all observed groups of SPDBs. We also analysed the UV/EUV observations to understand the evolution of magnetic connectivity during the flare. Results. The flare starts with a growing hot sigmoid observed in 131 Å. As the sigmoid evolves, it extends to and interacts with a half dome present within the active region. The evolving sigmoid reconnects at the respective hyperbolic flux tube, producing large-scale magnetic connections and an EUV swirl. Three groups of SPDBs are observed during this large-scale magnetic reconnection, along with a group of narrow-band type III bursts. The light curves of a kernel corresponding to the footpoint of spine line analogue show good agreement with the radio flux at 1190 MHz, indicating that the SPDBs are produced by the large-scale magnetic reconnection at the half dome. In addition, one of the kernels appeared in the neighbouring active region and also showed a similar evolution to the radio flux, implying that beams of accelerated particles can synchronize radio and UV/EUV light curves across relatively large distances.

Context. Shock waves in the solar corona are associated with solar flares and coronal mass ejections (CMEs). Type II solar bursts are radio signatures of shock waves in the solar corona. They are driven by solar flares or CMEs. Despite extensive studies, the intricate spectral patterns observed in type II solar bursts occasionally pose new challenges for the theory of electron acceleration in shocks. Aims. We study a newly identified feature in type II solar bursts called spectral cleaving. This feature is characterized by the actual branching of a type II radio emission lane in radio spectral data. Methods. We analyzed the type II burst exhibiting spectral cleaving in high-fidelity dynamic spectra obtained using the URAN-2 radio telescope (8.25–33 MHz; Poltava region, Ukraine) on 2011 February 14. The high-resolution spectrograms were examined to ascertain its spectral morphology. Results. Our research represents the first recognition of spectral cleaving as a peculiarity of type II bursts that is yet to be classified. This effect occurs due to the shift (or migration) of radio source(s) along a shock front, which in turn is caused by changes in the magnetic field orientation ahead of the propagating shock front. Conclusions. The spectral cleaving observed in solar type II bursts reveals a distinct phenomenon that indicates complex interactions between shock waves and magnetic fields in the solar corona. This discovery enhances our understanding of the mechanisms behind solar radio emissions and emphasizes the need for further observational studies to verify these findings.

We present a case study of a failed eruption that accompanied an M1.5 GOES class solar flare. It was observed by STIX onboard Solar Orbiter, HXI onboard the Advanced Space-based Solar Observatory, AIA onboard Solar Dynamics Observatory, and WAVES onboard the STEREO-A. The important input is from stereoscopic hard X-ray (HXR) observations obtained by HXI and STIX, whose vantage points were separated by $31.5^{\circ }$ 31.5 ∘ , allowing us to unfold the 3D geometry of the event. The eruption was a two-phase event. First, it started with the rope helical kink and then was slowed down, but with the structure still unstable, it erupted two minutes later due to ongoing reconnection in the interacting legs of the kinked structure. A Type III burst was observed in association with the eruption, indicating the acceleration of semirelativistic electrons into the heliosphere. During the second phase, a hot cloud was disconnected and confined in the overlying magnetic field, where the overlying loops connected two adjacent active regions. The estimated and corrected for real geometry velocities are in the range of 385 – 400 km s⁻¹, whereas acceleration reached 4.78 – 6.33 km s⁻². These extreme values are much more demanding from a perspective of conditions that are needed to stop the eruption. Images obtained simultaneously by HXI and STIX located in different vantage points showed that flare-related sources are not lying along a normal to the solar surface. The understanding of the eruption analyzed here has been highly enriched thanks to the stereoscopic information about HXR source locations.

Decimetric continua are commonly observed during long-lasting solar flares. Their frequency boundaries vary with time. We studied frequency boundary variations using the power spectrum analysis. Analyzing five decimetric continua, we found that their power spectra have a power-law form with the power-law index close to the Kolmogorov turbulence index −5/3. The same power index was also found in the power spectra of radio flux variations at frequencies in the range of the frequency boundary variations. Moreover, these frequency boundary variations were highly correlated with the radio flux ones. We interpret these results to be due to turbulent density variations in the reconnection plasma outflow to the termination shock formed above flare loops. In three cases of decimetric continua, we estimated the level of the plasma density turbulence to be 7.6 – 11.2% of the mean plasma density. We think that the analysis of variations of decimetric continua can be used in studies of the plasma turbulence in solar flares.

In this review we summarize results of our analysis of the observations of solar eruptive flares made by the Ondřejov radiospectrograph for more than twenty years. We also present some Potsdam-Tremsdorf radio spectra from our common studies. Considering a 3-dimensional model of eruptive flares together with the results of our magnetohydrodynamic and particle-in-cell simulations we show an importance of decimetric radio bursts for understanding of plasma processes in eruptive flares. We present drifting pulsation structures as signatures of plasmoids, an unusual zebra pattern in the very early flare stage, narrowband dm-spikes as the bursts generated in the reconnection plasma outflows, radio bursts indicating a merging of plasmoids, pair of decimetric type III bursts indicating the electron beams propagating upwards and downwards in the solar atmosphere from the acceleration site, and a special decimetric type III burst formed probably around the plasmoid. We present unusual radio bursts connected with the rising magnetic rope at the very beginning of eruptive flares. Furthermore, based on the analysis of decimetric continua we estimated the level of the plasma turbulence in a vicinity of the flare termination shock. Interpretations of all these bursts are based on models and time coincidences with observations in X-ray, UV and optical ranges; in most cases an information about positions of these radio sources is missing. To show an importance of positional information, we present a rare example of observations, where the drifting pulsation structure was observed simultaneously with the observations made by the EOVSA radiointerferometer. All the presented bursts are then summarized in a new scheme of bursts and compared with the schema commonly used.

Type II solar bursts are radio signatures of shock waves in the solar corona driven by solar flares or coronal mass ejections (CMEs). Therefore, these bursts present complex spectral morphologies in solar dynamic spectra. Here, we report meter–decameter radio observations of a type II burst on 2014 July 25 made with the Ukrainian radio telescopes UTR-2 (8.25–33 MHz) and GURT (8.25–78 MHz). The burst demonstrates fundamental and harmonic components, band splitting, a herringbone structure, and a spectral break. These specific spectral features, observed jointly in a single type II burst, are rarely detected. To contribute to our understanding of such puzzling type II events, we carried out a detailed analysis of the recorded type II dynamic spectrum. In particular, the herringbone pattern has been exploited to study electron density turbulence in the solar corona. We calculated the power spectral densities of the flux variations in selected herringbones. The spectral index is in the range of α = −1.69 to −2.00 with an average value of −1.897, which is slightly higher than the Kolmogorov spectral index of −5/3 for fully developed turbulence. We also recognized that the second type II burst consists of three drifting lanes. The lane onset times coincide with the spectral break in the first type II burst. We regard that the CME/shock passage through a streamer caused the spectral break and triggered the multilane type II radio emission. Thus, we support one of the proposed scenarios for type II burst occurrence as being the result of CME/shock–streamer interaction.

Energy spectra of X-ray solar flares observed by the Spectrometer-Telescope for Imaging X-rays (STIX) onboard the Solar Orbiter consist of both thermal and non-thermal parts. The thermal part is present in all solar events. When the non-thermal part of the energy spectrum begins to dominate, we can expect detection in interplanetary space of high-energy electron beams that have escaped the coronal loops. When hard X-ray flares are detected solar type III radio bursts are registered frequently with their numerous modifications like drift pairs, U-type, and structured bursts. The e-CALLISTO simple worldwide radio antenna stations allow us to identify the existence of non-thermal components in the energy spectra of strong X-ray flares. At the same time, some X-ray flares are accompanied by ejections of energetic ions including heavy ions. The specific features in X-ray bursts responsible for events with simultaneous light and heavy particle stream generation are still unclear compared with those with electron emission only. We present preliminary results of observations gathered in December 2022 and cross-analysis of data on energetic light and heavy particle fluxes and X-ray flare parameters. The end of 2022 was distinguished by moderate to high solar activity, the presence of three periods with enhanced proton and heavy-ion fluxes at the beginning of the month, in the middle, and on 25-26 December. We demonstrate also the presence of narrow directed electron beams detected by the Electron Proton Telescope (EPT) of EPD for selected events mentioned above, and heavy ions detected by the Suprathermal Ion Spectrograph (SIS) of EPD.