arXiv subjects

Marian Karlický

Publications and source records attributed to Marian Karlický.

16 recordsLinked to original sources

arXiv2025

Radio, X-ray, and EUV signatures of internal and external reconnection of an erupting flux rope

We analyse imaging (EUV, X-ray) and spectral (radio, X-ray) data obtained by ground based and space instruments on board space missions both on Earth (Fermi, Hinode, Solar Dynamics Observatory) and solar orbit (Solar Orbiter, STEREO-A), which provide a multi-directional view on the same event. The combination of EUV and X-ray images and X-ray spectra allowed us to identify hot loops in the vicinity of the filament before its eruption. We interpreted their interaction with the rising filament as a signature of an arcade-to-rope reconnection geometry. The subsequent EUV brightening within the filament revealed helical structure of the erupting rope. We explained co-temporal radio slowly positively drifting bursts as a result of beam acceleration within the magnetic rope and propagation along the helical structure. Corresponding X-ray spectra were consistent with a thermal origin. The filament rising was accompanied by co-temporal normal and reverse drift type III radio bursts. We interpreted them as a signature of a reconnection event and estimated electron density at the reconnection site. Further untwisting of the helical structure led to formation of a quasi-circular EUV structure seen from Earth and STEREO-A. Its occurrence was co-temporal with a unique tangle of radio U- and inverse U-bursts. We proposed that several accelerated beams propagate within that complex structure and generate the burst tangle. During the start of the flare hard X-ray emission was concentrated near the filament leg only suggesting predominant propagation of the beams towards its rooting. We collected multi-wavelength observations indicating interaction of the erupting magnetic flux rope with the overlying arcade and internal magnetic reconnection inside the rising flux rope.

astro-ph.SR↗

arXiv2024

Slowly positively drifting bursts generated by large-scale magnetic reconnection

The slowly positively drifting bursts (SPDBs) are rarely observed in radio emission of solar flares. 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). 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 A and 304 A 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. The flare starts with a growing hot sigmoid observed in 131 A. 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.

astro-ph.SR↗

arXiv2023

Radio bursts observed during solar eruptive flares and their schematic summary

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.

astro-ph.SR↗

arXiv2022

Zebra stripes with high gyro-harmonic numbers

Solar radio zebras are used in the determination of the plasma density and magnetic field in solar flare plasmas. Analyzing observed zebra stripes and assuming their generation by the double-plasma resonance (DPR) instability, high values of the gyro-harmonic number are found. In some cases, they exceed one hundred, in disagreement with the DPR growth rates computed up to now, which decrease with increasing gyro-harmonic number. We address the question of how the zebras with high values of the gyro-harmonic numbers $s$ are generated. For this purpose, we compute growth rates of the DPR instability in a very broad range of $s$, considering a loss-cone $κ$-distribution of superthermal electrons and varying the loss-cone angle, electron energies, and background plasma temperature. We numerically calculated dispersion relations and growth rates of the upper-hybrid waves and found that the growth rates increase with increasing gyro-harmonic numbers if the loss-cone angles are $\sim80^\circ$. The highest growth rates for these loss-cone angles are obtained for the velocity $v_κ= 0.15\,c$. The growth rates as function of the gyro-harmonic number still show well distinct peaks, which correspond to zebra-stripe frequencies. The contrast of the growth rate peaks to surrounding growth rate levels increases as the $κ$ index increases and the background temperature decreases. Zebras with high values of $s$ can be generated in regions where loss-cone distributions of superthermal electrons with large loss-cone angles ($\sim80^\circ$) are present. Furthermore, owing to the high values of $s$, the magnetic field is relatively weak and has a small spatial gradient in such regions.

astro-ph.SR↗

arXiv2021

Narrowband Spikes Observed during the 2013 November 7 Flare

Narrowband spikes are observed in solar flares for several decades. However, their exact origin is still discussed. To contribute to understanding of these spikes, we analyze the narrowband spikes observed in the 800-2000 MHz range during the impulsive phase of the November 7, 2013 flare. In the radio spectrum, the spikes started with typical broadband clouds of spikes, and then their distribution in frequencies changed into unique, very narrow bands having non-integer frequency ratios. We successfully fitted frequencies of these narrow spike bands by those, calculating dispersion branches and growth rates of the Bernstein modes. For comparison, we also analyzed the model, where the narrow bands of spikes are generated at the upper-hybrid frequencies. Using both models, we estimated the plasma density and magnetic field in spike sources. Then the models are discussed, and arguments in favor of the model with the Bernstein modes are presented. Analyzing frequency profiles of this spike event by the Fourier method, we found the power-law spectra with the power-law indices varying in the -0.8 -- -2.75 interval. Because at some times this power-law index was close to the Kolmogorov spectral index (-5/3), we propose that the spikes are generated through the Bernstein modes in turbulent plasma reconnection outflows or directly in the turbulent magnetic reconnection of solar flares.

astro-ph.SR↗

arXiv2021

Flare Expansion to a Magnetic Rope Accompanied by Rare Radio Bursts

We present multispectral analysis (radio, H$α$, UV/EUV, and hard X-ray) of a confined flare from 2015 March 12. This flare started within the active region NOAA 12 297 and then it expanded into a large preexisting magnetic rope embedded with a cold filament. The expansion started with several brightenings located along the rope. This process was accompanied by a group of slowly positively drifting bursts in the 0.8--2 GHz range. The frequency drift of these bursts was 45 -- 100 MHz s$^{-1}$. One of the bursts had an S-like form. During the brightening of the rope we observed an unique bright EUV structure transverse to the rope axis. The structure was observed in a broad range of temperatures and it moved along the rope with the velocity of about 240 km s$^{-1}$. When the structure dissipated, we saw a plasma further following twisted threads in the rope. The observed slowly positively drifting bursts were interpreted considering particle beams and we show that one with the S-like form could be explained by the beam propagating through the helical structure of the magnetic rope. The bright structure transverse to the rope axis was interpreted considering line-of-sight effects and the dissipation-spreading process, which we found to be more likely.

astro-ph.SR↗

arXiv2020

Expansion of Hot Plasma into Cold Plasma

The X-ray emission of coronal flare sources can be explained considering the kappa electron distribution. Motivated by this fact, we study the problem of how hot plasma with the kappa distribution of electrons is confined in these sources. For comparison, we analyze the same problem but with the Maxwellian distribution. We use a 3-D particle-in-cell code, which is large in one direction and thus effectively only one-dimensional, but describing all electromagnetic effects. In the case with the Maxwellian distribution, and in agreement with the previous studies, we show a formation of the double layer at the hot-cold transition region that suppresses the flux of hot electrons from hot plasma into the cold one. In the case with the kappa distribution, contrary to the Maxwellian case, we found that there are several fronts with the double layers in the hot-cold transition region. It is caused by a more extended tail in the kappa case than in the Maxwellian one. The electrons from the extended tail freely escape from the hot plasma into a cold one. They form a beam which generates the return current and also Langmuir turbulence, where at some locations Langmuir waves are accumulated. At these locations owing to the ponderomotive force, Langmuir waves generate density depressions, where the double layers with the thermal fronts, suppressing the hot electron flux, are formed. We also show how protons are accelerated in these processes. Finally, we compared the kappa and Maxwellian cases and discussed how these processes could be observed.

astro-ph.SR↗

arXiv2019

Growth rates of the electrostatic waves in radio zebra models

Zebras were observed not only in the solar radio emission but also in radio emissions of Jupiter and Crab Nebula pulsar. In their models, growth rates of the electrostatic waves play an important role. Considering the plasma composed from the thermal background plasma and hot and rare component with the Dory-Guest-Harris distribution, we compute the growth rates $γ$ and dispersion branches of the electrostatic waves in the $ω- k_\perp$ domain. We show complexity of the electrostatic wave branches in the upper-hybrid band. In order to compare the results, which we obtained using the kinetic theory and Particle-in-cell (PIC) simulations, we define and compute the integrated growth rate $Γ$, where the "characteristic width" of dispersion branches was considered. We found a very good agreement between the integrated growth rates and those from PIC simulations. For maximal and minimal $Γ$ we showed locations of dispersion branches in the $ω- k_\perp$ domain. We found that $Γ$ has a maximum when the dispersion branches not only cross the region with high growth rates $γ$, but when the dispersion branches in this region are sufficiently long and wide. We also mentioned the effects of changes in the background plasma and hot component temperatures.

physics.plasm-ph↗

arXiv2019

Growth Rates of the Upper-hybrid Waves for Power-law and Kappa Distributions with a Loss-cone Anisotropy

Fine structures of radio bursts play an important role in diagnostics of the solar flare plasma. Among them the zebras, which are prevalently assumed to be generated by the double plasma resonance instability, belong to the most important. In this paper we compute the growth rate of this instability for two types of the electron distribution: a) for the power-law distribution and b) for the kappa distribution, in the both cases with the loss-cone type anisotropy. We found that the growth rate of the upper-hybrid waves for the power-law momentum distribution strongly depends on the pitch-angle boundary. The maximum growth rate was found for the pitch-angle $θ_\mathrm{c} \approx$ 50$^\circ$. For small angles the growth rate profile is very flat and for high pitch-angles the wave absorption occurs. Furthermore, analyzing the growth rate of the upper hybrid waves for the kappa momentum distribution we found that a decrease of the characteristic momentum $p_κ$ shifts the maximum of the growth rate to lower values of the ratio of the electron-plasma and electron-cyclotron frequencies, and the frequency widths of the growth rate peaks are very broad. But, if we consider the kappa distribution which is isotropic up to some large momentum $p_m$ and anisotropic with loss-cone above this momentum then distinct peaks of the growth rate appear and thus distinct zebra stripes can be generated. It means that the restriction for small momenta for the anisotropic part of distributions is of principal importance for the zebra stripes generation. Finally, for the 1 August 2010 zebra stripes, the growth rates in dependence on radio frequency were computed. It was shown that in this case the growth rate peaks are more distinct than in usually presented dependencies of growth rates on the ratio of the plasma and cyclotron frequencies.

astro-ph.SR↗

arXiv2017

Double plasma resonance instability as a source of solar zebra emission

The double plasma resonance (DPR) instability plays a basic role in the generation of solar radio zebras. In the plasma, consisting of the loss-cone type distribution of hot electrons and much denser and colder background plasma, this instability generates the upper-hybrid waves, which are then transformed into the electromagnetic waves and observed as radio zebras. In the present paper we numerically study the double plasma resonance instability from the point of view of the zebra interpretation. We use a 3-dimensional electromagnetic particle-in-cell (3-D PIC) relativistic model. First using the multi-mode model, we study details of the double plasma resonance instability. We show how the distribution function of hot electrons changes during this instability. Then we show that there is a very good agreement between results obtained by the multi-mode and specific-mode models, which is caused by a dominance of the wave with the maximal growth rate. Therefore, for computations in a broad range of model parameters, we use the specific-mode model. We compute the maximal growth rates of the double plasma resonance instability. The results are compared with the analytical ones. We find a very good agreement between numerical and analytical growth rates. We also compute saturation energies of the upper-hybrid waves in a very broad range of parameters. We find that the saturation energies of the upper-hybrid waves show maxima and minima at almost the same values of $ω_\mathrm{UH}/ω_\mathrm{ce}$ as the growth rates. Furthermore, we find that the saturation energy of the upper-hybrid waves is proportional to the density of hot electrons. The maximum saturated energy can be up to one percent of the kinetic energy of hot electrons. All these findings can be used in the interpretation of solar radio zebras.

physics.plasm-ph↗

arXiv2017

Brightness Temperature of Radio Zebras and Wave Energy Densities in Their Sources

We estimated the brightness temperature of radio zebras (zebra pattern -- ZP), considering that ZPs are generated in loops having an exponential density profile in their cross-section. We took into account that when in plasma there is a source emitting in all directions, then in the escape process from the plasma the emission obtains a directional character nearly perpendicular to the constant-density profile. Owing to the high directivity of the plasma emission the region from which the emission escapes can be very small. We estimated the brightness temperature of three observed ZPs for two values of the density scale height (1 and 0.21 Mm) and two values of the loop width (1 and 2 arcsec). In all cases high brightness temperatures were obtained. For the higher value of the density scale height, the brightness temperature was estimated as 1.1 $\times$ 10$^{15}$ - 1.3 $\times$ 10$^{17}$ K, and for the lower value as 4.7 $\times$ 10$^{13}$ - 5.6 $\times$ 10$^{15}$ K. We also computed the saturation energy density of the upper-hybrid waves (which according to the double plasma resonance model are generated in the zebra source) using a 3D particle-in-cell model with the loss-cone type of distribution of hot electrons. We found that this saturated energy is proportional to the ratio of hot electron and background plasma densities. Thus, comparing the growth rate and collisional damping of the upper-hybrid waves, we estimated minimal densities of hot electrons as well as the minimal value of the saturation energy density of the upper-hybrid waves. Finally, we compared the computed energy density of the upper-hybrid waves with the energy density of the electromagnetic waves in the zebra source and thus estimated the efficiency of the wave transformation.

astro-ph.SR↗

arXiv2017

Temperature dependent growth rates of the upper-hybrid waves and solar radio zebra patterns

Context. The zebra patterns observed in solar radio emission are very important for are plasma diagnostics. The most promising model of these patterns is based on double plasma resonance instability, which generates upper-hybrid waves, which can be then transformed into the zebra emission. Aims. We aim to study in detail the double plasma resonance instability of hot electrons, together with a much denser thermal background plasma. In particular, we analyse how the growth rate of the instability depends on the temperature of both the hot plasma and background plasma components. Methods. We numerically integrated the analysed model equations, using Python and Wolfram Mathematica. Results. We found that the growth-rate maxima of the upper-hybrid waves for non-zero temperatures of both the hot and background plasma are shifted towards lower frequencies comparing to the zero temperature case. This shift increases with an increase of the harmonic number s of the electron cyclotron frequency and temperatures of both hot and background plasma components. We show how this shift changes values of the magnetic eld strength estimated from observed zebras. We confirmed that for a relatively low hot electron temperature, the dependence of growth rate vs. both the ratio of the electron plasma and electron cyclotron frequencies expresse distinct peaks, and by increasing this temperature these peaks become smoothed. We found that in some cases, the values of wave number vector components for the upper-hybrid wave for the maximal growth rate strongly deviate from their analytical estimations. We confirmed the validity of the assumptions used when deriving model equations.

astro-ph.SR↗

arXiv2014

Solar Radio Bursts with Spectral Fine Structures in Preflares

A good observation of preflare activities is important for us to understand the origin and triggering mechanism of solar flares, and to predict the occurrence of solar flares. This work presents the characteristics of microwave spectral fine structures as preflare activities of four solar flares observed by Ondřejov radio spectrograph in the frequency range of 0.8--2.0 GHz. We found that these microwave bursts which occurred 1--4 minutes before the onset of flares have spectral fine structures with relatively weak intensities and very short timescales. They include microwave quasi-periodic pulsations (QPP) with very short period of 0.1-0.3 s and dot bursts with millisecond timescales and narrow frequency bandwidths. Accompanying these microwave bursts, there are filament motions, plasma ejection or loop brightening on the EUV imaging observations and non-thermal hard X-ray emission enhancements observed by RHESSI. These facts may reveal certain independent non-thermal energy releasing processes and particle acceleration before the onset of solar flares. They may be conducive to understand the nature of solar flares and predict their occurrence.

astro-ph.SR↗

arXiv2014

Slipping magnetic reconnection during an X-class solar flare observed by SDO/AIA

We present SDO/AIA observations of an eruptive X-class flare of July 12, 2012, and compare its evolution with the predictions of a 3D numerical simulation. We focus on the dynamics of flare loops that are seen to undergo slipping reconnection during the flare. In the AIA 131A observations, lower parts of 10 MK flare loops exhibit an apparent motion with velocities of several tens of km/s along the developing flare ribbons. In the early stages of the flare, flare ribbons consist of compact, localized bright transition-region emission from the footpoints of the flare loops. A DEM analysis shows that the flare loops have temperatures up to the formation of Fe XXIV. A series of very long, S-shaped loops erupt, leading to a CME observed by STEREO. The observed dynamics are compared with the evolution of magnetic structures in the "standard solar flare model in 3D". This model matches the observations well, reproducing both the apparently slipping flare loops, S-shaped erupting loops, and the evolution of flare ribbons. All of these processes are explained via 3D reconnection mechanisms resulting from the expansion of a torus-unstable flux rope. The AIA observations and the numerical model are complemented by radio observations showing a noise storm in the metric range. Dm-drifting pulsation structures occurring during the eruption indicate plasmoid ejection and enhancement of reconnection rate. The bursty nature of radio emission shows that the slipping reconnection is still intermittent, although it is observed to persist for more than an hour.

astro-ph.SR↗

arXiv2011

Spontaneous current-layer fragmentation and cascading reconnection in solar flares: I. Model and analysis

Magnetic reconnection is commonly considered as a mechanism of solar (eruptive) flares. A deeper study of this scenario reveals, however, a number of open issues. Among them is the fundamental question, how the magnetic energy is transferred from large, accumulation scales to plasma scales where its actual dissipation takes place. In order to investigate this transfer over a broad range of scales we address this question by means of a high-resolution MHD simulation. The simulation results indicate that the magnetic-energy transfer to small scales is realized via a cascade of consecutive smaller and smaller flux-ropes (plasmoids), in analogy with the vortex-tube cascade in (incompressible) fluid dynamics. Both tearing and (driven) "fragmenting coalescence" processes are equally important for the consecutive fragmentation of the magnetic field (and associated current density) to smaller elements. At the later stages a dynamic balance between tearing and coalescence processes reveals a steady (power-law) scaling typical for cascading processes. It is shown that cascading reconnection also addresses other open issues in solar flare research such as the duality between the regular large-scale picture of (eruptive) flares and the observed signatures of fragmented (chaotic) energy release, as well as the huge number of accelerated particles. Indeed, spontaneous current-layer fragmentation and formation of multiple channelised dissipative/acceleration regions embedded in the current layer appears to be intrinsic to the cascading process. The multiple small-scale current sheets may also facilitate the acceleration of a large number of particles. The structure, distribution and dynamics of the embedded potential acceleration regions in a current layer fragmented by cascading reconnection are studied and discussed.

astro-ph.SR↗

arXiv2011

Spontaneous current-layer fragmentation and cascading reconnection in solar flares: II. Relation to observations

In the paper by Bárta et al. (arXive:astro-ph:/1011.4035, 2010) the authors addressed some open questions of the CSHKP scenario of solar flares by means of high-resolution MHD simulations. They focused, in particular, on the problem of energy transfer from large to small scales in decaying flare current sheet (CS). Their calculations suggest, that magnetic flux-ropes (plasmoids) are formed in full range of scales by a cascade of tearing and coalescence processes. Consequently, the initially thick current layer becomes highly fragmented. Thus, the tearing and coalescence cascade can cause an effective energy transfer across the scales. In the current paper we investigate whether this mechanism actually applies in solar flares. We extend the MHD simulation by deriving model-specific features that can be looked for in observations. The results of the underlying MHD model showed that the plasmoid cascade creates a specific hierarchical distribution of non-ideal/acceleration regions embedded in the CS. We therefore focus on the features associated with the fluxes of energetic particles, in particular on the structure and dynamics of emission regions in flare ribbons. We assume that the structure and dynamics of diffusion regions embedded in the CS imprint themselves into structure and dynamics of flare-ribbon kernels by means of magnetic-field mapping. Using the results of the underlying MHD simulation we derive the expected structure of ribbon emission and we extract selected statistical properties of the modelled bright kernels. Comparing the predicted emission and its properties with the observed ones we obtain a good agreement of the two.

astro-ph.SR↗