Miroslav Barta

Dynamics of plasmoids formed by the current sheet tearing

Context. Moving blob-like features observed in the soft X-ray and EUV range above flare-loops are often interpreted as signatures of plasmoids formed by the current sheet tearing in the flare-associated reconnection process. Aims. We investigate the evolution of the flare-associated current sheet numerically in order to analyse the kinematics and dynamics of plasmoids. The goal is to explain the broad diversity of kinematical properties of the plasmoid signatures recorded by various observational techniques. Methods. We performed a 2-dimensional resistive-MHD numerical simulation of the reconnection starting from the Harris-type current sheet. After identifying the plasmoids, we followed their motion to determine basic kinematical parameters (velocity and acceleration), and we analysed the associated magnetic field topology. Results. The simulation reveals a broad variety of the kinematical/dynamical properties of plasmoids – after formation, a plasmoid can move upward, downward, or can even change its direction of propagation. The highest velocities, in the range of the ambient Alfven speed, are found in the case of upward propagating plasmoids. The acceleration is determined by the net magnetic field tension of the reconnected field lines. Downwardly propagating plasmoids achieve only a fraction of the ambient Alfven speed. They strongly decelerate during the coalescence with low-lying flare-loops, when distinct energy-release peaks occur and loop system oscillations are excited. Conclusions. The presented results explain, qualitatively and quantitatively, the broad spectrum of kinematical properties of various observational features attributed to the current-sheet plasmoids.

Radio bursts with rapid frequency variations-Lace bursts

The Ondřejov radiospectrograph operating in the 0.8-2.0 GHz frequency range recorded in recent years (1998-2000), three (August 10, 1998; August 17, 1999; June 27, 2000) unique bursts with rapid frequency variations (lace bursts) lasting for several minutes. On August 17, 1999, the same burst was recorded simultaneously by the Brazilian Solar Spectroscope in the 1.0-2.5 GHz frequency range. The frequency variations of these bursts in four time intervals were analyzed by the Fourier method and power-law spectra with power-law indices close to -2 were found. The Fourier spectra show the presence of frequency variations in the 0.01-3.0 Hz interval which indicate fast changes of plasma parameters in the radio source. Due to the similarities in the line features of these bursts with zebra pattern lines, a model similar to that of the zebra pattern was suggested. The model radio spectra, computed using this model with a turbulent state of the solar flare atmosphere, are similar to those observed by the radiospectrographs.

Interference patterns in solar radio spectra : high-resolution structural analysis of the corona

Aims. We present a new method for high-resolution structural analysis of the solar corona. Methods. The relationship between the spectral features of various types of solar radio bursts and the physical properties of their sources have been extensively studied by many authors. On the other hand, it is plausible to accept that the spectral properties of the solar radio radiation received on the Earth are – besides the physics of the radio source – influenced by an inter-laying medium that radio waves propagate through. In particular, the regular structures in the solar corona – such as coronal waves, oscillations in shock fronts, the fine structures of coronal loops, streamer current sheets, etc. – might efficiently filter transferred radio radiation just as (broad-band) X-rays are filtered by a periodic atomic structure of crystals; the difference is only in the spatial scale. Using the wave optics methods, we investigate the prospective influence of considered coronal structures on the propagating radio waves originating in an external remote source. Results. Preliminary results have shown that the resulting modelled radio emission may recall the spectra of observed zebra patterns for the simple 1D density structure considered here and for a reasonable set of parameters. Conversely, it is suggested that the spectra of the zebra patterns might be used for an analysis of those coronal structures that made these traces on the radiation by methods similar to those used in crystallography. The possibility of the presence of such regular small scale structures in the solar corona is demonstrated. For completeness, a brief review of contemporary models of the zebra patterns is provided.

Time scales of the slowly drifting pulsating structure observed during the April 12, 2001 flare

First time scales of high-frequency (500−1500 MHz) slowly drifting pulsating structures observed during the April 12, 2001 flare by the Ondy (800−4500 MHz) and Potsdam (40−800 MHz) radiospectrographs and by the 1420 and 610 MHz Trieste radiopolarimeters (with high time resolution (1 ms)) are studied statistically. The Fourier method reveals pe- riods in the range of 0.9−7.5 s. For shorter periods the power spectra show a power-law form, especially in the interval of about 0.06−0.2 s, where the power-law index is in the 1.3−1.6 range. The results are interpreted using the flare model with plasmoid ejections. For the first time, the multi-scale cascading reconnection process is included in the interpretation. Corresponding time scales are estimated analytically. Further, magnetic reconnection in the bursting regime is simulated in a 2-D MHD model and variations of the dissipation power and radio radiation measure are computed. Fourier spectra of these numerical variations are determined and compared with those obtained from observations.

Fragmentation during merging of plasmoids in the magnetic field reconnection

Context. Application of the magnetic-reconnection theory onto large-scale events, such as solar flares, requires formation of very thin (kinetic-scale) current sheets within the rather thick flare current layer. Hence, some fragmentation/filamentation mechanisms has to be in action. Aims. We aim at identifying fragmentation mechanisms for magnetic field and current density structures. Namely, we focus at detailed study of the processes during the merging of plasmoids that had been formed in the current layer. Methods. A 2.5-D electromagnetic particle-in-cell model is used and its results analysed. Results. It is shown that the merging process of plasmoids is not a simple process as presented in some previous studies. On the contrary, this process leads to a complex fragmentation. We found two types of fragmentation processes: a) fragmentation in the current sheet generated between the merging plasmoids and b) fragmentation at the boundary of plasma outflow from the reconnection between these plasmoids. While the first type of fragmentation is generated by the tearing-mode (plasmoid) instability of the secondary current sheet, the second one looks to be connected with an increase of the plasma β parameter during these processes. Thus, sheared high-β plasma flows produce this additional fragmentation. Conclusions. The fragmentation and energy transport from large to small scales in a large-scale magnetic reconnection seem to be the result of interplay and positive feedback between instabilities driven by high gradients in both magnetic (intense current density) and velocity (high vorticity) fields.

Particle-in-cell simulations of return current in solar flares

Aims. We numerically study a formation of the return current generated in solar flares. Methods. For simulations of the return current in the beam-plasma system, a 3D particle-in-cell electromagnetic code is used. Results. In conditions of solar flares with the electron beam fluxes of EF = 9.1 × 10 9 −4.55 × 10 10 erg s −1 cm −2 , the beam-plasma interaction with the return current is studied. We found that the electron beam relaxes to the plateau distribution function as known from electrostatic simulations. Simultaneously, due to electromagnetic effects and the Buneman instability of the prescribed Maxwellshifted return current, the electron distribution function evolves to a new stationary state with a new form of the return current. In this final state the return current is formed not only by electrons in the bulk of the electron distribution function, but also by electrons in the extended tail. We use the results of simulations to estimate the critical beam fluxes for the processes under study in the low corona, the transition region and the upper chromosphere.

Energy Cascades in Large-Scale Solar Flare Reconnection

Very fast energy release is sometimes observed in large-scale current layers formed in astrophysical plasmas. Solar flares represent a clear example. The dissipation and changes of the magnetic field topology in the almost collision-less plasmas are inherently related to plasma-kinetic processes in very thin current sheets (CSs). Question arises, how the originally thick current layer is efficiently fragmented into these small-scale dissipative CSs. We investigated this question by means of high-resolution MHD simulations. In addition to the earlier considered chain plasmoid instability we identified other elementary process for energy transport from large to small scales – fragmenting coalescence of plasmoids. This result changes so far basically 1D picture of energy cascade in a large scale magnetic reconnection and reveals multi-dimensional nature of the fragmentation process. At the same moment it shows that plasmoid coalescence contributes – quite surprisingly – to the direct energy cascade.

Plasmoids in Solar Flares and Their Radio and X-ray Signatures

This review summarizes our recent results connected with the theoretical and observational aspects of plasmoids in solar flares. We show that the plasmoids play a very important role in the primary flare process – in the magnetic field reconnection. It is shown how the plasmoids are formed, how they move and interact, and how the flare current sheet is fragmented due to these processes. Furthermore, we present a successive merging of the plasmoids, which not only very efficiently accelerate particles, but also it can produce large plasmoids which are sometimes observed in the X-ray emission. Considering the plasmoids the radio drifting pulsating structures (DPSs), narrowband dm-spikes and the above-the-loop-top hard X-ray sources are interpreted. Some interesting radio spectra, relevant to this topic, are added.

ALMA and solar research

The ALMA (Atacama Large Millimeter/sub-millimeter Array) is the large interferometer that will consist up to 64 high-precision antennas operating in the 31.3 – 950 GHz frequency range. In this range unique observations in cosmology, cold universe, galaxies, stars and their formations, and so on are expected. Among these objectives there is a unique possibility to observe the Sun and to address outstanding issues of solar physics. The ALMA is shortly described and then the new ESO-ALMA European node (ARC) built at Ondřejov Observatory is presented. The new ARC is the only one in Europe oriented to solar physics. The requirements and limitations for ALMA solar observations, as well as some examples of possible solar-oriented ALMA projects, are mentioned.

Radio and X-ray diagnostics of electrons accelerated in solar flares

Starting from 2.5D MHD modelling of solar flares on a global scale we calculate (using the PIC and test-particle simulations) the radio and X-ray emissions generated in solar flare reconnection. Our results – the radio and X-ray spectra and brightness distributions, and their dynamics – are directly comparable with observations providing thus a test of particle acceleration models as well as of the ‘standard’ global flare scenario.