H. Mészárosová

Global statistics of 0.8-2.0 GHz radio bursts and fine structures observed during 1992-2000 by the Ondřejov radiospectrograph

681 solar radio events observed by the Ondřejov 0.8-2.0 GHz radiospectrograph during 1992-2000 are analyzed and corresponding bursts and fine structures classified into ten different classes. A new rare type of fine structure with rapid frequency variation we called lace pattern was included. Drifting pulsation structures, observed usually at the beginning of the impulsive flare phase, were recognized among pulsations. Furthermore, a new sub-class of zebra patterns with many zebra lines (~30) superimposed on fibers was identified. For all defined types of burst and fine structures basic characteristics of their parameters are presented. Distributions of various types of burst and fine structures in the years 1992-2000 in dependence on the changes of solar activity during the cycles 22 and 23, occurrences of studied types of burst in association with GOES class flares as well as their relationship to GOES flare maxima are shown. Finally, the association of the analyzed bursts with the metric type III bursts observed at Potsdam-Tremsdorf Observatory was studied.

High-frequency slowly drifting structures in solar flares

Radio emission of four solar flares with high-frequency slowly drifting structures is presented. Three sub-classes of these structures were recognized. It is shown that the April 15, 2001 X14.4 flare started with the slowly drifting structure associated with a plasmoid ejection observed by TRACE in the 171 A line. The August 18, 1998 event presents an example of the drifting pulsation structure (DPS) which is well limited in frequency extent at both sides. A further example of the DPS, but followed by clouds of the narrowband dm-spikes, was observed during the November 23, 2001 flare. Finally, in the case of the April 12, 2001 flare, the drifting pulsation-continuum structure was recorded at the same time as the metric type II radio burst, i.e. in different frequency ranges. The slowly drifting structures were analyzed and in two cases their relation to hard X-ray emission was studied. Possible underlying physical processes are discussed assuming the plasmoid ejection model of eruptive solar flares.

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.

Radio fiber bursts and fast magnetoacoustic wave trains

Aims. We present a model for dm-fiber bursts that is based on assuming fast sausage magnetoacoustic wave trains that propagate along a dense vertical filament or current sheet. Methods. Eight groups of dm-fiber bursts that were observed during solar flares were selected and analyzed by the wavelet analysis method. To model these fiber bursts we built a semi-empirical model. We also did magnetohydrodynamic simulations of a propagation of the magnetoacoustic wave train in a vertical and gravitationally stratified current sheet. Results. In the wavelet spectra of the fiber bursts computed at different radio frequencies we found the wavelet tadpoles, whose head maxima have the same frequency drift as the drift of fiber bursts. It indicates that the drift of these fiber bursts can be explained by the propagating fast sausage magnetoacoustic wave train. Using new semi-empirical and magnetohydrodynamic models with a simple radio emission model we generated the artificial radio spectra of the fiber bursts, which are similar to the observed ones.

Tadpoles in Wavelet Spectra of a Solar Decimetric Radio Burst

In the solar decimetric type IV radio event observed on 2001 June 13, we have found wavelet tadpole patterns for the first time. They were detected simultaneously at all radio frequencies in the 1.1-4.5 GHz frequency range. The characteristic period of the wavelet tadpole patterns was found to be 70.9 s. The parameters of the tadpoles on different frequencies are very similar and the correlations between individual radio fluxes are high. These tadpoles are interpreted as a signature of the magnetoacoustic wave train moving along the flare loop through the radio source and modulating its gyrosynchrotron emission.

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.

Drifting radio bursts and fine structures in the 0.8-7.6 GHz frequency range observed in the NOAA 9077 AR (July 10-14, 2000) solar flares

The 0.8{7.6 GHz global and detailed radio spectra of the four most intense flares observed in the NOAA 9077 active region (July 10{14, 2000) are presented. The radio bursts of these flares and their sequence reveal features indicative of topological similarities among the flares under study. The drifting pulsation structures were found to be the typical signatures of these flares. Furthermore, many other ne structures such as narrowband drifting lines, drifting harmonic structure with zebra patterns, drifting branches of narrowband dm-spikes, and structures with fast positively and negatively drifting bursts are shown in the context of the whole radio flares. Some of them were observed for the rst time. The relationships among them and the resulting interpretations are summarized. The characteristic periods of the drifting pulsation structures and the magnetic eld in the zebra radio source are determined.

Magnetoacoustic waves propagating along a dense slab and Harris current sheet and their wavelet spectra

Currently, there is a common endeavor to detect magnetoacoustic waves in solar flares. This paper contributes to this topic using an approach of numerical simulations. We studied a spatial and temporal evolution of impulsively generated fast and slow magnetoacoustic waves propagating along the dense slab and Harris current sheet using two-dimensional magnetohydrodynamic numerical models. Wave signals computed in numerical models were used for computations of the temporal and spatial wavelet spectra for their possible comparison with those obtained from observations. It is shown that these wavelet spectra allow us to estimate basic parameters of waveguides and perturbations. It was found that the wavelet spectra of waves in the dense slab and current sheet differ in additional wavelet components that appear in association with the main tadpole structure. These additional components are new details in the wavelet spectrum of the signal. While in the dense slab this additional component is always delayed after the tadpole head, in the current sheet this component always precedes the tadpole head. It could help distinguish a type of the waveguide in observed data. We present a technique based on wavelets that separates wave structures according to their spatial scales. This technique shows not only how to separate the magnetoacoustic waves and waveguide structure in observed data, where the waveguide structure is not known, but also how propagating magnetoacoustic waves would appear in observations with limited spatial resolutions. The possibilities detecting these waves in observed data are mentioned.

Magnetoacoustic waves in the narrowband dm-spikes sources

Aims. A new type of analysis of the narrowband dm-spikes in solar radio radiation is introduced to look for magnetoacoustic waves in their sources. Methods. The Fourier and wavelet methods were used. For the first time, the tadpole structures in the wavelet spectra of this radio emission were searched for. Results. Fifteen groups of the narrowband dm-spikes, observed during solar flares, were selected and analyzed by the Fourier and wavelet analysis methods. We found that the mean Fourier spectra of these spikes in frequency space are the powerlaws with a powerlaw index in the range −1. 2– −1.8. Furthermore, their wavelet spectra based on time series reveal tadpoles at some frequencies, which indicates the presence of magnetoacoustic waves. These waves are interpreted as propagating through a source of the narrowband dm-spikes. It is proposed that the spikes are generated by driven coalescence and fragmentation processes in turbulent reconnection outflow. This interpretation is supported by a simultaneous observation of drifting pulsating structures (DPSs) and spikes. Finally, modeling of the magnetoacoustic waves and tadpoles in the Harris current sheet supports this interpretation.

STATISTICS AND CLASSIFICATION OF THE MICROWAVE ZEBRA PATTERNS ASSOCIATED WITH SOLAR FLARES

The microwave zebra pattern (ZP) is the most interesting, intriguing, and complex spectral structure frequently observed in solar flares. A comprehensive statistical study will certainly help us to understand the formation mechanism, which is not exactly clear now. This work presents a comprehensive statistical analysis of a big sample with 202 ZP events collected from observations at the Chinese Solar Broadband Radio Spectrometer at Huairou and the Ondŕejov Radiospectrograph in the Czech Republic at frequencies of 1.00-7.60 GHz from 2000 to 2013. After investigating the parameter properties of ZPs, such as the occurrence in flare phase, frequency range, polarization degree, duration, etc., we find that the variation of zebra stripe frequency separation with respect to frequency is the best indicator for a physical classification of ZPs. Microwave ZPs can be classified into three types: equidistant ZPs, variable-distant ZPs, and growing-distant ZPs, possibly corresponding to mechanisms of the Bernstein wave model, whistler wave model, and double plasma resonance model, respectively. This statistical classification may help us to clarify the controversies between the existing various theoretical models and understand the physical processes in the source regions.