M. Karlický

The Magnetic Rope Structure and Associated Energetic Processes in the 2000 July 14 Solar Flare

In the reconstructed nonlinear force-free magnetic field of NOAA Active Region 9077 before the X5.7/3B (10:24 UT) flare on 2000 July 14, we reveal for the first time the presence of a magnetic rope from the extrapolation of the three-dimensional magnetic field structure. This magnetic rope is located in a space above the magnetic neutral lines of the filament. The calculated field lines of the rope rotate around its axis for more than three turns. Overlying the rope are multilayer magnetic arcades with different orientations. These arcades are in agreement with the Transition Region and Coronal Explorer observations. The estimated free magnetic energy in this rope system is about 1.6 × 1032 ergs. Such magnetic field structure provides a favorable model for the interpretation of the energetic flare processes as revealed by Hα, EUV, and radio observations. In particular, the intermittent cospatial brightening of the rope in EUV 1600 A image leading to the onset of the flare suggests that the rope instability may have triggered the flare event, and the drifting pulsation structure in the decimetric frequency range is considered to manifest the initial phase of the coronal mass ejection.

Multi-wavelength study of coronal waves associated with the CME-flare event of 3 November 2003

The large flare/CME event that occurred close to the west solar limb on 3 November 2003 launched a large-amplitude large-scale coronal wave that was observed in H α and Fe xii 195 A spectral lines, as well as in the soft X-ray and radio wavelength ranges. The wave also excited a complex decimeter-to-hectometer type II radio burst, revealing the formation of coronal shock(s). The back-extrapolation of the motion of coronal wave signatures and the type II burst sources distinctly marks the impulsive phase of the flare (the hard X-ray peak, drifting microwave burst, and the highest type III burst activity), favoring a flare-ignited wave scenario. On the other hand, comparison of the kinematics of the CME expansion with the propagation of the optical wave signatures and type II burst sources shows a severe discrepancy in the CME-driven scenario. However, the CME is quite likely associated with the formation of an upper-coronal shock revealed by the decameter-hectometer type II burst. Finally, some six minutes after the launch of the first coronal wave, another coronal disturbance was launched, exciting an independent (weak) decimeter-meter range type II burst. The back-extrapolation of this radio emission marks the revival of the hard X-ray burst, and since there was no CME counterpart, it was clearly ignited by the new energy release in the flare.

On the Solar Origin of Complex Type III-like Radio Bursts Observed at and below 1 MHz

By simultaneously analyzing decimetric, metric, and hectometric radio observations, we provide evidence that a class of hectometric type III-like radio events are associated with electrons accelerated during the primary flare energy release process. We do this by demonstrating that there is a good temporal correspondence between the hectometric and decimetric radio emissions that are believed to involve the acceleration of electrons in deep layers of the solar atmosphere at heights estimated to be from 10,000 to 30,000 km (0.014 to 0.043 R☉). This class of hectometric type III-like events may have simple or complex intensity-time profiles. When they have complex profiles of long duration, the decimeter emissions also have complex long-duration profiles. In this latter case they are also often associated with metric type II radio bursts and coronal mass ejections (CMEs). However, we argue on the basis of the observations that the hectometric radio emissions are not necessarily associated with the metric phenomena. Specifically, they are not likely generated by a secondary acceleration process associated with the shock generating a metric type II burst or with a shock driven ahead of the CME despite the fact that the hectometric emission time profiles would (erroneously) be morphologically classified as shock-accelerated (SA) events.

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.

SPONTANEOUS CURRENT-LAYER FRAGMENTATION AND CASCADING RECONNECTION IN SOLAR FLARES. II. RELATION TO OBSERVATIONS

In a paper by Barta et al., the authors addressed by means of high-resolution MHD simulations some open questions on the CSHKP scenario of solar flares. In particular, they focused on the problem of energy transfer from large to small scales in the decaying flare current sheet (CS). Their calculations suggest that magnetic flux ropes (plasmoids) are formed in a 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 this paper, we investigate whether this mechanism actually applies in solar flares. We extend the MHD simulation by deriving model-specific features that can be searched for in observations. The results of the underlying MHD model show 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 the 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 extract selected statistical properties of the modeled bright kernels. Comparing the predicted emission and its properties with the observed ones, we obtain a good agreement between the two.

Successive solar flares and coronal mass ejections on 2005 September 13 from NOAA AR 10808

We present a multiwavelength study of the 2005 September 13 eruption from NOAA AR 10808 that produced total four flares and two fast coronal mass ejections (CMEs) within ~1.5 hr. Our primary attention is paid to the fact that these eruptions occurred in close succession in time, and that all of them were located along an S-shaped magnetic polarity inversion line (PIL) of the active region. In our analysis, (1) the disturbance created by the first flare propagated southward along the PIL to cause a major filament eruption that led to the first CME and the associated second flare underneath. (2) The first CME partially removed the overlying magnetic fields over the northern ? spot to allow the third flare and the second CME. (3) The ribbon separation during the fourth flare would indicate reclosing of the overlying field lines opened by the second CME. It is thus concluded that these series of flares and CMEs are interrelated to each other via magnetic reconnections between the expanding magnetic structure and the nearby magnetic fields. These results complement previous works made on this event with the suggested causal relationship among the successive eruptions.

A shock associated (SA) radio event and related phenomena observed from the base of the solar corona to 1 AU

We present for the rst time an almost com- plete frequency coverage of a Shock Associated (SA) radio event and related phenomena observed on May 6, 1996 at 9:27 UT. It is observed from the base of the solar corona up to almost 1 Astronomical Unit (AU) from the Sun by the following radio astronomical instruments: the Ond rejov spectrometer operating between 4.5 GHz and 1 GHz (radi- ation produced near the chromosphere); the Thermopyles Artemis-IV spectrograph operating between 600 MHz and 110 MHz (distance range about 1.1-1.4R from sun center); the Nan cay Decameter Array operating between 75 and 25 MHz (distance range about 1.4-2 R); and the RAD2 and RAD1 radio receivers on the WIND spacecraft covering the range from 14 MHz to about 20 kHz (distance range be- tween 3 R and about 1 AU). Observations at the Nan cay Decameter Array clearly show that the SA event starts from a coronal type II radio burst which traces the progression of a shock wave through the corona above 1.8 R-2 R from the sun center. This SA event has no associated radio emis- sion in the decimetric-metric range, thus there is no evidence for electron injection in the low/middle corona. The SA event enigma: What does SA stand for? Type II and type III solar radio bursts result from the interaction of a disturbing agent {a beam of energetic elec- trons or a shock wave{ with the ambient plasma (Wild and Smerd, 1972). Radiation is produced near the fundamen- tal of the local plasma frequency f p (kHz) =9 n 1 = 2 e (cm 3 ) and/or its second harmonic through various plasma mech- anisms (see e.g. Robinson, 1997). The observed frequency can be converted into an altitude in the corona, assuming a density model and the radiated mode. Dierent frequency drifts reflect dierent velocities along the density gradient in the corona and interplanetary medium, helping us to charac- terize the nature of the exciter: 0.05-0.3c electron beam for

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.

Response of optical hydrogen lines to beam heating - I. Electron beams

Context. Observations of hydrogen Balmer lines in solar flares remain an important source of information on flare processes in the chromosphere during the impulsive phase of flares. The intensity profiles of optically thick hydrogen lines are determined by the temperature, density, and ionisation structure of the flaring atmosphere, by the plasma velocities and by the velocity distribution of particles in the line formation regions. Aims. We investigate the role of non-thermal electrons in the formation regions of H α , H β , and H γ lines in order to unfold their influence on the formation of these lines. We concentrate on pulse-beam heating varying on a subsecond timescale. Furthermore, we theoretically explore possibility that a new diagnostic tool exists indicating the presence of non-thermal electrons in the flaring chromosphere based on observations of optical hydrogen lines. Methods. To model the evolution of the flaring atmosphere and the time-dependent hydrogen excitation and ionisation, we used a 1-D radiative hydrodynamic code combined with a test-particle code that simulates the propagation, scattering, and thermalisation of a power-law electron beam in order to obtain the flare heating and the non-thermal collisional rates due to the interaction of the beam with the hydrogen atoms. To not bias the results by other effects, we calculate only short time evolutions of the flaring atmosphere and neglect the plasma velocities in the radiative transfer. Results. All calculated models have shown a time-correlated response of the modelled Balmer line intensities on a subsecond timescale, with a subsecond timelag behind the beam flux. Depending on the beam parameters, both line centres and wings can show pronounced intensity variations. The non-thermal collisional rates generally result in an increased emission from a secondary region formed in the chromosphere. Conclusions. Despite the clear influence of the non-thermal electron beams on the Balmer line intensity profiles, we were not able on the basis of our simulations to produce any unambiguous diagnostic of non-thermal electrons in the line-emitting region, which would be based on comparison of individual Balmer line intensity profiles. However, fast line intensity variations, well-correlated with the beam flux variations, represent an indirect indication of pulsating beams.