Drifting Pulsation Structures Research Articles

Oscillations and Waves in Radio Source of Drifting Pulsation Structures

Drifting pulsation structures (DPSs) are considered to be radio signatures of the plasmoids formed during magnetic reconnection in the impulsive phase of solar flares. In the present paper we analyze oscillations and waves in seven examples of drifting pulsation structures, observed by the 800-2000 MHz Ondrejov radiospectrograph. For their analysis we use a new type of oscillation maps which give us a much more information about processes in DPSs than that in previous analyzes. Based on these oscillation maps, made from radio spectra by the wavelet technique, we recognized quasi-periodic oscillations with periods ranging from about 1 to 108 s in all studied DPSs. This strongly supports the idea that DPSs are generated during a fragmented magnetic reconnection. Phases of most the oscillations in DPSs, especially for the period around 1 s, are synchronized ("infinite" frequency drift) in the whole frequency range of DPSs. For longer periods in some DPSs we found that the phases of the oscillations drift with the frequency drift in the interval from -17 to +287 MHz\s. We propose that these drifting phases can be caused (a) by the fast or slow magnetosonic waves generated during the magnetic reconnection and propagating through the plasmoid, (b) by a quasi-periodic structure in the plasma inflowing to the reconnection forming a plasmoid, and (c) by a quasi-periodically varying reconnection rate in the X-point of the reconnection close to the plasmoid.

Chains of type-I radio bursts and drifting pulsation structures

Aims. Owing to similarities of chains of type-I radio bursts and drifting pulsation structures the question arises as to whether both these radio bursts are generated by similar processes.Methods. Characteristics and parameters of both these radio bursts are compared.Results. We present examples of both types of bursts and show their similarities and differences. Then, for chains of type-I bursts, a similar model as for drifting pulsation structures (DPSs) is proposed. We show that, similar to the DPS model, the chains of type-I bursts can be generated by the fragmented magnetic reconnection associated with plasmoid interactions. To support this new model of chains of type-I bursts, we present an effect of merging two plasmoids to form one larger plasmoid on the radio spectrum of DPS. This process can also explain the ‘wavy’ appearance of some chains of type-I bursts. Further, we show that the chains of type-I bursts with the wavy appearance can be used for estimation of the magnetic field strength in their sources. We think that differences of chains of type-I bursts and DPSs are mainly owing to different regimes of the magnetic field reconnection. While in the case of chains of type-I bursts, the magnetic reconnection and plasmoid interactions are in the quasi-separatrix layer of the active region in more or less quasi-saturated regime, in the case of DPSs, observed in the impulsive phase of eruptive flares, the magnetic reconnection and plasmoid interactions are in the current sheet formed under the flare magnetic rope, which moves upwards and forces this magnetic reconnection.

PARTICLE ACCELERATION IN PLASMOID EJECTIONS DERIVED FROM RADIO DRIFTING PULSATING STRUCTURES

We report observations of slowly drifting pulsating structures (DPS) in the 0.8-4.5 GHz frequency range of the RT4 and RT5 radio spectrographs at Ondrejov observatory, between 2002 and 2012. We found 106 events of drifting pulsating structures, which we classified into 4 cases: (I) single events with a constant frequency drift [12 events], (II) multiple events occurring in the same flare with constant frequency drifts [11 events], (III) single or multiple events with increasing or decreasing frequency drift rates [52 events], and (IV) complex events containing multiple events occurring at the same time in the different frequency range [31 events]. Many DPSs are associated with hard X-ray bursts (15-25 keV) and soft X-ray gradient peaks, as they typically occurred at the beginning of the hard X-ray peaks. This indicates that DPS events are related to the processes of fast energy release and particle acceleration. Furthermore, interpreting DPSs as signatures of plasmoids, we measured their ejection velocity, their width and their height from the DPS spectra, from which we also estimated the reconnection rate and the plasma beta. In this interpretation, constant frequency drift indicates a constant velocity of a plasmoid, and an increasing/decreasing frequency drift indicates a deceleration/acceleration of a plasmoid ejection. The reconnection rate shows a good positive correlation with the plasmoid velocity. Finally we confirmed that some DPS events show plasmoid counterparts in AIA/SDO images.

Solar flares: radio and X-ray signatures of magnetic reconnection processes

This review summarizes new trends in studies of magnetic reconnection in solar flares. It is shown that plasmoids play a very important role in this primary flare process. Using the results of magnetohydrodynamic and particle-in-cell simulations, we describe how the plasmoids are formed, how they move and interact, and how a flare current sheet is fragmented into a cascade of plasmoids. Furthermore, it is shown that during the interactions of these plasmoids electrons are not only very efficiently accelerated and heated, but electromagnetic (radio) emission is also produced. We also describe possible mechanisms for the triggering of magnetic reconnection. The relevant X-ray and radio signatures of these processes (such as radio drifting pulsation structures, narrowband dm-spikes, and the loop-top and above-the-loop-top X-ray sources) are then described. It is shown that plasmoids can also be formed in kinked magnetic ropes. A mapping of X-points of the magnetic reconnection on the chromosphere (as e.g. a splitting of flare ribbons) is mentioned. Supporting EUV and white-light observations of plasmoids are added. The significance of all these processes for the fast magnetic reconnection and electron acceleration is outlined. Their role in fusion experiments is briefly mentioned.

Simultaneous EUV and radio observations of bidirectional plasmoids ejection during magnetic reconnection

We present a multiwavelength study of the X-class flare, which occurred in active region (AR) NOAA 11339 on 3 November 2011. The EUV images recorded by SDO/AIA show the activation of a remote filament (located north of the AR) with footpoint brightenings about 50 min prior to the flare occurrence. The kinked filament rises-up slowly and after reaching a projected height of ~49 Mm, it bends and falls freely near the AR, where the X-class flare was triggered. Dynamic radio spectrum from the Green Bank Solar Radio Burst Spectrometer (GBSRBS) shows simultaneous detection of both positive and negative drifting pulsating structures (DPSs) in the decimetric radio frequencies (500-1200 MHz) during the impulsive phase of the flare. The global negative DPSs in solar flares are generally interpreted as a signature of electron acceleration related to the upward moving plasmoids in the solar corona. The EUV images from AIA 94 \AA reveal the ejection of multiple plasmoids, which move simultaneously upward and downward in the corona during the magnetic reconnection. The estimated speeds of the upward and downward moving plasmoids are ~152-362 and ~83-254 km/s, respectively. These observations strongly support the recent numerical simulations of the formation and interaction of multiple plasmoids due to tearing of the current-sheet structure. On the basis of our analysis, we suggest that the simultaneous detection of both the negative and positive DPSs is most likely generated by the interaction/coalescence of the multiple plasmoids moving upward and downward along the current-sheet structure during the magnetic reconnection process. Moreover, the differential emission measure (DEM) analysis of the active region reveals presence of a hot flux-rope structure (visible in AIA 131 and 94 \AA) prior to the flare initiation and ejection of the multi-temperature plasmoids during the flare impulsive phase.

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 power-law 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.

Separation of drifting pulsating structures in a complex radio spectrum of the 2001 April 11 event

Aims. We present new method of separating a complex radio spectrum into single radio bursts. The method is used in the analysis of the 0.8–2.0 GHz radio spectrum of the 2001 April 11 event, which was rich in drifting pulsating structures. Methods. The method is based on the wavelet analysis technique, which separates different spatial-temporal components (radio bursts) that are difficult to recognize in the original radio spectrum. Results. We show with this method that the complex radio spectrum observed during the 2001 April 11 event consists of at least four drifting pulsating structures (DPSs). These structures were separated with respect to their different frequency drifts. The DPSs indicate at least four plasmoids that are supposed to be formed in a flaring current sheet.

Separation of solar radio bursts in a complex spectrum

AbstractRadio spectra, observed during solar flares, are usually very complex (many bursts and fine structures). We have developed a new method to separate them into individual bursts and analyze them separately. The method is used in the analysis of the 0.8–2.0 GHz radio spectrum of the April 11, 2001 event, which was rich in drifting pulsating structures (DPSs). Using this method we showed that the complex radio spectrum consists of at least four DPSs separated with respect to their different frequency drifts (−115, −36, −23, and −11 MHz s−1). These DPSs indicate a presence of at least four plasmoids expected to be formed in a flaring current sheet. These plasmoids produce the radio emission on close frequencies giving thus a mixture of superimposed DPSs observed in the radio spectrum.

Radio spectra generated during coalescence processes of plasmoids in a flare current sheet

Aims. Motivated by observations of the drifting pulsating structures (DPSs) in solar radio spectra, we study the electromagnetic (radio) emission generated during tearing and coalescence processes in a flare current sheet.Methods. For numerical simulations, we used a 2.5-D particle-in-cell electromagnetic relativistic code. Numerical data were analyzed by the wavelet methods.Results. It is found that the electromagnetic emission is generated during a coalescence of plasmoids, and it has a quasi-periodic character. Detailed analysis reveals that the electromagnetic emission is produced around the interacting plasmoids just before their coalescence into a larger one. The period in variations of electromagnetic emission corresponds to that of magnetic field at the same region. Reflections of the electromagnetic waves between interacting plasmoids are recognized. The computed and observed periodicities are discussed. The similarity of the DPSs with some radio bursts observed during star flares indicates a broader applicability for this model.

Tearing, coalescence and fragmentation processes in solar flare current sheet and drifting pulsating structures

The paper presents a summary of results from two different simulations which study the tearing, coalescence and fragmentation of current sheets, the associated production of energetic electrons and of plasma waves from these electrons which could explain drifting pulsation structures observed at radio wavelengths. Using a 2.5-D particle-in-cell (PIC) model of the current sheet it is shown that due to the tearing mode instability the current sheet tears into plasmoids and these plasmoids later on coalesce into larger ones. During these processes electrons are accelerated and they produce observable electromagnetic waves. Furthermore, the 3-D PIC model with two current sheets extended in the electric current direction shows their fast fragmentation associated with the exponential dissipation of the free magnetic field energy. An example of the drifting pulsating structure which is considered to be a radio signature of the above mentioned processes in solar flares is shown.

Plasmoid Dynamics in Flare Reconnection and the Frequency Drift of the Drifting Pulsating Structure

In the paper by Kliem, Karlický, and Benz (Astron. Astrophys.360, 715, 2000) it was suggested, that plasmoids formed during the bursty regime of solar flare reconnection can be “visualised” in the radio spectra as drifting pulsating structures via accelerated particles trapped inside the plasmoid. In the present paper we investigate this idea in detail. First, simple statistical analysis supporting this hypothesis is presented. Then, by using the 2.5-D MHD (including gravity) model solar flare reconnection in the inhomogeneous, stratified atmosphere is simulated and the formation and subsequent ejection of the plasmoid is demonstrated. The ejected plasmoid, which is considered to be a trap for accelerated electrons, is traced and its plasma parameters are computed. To estimate the associated plasma radio emission we need to know locations of accelerated electrons and corresponding plasma frequencies. General considerations predict that these electrons should be distributed mainly along the magnetic separatrix surfaces and this was confirmed by using a particle-in-cell simulation. Finally, under some simplifying assumptions the model dynamic radio spectrum is constructed. The relation between the global frequency drift and the plasmoid motion in the inhomogeneous ambient atmosphere is studied. The results are discussed with respect to the observed drifting pulsation structures and their possible utilisation for flare magnetic field diagnostics.

Statistical study of radio drifting pulsation structures with associated CMEs and other observations

Solar radio burst, especially the fine structures (FSs) and the drifting pulsation structures (DPSs), may be used as an important diagnostics tool to draw the evolution map of the flare loop in the initial phase of solar flares. In this work, 52 radio events were found accompanying with DPSs. They were all observed with the Solar Radio Spectrometers (0.625–7.6 GHz) of China during 1998–2004. Combining the radio observations with LASCO-C2, Goes-8 SXR, H α, EUV and Trace observations, we analyzed all these events and obtained some statistic conclusions: First, 88% DPSs take place at the initial phase of the radio burst, and their rich spectrum characteristics are helpful to understand the events further. Second, 83% DPSs are associated with CMEs or ejection events, and all the events are accompanied by Goes SXR flare. Third, for CMEs and DPSs, which take the first step, there is no significant predominance of either of them. The relationship between the DPSs and CMEs is still not clear in this study because of the lack of spatial resolution in the centimeter–decimeter band. However, the EIT or Trace ejection happened during the onset/end time of DPSs. They are signatures of the initial phase of CMEs. Two events will be illustrated to explain this.

Multi-Wavelength Radio Features Associated with Large CMEs on Oct. 26–28, 2003

We study in focus to the multi-wavelength radio spectra associated with quite strong CME events observed in the time interval of October 26–28, 2003. Using multi-wavelength observations recorded by WIND/WAVES experiment, Learmouth Spectrograph (Australian), PHOENIX-2 (Switzerland), Hiraiso (Japan), DAM/Nançay (France), Izmiran (Russia) and Huairou/NAOC (China), an analyzing of radio bursts was performed for the those events over a large coronal region across the microwave, the decimetric wavelength, the metric wavelength and even to much longer wavelength. The composite spectra indicate there were many complicated structures of radio bursts, including type II bursts, type IV bursts, type III bursts, drifting pulsation structures (DPS) and many radio fine structures (FS).To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

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.

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.