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Dynamics of plasmoids formed by the current sheet tearing  

Miroslav Barta   Submitted: 2008-09-30 03:01

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 Alfvén 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 Alfvén 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.

Authors: Bárta, M.; Vr?nak, B.; Karlický, M.
Projects: None

Publication Status: A&A 477, Issue 2, January II 2008, pp.649-655
Last Modified: 2008-09-30 09:31
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Interference patterns in solar radio spectra: high-resolution structural analysis of the corona  

Miroslav Barta   Submitted: 2006-01-04 05:47

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. 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.

Authors: Barta, M., Karlický, M.
Projects: None

Publication Status: A&A (accepted)
Last Modified: 2006-01-04 05:47
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Radio signatures of solar flare reconnection  

Miroslav Barta   Submitted: 2006-01-04 05:40

The time evolution of plasma parameters (density, magnetic field etc.) in a 2-D MHD model of solar flare reconnection is computed numerically. Then assuming plasma radio emission from locations where the ``double-resonance'' instability generates upper-hybrid waves due to unstable distribution function of suprathermal electrons, the radio spectra and spatial source structures in the reconnection region are modeled. By comparison of the modeled and observed spectra a remarkable similarity has been found between the computed narrow-band emission and the observed lace bursts. Finally, a new radio diagnostic of the reconnection process based on the model is proposed.

Authors: Barta, M., Karlický, M.
Projects: None

Publication Status: ApJ 631, p. 612 (2005)
Last Modified: 2006-01-04 05:40
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Turbulent plasma model of the narrowband dm-spikes  

Miroslav Barta   Submitted: 2002-02-01 05:52

Starting from radio spectrum features of the narrowband dm-spikes, it is suggested that they are generated at positions in the solar flare atmosphere where the upper hybrid frequency is equal to the low harmonics of the electron cyclotron frequency. It is assumed that a radio source plasma is in a turbulent state. The upper hybrid waves are generated by an anisotropic electron beam which propagates along magnetic loop lines. Collisional damping and turbulent changes of plasma parameters in the radio source cause strong intensity-frequency variations of the upper hybrid waves. These waves are then transformed into observed narrowband dm-spikes. Using this model, the artificial radio spectra are computed. It is shown that a chain of narrowband spikes is generated at a one cyclotron harmonic, in one turbulent magnetic flux tube. On the other hand, a cloud of spikes (or their harmonic clouds) results from a superposition of many chains of spikes generated in many turbulent magnetic flux tubes (in several cyclotron harmonics). Finally, further features of the narrowband dm-spikes are discussed in the framework of the proposed model.

Authors: Barta, M. and Karlický, M.
Projects:

Publication Status: A&A 379, 1045 - 1051 (2001)
Last Modified: 2002-02-01 05:52
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Abstracts by Author
Dynamics of plasmoids formed by the current sheet tearing
Interference patterns in solar radio spectra: high-resolution structural analysis of the corona
Radio signatures of solar flare reconnection
Turbulent plasma model of the narrowband dm-spikes

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