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Oscillations Excited by Plasmoids Formed During Magnetic Reconnection in a Vertical Gravitationally Stratified Current Sheet  

Petr Jelinek   Submitted: 2017-10-03 08:13

Using the FLASH code, which solves the full set of the 2D non-ideal (resistive) time-dependent magnetohydrodynamic (MHD) equations, we study processes during the magnetic reconnection in a vertical gravitationally stratified current sheet. We show that during these processes, which correspond to processes in solar flares, plasmoids are formed due to the tearing mode instability of the current sheet. These plasmoids move upward or downward along the vertical current sheet and some of them merge into larger plasmoids. We study the density and temperature structure of these plasmoids and their time evolution in detail. We found that during the merging of two plasmoids, the resulting larger plasmoid starts to oscillate with a period largely determined by L/cA, where L is the size of the plasmoid and cA is the Alfvén speed in the lateral parts of the plasmoid. In our model, L/cA evaluates to ~ 25 s. Furthermore, the plasmoid moving downward merges with the underlying flare arcade, which causes oscillations of the arcade. In our model, the period of this arcade oscillation is ~ 35 s, which also corresponds to L/cA, but here L means the length of the loop and cA is the average Alfvén speed in the loop. We also show that the merging process of the plasmoid with the flare arcade is a complex process as presented by complex density and temperature structures of the oscillating arcade. Moreover, all these processes are associated with magnetoacoustic waves produced by the motion and merging of plasmoids.

Authors: P. Jelínek, M. Karlický, T. Van Doorsselaere, M. Bárta
Projects:

Publication Status: The Astrophysical Journal, Volume 847, Number 2
Last Modified: 2017-10-18 11:33
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Quasi-periodic processes in the flare loop generated by sudden temperature enhancements at loop footpoints  

Petr Jelinek   Submitted: 2016-04-28 12:51

Aims. During the impulsive flare phase, the plasma at the flare loop footpoints is rapidly heated by particle beams. In the present paper, we study processes that occur after this sudden heating in a two-dimensional magnetic loop. Methods. We adopt a 2D magnetohydrodynamic (MHD) model, in which we solve a full set of the ideal time-dependent MHD equations by means of the FLASH code, using the adaptive mesh refinement (AMR) method. Periods in the processes are estimated by the wavelet analysis technique. Results. We consider a model of the solar atmosphere with a symmetric magnetic loop. The length of this loop in the corona is approximately 21.5 Mm. At both loop footpoints, at the transition region, we initiate the Gaussian temperature (pressure) perturbation with the maximum temperature 14, 7, or 3.5 times higher than the unperturbed temperature. In the corona, the perturbations produce supersonic blast shocks with the Mach number of about 1.1, but well below Alfvén velocities. We consider cases with the same perturbations at both footpoints (symmetric case) and one with different perturbations (asymmetric case). In the symmetric case, the shocks move along both loop legs upwards to the top of the loop, where they interact and form a transient compressed region. Then they continue in their motion to the transition region at the opposite side of the loop, where they are reflected upwards, and so on. At the top of the loop, the shock appears periodically with the period of about 170 s. In the loop legs during this period, a double peak of the plasma parameters, which is connected with two arrivals of shocks, is detected: firstly, when the shock moves up and then when the shock, propagating from the opposite loop leg, moves down. Increasing the distance of the detection point in the loop leg from the top of the loop, the time interval between these shock arrivals increases. Thus, at these detection points, the processes with shorter periods can be detected. After ~500 s the process with the periodically interacting shocks slowly changes to slow mode magnetosonic free oscillation. Furthermore, we detected quasi-periodic processes, even in the chromosphere under the location of the pressure perturbation. These processes can be observed in intensities and Doppler shifts of optical chromospheric lines. In the case with the asymmetric perturbations, we found that the processes are even more complex.

Authors: M. Karlický and P. Jelínek
Projects: None

Publication Status: A&A 590, A4 (2016)
Last Modified: 2016-04-29 16:21
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Electric current filamentation at a non-potential magnetic null-point due to pressure perturbation  

Petr Jelinek   Submitted: 2015-10-14 00:52

An increase of electric current densities due to filamentation is an important process in any flare. We show that the pressure perturbation, followed by an entropy wave, triggers such a filamentation in the non-potential magnetic null-point. In the two-dimensional (2D), non-potential magnetic null-point, we generate the entropy wave by a negative or positive pressure pulse that is launched initially. Then, we study its evolution under the influence of the gravity field. We solve the full set of 2D time dependent, ideal magnetohydrodynamic equations numerically, making use of the FLASH code. The negative pulse leads to an entropy wave with a plasma density greater than in the ambient atmosphere and thus this wave falls down in the solar atmosphere, attracted by the gravity force. In the case of the positive pressure pulse, the plasma becomes evacuated and the entropy wave propagates upward. However, in both cases, owing to the Rayleigh?Taylor instability, the electric current in a non-potential magnetic null-point is rapidly filamented and at some locations the electric current density is strongly enhanced in comparison to its initial value. Using numerical simulations, we find that entropy waves initiated either by positive or negative pulses result in an increase of electric current densities close to the magnetic null-point and thus the energy accumulated here can be released as nanoflares or even flares.

Authors: P. Jelinek, M. Karlický, K. Murawski
Projects: None

Publication Status: ApJ 812, 105, 2015
Last Modified: 2015-10-15 07:27
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Spectroscopic observations and modelling of impulsive Alfvén waves along a polar coronal jet  

Petr Jelinek   Submitted: 2015-09-29 13:33

Context. The magnetic reconnection in the solar corona results in impulsively generated Alfvén waves, which drive a polar jet. Aims. Using the Hinode/EIS 2′′ spectroscopic observations, we study the intensity, velocity, and full width at half maximum (FWHM) variations of the strongest Fe XII 195.12 Å line along the jet to find the signature of Alfvén waves. We numerically simulate the impulsively generated Alfvén waves within the vertical Harris current sheet, forming the jet plasma flows, and mimicking their observational signatures. Methods. Using the FLASH code and an atmospheric model with an embedded, weakly expanding magnetic field configuration within a vertical Harris current sheet, we solve the 2.5-dimensional (2.5D) ideal magnetohydrodynamic (MHD) equations to study the evolution of Alfvén waves and vertical flows forming the plasma jet. Results. At a height of ~5 Mm from the base of the jet, the red-shifted velocity component of Fe XII 195.12 Å line attains its maximum (5 km s-1), which converts into a blue-shifted velocity component between the altitude of 5-10 Mm. The spectral intensity continuously increases up to 10 Mm, while the FWHM still exhibits low values with an almost constant trend. This indicates that the reconnection point within the jet's magnetic field topology lies in the corona 5-10 Mm from its footpoint anchored in the Sun's surface. Beyond this height, the FWHM shows a growing trend. This may be the signature of Alfvén waves that impulsively evolve, due to reconnection, and propagate along the jet. From our numerical data, we evaluate space- and time- averaged Alfvén waves velocity amplitudes at different heights in the jet's current sheet, which contribute to the non-thermal motions and spectral line broadening. The synthetic width of Fe XII 195.12 Å line exhibits a similar trend of increment as in the observational data, possibly proving the existence of Alfvén waves, impulsively generated by reconnection, that propagate along the jet. Conclusions. The numerical simulations show that the impulsive perturbations in the transversal component of velocity at the reconnection point can excite the Alfvén waves. These waves can power the plasma jet higher into the polar coronal hole, as vertical plasma flows are also associated with these waves due to pondermotive force. The simulated Alfvén waves match well with the observed non-thermal broadening along the jet, which may provide direct spectroscopic evidence of the impulsively excited Alfvén waves within the polar jet.

Authors: P. Jelínek, A. K. Srivastava, K. Murawski, P. Kayshap and B. N. Dwivedi
Projects: None

Publication Status: A&A 581, A131, 2015
Last Modified: 2015-09-30 14:44
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Numerical simulations of magnetoacoustic?gravity waves in the solar coronal curved magnetic field lines structure  

Petr Jelinek   Submitted: 2013-07-25 00:38

We present a two-dimensional (2D) magnetohydrodynamic (MHD) model of magneto-acoustic?gravity waves in the gravitationally stratified solar corona that is shaped by a realistic (VAL-C, Vernazza Avrett Loeser model C) temperature profile and curved magnetic field lines. These waves are triggered by an initial Gaussian pulse in the horizontal component of velocity, that is, launched either just below or above the transition region. The time-dependent ideal MHD equations are solved numerically with the use of the flash code. The numerical results reveal conversion of a horizontal flow into its vertical counterpart, oscillations of the transition region and vertical jets of cold plasma penetrating the solar corona. The wavelet analysis of the mass-density variations at a fixed detection point leads to the oscillation period of about 180 s, which corresponds to 3-min oscillations observed in solar active regions.

Authors: P. Jelínek, K. Murawski
Projects: None

Publication Status: Published in MNRAS
Last Modified: 2013-07-25 09:55
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Magnetoacoustic waves in a vertical flare current-sheet in a gravitationally stratified solar atmosphere  

Petr Jelinek   Submitted: 2012-10-02 09:29

We numerically studied evolution of impulsively generated magnetoacoustic waves in the vertical flare current-sheet that is embedded in the gravitationally stratified solar atmosphere and compared it with its gravity-free counterpart. We adopted a two-dimensional (2D) magnetohydrodynamic (MHD) model, in which we solved a full set of ideal time-dependent MHD equations by means of the FLASH code, using the adaptive mesh refinement (AMR) method. To initiate the fast sausage magnetoacoustic waves, we used axisymmetric Gaussian velocity perturbation. As a diagnostic tool of these magnetoacoustic waves, we used the wavelet analysis method. We present a model of magnetoacoustic wave propagation with a gravity that is more realistic than that presented in previous studies. We compare our results with those of a gravity-free case. In equilibrium the current-sheet with gravity requires a non-zero horizontal component of the magnetic field, contrary to the gravity-free case. This causes differences in the parameters of the wave signal that propagates along the current sheet. In addition to these differences we find that wave signal variations and their wavelet tadpoles are more complex in the case with gravity than in the gravity-free case. Furthermore, for a shorter scale-height we found a prolongation of the wavelet tadpoles. These differences result from a variation of the dispersive properties and group velocities of the propagating magnetoacoustic waves with height in the solar atmosphere in the gravitational case. We show that these results can affect the diagnostics of physical processes in solar flares.

Authors: P. Jelinek, M. Karlický, K. Murawski
Projects: None

Publication Status: Astron. Astrophys. 546, A49 (2012)
Last Modified: 2012-10-02 10:51
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Magnetoacoustic waves in diagnostics of the flare current sheets  

Petr Jelinek   Submitted: 2012-01-09 02:53

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Jelinek, P., Karlický, M.
Projects: None

Publication Status: Astron. Astrophys. 537 (2012), A46
Last Modified: 2012-01-09 07:28
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Magnetoacoustic waves in the narrowband dm-spikes sources  

Petr Jelinek   Submitted: 2011-04-27 08:34

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.


Authors: M. Karlický, P. Jelinek and H. Meszarosova
Projects: None

Publication Status: Astron. Astrophys., 529 (2011), A96
Last Modified: 2011-04-27 11:00
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Impulsively Generated Wave Trains in a Solar Coronal Loop  

Petr Jelinek   Submitted: 2010-09-30 06:54

Impulsively generated fast magnetoacoustic wave trains in a solar coronal loop are numerically studied. The problem is considered as 2-D in space, and for the description, the full set of magnetohydrodynamic (MHD) equations is used. The numerical solution of the MHD equations is performed by means of the Lax?Wendroff algorithm on a uniformly structured mesh. The wavelet analysis of the obtained wave trains shows out the typical tadpole shapes, i.e., a narrow tail followed by a broadband head. In this paper, we discuss the propagation speed and periods of the wave trains as well as the shapes of the tadpoles in dependence on the plasma beta parameter. These studies are very important in connection with the observations because the tadpole signatures, firstly discovered during the solar eclipse in 1999 by the SECIS instrument, have been recently recognized also in decimetric type IV radio events by the Ondřejov; radiospectrograph.

Authors: Petr Jelinek and Marian Karlický
Projects: None

Publication Status: IEEE Trans. Plasma Sci., 38(9), 2243
Last Modified: 2010-09-30 10:58
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Abstracts by Author
Oscillations Excited by Plasmoids Formed During Magnetic Reconnection in a Vertical Gravitationally Stratified Current Sheet
Quasi-periodic processes in the flare loop generated by sudden temperature enhancements at loop footpoints
Electric current filamentation at a non-potential magnetic null-point due to pressure perturbation
Spectroscopic observations and modelling of impulsive Alfv?n waves along a polar coronal jet
Numerical simulations of magnetoacoustic?gravity waves in the solar coronal curved magnetic field lines structure
Magnetoacoustic waves in a vertical flare current-sheet in a gravitationally stratified solar atmosphere
Magnetoacoustic waves in diagnostics of the flare current sheets
Magnetoacoustic waves in the narrowband dm-spikes sources
Impulsively Generated Wave Trains in a Solar Coronal Loop

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