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

Petr Jelinek Submitted: 20171003 08:13
Using the FLASH code, which solves the full set of the 2D nonideal (resistive) timedependent 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/c_{A}, where L is the size of the plasmoid and c_{A} is the Alfvén speed in the lateral parts of the plasmoid. In our model, L/c_{A} 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/c_{A}, but here L means the length of the loop and c_{A} 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: 20171018 11:33



Quasiperiodic processes in the flare loop generated by sudden temperature enhancements at loop footpoints 

Petr Jelinek Submitted: 20160428 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 twodimensional magnetic loop.
Methods. We adopt a 2D magnetohydrodynamic (MHD) model, in which we solve a full set of the ideal timedependent 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 quasiperiodic 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: 20160429 16:21



Electric current filamentation at a nonpotential magnetic nullpoint due to pressure perturbation 

Petr Jelinek Submitted: 20151014 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 nonpotential magnetic nullpoint. In the twodimensional (2D), nonpotential magnetic nullpoint, 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 nonpotential magnetic nullpoint 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 nullpoint 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: 20151015 07:27



Spectroscopic observations and modelling of impulsive Alfvén waves along a polar coronal jet 

Petr Jelinek Submitted: 20150929 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 2.5dimensional (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 redshifted velocity component of Fe XII 195.12 Å line attains its maximum (5 km s^{1}), which converts into a blueshifted velocity component between the altitude of 510 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 510 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 nonthermal 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 nonthermal 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: 20150930 14:44




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