Quasi-periodic processes in the flare loop generated by sudden temperature enhancements at loop footpoints

Abstract

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.

Pulse-beam heating of deep atmospheric layers, their oscillations and shocks modulating the flare reconnection

“…Jelínek, e-mail: pjelinek@prf.jcu.cz 1985; Fisher et al 1985a,b,c;Mariska et al 1989;Karlický 1990;Karlický & Henoux 1992;Hawley & Fisher 1994;Abbett & Hawley 1999;Allred et al 2005;Varady et al 2014;Karlický & Jelínek 2016).…”

First Evidence of the Frequency Filtering of Magnetoacoustic Waves in the Flaring Star EK Dra

“…Magnetohydrodynamic (MHD) waves and oscillation are important physical phenomena arose in the magnetized plasma that provide a significant diagnostics (e.g., magnetic field, density and magnetic scale heights, transport phenomena, etc) of the localized solar and stellar coronae (Nakariakov & Verwichte 2005;Nakariakov & Melnikov 2009). The waves and oscillations are ubiquitous in the solar atmosphere, and are studied both observationally (e.g., Wang & Solanki 2004;Srivastava et al 2008;Jess et al 2009;Aschwanden & Schrijver 2011;White et al 2012;Srivastava & Goossens 2013;Srivastava et al 2017, references cited therein) and theoretically (e.g., König et al 2005;Goossens et al 2011;Jelínek & Murawski 2013;Pascoe 2014;Karlický & Jelínek 2016, references cited therein) in different magnetic structures. There are several pieces of evidence that they also exist in the atmosphere of similar type magnetically active stars (e.g., Mathioudakis et al 2003;Mitra-Kraev et al 2005;Mathioudakis et al 2006;Pandey & Srivastava 2009;Doyle et al 2018, and references cited there).…”