Quasi-Periodic Pulsations Driven by Structural Oscillations in a Kink-Unstable Flaring Coronal Loop

TL;DR

This study addresses how quasi-periodic pulsations (QPPs) observed in solar flares relate to structural oscillations in kink-unstable coronal loops. It employs 3D resistive MHD simulations of both straight and curved loops undergoing kink instability, coupled with forward gyrosynchrotron radiation modeling, and introduces a novel edge-detection/alpha-shape method to quantify loop-top structural oscillations. The results show that sausage- and kink-mode oscillations drive microwave GS fluctuations, but the mapping to observed QPP periods is complex and can involve mode interference and radiative transfer effects; curvature can introduce additional dynamics such as potential acoustic modes. Overall, the work advances flare timing analyses and lays groundwork for seismological diagnostics of loop parameters from QPP observations, by linking MHD oscillations to observable microwave signatures through a robust structural-oscillation framework.

Abstract

Twisted coronal loops in the solar atmosphere may become kink-unstable when their magnetic field lines are sufficiently twisted. This instability can trigger magnetic reconnection, leading to the emission of electromagnetic radiation, which manifests as a solar flare. Previous research has demonstrated that oscillations in microwave emissions, resembling observed quasi-periodic pulsations (QPPs), can be generated by the reconnecting loop. Our aim is to investigate the relationship between the oscillations of the loop and these microwave pulsations. Using 3D magnetohydrodynamical simulations, we examine two models: a straight loop in a uniform-density atmosphere and a curved loop in a gravitationally stratified atmosphere. Using new methodology, we extract the reconnecting loop-top from both models and identify structural oscillations. We then compare these oscillations with the gyrosynchrotron (GS) radiation emitted from the simulations, which is forward-modelled using a radiative transfer code. We find that oscillations in the GS emissions are driven by sausage and kink-mode oscillations. However, the relationship between the oscillation frequencies of the GS emission and the identified loop oscillation modes is complex. The dominant mode in the former may result from interference between sausage-mode and kink-mode oscillations or entirely different mechanisms. Results such as these increase our understanding of the time-dependent behaviour of solar flares and lay the groundwork for potential diagnostic tools that could be used to determine physical parameters within a flaring loop

Figures (15)

• Figure 1: Demonstration of the edge-detection algorithm in action. The top-left image shows an unprocessed input image (a 2D slice of pressure at the loop-top within the midplane of an evolving straight loop), while the top-right image displays the input image with outer edges detected (along with some inner edge artifacts). The bottom-left image illustrates Delaunay triangulation, with black triangles representing the triangles plotted between each point in the edge dataset. The solid yellow circles depict the circumcircles of the edge points associated with the convex hull, and the white dashed circles outline the alpha-shape of these circumcircles. The bottom-right image exclusively features the outer edge points and a fitted ellipse.
• Figure 2: The evolution of the straight loop's interior magnetic field lines. The blue field lines originate from the foot-point at $y = -10.0$, while the red field lines originate from the foot-point at $y = 10.0$.
• Figure 3: The temporal evolution of the average twist (top-left), total magnetic energy (top-right), kinetic energy (bottom-left) if the system, and cumulative Ohmic heating (bottom-right) for the straight loop. A black dashed line has been marked on each graph at the time at which the total magnetic energy starts to drop, differentiating between different phases of the loop's evolution.
• Figure 4: Colour maps illustrating the evolution of the in-plane magnetic field, density, and temperature of the straight loop in the X-Z plane at the midplane of the loop ($y=0$) over time.
• Figure 5: Analysis of the structural oscillations observed in the straight loop model. The first row illustrates how the fitted ellipses cross-sectional area, central coordinates ($x_c$ and $z_c$), and how the average twist of the loop evolves over time. A dashed black line separates data points before and after 322.5s. In the second row, the post 322.5s data, with the removal of its moving average, is presented. The third row showcases the periods of the power spectrum of this processed data. Finally, the fourth row maps Gaussian peaks to the corresponding peak periods identified in power spectra.