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* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

Standing kink waves in sigmoid solar coronal loops: implications for coronal seismology  

Norbert Magyar   Submitted: 2020-04-30 02:28

Using full three-dimensional magnetohydrodynamic numerical simulations, we study the effects of magnetic field sigmoidity or helicity on the properties of the fundamental kink oscillation of solar coronal loops. Our model consists of a single denser coronal loop, embedded in a plasma with dipolar force-free magnetic field with a constant α parameter. For the loop with no sigmoidity, we find that the numerically determined oscillation period of the fundamental kink mode matches the theoretical period calculated using WKB theory. In contrast, with increasing sigmoidity of the loop, the actual period is increasingly smaller than the one estimated by WKB theory. Translated through coronal seismology, increasing sigmoidity results in magnetic field estimates which are increasingly shifting towards higher values, and even surpassing the average value for the highest α value considered. Nevertheless, the estimated range of the coronal magnetic field value lies within the mimimal/maximal limits, proving the robustness coronal seismology. We propose that the discrepancy in the estimations of the absolute value of the force-free magnetic field could be exploited seismologically to determine the free energy of coronal loops, if averages of the internal magnetic field and density can be reliably estimated by other methods.

Authors: N. Magyar; V. M. Nakariakov
Projects: None

Publication Status: Accepted for publication
Last Modified: 2020-05-03 13:58
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Damping of nonlinear standing kink oscillations: a numerical study  

Norbert Magyar   Submitted: 2016-09-23 01:41

We aim to study the standing fundamental kink mode of coronal loops in the nonlinear regime, investigating the changes in energy evolution in the cross-section and oscillation amplitude of the loop which are related to nonlinear effects, in particular to the development of the Kelvin-Helmholtz instability (KHI). We run idea, high-resolution three-dimensional (3D) magnetohydrodynamics (MHD) simulations, studying the influence of the initial velocity amplitude and the inhomogeneous layer thickness. We model the coronal loop as a straight, homogeneous magnetic flux tube with an outer inhomogeneous layer, embedded in a straight, homogeneous magnetic field. We find that, for low amplitudes which do not allow for the KHI to develop during the simulated time, the damping time agrees with the theory of resonant absorption. However, for higher amplitudes, the presence of KHI around the oscillating loop can alter the loop's evolution, resulting in a significantly faster damping than predicted by the linear theory in some cases. This questions the accuracy of seismological methods applied to observed damping profiles, based on linear theory.

Authors: N. Magyar, T. Van Doorsselaere
Projects: None

Publication Status: Accepted
Last Modified: 2016-09-23 15:45
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The instability and non-existence of multi-stranded loops, when driven by transverse waves  

Norbert Magyar   Submitted: 2016-04-15 02:51

In recent years, omni-present transverse waves have been observed in all layers of the solar atmosphere. Coronal loops are often modeled as a collection of individual strands, in order to explain their thermal behaviour and appearance. We perform 3D ideal MHD simulations to study the effect of a continuous small amplitude transverse footpoint driving on the internal structure of a coronal loop composed of strands. The output is also converted to synthetic images, corresponding to the AIA 171 Å and 193 Å passbands, using FoMo. We show that the multi-stranded loop ceases to exist in the traditional sense of the word, because the plasma is efficiently mixed perpendicularly to the magnetic field, with the Kelvin-Helmholtz instability acting as the main mechanism. The final product of our simulation is mixed loop with density structures on a large range of scales, resembling a power-law. Thus, multi-stranded loops are unstable to driving by transverse waves, and this raises a strong doubt on the usability and applicability of coronal loop models consisting of independent strands.

Authors: N. Magyar, T. Van Doorsselaere
Projects: None

Publication Status: ApJ (in press)
Last Modified: 2016-04-18 21:09
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Abstracts by Author
Standing kink waves in sigmoid solar coronal loops: implications for coronal seismology
Damping of nonlinear standing kink oscillations: a numerical study
The instability and non-existence of multi-stranded loops, when driven by transverse waves

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