arXiv subjects

N. Magyar

Publications and source records attributed to N. Magyar.

14 recordsLinked to original sources

arXiv2025

Uniturbulence and Alfv\'en Wave Solar Model in MPI-AMRVAC

The coronal heating problem remains a fundamental challenge in solar physics. While AWSoM-type models (Alfv\'en Wave Solar Model) have proven highly successful in reproducing the large-scale structure of the solar corona, they inherently neglect contributions from additional wave modes that arise when the effects of transverse structuring is fully incorporated into the magnetohydrodynamic (MHD) equations. In this paper, we compare the roles of kink wave- and Alfv\'en wave-driven heating in sustaining a region of the solar atmosphere, using newly developed physics and radiative cooling modules within MPI-AMRVAC. We extend the existing MHD physics module in MPI-AMRVAC by incorporating additional Alfv\'en and kink wave energy contributions to the MHD equations. We examine their roles in heating the solar atmosphere and driving the solar wind. To validate our approach, we compare numerical results from Python-based simulations with those obtained using the UAWSoM module in MPI-AMRVAC. Furthermore, we assess the heating efficiency of kink waves relative to that of pure Alfv\'en waves through two parameter studies: (1) exploring how different Alfv\'en wave reflection rates impact the simulated atmosphere, and (2) varying the relative magnitudes of Alfv\'en and kink wave energy injections. Finally, we present results from a larger-scale domain, sustained entirely by kink wave-driven heating. Our results show that kink wave-driven (UAWSoM) models are able to sustain a stable atmosphere without requiring any artificial background heating terms, unlike traditional Alfv\'en-only models. We attribute this to the increased heating rate associated with kink waves compared with Alfv\'en waves, given the same energy injection. Kink waves can sustain a model plasma with temperature and density values representative of coronal conditions without resorting to ad hoc heating terms.

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arXiv2021

The dynamics and observability of circularly polarized kink waves

Context. Kink waves are routinely observed in coronal loops. Resonant absorption is a well-accepted mechanism that extracts energy from kink waves. Nonlinear kink waves are know to be affected by the Kelvin-Helmholtz instability. However, all previous numerical studies consider linearly polarized kink waves. Aims. We study the properties of circularly polarized kink waves on straight plasma cylinders, for both standing and propagating waves, and compare them to the properties of linearly polarized kink waves. Methods. We use the code MPI-AMRVAC to solve the full 3D Magnetohydrodynamic (MHD) equations for a straight magnetic cylinder, excited by both standing and propagating circularly polarized kink (m = 1) modes. Results. The damping due to resonant absorption is independent of the polarization state. The morphology or appearance of the induced resonant flow is different for the two polarizations, however, there are essentially no differences in the forward-modeled Doppler signals. For nonlinear oscillations, the growth rate of small scales is determined by the total energy of the oscillation rather than the perturbation amplitude. We discuss possible implications and seismological relevance.

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arXiv2021

Kink oscillations of coronal loops

Kink oscillations of coronal loops, i.e., standing kink waves, is one of the most studied dynamic phenomena in the solar corona. The oscillations are excited by impulsive energy releases, such as low coronal eruptions. Typical periods of the oscillations are from a few to several minutes, and are found to increase linearly with the increase in the major radius of the oscillating loops. It clearly demonstrates that kink oscillations are natural modes of the loops, and can be described as standing fast magnetoacoustic waves with the wavelength determined by the length of the loop. Kink oscillations are observed in two different regimes. In the rapidly decaying regime, the apparent displacement amplitude reaches several minor radii of the loop. The damping time which is about several oscillation periods decreases with the increase in the oscillation amplitude, suggesting a nonlinear nature of the damping. In the decayless regime, the amplitudes are smaller than a minor radius, and the driver is still debated. The review summarises major findings obtained during the last decade, and covers both observational and theoretical results. Observational results include creation and analysis of comprehensive catalogues of the oscillation events, and detection of kink oscillations with imaging and spectral instruments in the EUV and microwave bands. Theoretical results include various approaches to modelling in terms of the magnetohydrodynamic wave theory. Properties of kink oscillations are found to depend on parameters of the oscillating loop, such as the magnetic twist, stratification, steady flows, temperature variations and so on, which make kink oscillations a natural probe of these parameters by the method of magnetohydrodynamic seismology.

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arXiv2021

MHD Waves in open coronal structures

Modern observatories have revealed the ubiquitous presence of magnetohydrodynamic waves in the solar corona. The propagating waves (in contrast to the standing waves) are usually originated in the lower solar atmosphere which makes them particularly relevant to coronal heating. Furthermore, open coronal structures are believed to be the source regions of solar wind, therefore, the detection of MHD waves in these structures is also pertinent to the acceleration of solar wind. Besides, the advanced capabilities of the current generation telescopes have allowed us to extract important coronal properties through MHD seismology. The recent progress made in the detection, origin, and damping of both propagating slow mangetoacoustic waves and kink (Alfvénic) waves is presented in this review article especially in the context of open coronal structures. Where appropriate, we give an overview on associated theoretical modelling studies. A few of the important seismological applications of these waves are discussed. The possible role of Aflvénic waves in the acceleration of solar wind is also touched upon.

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arXiv2020

Three-dimensional simulations of the inhomogeneous Low Solar Wind

In the near future, Parker Solar Probe will put theories about the dynamics and nature of the transition between the solar corona and the solar wind to stringent tests. The most popular mechanism aimed to explain the dynamics of the nascent solar wind, including its heating and acceleration is magnetohydrodynamic (MHD) turbulence. Most of the previous models focus on nonlinear cascade induced by interactions of outgoing Alfvén waves and their reflections, ignoring effects that might be related to perpendicular structuring of the solar coronal plasma, despite overwhelming evidence for it. In this paper, for the first time, we analyse through 3D MHD numerical simulations the dynamics of the perpendicularly structured solar corona and solar wind, from the low corona to 15 R_Sun. We find that background structuring has a strong effect on the evolution of MHD turbulence, on much faster time scales than in the perpendicularly homogeneous case. On time scales shorter than nonlinear times, linear effects related to phase mixing result in a 1/f perpendicular energy spectrum. As the turbulent cascade develops, we observe a perpendicular (parallel) energy spectrum with the power law index of -3/2 or -5/3 (-2), a steeper perpendicular magnetic field than velocity spectrum, and a strong build-up of negative residual energy. We conclude that the turbulence is most probably generated by the self-cascade of the driven transverse kink waves, referred to previously as `uniturbulence', which might represent the dominant nonlinear energy cascade channel in the pristine solar wind.

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arXiv2020

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

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 alpha-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 alpha 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.

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arXiv2019

Understanding uniturbulence: self-cascade of MHD waves in the presence of inhomogeneities

It is widely accepted in the MHD turbulence community that the nonlinear cascade of wave energy requires counter-propagating Alfvénic wave-packets, along some mean magnetic field. This fact is an obvious outcome of the MHD equations under the assumptions of incompressibility and homogeneity. Despite attempts to relax these assumptions in the context of MHD turbulence, the central idea of turbulence generation persists. However, once the assumptions of incompressiblity and homogeneity break down, the generally accepted picture of turbulent cascade generation is not universal. In this paper, we show that perpendicular inhomogeneities (across the mean magnetic field) lead to propagating wave solutions which are necessarily described by co-propagating Elsässer fields, already in the incompressible case. One simple example of these wave solutions is the surface Alfvén wave on a planar discontinuity across the magnetic field. We show through numerical simulations how the nonlinear self-deformation of these unidirectionally propagating waves leads to a cascade of wave energy across the magnetic field. The existence of this type of unidirectional cascade might have an additional strong effect on the turbulent dissipation rate of dominantly outward propagating Alfvénic waves in structured plasma, as in the solar corona and solar wind.

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arXiv2019

The nature of Elsässer variables in compressible MHD

The Elsässer variables are often used in studies of plasma turbulence, in helping differentiate between MHD waves propagating parallel or anti-parallel to the main magnetic field. While for pure Alfvén waves in a homogeneous plasma the method is strictly valid, we show that compressible, magnetoacoustic waves are in general described by both Elsässer variables. Furthermore, in a compressible and inhomogeneous plasma, the pure MHD waves (Alfvén, fast and slow) are no longer normal modes, but waves become linearly coupled or display mixed properties of Alfvén and magnetoacoustic nature. These waves are necessarily described by both Elsässer variables and therefore the Elsässer formalism cannot be used to strictly separate parallel and anti-parallel propagating waves. Nevertheless, even in an inhomogeneous plasma, for a highly Alfvénic wave the Elsässer variable corresponding to the propagation direction appears still dominating. We suggest that for Alfvénic waves, the relative amplitude of Elsässer variables depends on the local degree of inhomogeneity and other plasma and wave properties. This finding has implications for turbulence studies in inhomogeneous and compressible plasmas, such as the solar corona and solar wind.

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arXiv2018

The polytropic index of solar coronal plasma in sunspot fan loops and its temperature dependence

Observations of slow magneto-acoustic waves have been demonstrated to possess a number of applications in coronal seismology. Determination of the polytropic index ($γ$) is one such important application. Analysing the amplitudes of oscillations in temperature and density corresponding to a slow magneto-acoustic wave, the polytropic index in the solar corona has been calculated and based on the obtained value it has been inferred that thermal conduction is highly suppressed in a very hot loop in contrast to an earlier report of high thermal conduction in a relatively colder loop. In this study, using SDO/AIA data, we analysed slow magneto-acoustic waves propagating along sunspot fan loops from 30 different active regions and computed polytropic indices for several loops at multiple spatial positions. The obtained $γ$ values vary from 1.04$\pm$0.01 to 1.58$\pm$0.12 and most importantly display a temperature dependence indicating higher $γ$ at hotter temperatures. This behaviour brings both the previous studies to agreement and perhaps implies a gradual suppression of thermal conduction with increase in temperature of the loop. The observed phase shifts between temperature and density oscillations, however, are substantially larger than that expected from a classical thermal conduction and appear to be influenced by a line-of-sight integration effect on the emission measure.

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arXiv2018

Assessing the Capabilities of Dynamic Coronal Seismology of Alfvénic Waves through Forward Modeling

Coronal seismology is a diagnostic tool used in solar physics for measuring parameters that are otherwise hard to measure; of these parameters, magnetic field values are arguably the most important. The parameters are inferred by combining observations of waves with magnetohydrodynamic (MHD) wave theory. To date, coronal seismology has successfully been applied to various single-oscillation events. Such events are relatively rare, resulting in rare occasions to use diagnostics. Ubiquitous waves in the solar atmosphere might, however, allow for the possibility of dynamic coronal seismology, which involves the continuous inversions of coronal parameters and would constitute a huge leap forward in many areas of solar physics. In this paper, we investigate the robustness and accuracy of magnetic field diagnostics applied to forward-modeled 3D MHD simulations of propagating Alfvénic waves. We find that the seismologically measured magnetic field values are reassuringly close to the input value (within 20%) for a range of setups studied, providing encouragement and confidence for the further development of dynamic coronal seismology.

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arXiv2017

Effect of magnetic twist on nonlinear transverse kink oscillations

Magnetic twist is thought to play an important role in many structures of the solar atmosphere. One of the effects of twist is to modify the properties of the eigenmodes of magnetic tubes. In the linear regime standing kink solutions are characterized by a change in polarization of the transverse displacement along the twisted tube. In the nonlinear regime, magnetic twist affects the development of shear instabilities that appear at the tube boundary when it is oscillating laterally. These Kelvin-Helmholtz instabilities (KHI) are produced either by the jump in the azimuthal component of the velocity at the edge of the sharp boundary between the internal and external part of the tube, or either by the continuous small length-scales produced by phase-mixing when there is a smooth inhomogeneous layer. In this work the effect of twist is consistently investigated by solving the time-dependent problem including the process of energy transfer to the inhomogeneous layer. It is found that twist always delays the appearance of the shear instability but for tubes with thin inhomogeneous layers the effect is relatively small for moderate values of twist. On the contrary, for tubes with thick layers, the effect of twist is much stronger. This can have some important implications regarding observations of transverse kink modes and the KHI itself.

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arXiv2016

Damping of nonlinear standing kink oscillations: a numerical study

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.

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arXiv2016

The instability and non-existence of multi-stranded loops, when driven by transverse waves

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 A and 193 A 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.

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arXiv2015

Numerical simulations of transverse oscillations in radiatively cooling coronal loops

We aim to study the influence of radiative cooling on the standing kink oscillations of a coronal loop. Using the FLASH code, we solved the 3D ideal magnetohydrodynamic equations. Our model consists of a straight, density enhanced and gravitationally stratified magnetic flux tube. We perturbed the system initially, leading to a transverse oscillation of the structure, and followed its evolution for a number of periods. A realistic radiative cooling is implemented. Results are compared to available analytical theory. We find that in the linear regime (i.e. low amplitude perturbation and slow cooling) the obtained period and damping time are in good agreement with theory. The cooling leads to an amplification of the oscillation amplitude. However, the difference between the cooling and non-cooling cases is small (around 6% after 6 oscillations). In high amplitude runs with realistic cooling, instabilities deform the loop, leading to increased damping. In this case, the difference between cooling and non-cooling is still negligible at around 12%. A set of simulations with higher density loops are also performed, to explore what happens when the cooling takes place in a very short time (tcool = 100 s). We strengthen the results of previous analytical studies that state that the amplification due to cooling is ineffective, and its influence on the oscillation characteristics is small, at least for the cases shown here. Furthermore, the presence of a relatively strong damping in the high amplitude runs even in the fast cooling case indicates that it is unlikely that cooling could alone account for the observed, flare-related undamped oscillations of coronal loops. These results may be significant in the field of coronal seismology, allowing its application to coronal loop oscillations with observed fading-out or cooling behaviour.

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