DR. NORBERT MAGYAR
Previous and Current Work
Coronal loop oscillations
I have investigated numerically, through three-dimensional magnetohydrodynamic simulations, large-amplitude oscillations of solar coronal loops, considering various effects, with results published in a number of papers, listed below.
Numerical simulations of transverse oscillations in radiatively cooling coronal loops
Authors: Magyar, N.;​ Van Doorsselaere, T.;​ Marcu, A.
Aims: We aim to study the influence of radiative cooling on the standing kink oscillations of a coronal loop.
Methods: 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.
Results: 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). In this case, the difference in amplitude after nearly 3 oscillation periods for the low amplitude case is 21% between cooling and non-cooling cases. 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.
Damping of nonlinear standing kink oscillations: a numerical study
Authors: Magyar, N.;​ Van Doorsselaere, T.;
Aims: 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).
Methods: We run ideal, high-resolution three-dimensional (3D) magnetohydrodynamic (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.
Results: 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.
Standing Kink Waves in Sigmoid Solar Coronal Loops: Implications for Coronal Seismology
Authors: Magyar, N.;​ ​Nakariakov, V. M.
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 that are increasingly shifting toward 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 minimal/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.
Kink wave turbulence
We have realized in 2017 that unidirectionally-propagating MHD waves (surface Alfvén waves, Alfvénic waves, kink waves), can self-cascade nonlinearly, without the need for counterpropagating waves as Alfvén waves. This constitutes a paradigm shift in MHD turbulence phenomenology, which was dominated by Alfvén wave turbulence since the 60's. We have names this new turbulence generation mechanism "uniturbulence", and we have explored it in a number of papers listed below.
The Instability and Non-existence of Multi-stranded Loops When Driven by Transverse Waves
Authors: Magyar, N.;​ ​Van Doorsselaere, T.
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 behavior and appearance. We perform three-dimensional (3D) ideal magnetohydrodynamics 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 into synthetic images, corresponding to the AIA 171 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 a 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 strong doubts on the usability and applicability of coronal loop models consisting of independent strands.
Generalized phase mixing: Turbulence-like behaviour from unidirectionally propagating MHD waves
Authors: Magyar, N.;​ ​Van Doorsselaere, T.; Goossens, M.
We present the results of three-dimensional (3D) ideal magnetohydrodynamics (MHD) simulations on the dynamics of a perpendicularly inhomogeneous plasma disturbed by propagating Alfvénic waves. Simpler versions of this scenario have been extensively studied as the phenomenon of phase mixing. We show that, by generalizing the textbook version of phase mixing, interesting phenomena are obtained, such as turbulence-like behavior and complex current-sheet structure, a novelty in longitudinally homogeneous plasma excited by unidirectionally propagating waves. This study is in the setting of a coronal hole. However, it constitutes an important finding for turbulence-related phenomena in astrophysics in general, relaxing the conditions that have to be fulfilled in order to generate turbulent behavior.
Understanding uniturbulence: self-cascade of MHD waves in the presence of inhomogeneities
Authors: Magyar, N.;​ ​Van Doorsselaere, T.; Goossens, M.
It is a generally accepted fact in the MHD turbulence community that the nonlinear cascade of wave energy requires the existence of counter-propagating Alfvénic wave-packets, along some mean magnetic field. This fact is an obvious outcome of the MHD equations when assuming an incompressible and homogenoeus plasma. There have been relatively few attempts to relax these assumptions in the context of MHD turbulence studies. However, it should be clear that once these assumptions brake down, the generally accepted picture of turbulent cascade generation is not universal. In the context of longitudinally stratified plasmas (i.e. gravitationally stratified coronal holes), it has been known since the 70's that inhomogeneities along the mean magnetic field lead to the linear coupling of sunward and anti-sunward propagating waves. This leads to co-propagating disturbances of Elsasser fields, which can interact coherently to initiate a nonlinear cascade. The alternative case of perpendicular inhomogeneity (across the mean magnetic field) was even less studied in the context of MHD turbulence. In this study we show that these type of inhomogeneities lead also to co-propagating Elsasser fields, already in the incompressible case. We show how the nonlinear self-deformation of these unidirectionally propagating waves leads to a cascade in k-space 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 solar coronal holes.
Switchback propagation
Switchbacks are one of the highlight (re)discoveries of the Parker Solar Probe satellite, and they represent localized deflections or bends of the magnetic field lines. Their origins and nature are not entirely understood. In these series of two papers, we have investigated whether switchbacks can originate in the lower solar atmosphere.
Could Switchbacks Originate in the Lower Solar Atmosphere? I. Formation Mechanisms of Switchbacks
Authors: Magyar, N.;​​ Utz, D.; Erdélyi, R.; Nakariakov, V. M.
The recent rediscovery of magnetic field switchbacks or deflections embedded in the solar wind flow by the Parker Solar Probe mission lead to a huge interest in the modeling of the formation mechanisms and origin of these switchbacks. Several scenarios for their generation were put forth, ranging from lower solar atmospheric origins by reconnection, to being a manifestation of turbulence in the solar wind, and so on. Here we study some potential formation mechanisms of magnetic switchbacks in the lower solar atmosphere, using three-dimensional magnetohydrodynamic (MHD) numerical simulations. The model is that of an intense flux tube in an open magnetic field region, aiming to represent a magnetic bright point opening up to an open coronal magnetic field structure, e.g., a coronal hole. The model is driven with different plasma flows in the photosphere, such as a fast up-shooting jet, as well as shearing flows generated by vortex motions or torsional oscillations. In all scenarios considered, we witness the formation of magnetic switchbacks in regions corresponding to chromospheric heights. Therefore, photospheric plasma flows around the foot-points of intense flux tubes appear to be suitable drivers for the formation of magnetic switchbacks in the lower solar atmosphere. Nevertheless, these switchbacks do not appear to be able to enter the coronal heights of the simulation in the present model. In conclusion, based on the presented simulations, switchbacks measured in the solar wind are unlikely to originate from photospheric or chromospheric dynamics.
Authors: Magyar, N.;​​ Utz, D.; Erdélyi, R.; Nakariakov, V. M.
The magnetic switchbacks observed recently by the Parker Solar Probe have raised the question about their nature and origin. One of the competing theories of their origin is the interchange reconnection in the solar corona. In this scenario, switchbacks are generated at the reconnection site between open and closed magnetic fields, and are either advected by an upflow or propagate as waves into the solar wind. In this paper we test the wave hypothesis, numerically modeling the propagation of a switchback, modeled as an embedded Alfvén wave packet of constant magnetic field magnitude, through the gravitationally stratified solar corona with different degrees of background magnetic field expansion. While switchbacks propagating in a uniform medium with no gravity are relatively stable, as reported previously, we find that gravitational stratification together with the expansion of the magnetic field act in multiple ways to deform the switchbacks. These include WKB effects, which depend on the degree of magnetic field expansion, and also finite-amplitude effects, such as the symmetry breaking between nonlinear advection and the Lorentz force. In a straight or radially expanding magnetic field the propagating switchbacks unfold into waves that cause minimal magnetic field deflections, while a super-radially expanding magnetic field aids in maintaining strong deflections. Other important effects are the mass uplift the propagating switchbacks induce and the reconnection and drainage of plasmoids contained within the switchbacks. In the Appendix, we examine a series of setups with different switchback configurations and parameters, which broaden the scope of our study.
Solar wind modelling
Recently, fully compressible MHD models of the solar corona and solar wind were published, which represent the state-of-the-art in realistic solar wind modelling. We have implemented a fully compressible MHD model of the solar corona and wind with inhomogeneities across the magnetic field, to study its effects on the overall dynamics, including by uniturbulence.
Three-dimensional Simulations of the Inhomogeneous Low Solar Wind
Authors: Magyar, N.;​​ Nakariakov, V. M.
In the near future, the 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 focused 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 analyze through 3D MHD numerical simulations the dynamics of the perpendicularly structured solar corona and solar wind, from the low corona to 15 R⊙. We find that background structuring has a strong effect on the evolution of MHD turbulence, on much faster timescales than in the perpendicularly homogeneous case. On timescales 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 a 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.