arXiv subjects

Tom Van Doorsselaere

Publications and source records attributed to Tom Van Doorsselaere.

At least 37 records · Page 2Linked to original sources

arXiv2025

Exploring the variation in the dynamic rotation profile of the hotter solar atmosphere using mutliwavelength data

The global rotational profile of the solar atmosphere and its variation at different layers, although crucial for a comprehensive understanding of the dynamics of the solar magnetic field, has been a subject to contradictory results throughout the past century. In this study, we thereby unify the results for different parts of the multi-thermal Solar atmosphere by utilizing 13 years of data in 7 wavelength channels of the Atmospheric Imaging Assembly (AIA) atop the Solar Dynamic Observatory (SDO). Using the method of image correlation, we find that the solar atmosphere exhibits a rotational profile that is up to 4.18% and 1.92% faster at the equator and comparatively less differential than that of the photosphere, as derived from Doppler measurements and sunspots, respectively and exhibits variation at different respective heights. Additionally, we find results suggestive of the role played by the rooting of different magnetic field structures on a comparison with helioseismology data.

astro-ph.SR↗

arXiv2025

Spectra of standing kink waves in loops and the effects of the lower solar atmosphere

Understanding the effects of the lower solar atmosphere on the spectrum of standing kink oscillations of coronal loops, in both the decaying and decayless regime, is essential for developing more advanced tools for coronal seismology. We aim to reveal the effects of the chromosphere on the spatial profiles and frequencies of the standing kink modes, create synthetic emission maps to compare with observations, and study the results using spatial and temporal coronal seismology techniques. We excited transverse oscillations in a 3D straight flux tube using (a) a broadband footpoint driver, (b) a sinusoidal velocity pulse, and (c) an off-centre Gaussian velocity pulse, using the PLUTO code. The flux tube is gravitationally stratified, with footpoints embedded in chromospheric plasma. Using the FoMo code, we created synthetic observations of our data in the Fe IX 17.1 nm line and calculated the spectra with the Auto-NUWT code. We also numerically solved the generalised eigenvalue system for the 1D wave equation to determine the effects of the stratification on the kink modes of our system. The synthetic observations of the loops perturbed by the velocity pulses show a single dominant mode that our 1D analysis reveals to be the third harmonic of the system. For the broadband driver, the synthetic emission shows multiple frequency bands, associated with both the loop and the driver. Finally, using seismological techniques, we highlight the possibility of misidentifying the observed third, sixth, and ninth harmonics with the first, second, and third harmonics of the coronal part of longer loops. Unless more advanced techniques of spatial seismology are used with many data points from observations along the entire loop length, this misidentification can result in overestimating the mean magnetic field by a factor equal to the period ratio of the fundamental over the third harmonic.

astro-ph.SR↗

arXiv2024

On the Existence of Long-Period Decayless Oscillations in Short Active Region Loops

Decayless kink oscillations, characterized by their lack of decay in amplitude, have been detected in coronal loops of varying scales in active regions, quiet Sun and coronal holes. Short-period (< 50 s) decayless oscillations have been detected in short loops (< 50 Mm) within active regions. Nevertheless, long-period decayless oscillations in these loops remain relatively unexplored and crucial for understanding the wave modes and excitation mechanisms of decayless oscillations. We present the statistical analysis of decayless oscillations from two active regions observed by the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter. The average loop length and period of the detected oscillations are 19 Mm and 151 seconds, respectively. We find 82 long-period and 23 short-period oscillations in these loops. We do not obtain a significant correlation between loop length and period. We discuss the possibility of different wave modes in short loops, although standing waves can not be excluded from possible wave modes. Furthermore, a different branch exists for active region short loops in the loop length vs period relation, similar to decayless waves in short loops in quiet Sun and coronal holes. The magnetic fields derived from MHD seismology, based on standing kink modes, show lower values for multiple oscillations compared to previous estimates for long loops in active regions. Additionally, the comparison of period distributions in short loops across different coronal regions indicates that different excitation mechanisms may trigger short-period kink oscillations in active regions compared to the quiet Sun and coronal holes.

astro-ph.SR↗

arXiv2024

Measurements of the solar coronal magnetic field based on coronal seismology with propagating Alfvenic waves: forward modeling

Recent observations have demonstrated the capability of mapping the solar coronal magnetic field using the technique of coronal seismology based on the ubiquitous propagating Alfvenic/kink waves through imaging spectroscopy. We established a magnetohydrodynamic (MHD) model of a gravitationally stratified open magnetic flux tube, exciting kink waves propagating upwards along the tube. Forward modeling was performed to synthesize the Fe XIII 1074.7 and 1079.8 nm spectral line profiles, which were then used to determine the wave phase speed, plasma density, and magnetic field with seismology method. A comparison between the seismologically inferred results and the corresponding input values verifies the reliability of the seismology method. In addition, we also identified some factors that could lead to errors during magnetic field measurements. Our results may serve as a valuable reference for current and future coronal magnetic field measurements based on observations of propagating kink waves.

astro-ph.SR↗

arXiv2024

Coronal heating

Coronal heating refers to the physical processes that shape and structure the corona of the Sun and are responsible for its multi-million Kelvin temperatures. These processes are revealed in a number of different observational manifestations and have been studied on theoretical grounds in great detail over the last eight decades. The aim of this Chapter is to give an account of some of those manifestations and to discuss relevant physics that we believe is responsible for them. Coronal heating is closely connected to other magnetohydrodynamic (MHD) processes occurring in the solar plasma and described in this book such as waves, shocks, instabilities, and magnetic reconnection.

astro-ph.SR↗

arXiv2024

Undersampling effects on observed periods of coronal oscillations

Context. Recent observations of decayless transverse oscillations have shown two branches in the relationship between periods and loop lengths. One is a linear relationship, interpreted as a standing mode. The other shows almost no correlation and has not yet been interpreted conclusively. Aims. We investigated the undersampling effect on observed periods of decayless oscillations. Methods. We considered oscillating coronal loops that closely follow the observed loop length distribution. Assuming that all oscillations are standing waves, we modeled a signal that represents decayless oscillations where the period is proportional to the loop length and the amplitude and phase are randomly drawn. A downsampled signal was generated from the original signal by considering different sample rates that mimic temporal cadences of telescopes, and periods for sampled signals were analysed using the fast Fourier transform. Results. When the sampling cadence is getting closer to the actual oscillation period, a tendency for overestimating periods in short loops is enhanced. The relationship between loop lengths and periods of the sampled signals shows the two branches as in the observation. Conclusions. We find that long periods of decayless oscillations occurring in short loops could be the result of undersampling.

astro-ph.SR↗

arXiv2024

Exploring the dynamic rotational profile of the hotter solar atmosphere: A multi-wavelength approach using SDO/AIA data

Understanding the global rotational profile of the solar atmosphere and its variation is fundamental to uncovering a comprehensive understanding of the dynamics of the solar magnetic field and the extent of coupling between different layers of the Sun. In this study, we employ the method of image correlation to analyze the extensive dataset provided by the Atmospheric Imaging Assembly of the Solar Dynamic Observatory in different wavelength channels. We find a significant increase in the equatorial rotational rate ($A$) and a decrease in absolute latitudinal gradient ($|B|$) at all temperatures representative of the solar atmosphere, implying an equatorial rotation up to $4.18\%$ and $1.92\%$ faster and less differential when compared to the rotation rates for the underlying photosphere derived from Doppler measurement and sunspots respectively. In addition, we also find a significant increase in equatorial rotation rate ($A$) and a decrease in differential nature ($|B|$ decreases) at different layers of the solar atmosphere. We also explore a possible connection from the solar interior to the atmosphere and interestingly found that $A$ at $r=0.94\,\mathrm{R}_{\odot}, 0.965\,\mathrm{R}_{\odot}$ show an excellent match with 171 Angstrom, 304 Angstrom and 1600 Angstrom, respectively. Furthermore, we observe a positive correlation between the rotational parameters measured from 1600 Angstrom, 131 Angstrom, 193 Angstrom and 211 Angstrom with the yearly averaged sunspot number, suggesting a potential dependence of the solar rotation on the appearance of magnetic structures related to the solar cycle or the presence of cycle dependence of solar rotation in the solar atmosphere.

astro-ph.SR↗

arXiv2024

Propagating Kink Waves in an Open Coronal Magnetic Flux Tube with Gravitational Stratification: Magnetohydrodynamic Simulation and Forward Modelling

Context. In the coronal open-field regions, such as coronal holes, there are many transverse waves propagating along magnetic flux tubes, generally interpreted as kink waves. Previous studies have highlighted their potential in coronal heating, solar wind acceleration, and seismological diagnostics of various physical parameters. Aims. This study aims to investigate propagating kink waves, considering both vertical and horizontal density inhomogeneity, using three-dimensional magnetohydrodynamic (MHD) simulations. Methods. We establish a 3D MHD model of a gravitationally stratified open flux tube, incorporating a velocity driver at the lower boundary to excite propagating kink waves. Forward modelling is conducted to synthesise observational signatures of the Fe ix 17.1 nm line. Results. It is found that resonant absorption and density stratification both affect the wave amplitude. When diagnosing the relative density profile with velocity amplitude, resonant damping needs to be properly considered to avoid possible underestimation. In addition, unlike standing modes, propagating waves are believed to be Kelvin-Helmholtz stable. In the presence of vertical stratification, however, phase mixing of transverse motions around the tube boundary can still induce small scales, partially dissipating wave energy and leading to a temperature increase, especially at higher altitudes. Moreover, forward modeling is conducted to synthesise observational signatures, revealing the promising potential of future coronal imaging spectrometers such as MUSE in resolving these wave-induced signatures. Also, the synthesised intensity signals exhibit apparent periodic variations, offering a potential method to indirectly observe propagating kink waves with current EUV imagers.

astro-ph.SR↗

arXiv2024

Modelling the connection between propagating disturbances and solar spicules

Propagating (intensity) disturbances (PDs) are well reported in observations of coronal loops and polar plumes in addition to recent links with co-temporal spicule activity in the solar atmosphere. However, despite being reported in observations, they are yet to be studied in depth and understood from a modelling point of view. In this work, we present results from a 3D MHD numerical model featuring a stratified solar atmosphere which is perturbed by a p-mode wave driver at the photosphere, subsequently forming spicules described by the rebound shock model. Features with striking characteristics to those of detected PDs appear consistent with the co-temporal transition region dynamics and spicular activity resulting from nonlinear wave steepening and shock formation. Furthermore, the PDs can be interpreted as slow magnetoacoustic pulses propagating along the magnetic field, rather than high speed plasma upflows, carrying sufficient energy flux to at least partially heat the lower coronal plasma. Using forward modelling, we demonstrate the similarities between the PDs in the simulations and those reported in observations from IRIS and SDO/AIA. Our results suggest that, in the presented model, the dynamical movement of the transition region is a result of wave dynamics and shock formation in the lower solar atmosphere, and that PDs are launched co-temporally with the rising of the transition region, regardless of the wave-generating physical mechanisms occurring in the underlying lower solar atmosphere. However, it is clear that signatures of PDs appear much clearer when a photospheric wave driver is included. Finally, we present the importance of PDs in the context of providing a source for powering the (fast) solar wind

astro-ph.SR↗

arXiv2024

Kelvin-Helmholtz instability and heating in oscillating loops perturbed by power-law transverse wave drivers

Instabilities in oscillating loops are believed to be essential for dissipating the wave energy and heating the solar coronal plasma. Our aim is to study the development of the Kelvin-Helmholtz (KH) instability in an oscillating loop that is driven by random footpoint motions. Using the PLUTO code, we performed 3D simulations of a straight gravitationally stratified flux tube. The loop footpoints are embedded in chromospheric plasma, in the presence of thermal conduction and an artificially broadened transition region. Using drivers with a power-law spectrum, one with a red noise spectrum and one with the low-frequency part subtracted, we excited standing oscillations and the KH instability in our loops, after one-and-a-half periods of the oscillation. We see that our broadband drivers lead to fully deformed, turbulent loop cross-sections over the entire coronal part of the loop due to the spatially extended KH instability. The low RMS velocity of our driver without the low-frequency components supports the working hypothesis that the KH instability can easily manifest in oscillating coronal loops. We report for the first time in driven transverse oscillations of loops the apparent propagation of density perturbations due to the onset of the KH instability, from the apex towards the footpoints. Both drivers input sufficient energy to drive enthalpy and mass flux fluctuations along the loop, while also causing heating near the driven footpoint of the oscillating loop, which becomes more prominent when a low-frequency component is included in the velocity driver. Finally, our power-law driver with the low-frequency component provides a RMS input Poynting flux of the same order as the radiative losses of the quiet-Sun corona, giving us promising prospects for the contribution of decayless oscillations in coronal heating.

astro-ph.SR↗

arXiv2024

Characteristics and Energy Flux Distributions of Decayless Transverse Oscillations Depending on Coronal Regions

Lim et al. (2023) have recently proposed that the slope ($δ$) of the power law distribution between the energy flux and oscillation frequency could determine whether high-frequency transverse oscillations give a dominant contribution to the heating ($δ<1$). A meta-analysis of decayless transverse oscillations revealed that high-frequency oscillations potentially play a key role in heating the solar corona. We aim to investigate how (whether) the distributions of the energy flux contained in transverse oscillations, and their slopes, depend on the coronal region in which the oscillation occurs. We analyse transverse oscillations from 41 quiet Sun (QS) loops and 22 active region (AR) loops observed by Solar Orbiter/Extreme Ultraviolet Imager (SolO/EUI) HRIEUV. The energy flux and energy are estimated using analysed oscillation parameters and loop properties, such as periods, displacement amplitudes, loop lengths, and minor radii of the loops. It is found that about 71% of QS loops and 86% of AR loops show decayless oscillations. We find that the amplitude does not change depending on different regions, but the difference in the period is more pronounced. Although the power law slope ($δ=-1.79$) in AR is steeper than that ($δ=-1.59$) in QS, both of them are significantly less than the critical slope of 1. Our statistical study demonstrates that high-frequency transverse oscillations can heat the QS. For ARs, the total energy flux is insufficient unless yet-unobserved oscillations with frequencies up to 0.17 Hz are present. Future EUI campaigns will be planned to confirm this.

astro-ph.SR↗

arXiv2024

Solar wind data analysis aided by synthetic modeling: a better understanding of plasma-frame variations from temporal data

In-situ measurements of the solar wind, a turbulent and anisotropic plasma flow originating at the Sun, are mostly carried out by single spacecraft, resulting in one-dimensional time series. The conversion of these measurements to the spatial frame of the plasma is a great challenge, but required for direct comparison of the measurements with MHD turbulence theories. Here we present a toolkit, based on the synthetic modeling of solar wind fluctuations as two-dimensional noise maps with adjustable spectral and power anisotropy, that can help with the temporal-spatial conversion of real data. Specifically, by following the spacecraft trajectory through a noise map (relative velocity and angle relative to some mean magnetic field) with properties tuned to mimic those of the solar wind, the likelihood that the temporal data fluctuations represent parallel or perpendicular fluctuations in the plasma frame can be quantified by correlating structure functions of the noise map. Synthetic temporal data can also be generated, which can provide a testing ground for analysis applied to the solar wind data. We demonstrate this tool by investigating Parker Solar Probe's E7 trajectory and data, and showcase several possible ways in which it can be used. We find that whether temporal variations in the spacecraft frame come from parallel or perpendicular variations in the plasma frame strongly depends on the spectral and power anisotropy of the measured wind. Data analysis assisted by such underlying synthetic models as presented here could open up new ways to interpret measurements in the future, specifically in the more reliable determination of plasma frame quantities from temporal measurements.

astro-ph.SR↗

arXiv2024

Horizontally and vertically polarized kink oscillations in curved solar coronal loops

Kink oscillations are frequently observed in coronal loops. This work aims to numerically clarify the influence of loop curvature on horizontally and vertically polarized kink oscillations. Working within the framework of ideal MHD, we conduct 3D simulations of axial fundamental kink oscillations in curved density-enhanced loops embedded in a potential magnetic field. Both horizontal and vertical polarizations are examined, and their oscillation frequencies are compared with WKB expectations. We discriminate two different density specifications. In the first (dubbed"uniform-density"), the density is axially uniform and varies continuously in the transverse direction toward a uniform ambient corona. Some further stratification is implemented in the second specification (dubbed"stratified"), allowing us to address the effect of evanescent barriers. Examining the oscillating profiles of the initially perturbed uniform-density loops, we found that the frequencies for both polarizations deviate from the WKB expectation by $\sim 10\%$. In the stratified loop, however, the frequency of the horizontal polarization deviates to a larger extent ($\sim 25\%$). We illustrate the lateral leakage of kink modes through wave tunnelling in 3D simulations, for the first time. Despite this, in both loops, the damping time-to-period ratios are similar and close to the analytical predictions for straight configurations under the thin-tube-thin-boundary (TTTB) assumption. The WKB expectation for straight configurations can reasonably describe the eigenfrequency of kink oscillations only in loops without an asymmetrical cross-loop density profile perpendicular to the oscillating direction. Lateral leakage via wave tunnelling is found to be less efficient than resonant absorption, meaning that the latter remains a robust damping mechanism for kink motions even when loop curvature is included.

astro-ph.SR↗

arXiv2024

A Statistical Investigation of Decayless Oscillations in Small-scale Coronal Loops Observed by Solar Orbiter/EUI

Decayless kink oscillations are omnipresent in the solar atmosphere and a viable candidate for coronal heating. Though there have been extensive studies of decayless oscillations in coronal loops with a few hundred Mm lengths, the properties of these oscillations in small-scale ($\sim$10 Mm) loops are yet to be explored. In this study, we present the properties of decayless oscillations in small loops embedded in the quiet corona and coronal holes. We use high resolution observations from the Extreme Ultraviolet Imager onboard Solar Orbiter with pixel scales of 210 km and 5 s cadence or better. We find 42 oscillations in 33 coronal loops with loop lengths varying between 3 to 23 Mm. The average displacement amplitude is found to be 136 km. The oscillations period has a range of 27 to 276 s, and the velocity amplitudes range from 2.2 to 19.3 km s$^{-1}$. The observed kink speeds are lower than those observed in active region coronal loops. The variation of loop length with the period does not indicate a strong correlation. Coronal seismology technique indicated an average magnetic field value of 2.1 G. We estimate the energy flux with a broad range of 0.6-314 W m$^{-2}$. Moreover, we note that the short-period decayless oscillations are not prevalent in the quiet Sun and coronal holes. Therefore, our study suggests that decayless oscillations in small-scale coronal loops are unlikely to provide enough energy to heat the quiet Sun and accelerate solar wind in the coronal holes.

astro-ph.SR↗

arXiv2024

Multithermal apparent damping of slow waves due to strands with a Gaussian temperature distribution

Context. Slow waves in solar coronal loops are strongly damped. The current theory of damping by thermal conduction cannot explain some observational features.\n Aims. We investigate the propagation of slow waves in a coronal loop built up from strands of different temperatures. \n Methods. We consider the loop to have a multithermal, Gaussian temperature distribution. The different propagation speeds in different strands lead to an multithermal apparent damping of the wave, similar to observational phase mixing. We use an analytical model to predict the damping length and propagation speed for the slow waves, including in imaging with filter telescopes. \n Results. We compare the damping length due to this multithermal apparent damping with damping due to thermal conduction and find that the multithermal apparent damping is more important for shorter period slow waves. We have found the influence of instrument filters on the wave's propagation speed and damping. This allows us to compare our analytical theory to forward models of numerical simulations. \n Conclusions. We find that our analytical model matches the numerical simulations very well. Moreover, we offer an outlook for using the slow wave properties to infer the loop's thermal properties.

astro-ph.SR↗

arXiv2024

The magnetohydrodynamic equations in terms of waveframe variables

Generalising the Elsasser variables, we introduce the Q-variables. These are more flexible than the Elsasser variables, because they also allow to track waves with phase speeds different than the Alfven speed. We rewrite the MHD equations with these Q-variables. We consider also the linearised version of the resulting MHD equations in a uniform plasma, and recover the classical Alfven waves, but also separate the fast and slow magnetosonic waves in upward and downward propagating waves. Moreover, we show that the Q-variables may also track the upward and downward propagating surface Alfven waves in a non-uniform plasma, displaying the power of our generalisation. In the end, we lay the mathematical framework for driving solar wind models with a multitude of wave drivers.

physics.plasm-ph↗

arXiv2024

Mode conversion and energy flux absorption in the structured solar atmosphere

Context. Structuring in the solar atmosphere, in the form of inhomogeneities transverse to the magnetic field, is believed to play a vital role in wave propagation, conversion, and absorption. Aims. We investigated the effect of transverse structuring on the processes of mode conversion and wave energy flux absorption using a 3D ideal magnetohydrodynamic simulation featuring an expanding coronal loop in a gravitationally stratified atmosphere. Methods. Multiple wave drivers were modelled. The location of the driver at the photospheric base was allowed to vary so that we could study how the driven waves interact with the transverse structuring, provided by the magnetic field, as well as with the vertical structuring due to gravity. Results. We find that the transverse structuring acts as a conduit for Alfvén wave energy flux through the transition region and into the solar corona. Moreover, in regions of strong transverse gradients, the reflection of Alfvén waves at the transition region is greatly reduced, supporting results from recent studies. Finally, we investigated the efficiency of the loop structuring at absorbing energy flux from externally driven waves and find that the loop is extremely effective at channelling wave energy flux to the loop apex in the corona; in some cases, it can absorb over a third of the externally driven wave energy flux. Conclusions. These results may have important consequences in the context of decayless loop oscillations as they suggest that the oscillations are driven by acoustic waves outside of the existing loop structure.

astro-ph.SR↗

arXiv2024

Decayless oscillations in 3D coronal loops excited by a power-law driver

Aims. We studied the manifestation of decayless oscillations in 3D simulations of coronal loops, driven by random motions. Methods. Using the PLUTO code, we ran magnetohydrodynamic (MHD) simulations of a straight gravitationally stratified flux tube, with its footpoints embedded in chromospheric plasma. We consider transverse waves drivers with a horizontally polarised red noise power-law spectrum. Results. Our broadband drivers lead to the excitation of standing waves with frequencies equal to the fundamental standing kink mode and its harmonics. These standing kink oscillations have non-decaying amplitudes, and spectra that depend on the characteristics of the loops, with the latter amplifying the resonant frequencies from the drivers. We thus report for the first time in 3D simulations the manifestation of decayless oscillations from broadband drivers. The spatial and temporal evolution of our oscillation spectra reveals the manifestation of a half harmonic, which exhibits half the frequency of the identified fundamental mode with a similar spatial profile. Our results suggest that this mode is related to the presence of the transition region in our model and could be interpreted as being the equivalent to the fundamental mode of standing sound waves driven on pipes closed at one end. Conclusions. The potential existence of this half harmonic has important implications for coronal seismology, since misinterpreting it for the fundamental mode of the system can lead to false estimations of the average kink speed profile along oscillating loops. Finally, its detection could potentially give us a tool for distinguishing between different excitation and driving mechanisms of decayless oscillations in observations.

astro-ph.SR↗