Author name code: verth ADS astronomy entries on 2022-09-14 author:"Verth, Gary" ------------------------------------------------------------------------ Title: Comparison of exact and approximate MHD slow body mode solutions in photospheric waveguides Authors: Aldhafeeri, Anwar; Verth, Gary; Fedun, Viktor; Lennard, Matthew; Ballai, Istvan Bibcode: 2022arXiv220904689A Altcode: In this study we explore the possibility of simplifying the modeling of magnetohydrodynamic (MHD) slow body modes observed in photospheric magnetic structure such as the umbrae of sunspots and pores. The simplifying approach assumes that the variation of the eigenvalues of slow body waves can be derived by imposing that the longitudinal component of velocity with respect to the tube axis is zero at the boundary of the magnetic flux tube, which is in a good agreement with observations. To justify our approach we compare the results of our simplified model for slow body modes in cylindrical flux tubes with the model prediction obtained by imposing the continuity of the radial component of the velocity and total pressure at the boundary of the flux tube. Our results show that, to a high accuracy (less than 1\% for the considered model), the conditions of continuity of the component of transversal velocity and pressure at the boundary can be neglected when modelling slow body modes under photospheric conditions. Title: The properties of magnetoacoustic waves in magnetic flux tubes with background rotational flows Authors: Skirvin, Samuel; Fedun, Viktor; van Doorsselaere, Tom; Goossens, Marcel; Verth, Gary; Claes, Niels Bibcode: 2022cosp...44.2547S Altcode: Rotational flows are commonly observed features in structures within the lower solar atmosphere, for example in solar vortices and within intergranular lanes. It is to be expected that the presence of an equilibrium rotational flow inside a magnetic flux tube will modify the behaviour of propagating magnetoacoustic (MA) waves inside the structure. Using a previously developed numerical eigensolver, we obtain the eigenvalues for both the sausage and kink modes of the magnetic flux tube in the presence of a background rotational flow under photospheric conditions. The effect of both the amplitude of the rotational flow and the flow's radial structure are investigated. It is found that when the rotational flow is linear, the modified slow continuum, shifted due to the background flow, reduces to single-point values. However, in the case when the flow is non linear, the modified slow continuum now occupies a band of frequencies. The radial structure of the background rotational flow has important consequences for determining which wave modes are absorbed into the continua. Furthermore, we present for the first time 2D velocity field visualisations showing the resulting wave perturbations alongside incorporation of the additional background flow for both the sausage and kink modes. For both cases of linear and nonlinear background rotational flow, it is shown that the velocity fields with the additional background flow display different characteristics when compared to the perturbation alone, which may be useful for observers when interpreting high resolution data from e.g. DKIST. Finally, we present initial results investigating the nature of MA waves in rotating magnetic flux tubes with an additional vertical flow component, a configuration commonly seen in simulations of solar vortices. Title: Small-scale solar jet formation and their associated waves and instabilities Authors: Skirvin, Samuel; Verth, Gary; Juan González-Avilés, José; Shelyag, Sergiy; Sharma, Rahul; Guzmán, Fransisco; Ballai, Istvan; Scullion, Eamon; Silva, Suzana S. A.; Fedun, Viktor Bibcode: 2022arXiv220509598S Altcode: Studies on small-scale jets' formation, propagation, evolution, and role, such as type I and II spicules, mottles, and fibrils in the lower solar atmosphere's energetic balance, have progressed tremendously thanks to the combination of detailed observations and sophisticated mathematical modelling. This review provides a survey of the current understanding of jets, their formation in the solar lower atmosphere, and their evolution from observational, numerical, and theoretical perspectives. First, we review some results to describe the jet properties, acquired numerically, analytically and through high-spatial and temporal resolution observations. Further on, we discuss the role of hydrodynamic and magnetohydrodynamic instabilities, namely Rayleigh-Taylor and Kelvin-Helmholtz instabilities, in jet evolution and their role in the energy transport through the solar atmosphere in fully and partially ionised plasmas. Finally, we discuss several mechanisms of magnetohydrodynamic wave generation, propagation, and energy transport in the context of small-scale solar jets in detail. This review identifies several gaps in the understanding of small-scale solar jets and some misalignments between the observational studies and knowledge acquired through theoretical studies and numerical modelling. It is to be expected that these gaps will be closed with the advent of high-resolution observational instruments, such as Daniel K. Inouye Solar Telescope, Solar Orbiter, Parker Solar Probe, and Solar CubeSats for Linked Imaging Spectropolarimetry, combined with further theoretical and computational developments. Title: Waves in weakly ionized solar plasmas Authors: Alharbi, A.; Ballai, I.; Fedun, V.; Verth, G. Bibcode: 2022MNRAS.511.5274A Altcode: 2022MNRAS.tmp..445A; 2022arXiv220207387A Here, we study the nature and characteristics of waves propagating in partially ionized plasmas in the weakly ionized limit, typical for the lower part of the solar atmosphere. The framework in which the properties of waves are discussed depends on the relative magnitude of collisions between particles, but also on the relative magnitude of the collisional frequencies compared to the gyro-frequency of charged particles. Our investigation shows that the weakly ionized solar atmospheric plasma can be divided into two regions, and this division occurs, roughly, at the base of the chromosphere. In the solar photosphere, the plasma is non-magnetized and the dynamics can described within the three-fluid framework, where acoustic waves associated to each species can propagate. Due to the very high concentration of neutrals, the neutral sound waves propagates with no damping, while for the other two modes the damping rate is determined by collisions with neutrals. The ion- and electron-related acoustic modes propagate with a cut-off determined by the collisional frequency of these species with neutrals. In the weakly ionized chromosphere, only electrons are magnetized, however, the strong coupling of charged particles reduces the working framework to a two-fluid model. The disassociation of charged particles creates electric currents that can influence the characteristic of waves. The propagation properties of waves with respect to the angle of propagation are studied with the help of polar diagrams. Title: New Approach for Analyzing Dynamical Processes on the Surface of Photospheric Vortex Tubes Authors: Aljohani, Yasir; Fedun, Viktor; Ballai, Istvan; Silva, Suzana S. A.; Shelyag, Sergiy; Verth, Gary Bibcode: 2022ApJ...928....3A Altcode: 2022arXiv220209332A The majority of studies on multi-scale vortex motions employ a two-dimensional geometry by using a variety of observational and numerical data. This approach limits the understanding the nature of physical processes responsible for vortex dynamics. Here, we develop a new methodology to extract essential information from the boundary surface of vortex tubes. 3D high-resolution magneto-convection MURaM numerical data has been used to analyze photospheric intergranular velocity vortices. The Lagrangian averaged vorticity deviation technique was applied to define the centers of vortex structures and their boundary surfaces based on the advection of fluid elements. These surfaces were mapped onto a constructed envelope grid that allows the study of the key plasma parameters as functions of space and time. Quantities that help in understanding the dynamics of the plasma, e.g., Lorentz force, pressure force, and plasma-β were also determined. Our results suggest that, while density and pressure have a rather global behavior, the other physical quantities undergo local changes, with their magnitude and orientation changing in space and time. At the surface, the mixing in the horizontal direction is not efficient, leading to appearance of localized regions with higher/colder temperatures. In addition, the analysis of the MHD Poynting flux confirms that the majority of the energy is directed in the horizontal direction. Our findings also indicate that the pressure and magnetic forces that drive the dynamics of the plasma on vortex surfaces are unbalanced and therefore the vortices do not rotate as a rigid body. Title: Magnetohydrodynamic Wave Mode Identification in Circular and Elliptical Sunspot Umbrae: Evidence for High-order Modes Authors: Albidah, A. B.; Fedun, V.; Aldhafeeri, A. A.; Ballai, I.; Brevis, W.; Jess, D. B.; Higham, J.; Stangalini, M.; Silva, S. S. A.; Verth, G. Bibcode: 2022ApJ...927..201A Altcode: 2022arXiv220200624A In this paper, we provide clear direct evidence of multiple concurrent higher-order magnetohydrodynamic (MHD) modes in circular and elliptical sunspots by applying both proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) techniques on solar observational data. These techniques are well documented and validated in the areas of fluid mechanics, hydraulics, and granular flows but are relatively new to the field of solar physics. While POD identifies modes based on orthogonality in space and provides a clear ranking of modes in terms of their contribution to the variance of the signal, DMD resolves modes that are orthogonal in time. The clear presence of the fundamental slow sausage and kink body modes, as well as higher-order slow sausage and kink body modes, have been identified using POD and DMD analysis of the chromospheric Hα line at 6562.808 Å for both the circular and elliptical sunspots. Additionally, for the various slow body modes, evidence for the presence of the fast surface kink mode was found in the circular sunspot. All of the MHD mode patterns were cross-correlated with their theoretically predicted counterparts, and we demonstrated that ellipticity cannot be neglected when interpreting MHD wave modes. The higher-order MHD wave modes are even more sensitive to irregularities in umbral cross-sectional shapes; hence, this must be taken into account for more accurate modeling of the modes in sunspots and pores. Title: The Importance of Horizontal Poynting Flux in the Solar Photosphere Authors: Silva, Suzana S. A.; Murabito, Mariarita; Jafarzadeh, Shahin; Stangalini, Marco; Verth, Gary; Ballai, Istvan; Fedun, Viktor Bibcode: 2022ApJ...927..146S Altcode: 2022arXiv220301221S The electromagnetic energy flux in the lower atmosphere of the Sun is a key tool to describe the energy balance of the solar atmosphere. Current investigations on energy flux in the solar atmosphere focus primarily on the vertical electromagnetic flux through the photosphere, ignoring the Poynting flux in other directions and its possible contributions to local heating. Based on a realistic Bifrost simulation of a quiet-Sun (coronal hole) atmosphere, we find that the total electromagnetic energy flux in the photosphere occurs mainly parallel to the photosphere, concentrating in small regions along intergranular lanes. Thereby, it was possible to define a proxy for this energy flux based on only variables that can be promptly retrieved from observations, namely, horizontal velocities of the small-scale magnetic elements and their longitudinal magnetic flux. Our proxy accurately describes the actual Poynting flux distribution in the simulations, with the electromagnetic energy flux reaching 1010 erg cm-2 s-1. To validate our findings, we extended the analysis to SUNRISE/IMaX data. First, we show that Bifrost realistically describes photospheric quiet-Sun regions, as the simulation presents similar distributions for line-of-sight magnetic flux and horizontal velocity field. Second, we found very similar horizontal Poynting flux proxy distributions for the simulated photosphere and observational data. Our results also indicate that the horizontal Poynting flux in the observations is considerably larger than the vertical electromagnetic flux from previous observational estimates. Therefore, our analysis confirms that the electromagnetic energy flux in the photosphere is mainly horizontal and is most intense in localized regions along intergranular lanes. Title: II. The effect of axisymmetric and spatially varying equilibria and flow on MHD wave modes: cylindrical geometry Authors: Skirvin, S. J.; Fedun, V.; Silva, Suzana S. A.; Verth, G. Bibcode: 2022MNRAS.510.2689S Altcode: 2021arXiv211204427S; 2021MNRAS.tmp.3327S Magnetohydrodynamic (MHD) waves are routinely observed in the solar atmosphere. These waves are important in the context of solar physics as it is widely believed they can contribute to the energy budget of the solar atmosphere and are a prime candidate to contribute towards coronal heating. Realistic models of these waves are required representing observed configurations such that plasma properties can be determined more accurately, since they cannot be measured directly. This work utilizes a previously developed numerical technique to find permittable eigenvalues under different non-uniform equilibrium conditions in a Cartesian magnetic slab geometry. Here, we investigate the properties of magnetoacoustic waves under non-uniform equilibria in a cylindrical geometry. Previously obtained analytical results are retrieved to emphasize the power and applicability of this numerical technique. Further case studies investigate the effect that a radially non-uniform plasma density and non-uniform plasma flow, modelled as a series of Gaussian profiles, have on the properties of different MHD waves. For all cases the dispersion diagrams are obtained and spatial eigenfunctions calculated which display the effects of the equilibrium inhomogeneity. It is shown that as the equilibrium non-uniformity is increased, the radial spatial eigenfunctions are affected and extra nodes introduced, similar to the previous investigation of a magnetic slab. Furthermore, azimuthal perturbations are increased with increasing inhomogeneity introducing vortical motions inside the waveguide. Finally, 2D and 3D representations of the velocity fields are shown which may be useful for observers for wave mode identification under realistic magnetic waveguides with ever increasing instrument resolution. Title: Large scale coherent magnetohydrodynamic oscillations in a sunspot Authors: Stangalini, M.; Verth, G.; Fedun, V.; Aldhafeeri, A. A.; Jess, D. B.; Jafarzadeh, S.; Keys, P. H.; Fleck, B.; Terradas, J.; Murabito, M.; Ermolli, I.; Soler, R.; Giorgi, F.; MacBride, C. D. Bibcode: 2022NatCo..13..479S Altcode: Although theoretically predicted, the simultaneous excitation of several resonant modes in sunspots has not been observed. Like any harmonic oscillator, a solar magnetic flux tube can support a variety of resonances, which constitute the natural response of the system to external forcing. Apart from a few single low order eigenmodes in small scale magnetic structures, several simultaneous resonant modes were not found in extremely large sunspots. Here we report the detection of the largest-scale coherent oscillations observed in a sunspot, with a spectrum significantly different from the Sun's global acoustic oscillations, incorporating a superposition of many resonant wave modes. Magnetohydrodynamic numerical modeling agrees with the observations. Our findings not only demonstrate the possible excitation of coherent oscillations over spatial scales as large as 30-40 Mm in extreme magnetic flux regions in the solar atmosphere, but also paves the way for their diagnostic applications in other astrophysical contexts. Title: The Solar Magnetic Vortices Authors: Silva, Suzana; Verth, Gary; Rempel, Erico; Shelyag, Sergiy; Schiavo, Luiz C. A.; Fedun, Viktor Bibcode: 2021AGUFMSH45B2374S Altcode: Photospheric flows play a crucial role in the dynamical evolution of the solar atmosphere as they are inherently coupled to magnetic fields. This leads to a wealth class of events, like solar tornados and flares. One key magnetic structure is the twisted flux tubes, linked to eruptive phenomena, plasma jets and heating. However, the lack of a universal definition precluded the automatic detection of twisted flux tubes and, thus, there were no available databases for significant statistical analysis of their morphology and properties.This work introduces the magnetic flux tube as a magnetic vortex (M-vortex): a new typology of a solar vortex. We apply a formal definition based on a recently developed magnetic coherent structure detection technique, the Integrated Averaged Current Deviation (IACD) method, to define the M-vortex boundary. By applying this methodology to the realistic magnetoconvection simulations of a solar plage it was found that M-vortices appear in the intergranular regions where shear flows and high value of plasma beta are present. Their lifetime is, on average, around a minute and they locally concentrate the vertical magnetic field and current density. Based on M-vortex magnetic to kinetic energy ratio, our results indicate two distinct types of them. Type (i): when magnetic energy dominates, the twisted magnetic flux tube is mainly oriented perpendicular to the solar surface. Type (ii): when plasma inside of the M-vortex has more considerable kinetic energy, the magnetic structure can present various shapes, sizes and magnetic field geometry. It was also found that magnetic and flow vortices (kinetic vortices, i.e. K-vortices) are different structures, but they may intersect, interfering in each other's dynamics. The M-vortices may appear if two conditions are simultaneously found in the plasma flow: (i) shear and (ii) plasma-beta>1. Our results show that the presence of a K-vortex is not a necessary condition for the formation of M-vortices. Title: Using Machine Learning Tools To Estimate Photospheric Velocity Fields Prior To The Formation Of Active Regions. Authors: Lennard, Matthew; Tremblay, Benoit; Asensio Ramos, Andres; Hotta, Hideyuki; Iijima, Haruhisa; Park, Sung-Hong; Silva, Suzana; Verth, Gary; Fedun, Viktor Bibcode: 2021AGUFMSH45B2371L Altcode: In recent years a number of major advances have been made using numerical modelling to better our understanding of magnetic structures and the evolution of active regions (AR, see e.g. Hotta & Iijima, 2020; Chen et al, 2021). In particular, these high resolution simulations provide us with the means to study the photospheric flows associated with the aforementioned magnetic structures. In practice, plasma flows at the solar surface cannot be directly recovered; the component transverse to the line-of-sight must be inferred from observational data. Inferences depend on the method, the observational data used as input, the spatial resolution of the data and its cadence. Tracking methods such as local correlation tracking (LCT) are promising, but the flows they measure are optical and cannot be used to estimate the flow patterns of an AR until shortly before the emergence of flux. Another issue with applying LCT to estimate AR flows is that the recovered velocity field is usually not smooth, which precludes advanced flow analysis. Besides, depending on the data, there is a considerable chance of having 'holes' in the velocity field. Therefore, although LCT methodology can help give a hint on general flow properties, a more sophisticated technique is necessary to perform proper analysis on the flow topology. There are also limitations in the region of the Sun in which we can accurately track flows as well as problems with accurately extracting longitudinal and latitudinal velocities. Recently, deep learning has shown promise in capturing subtleties in Quiet Sun flows at spatial and temporal scales that typically cannot be recovered by tracking methods (Asensio Ramos et al, 2017). The DeepVel neural network is trained to infer plasma flows from surface data using examples from detailed numerical models (i.e., supervised learning). Using a version of DeepVel that was trained using a high-resolution numerical simulation of the evolution of an AR (e.g. Hotta & Iijima, 2020) we developed the algorithm for predicting flow trajectories from high resolution observational data. This method was directly compared with previous contenders for tracking flows and shows more realistic plasma flow field estimation as well as an increase of reconstruction efficiency. Title: On horizontal Poynting flux in the solar photosphere Authors: Silva, Suzana; Murabito, Mariarita; Jafarzadeh, Shahin; Stangalini, Marco; Verth, Gary; Ballai, Istvan; Fedun, Viktor Bibcode: 2021AGUFMSH44A..03S Altcode: Describing the solar atmospheric energy balance and transport is an essential step to understanding the high temperatures of the upper atmosphere. This work analyses the 3D electromagnetic energy flux in the lower atmosphere by combining Bifrost radiative MHD simulations and Sunrise/IMaX data. Based on a simulated quiet Sun atmosphere, it was found that only a minor fraction of the Poynting flux propagates upwards in the photosphere. Most of the total electromagnetic energy flows parallel to the solar surface, concentrating energy in small regions along the intergranular lanes. The dominance of the horizontal component of the electromagnetic energy flux allows an approximation for the horizontal Poynting flux, which is based solely on the horizontal velocity and the vertical magnetic field. The proxy to the horizontal Poynting flux provides a very similar distribution of the total Poynting flux and can describe the total flux for most of the photosphere with a small relative error, <30%, in regions with an intense concentration of electromagnetic energy. The results of the numerical data analysis were validated by using observational data. First, it was shown that both Bifrost and IMAX/sunrise data presents similar distributions for line-of-sight magnetic field and velocity field, indicating that the simulation realistically describes a quiet Sun region. The horizontal Poynting flux proxy provided very similar distributions for the numerical and observational data, which is considerably larger than previous observational estimates for upwards electromagnetic flux. Thereby, those findings corroborate that the electromagnetic energy flux in the photosphere is mainly parallel to the solar surface and can be properly described by approximated Poynting flux, based only on the horizontal plasma flow and the vertical magnetic field. The results also indicate that small scale intergranular motion may be important to describe the properties of Poynting flux. Therefore, further investigations based on the high-resolution data from DKIST will be important for a valuable description of the energy transport in the lower atmosphere. Title: Propagation of Magnetoacoustic Waves in Symmetrically Structured Non-Uniform Solar Waveguides with Spatially Varying Equilibria and Flow Authors: Skirvin, Samuel; Fedun, Viktor; Verth, Gary Bibcode: 2021AGUFMSH45B2366S Altcode: The modern ground- and space-based instruments (DST, SST, DKIST, SDO, Hinode, Solar Orbiter) provide solar physicists an ample of observations of solar plasma processes, i.e. magnetic bright points, spicules, plasma flows, structure of magnetic fields etc. at different time and spatial scales. Therefore, advanced theoretical modelling becomes essential to explain observational results and provide more accurate information about MHD wave propagation and solar atmospheric plasma properties. In this work, we discuss a variety of theoretically constructed 2-3D MHD equilibria obtained by considering different internal density and flow profiles. To obtain the numerical solution, the shooting method has been used to match necessary boundary conditions on continuity of displacement and total pressure of the waveguide. The analytic dispersion relation is not required, making this numerical approach a very powerful tool. The proposed methodology has been successfully tested against well-known analytical results obtained for uniform slab and cylinder. Considering both a magnetic slab (Skirvin et al. MNRAS 2021) and cylindrical geometry, the dispersion diagrams and eigenfunctions were obtained numerically for the case where the equilibrium plasma density is modelled as a series of Gaussian profiles. Furthermore, an additional analysis was conducted investigating a non-uniform background plasma flow. We have found that under coronal conditions, with increasing inhomogeneity in the initial equilibria, additional nodes appear in the resulting eigenfunctions for slow body sausage and kink modes. This could lead to an alternative interpretation of some existing observational results and may be used as a new technique for inhomogeneity of density and flow detection in the solar magnetic structures. Furthermore, in a cylindrical geometry, we discuss the effects that a non-uniform equilibrium has on the physical appearance of the perturbed boundary and resulting velocity and vorticity fields. Title: Kink Oscillations of Coronal Loops Authors: Nakariakov, V. M.; Anfinogentov, S. A.; Antolin, P.; Jain, R.; Kolotkov, D. Y.; Kupriyanova, E. G.; Li, D.; Magyar, N.; Nisticò, G.; Pascoe, D. J.; Srivastava, A. K.; Terradas, J.; Vasheghani Farahani, S.; Verth, G.; Yuan, D.; Zimovets, I. V. Bibcode: 2021SSRv..217...73N Altcode: 2021arXiv210911220N 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. Title: Solar Vortex Tubes. II. On the Origin of Magnetic Vortices Authors: Silva, Suzana S. A.; Verth, Gary; Rempel, Erico L.; Shelyag, Sergiy; Schiavo, Luiz A. C. A.; Fedun, Viktor Bibcode: 2021ApJ...915...24S Altcode: The solar atmosphere presents a wealth of dynamics due to a constant interplay between the plasma flows and magnetic fields. Twisted flux tubes are an essential magnetic structure, believed to be driven by the rotational surface's motions and linked to plasma heating, jets, and eruptive phenomena. Despite extensive investigations, twisted magnetic flux tubes lack a proper mathematical definition, precluding their automatic detection. This work addresses this issue by defining them as magnetic vortices and introduces a formal definition that is based on a recently developed magnetic vortex detection technique, the integrated averaged current deviation method. We applied this method and a kinetic vortex identification technique to realistic magnetoconvection simulations obtained from the MURaM code. The preferential site for these two types of vortices is the intergranular downflow, but while the magnetic vortices are found mostly in the small areas where plasma-β > 1, the rotational flow structures (the kinetic vortices), were detected in locations where plasma-β < 1. The magnetic vortices locally concentrate the magnetic field's vertical components and current, lasting, on average, around a minute. Two types of magnetic vortices are introduced based on their magnetic-to-kinetic energy ratio. For the first type, the magnetic energy prevails, and the magnetic vortices are mostly vertical. The second type of magnetic vortex presents distinct shapes and a lower magnetic-to-kinetic energy ratio. We have found that magnetic vortices may appear if two conditions are simultaneously present: (i) shear flow and (ii) plasma-β > 1. The presence of rotational motion is not necessary. Title: I. The effect of symmetric and spatially varying equilibria and flow on MHD wave modes: slab geometry Authors: Skirvin, S. J.; Fedun, V.; Verth, G. Bibcode: 2021MNRAS.504.4077S Altcode: 2021MNRAS.tmp.1155S Realistic theoretical models of magnetohydrodynamic wave propagation in the different solar magnetic configurations are required to explain observational results, allowing magnetoseismology to be conducted and provide more accurate information about local plasma properties. The numerical approach described in this paper allows a dispersion diagram to be obtained for any arbitrary symmetric magnetic slab model of solar atmospheric features. This proposed technique implements the shooting method to match necessary boundary conditions on continuity of displacement and total pressure of the waveguide. The algorithm also implements fundamental physical knowledge of the sausage and kink modes such that both can be investigated. The dispersion diagrams for a number of analytic cases that model magnetohydrodynamic waves in a magnetic slab were successfully reproduced. This work is then extended by considering density structuring modelled as a series of Gaussian profiles and a sinc(x) function. A further case study investigates properties of MHD wave modes in a coronal slab with a non-uniform background plasma flow, for which the governing equations are derived. It is found that the dispersive properties of slow body modes are more greatly altered than those of fast modes when any equilibrium inhomogeneity is increased, including background flow. The spatial structure of the eigenfunctions is also modified, introducing extra nodes and points of inflexion that may be of interest to observers. Title: Magnetohydrodynamic Wave Modes of Solar Magnetic Flux Tubes with an Elliptical Cross Section Authors: Aldhafeeri, Anwar A.; Verth, Gary; Brevis, Wernher; Jess, David B.; McMurdo, Max; Fedun, Viktor Bibcode: 2021ApJ...912...50A Altcode: The purpose of this paper is to study the behavior of magnetohydrodynamic (MHD) wave modes that propagate in compressible magnetic flux tubes with an elliptical cross section embedded in a magnetic environment. The dispersion relation that describes the behavior of MHD wave modes permitted in an elliptical magnetic flux tube is solved numerically. Distortion of the spatial structure of the purely real eigenmodes from the well-known circular flux tube model has been considered. It has been studied under both photospheric and coronal conditions. It has been shown that (i) solutions in the form of even Mathieu functions are more sensitive to the value of eccentricity than solutions with the form of odd Mathieu functions; (ii) if the ellipticity of the cross section of the magnetic flux tube increases, a sausage mode (m = 0) cannot be easily identified; (iii) even solutions that correspond to the fluting mode (m = 3) can be misinterpreted as a kink mode (m = 1) due to their similarities. In contrast to the fluting modes that are polarized along the major axis and strongly depend on the ellipticity of the magnetic flux tube, the kink and sausage surface modes are practically unaffected by ellipticity. Several examples of the spatial structure of the eigenmodes permitted in the pores and sunspots have been visualized. The solutions obtained in the approximation of cylindrical symmetry are in agreement with previous studies. Title: A novel approach to identify resonant MHD wave modes in solar pores and sunspot umbrae: B − ω analysis Authors: Stangalini, M.; Jess, D. B.; Verth, G.; Fedun, V.; Fleck, B.; Jafarzadeh, S.; Keys, P. H.; Murabito, M.; Calchetti, D.; Aldhafeeri, A. A.; Berrilli, F.; Del Moro, D.; Jefferies, S. M.; Terradas, J.; Soler, R. Bibcode: 2021A&A...649A.169S Altcode: 2021arXiv210311639S The umbral regions of sunspots and pores in the solar photosphere are generally dominated by 3 mHz oscillations, which are due to p-modes penetrating the magnetic region. In these locations, wave power is also significantly reduced with respect to the quiet Sun. However, here we study a pore where not only is the power of the oscillations in the umbra comparable to, or even larger than, that of the quiet Sun, but the main dominant frequency is not 3 mHz as expected, but instead 5 mHz. By combining Doppler velocities and spectropolarimetry and analysing the relationship between magnetic field strength and frequency, the resultant B − ω diagram reveals distinct ridges that are remarkably clear signatures of resonant magneto-hydrodynamic (MHD) oscillations confined within the pore umbra. We demonstrate that these modes, in addition to velocity oscillations, are also accompanied by magnetic oscillations, as predicted from MHD theory. The novel technique of B − ω analysis proposed in this article opens up an exciting new avenue for identifying MHD wave modes in the umbral regions of both pores and sunspots. Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST) Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio, Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart; Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa, Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler, Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun, Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres, Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.; Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini, Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena; Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor; Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael; Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli, Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys, Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.; Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis, Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson, Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.; Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.; Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava, Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas, Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST Instrument Scientists; DKIST Science Working Group; DKIST Critical Science Plan Community Bibcode: 2021SoPh..296...70R Altcode: 2020arXiv200808203R The National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute. Title: Slow magnetoacoustic waves in gravitationally stratified two-fluid plasmas in strongly ionized limit Authors: Alharbi, A.; Ballai, I.; Fedun, V.; Verth, G. Bibcode: 2021MNRAS.501.1940A Altcode: 2020MNRAS.tmp.3613A; 2020arXiv201205789A The plasma dynamics at frequencies comparable with collisional frequency between various species has to be described in multifluid framework, where collisional interaction between particles is an important ingredient. In our study, we will assume that charged particles are strongly coupled, meaning that they form a single fluid that interacts with neutrals, therefore we will employ a two-fluid model. Here, we aim to investigate the evolutionary equation of slow sausage waves propagating in a gravitationally stratified flux tube in the two-fluid solar atmosphere in a strongly ionized limit using an initial value analysis. Due to the collisional interaction between massive particles (ions and neutrals), the governing equations are coupled. Solutions are sought in the strongly ionized limit and the density ratio between neutrals and charged particles is a small parameter. This limit is relevant to the upper part of the chromosphere. Our results show that slow sausage waves associated with charged particles propagate such that their possible frequency is affected by a cut-off due to the gravitational stratification. In contrast, for neutral acoustic waves the cut-off value applies on their wavelength and only small wavelength waves are able to propagate. Slow modes associated with neutrals are driven by the collisional coupling with ions. Title: Proper orthogonal and dynamic mode decomposition of sunspot data Authors: Albidah, A. B.; Brevis, W.; Fedun, V.; Ballai, I.; Jess, D. B.; Stangalini, M.; Higham, J.; Verth, G. Bibcode: 2021RSPTA.37900181A Altcode: 2020arXiv201008530A High-resolution solar observations show the complex structure of the magnetohydrodynamic (MHD) wave motion. We apply the techniques of proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) to identify the dominant MHD wave modes in a sunspot using the intensity time series. The POD technique was used to find modes that are spatially orthogonal, whereas the DMD technique identifies temporal orthogonality. Here, we show that the combined POD and DMD approaches can successfully identify both sausage and kink modes in a sunspot umbra with an approximately circular cross-sectional shape.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Spicule Jets in the Solar Atmosphere Modeled with Resistive MHD and Thermal Conduction Authors: González, J. J. J.; Guzmán, F.; Fedun, V.; Verth, G. Bibcode: 2020AGUFMSH0290007G Altcode: Using numerical simulations, we study the effects of magnetic resistivity and thermal conductivity in the dynamics and properties of solar jets with characteristics of Type II spicules and cool coronal jets. The resistive MHD equations govern the jets' dynamic evolution with thermal conduction along the magnetic field lines on a 2.5D slice. The magnetic field configuration consists of two symmetric neighboring loops with opposite polarity, used to support reconnection and followed by the plasma jet formation. In total, ten simulations were carried out with different values of resistivity and thermal conductivity that produce jets with different morphological and thermal properties we quantify. We find that an increase in magnetic resistivity does not significantly affect the morphology, velocity, and temperature of the jets. However, thermal conductivity affects both temperature and morphology of the jets. In particular, thermal conductivity causes jets to reach greater heights and increases the temperature of the jet-apex. Also, heat flux maps indicate the jet-apex and corona interchange energy more efficiently than the jet's body. These results could potentially open a new avenue for plasma diagnostics in the Sun's atmosphere. Title: Generation of Low-Frequency Kinetic Waves at the Footpoints of Pre-Flare Coronal Loops Authors: Kryshtal, Alexandr; Voitsekhovska, Anna; Cheremnykh, Oleg; Ballai, Istvan; Verth, Gary; Fedun, Viktor Bibcode: 2020SoPh..295..162K Altcode: In this study we discuss the excitation of low-frequency plasma waves in the lower-middle chromosphere region of loop footpoints for the case when the plasma can be considered to be in a pre-flare state. It is shown that among the well-known semi-empirical models of the solar atmosphere, only the VAL (F) model together with a particular set of basic plasma parameters and amplitudes of the electric and magnetic fields supports generation of low-frequency wave instability. Our results show that it is possible to predict the onset of the flare process in the active region by using the interaction of kinetic Alfvén and kinetic ion-acoustic waves, which are solutions of the derived dispersion equation. The VAL (F) model allows situations when the main source of the aforementioned instability can be a sub-Dreicer electric field and drift plasma movements due to presence of spatial inhomogeneities. We also show that the generation of kinetic Alfvén and kinetic ion-acoustic waves can occur both, in plasma with a purely Coulomb conductivity and in the presence of small-scale Bernstein turbulence. The excitation of the small amplitude kinetic waves due to the development of low threshold instability in plasma with relatively low values of the magnetic field strength is also discussed. Title: Solar Vortex Tubes: Vortex Dynamics in the Solar Atmosphere Authors: Silva, Suzana S. A.; Fedun, Viktor; Verth, Gary; Rempel, Erico L.; Shelyag, Sergiy Bibcode: 2020ApJ...898..137S Altcode: 2020arXiv200704371S In this work, a state-of-the-art vortex detection method, Instantaneous Vorticity Deviation, is applied to locate three-dimensional vortex tube boundaries in numerical simulations of solar photospheric magnetoconvection performed by the MURaM code. We detected three-dimensional vortices distributed along intergranular regions and displaying coned shapes that extend from the photosphere to the low chromosphere. Based on a well-defined vortex center and boundary, we were able to determine averaged radial profiles and thereby investigate the dynamics across the vortical flows at different height levels. The solar vortex tubes present nonuniform angular rotational velocity, and, at all height levels, there are eddy viscosity effects within the vortices, which slow down the plasma as it moves toward the center. The vortices impact the magnetic field as they help to intensify the magnetic field at the sinking points, and in turn, the magnetic field ends up playing an essential role in the vortex dynamics. The magnetic field was found to be especially important to the vorticity evolution. On the other hand, it is shown that, in general, kinematic vortices do not give rise to magnetic vortices unless their tangential velocities at different height levels are high enough to overcome the magnetic tension. Title: Spicule Jets in the Solar Atmosphere Modeled with Resistive MHD and Thermal Conduction Authors: González-Avilés, J. J.; Guzmán, F. S.; Fedun, V.; Verth, G. Bibcode: 2020ApJ...897..153G Altcode: 2020arXiv200513647G Using numerical simulations, we study the effects of magnetic resistivity and thermal conductivity in the dynamics and properties of solar jets with characteristics of Type II spicules and cool coronal jets. The dynamic evolution of the jets is governed by the resistive MHD equations with thermal conduction along the magnetic field lines on a 2.5D slice. The magnetic field configuration consists of two symmetric neighboring loops with opposite polarity, used to support reconnection and followed by the plasma jet formation. In total, 10 simulations were carried out with different values of resistivity and thermal conductivity that produce jets with different morphological and thermal properties we quantify. We find that an increase in magnetic resistivity does not produce significant effects on the morphology, velocity, and temperature of the jets. However, thermal conductivity affects both temperature and morphology of the jets. In particular, thermal conductivity causes jets to reach greater heights and increases the temperature of the jet-apex. Also, heat flux maps indicate the jet-apex and corona interchange energy more efficiently than the body of the jet. These results could potentially open a new avenue for plasma diagnostics in the Sun's atmosphere. Title: The Effect of the 21 August 2017 Total Solar Eclipse on the Phase of VLF/LF Signals Authors: Rozhnoi, A.; Solovieva, M.; Shalimov, S.; Ouzounov, D.; Gallagher, P.; Verth, G.; McCauley, J.; Shelyag, S.; Fedun, V. Bibcode: 2020E&SS....700839R Altcode: An experimental study of the phase and amplitude observations of sub-ionospheric very low and low frequency (VLF/LF) signals is performed to analyze the response of the lower ionosphere during the 21 August 2017 total solar eclipse in the United States of America. Three different sub-ionospheric wave paths are investigated. The length of the paths varies from 2,200 to 6,400 km, and the signal frequencies are 21.4, 25.2, and 40.75 kHz. The two paths cross the region of the total eclipse, and the third path is in the region of 40-60% of obscuration. None of the signals reveal any noticeable amplitude changes during the eclipse, while negative phase anomalies (from -33° to -95°) are detected for all three paths. It is shown that the effective reflection height of the ionosphere in low and middle latitudes is increased by about 3-5 km during the eclipse. Estimation of the electron density change in the lower ionosphere caused by the eclipse, using linear recombination law, shows that the average decrease is by 2.1 to 4.5 times. Title: New Elliptical Flux Tube Model to Explain Sunspot Oscillations Authors: Ali Aldhafeeri, A.; Fedun, V.; Jess, D. B.; Brevis, W.; Verth, G. Bibcode: 2019AGUFMSH41F3322A Altcode: This work is motivated by elliptical sunspot oscillation data that cannot be explained by the standard cylinder model. We obtained a new numerical solution of the dispersion relation for a compressible magnetic flux tube with an elliptical cross-section embedded in a magnetic environment. We applied the bisection method to solve this equation and investigated the behaviour of oscillations which arise under both photospheric and coronal conditions. From our solutions we obtain an approximation of a circle like shape which is in perfect agreement with a previous well known results. Our model also explains a mode detected in an elliptical sunspot using the ROSA (Rapid Oscillations in the Solar Atmosphere) instrument based at the Dunn Solar Telescope. Title: Spicule jets in the chromosphere-corona interface modeled with resistive MHD with thermal conductivity Authors: González, J. J.; Guzmán, F.; Fedun, V.; Verth, G. Bibcode: 2019AGUFMSH11D3383G Altcode: We study the effects of resistivity, thermal conductivity and numerical methods in the dynamics and properties of solar jets. We focus on the generation and evolution of type II spicules with a suitable magnetic field configuration consisting of two loops. The dynamics is assumed to be governed by the resistive magnetohydrodynamics equations with thermal conductivity on a 2.5D slice. The equations are solved for a successfully triggered jet using High Resolution Shock Capturing methods that use HLLE and HLLC flux formulae, together with first and second order reconstructors. We quantify differences of height, thickness, time-life and velocity of the jet excited in terms of solar atmosphere and numerical parameters applied. Title: Sheffield students win US rocket competition Authors: Seniuc, Iulius-Vladimir; Kalra, Ankita; Lim, Jae Hyun; Schiona, Andrea; Narayanankutty, Gopika; Birakasan, Vishan Nair; Rontogiannis, Georgios; Ioannou, Konstantinos; Verth, Gary; Fedun, Viktor; Simpson, Charles Bibcode: 2019A&G....60c3.36S Altcode: Viktor Fedun reports on success for SunrIde, a team of students from the University of Sheffield who designed and built a rocket. Title: In situ generation of coronal Alfvén waves by jets Authors: González-Avilés, J. J.; Guzmán, F. S.; Fedun, V.; Verth, G.; Sharma, R.; Shelyag, S.; Regnier, S. Bibcode: 2019MNRAS.484.1936G Altcode: 2019MNRAS.tmp...67G; 2018arXiv180704224G Within the framework of 3D resistive magnetohydrodynamic, we simulate the formation of a plasma jet with the morphology, upward velocity up to 130 km s-1, and time-scale formation between 60 and 90 s after beginning of simulation, similar to those expected for type II spicules. Initial results of this simulation were published in paper by, e.g. González-Avilés et al. (2018), and present paper is devoted to the analysis of transverse displacements and rotational-type motion of the jet. Our results suggest that 3D magnetic reconnection may be responsible for the formation of the jet in paper by González-Avilés et al. (2018). In this paper, by calculating times series of the velocity components vx and vy in different points near to the jet for various heights we find transverse oscillations in agreement with spicule observations. We also obtain a time-distance plot of the temperature in a cross-cut at the plane x = 0.1 Mm and find significant transverse displacements of the jet. By analysing temperature isosurfaces of 104 K with the distribution of vx, we find that if the line-of-sight (LOS) is approximately perpendicular to the jet axis then there is both motion towards and away from the observer across the width of the jet. This red-blue shift pattern of the jet is caused by rotational motion, initially clockwise and anti-clockwise afterwards, which could be interpreted as torsional motion and may generate torsional Alfvén waves in the corona region. From a nearly vertical perspective of the jet the LOS velocity component shows a central blue-shift region surrounded by red-shifted plasma. Title: Vortex Flows in the Solar Atmosphere: Automated Identification and Statistical Analysis Authors: Giagkiozis, Ioannis; Fedun, Viktor; Scullion, Eamon; Jess, David B.; Verth, Gary Bibcode: 2018ApJ...869..169G Altcode: Vortices on the photosphere are fundamentally important as these coherent flows have the potential to form coherent magnetic field structures in the solar atmosphere, e.g., twisted magnetic flux tubes. These flows have traditionally been identified by tracking magnetic bright points (BPs) using primarily visual inspection. This approach has the shortcoming that it introduces bias into the statistical analyses. In this work we fully automate the process of vortex identification using an established method from hydrodynamics for the study of eddies in turbulent flows. For the first time, we apply this to detect intergranular photospheric intensity vortices. Using this automated approach, we find that the expected lifetime of intensity vortices is much shorter (≈17 s) compared with previously observed magnetic BP swirls. We suggest that at any time there are 1.48 × 106 such small-scale intensity vortices covering about 2.8% of the total surface of the solar photosphere. Lastly, we compare our results with previous works and speculate what this could imply with regards to estimating the global energy flux due magnetic tornadoes in the solar atmosphere with future higher resolution instrumentation. Title: Flows and magnetic field structures in reconnection regions of simulations of the solar atmosphere: Do flux pile-up models work? Authors: Shelyag, S.; Litvinenko, Y. E.; Fedun, V.; Verth, G.; González-Avilés, J. J.; Guzmán, F. S. Bibcode: 2018A&A...620A.159S Altcode: 2018arXiv180900587S
Aims: We study the process of magnetic field annihilation and reconnection in simulations of magnetised solar photosphere and chromosphere with magnetic fields of opposite polarities and constant numerical resistivity.
Methods: Exact analytical solutions for reconnective annihilations were used to interpret the features of magnetic reconnection in simulations of flux cancellation in the solar atmosphere. We used MURaM high-resolution photospheric radiative magneto-convection simulations to demonstrate the presence of magnetic field reconnection consistent with the magnetic flux pile-up models. Also, a simulated data-driven chromospheric magneto-hydrodynamic simulation is used to demonstrate magnetic field and flow structures, which are similar to the theoretically predicted ones.
Results: Both simulations demonstrate flow and magnetic field structures roughly consistent with accelerated reconnection with magnetic flux pile-up. The presence of standard Sweet-Parker type reconnection is also demonstrated in stronger photospheric magnetic fields. Title: On the Stability of Incompressible MHD Modes in Magnetic Cylinder with Twisted Magnetic Field and Flow Authors: Cheremnykh, Oleg; Fedun, Viktor; Ladikov-Roev, Yu.; Verth, Gary Bibcode: 2018ApJ...866...86C Altcode: In this work, we studied MHD modes in a magnetically twisted flux tube with a twisted flow that is embedded in the uniform magnetic field. We consider when the azimuthal magnetic field and velocity are linear functions of radius (case i) and also more generally when they are arbitrary functions of radius (case ii). Under these assumptions, we obtain the dispersion equation in the incompressible limit. This solution can also be used to describe the MHD perturbations in plasma pinches and vortices. The dispersion equation is simplified by implementing the thin flux tube approximation. It is shown that sausage modes (m = 0) become unstable for large enough azimuthal flow speeds. Also, we obtained the unstable modes for m > 0. It is shown that the stability criterion of the m = 1 mode (for case i) is independent of the background azimuthal components of the plasma velocity and magnetic field. These criteria fully coincide with the result that was previously obtained by Syrovatskiy for a plane interface. Moreover, this result even remains valid when the azimuthal magnetic field and velocity have an arbitrary dependence on radius (case ii). A criterion for the stability of the m ≥ 2 modes is also obtained. It was found that instability of these modes is determined by both longitudinal and azimuthal flows. It is shown that if there is sufficient azimuthal background flow, then all modes with m ≥ 2 will become unstable. Title: Photospheric Observations of Surface and Body Modes in Solar Magnetic Pores Authors: Keys, Peter H.; Morton, Richard J.; Jess, David B.; Verth, Gary; Grant, Samuel D. T.; Mathioudakis, Mihalis; Mackay, Duncan H.; Doyle, John G.; Christian, Damian J.; Keenan, Francis P.; Erdélyi, Robertus Bibcode: 2018ApJ...857...28K Altcode: 2018arXiv180301859K Over the past number of years, great strides have been made in identifying the various low-order magnetohydrodynamic wave modes observable in a number of magnetic structures found within the solar atmosphere. However, one aspect of these modes that has remained elusive, until now, is their designation as either surface or body modes. This property has significant implications for how these modes transfer energy from the waveguide to the surrounding plasma. Here, for the first time to our knowledge, we present conclusive, direct evidence of these wave characteristics in numerous pores that were observed to support sausage modes. As well as outlining methods to detect these modes in observations, we make estimates of the energies associated with each mode. We find surface modes more frequently in the data, as well as that surface modes appear to carry more energy than those displaying signatures of body modes. We find frequencies in the range of ∼2-12 mHz, with body modes as high as 11 mHz, but we do not find surface modes above 10 mHz. It is expected that the techniques we have applied will help researchers search for surface and body signatures in other modes and in differing structures from those presented here. Title: Magnetic Shocks and Substructures Excited by Torsional Alfvén Wave Interactions in Merging Expanding Flux Tubes Authors: Snow, B.; Fedun, V.; Gent, F. A.; Verth, G.; Erdélyi, R. Bibcode: 2018ApJ...857..125S Altcode: 2018arXiv180306112S Vortex motions are frequently observed on the solar photosphere. These motions may play a key role in the transport of energy and momentum from the lower atmosphere into the upper solar atmosphere, contributing to coronal heating. The lower solar atmosphere also consists of complex networks of flux tubes that expand and merge throughout the chromosphere and upper atmosphere. We perform numerical simulations to investigate the behavior of vortex-driven waves propagating in a pair of such flux tubes in a non-force-free equilibrium with a realistically modeled solar atmosphere. The two flux tubes are independently perturbed at their footpoints by counter-rotating vortex motions. When the flux tubes merge, the vortex motions interact both linearly and nonlinearly. The linear interactions generate many small-scale transient magnetic substructures due to the magnetic stress imposed by the vortex motions. Thus, an initially monolithic tube is separated into a complex multithreaded tube due to the photospheric vortex motions. The wave interactions also drive a superposition that increases in amplitude until it exceeds the local Mach number and produces shocks that propagate upward with speeds of approximately 50 km s-1. The shocks act as conduits transporting momentum and energy upward, and heating the local plasma by more than an order of magnitude, with a peak temperature of approximately 60,000 K. Therefore, we present a new mechanism for the generation of magnetic waveguides from the lower solar atmosphere to the solar corona. This wave guide appears as the result of interacting perturbations in neighboring flux tubes. Thus, the interactions of photospheric vortex motions is a potentially significant mechanism for energy transfer from the lower to upper solar atmosphere. Title: I. Jet Formation and Evolution Due to 3D Magnetic Reconnection Authors: González-Avilés, J. J.; Guzmán, F. S.; Fedun, V.; Verth, G.; Shelyag, S.; Regnier, S. Bibcode: 2018ApJ...856..176G Altcode: 2017arXiv170905066G Using simulated data-driven, 3D resistive MHD simulations of the solar atmosphere, we show that 3D magnetic reconnection may be responsible for the formation of jets with the characteristics of Type II spicules. We numerically model the photosphere-corona region using the C7 equilibrium atmosphere model. The initial magnetic configuration is a 3D potential magnetic field, extrapolated up to the solar corona region from a dynamic realistic simulation of the solar photospheric magnetoconvection model that mimics the quiet-Sun. In this case, we consider a uniform and constant value of the magnetic resistivity of 12.56 Ω m. We have found that the formation of the jet depends on the Lorentz force, which helps to accelerate the plasma upward. Analyzing various properties of the jet dynamics, we found that the jet structure shows a Doppler shift close to regions with high vorticity. The morphology, the upward velocity covering a range up to 130 km s-1, and the timescale formation of the structure between 60 and 90 s, are similar to those expected for Type II spicules. Title: Evolution of Complex 3D Motions in Spicules Authors: Sharma, Rahul; Verth, Gary; Erdélyi, Robertus Bibcode: 2018ApJ...853...61S Altcode: Ubiquitous transverse oscillations observed in spicular waveguides, identified as the kink wave-mode had previously been reported along with periodic structural distortions of the flux tubes, observed as cross-sectional width and associated photometric variations. Previous studies identified these perturbations as the observed signatures of concurrent kink and sausage wave-modes. High-resolution Hα imaging-spectroscopy data from the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope are used to analyze the off-limb spicular structures. For the first time, the evolution of the resultant transverse displacement of the flux-tube structure, estimated from the perpendicular velocity components, is analyzed along with longitudinal, cross-sectional width, photometric, and azimuthal shear/torsion variations. The pulse-like nonlinear kink wave-mode shows strong coupling with these observables, with a period-doubling, -tripling aspect, supported by mutual phase relations concentrated around 0° and +/- 180^\circ . The three-dimensional ensemble of the observed dynamical components revealed complexities pertinent to the accurate identification and interpretation of, e.g., linear/nonlinear, coupled/uncoupled magnetohydrodynamical wave-modes in spicules. Title: MHD Wave Modes of Twisted Magnetic Flux Tube Authors: Fedun, V.; Cheremnykh, O.; Kryshtal, A.; Verth, G. Bibcode: 2017AGUFMSH42B..05F Altcode: The theoretical study of MHD wave propagation in the solar magnetic flux tubes is an important problem in modern solar physics. Recent space- and ground-base observations have shown that that twisted magnetic fields occur naturally, and indeed are omnipresent in the Sun's atmosphere. In the framework of ideal MHD, in this talk, we will discuss small amplitude incompressible wave modes with azimuthal wave number m=1 and m>1 of twisted magnetic flux tubes. Special attention will be given to the problem of finding the eigenvalues and eigenfunctions of modes with m=1 in the long wavelength limit. We will show that the dispersion relation for kink mode is completely unaffected by the radial profile of background internal magnetic twist. However, fluting modes are sensitive to the particular radial profile of magnetic twist chosen. Title: I. Jet Formation and Evolution due to 3D Magnetic Reconnection Authors: González, J. J.; Guzmán, F.; Fedun, V.; Verth, G.; Shelyag, S.; Regnier, S. Bibcode: 2017AGUFMSH43A2807G Altcode: Using simulated data-driven three-dimensional resistive MHD simulations of the solar atmosphere, we show that magnetic reconnection can be responsible of the formation of jets with characteristic of Type II spicules. For this, we numerically model the photosphere-corona region using the C7 equilibrium atmosphere model. The initial magnetic configuration is a 3D potential magnetic field, extrapolated up to the solar corona region from a dynamic realistic simulation of solar photospheric magnetoconvection model which is mimicking quiet-Sun. In this case we consider a uniform and constant value of the magnetic resistivity of 12.56 Ω m. We have found that formation of the jets depends on the Lorentz force, which helps to accelerate the plasma upwards. Analyzing various properties of the jet dynamics, we found that the jet structure shows Doppler shift near to regions with high vorticity. The morphology, upward velocity, covering a range up to 100 km s-1, and life-time of the estructure, bigger than 100 s, are similar to those expected for Type II spicules. Title: The Frequency-dependent Damping of Slow Magnetoacoustic Waves in a Sunspot Umbral Atmosphere Authors: Krishna Prasad, S.; Jess, D. B.; Van Doorsselaere, T.; Verth, G.; Morton, R. J.; Fedun, V.; Erdélyi, R.; Christian, D. J. Bibcode: 2017ApJ...847....5K Altcode: 2017ApJ...847....5P; 2017arXiv170804835K High spatial and temporal resolution images of a sunspot, obtained simultaneously in multiple optical and UV wavelengths, are employed to study the propagation and damping characteristics of slow magnetoacoustic waves up to transition region heights. Power spectra are generated from intensity oscillations in sunspot umbra, across multiple atmospheric heights, for frequencies up to a few hundred mHz. It is observed that the power spectra display a power-law dependence over the entire frequency range, with a significant enhancement around 5.5 mHz found for the chromospheric channels. The phase difference spectra reveal a cutoff frequency near 3 mHz, up to which the oscillations are evanescent, while those with higher frequencies propagate upward. The power-law index appears to increase with atmospheric height. Also, shorter damping lengths are observed for oscillations with higher frequencies suggesting frequency-dependent damping. Using the relative amplitudes of the 5.5 mHz (3 minute) oscillations, we estimate the energy flux at different heights, which seems to decay gradually from the photosphere, in agreement with recent numerical simulations. Furthermore, a comparison of power spectra across the umbral radius highlights an enhancement of high-frequency waves near the umbral center, which does not seem to be related to magnetic field inclination angle effects. Title: Incompressible magnetohydrodynamic modes in the thin magnetically twisted flux tube Authors: Cheremnykh, O. K.; Fedun, V.; Kryshtal, A. N.; Verth, G. Bibcode: 2017A&A...604A..62C Altcode: 2017arXiv170703272C Context. Observations have shown that twisted magnetic fields naturally occur, and indeed are omnipresent in the Sun's atmosphere. It is therefore of great theoretical interest in solar atmospheric waves research to investigate the types of magnetohydrodynamic (MHD) wave modes that can propagate along twisted magnetic flux tubes.
Aims: Within the framework of ideal MHD, the main aim of this work is to investigate small amplitude incompressible wave modes of twisted magnetic flux tubes with m ≥ 1. The axial magnetic field strength inside and outside the tube will be allowed to vary, to ensure the results will not be restricted to only cold plasma equilibria conditions.
Methods: The dispersion equation for these incompressible linear MHD wave modes was derived analytically by implementing the long wavelength approximation.
Results: It is shown, in the long wavelength limit, that both the frequency and radial velocity profile of the m = 1 kink mode are completely unaffected by the choice of internal background magnetic twist. However, fluting modes with m ≥ 2 are sensitive to the particular radial profile of magnetic twist chosen. Furthermore, due to background twist, a low frequency cut-off is introduced for fluting modes that is not present for kink modes. From an observational point of view, although magnetic twist does not affect the propagation of long wavelength kink modes, for fluting modes it will either work for or against the propagation, depending on the direction of wave travel relative to the sign of the background twist. Title: An Inside Look at Sunspot Oscillations with Higher Azimuthal Wavenumbers Authors: Jess, David B.; Van Doorsselaere, Tom; Verth, Gary; Fedun, Viktor; Krishna Prasad, S.; Erdélyi, Robert; Keys, Peter H.; Grant, Samuel D. T.; Uitenbroek, Han; Christian, Damian J. Bibcode: 2017ApJ...842...59J Altcode: 2017arXiv170506282J Solar chromospheric observations of sunspot umbrae offer an exceptional view of magnetohydrodynamic wave phenomena. In recent years, a wealth of wave signatures related to propagating magneto-acoustic modes have been presented, which demonstrate complex spatial and temporal structuring of the wave components. Theoretical modeling has demonstrated how these ubiquitous waves are consistent with an m = 0 slow magneto-acoustic mode, which is excited by trapped sub-photospheric acoustic (p-mode) waves. However, the spectrum of umbral waves is broad, suggesting that the observed signatures represent the superposition of numerous frequencies and/or modes. We apply Fourier filtering, in both spatial and temporal domains, to extract chromospheric umbral wave characteristics consistent with an m = 1 slow magneto-acoustic mode. This identification has not been described before. Angular frequencies of 0.037+/- 0.007 {rad} {{{s}}}-1 (2.1+/- 0.4 \deg {{{s}}}-1, corresponding to a period ≈170 s) for the m = 1 mode are uncovered for spatial wavenumbers in the range of 0.45< k< 0.90 arcsec-1 (5000-9000 km). Theoretical dispersion relations are solved, with corresponding eigenfunctions computed, which allows the density perturbations to be investigated and compared with our observations. Such magnetohydrodynamic modeling confirms our interpretation that the identified wave signatures are the first direct observations of an m = 1 slow magneto-acoustic mode in the chromospheric umbra of a sunspot. Title: Vortex Flows in the Solar Atmoshpere: Automated Identification and Statistical Analysis Authors: Giagkiozis, Ioannis; Fedun, Viktor; Scullion, Eamon; Verth, Gary Bibcode: 2017arXiv170605428G Altcode: Aims. Due to the fundamental importance of vortices on the photosphere, in this work we aim to fully automate the process of intensity vortex identification to facilitate a more robust statistical analysis of their properties. Methods. Using on-disk observational data of the Fe I continuum, the process of vortex identification is fully automated, for the first time in solar physics, with the help of an established method from hydrodynamics initially employed for the study of eddies in turbulent flows (Graftieaux et al. 2001). Results. We find that the expected lifetime of intensity vortices is much shorter (~ 17s) compared with previously observed magnetic bright point swirls. Our findings suggest that at any time there are 1.4e6 such small-scale intensity vortices covering about 2.8% of the total surface of the solar photosphere. Title: Dynamic Behavior of Spicules Inferred from Perpendicular Velocity Components Authors: Sharma, Rahul; Verth, Gary; Erdélyi, Robertus Bibcode: 2017ApJ...840...96S Altcode: Understanding the dynamic behavior of spicules, e.g., in terms of magnetohydrodynamic (MHD) wave mode(s), is key to unveiling their role in energy and mass transfer from the photosphere to corona. The transverse, torsional, and field-aligned motions of spicules have previously been observed in imaging spectroscopy and analyzed separately for embedded wave-mode identification. Similarities in the Doppler signatures of spicular structures for both kink and torsional Alfvén wave modes have led to the misinterpretation of the dominant wave mode in these structures and is a subject of debate. Here, we aim to combine line- of-sight (LOS) and plane-of-sky (POS) velocity components using the high spatial/temporal resolution Hα imaging-spectroscopy data from the CRisp Imaging SpectroPolarimeter based at the Swedish Solar Telescope to achieve better insight into the underlying nature of these motions as a whole. The resultant three-dimensional velocity vectors and the other derived quantities (e.g., magnetic pressure perturbations) are used to identify the MHD wave mode(s) responsible for the observed spicule motion. We find a number of independent examples where the bulk transverse motion of the spicule is dominant either in the POS or along the LOS. It is shown that the counterstreaming action of the displaced external plasma due to spicular bulk transverse motion has a similar Doppler profile to that of the m = 0 torsional Alfvén wave when this motion is predominantly perpendicular to the LOS. Furthermore, the inferred magnetic pressure perturbations support the kink wave interpretation of observed spicular bulk transverse motion rather than any purely incompressible MHD wave mode, e.g., the m = 0 torsional Alfvén wave. Title: Project SunbYte: solar astronomy on a budget Authors: Alvarez Gonzalez, F.; Badilita, A. -M.; Baker, A.; Cho, Y. -H.; Dhot, N.; Fedun, V.; Hare, C.; He, T.; Hobbs, M.; Javed, M.; Lovesey, H.; Lord, C.; Panoutsos, G.; Permyakov, A.; Pope, S.; Portnell, M.; Rhodes, L.; Sharma, R.; Taras, P.; Taylor, J.; Tilbrook, R.; Verth, G.; Wrigley, S. N.; Yaqoob, M.; Cook, R.; McLaughlin, J.; Morton, R.; Scullion, E.; Shelyag, S.; Hamilton, A.; Zharkov, S.; Jess, D.; Wrigley, M. Bibcode: 2017A&G....58d2.24A Altcode: The Sheffield University Nova Balloon Lifted Solar Telescope (SunbYte) is a high-altitude balloon experiment devised and run largely by students at the University of Sheffield, and is scheduled for launch in October 2017. It was the only UK project in 2016 to be selected for the balloon side of the Swedish-German student programme REXUS/BEXUS (Rocket and Balloon Experiments for University Students; see box on p2.25). The success of the SunbYte team in the REXUS/BEXUS selection process is an unprecedented opportunity for the students to gain valuable experience working in the space engineering industry, using their theoretical knowledge and networking with students and technology companies from all over Europe. Title: Resonant Absorption of Axisymmetric Modes in Twisted Magnetic Flux Tubes Authors: Giagkiozis, I.; Goossens, M.; Verth, G.; Fedun, V.; Van Doorsselaere, T. Bibcode: 2016ApJ...823...71G Altcode: 2017arXiv170609665G It has been shown recently that magnetic twist and axisymmetric MHD modes are ubiquitous in the solar atmosphere, and therefore the study of resonant absorption for these modes has become a pressing issue because it can have important consequences for heating magnetic flux tubes in the solar atmosphere and the observed damping. In this investigation, for the first time, we calculate the damping rate for axisymmetric MHD waves in weakly twisted magnetic flux tubes. Our aim is to investigate the impact of resonant damping of these modes for solar atmospheric conditions. This analytical study is based on an idealized configuration of a straight magnetic flux tube with a weak magnetic twist inside as well as outside the tube. By implementing the conservation laws derived by Sakurai et al. and the analytic solutions for weakly twisted flux tubes obtained recently by Giagkiozis et al. we derive a dispersion relation for resonantly damped axisymmetric modes in the spectrum of the Alfvén continuum. We also obtain an insightful analytical expression for the damping rate in the long wavelength limit. Furthermore, it is shown that both the longitudinal magnetic field and the density, which are allowed to vary continuously in the inhomogeneous layer, have a significant impact on the damping time. Given the conditions in the solar atmosphere, resonantly damped axisymmetric modes are highly likely to be ubiquitous and play an important role in energy dissipation. We also suggest that, given the character of these waves, it is likely that they have already been observed in the guise of Alfvén waves. Title: Magnetohydrodynamic Oscillations in the Solar Corona and Earth's Magnetosphere: Towards Consolidated Understanding Authors: Nakariakov, V. M.; Pilipenko, V.; Heilig, B.; Jelínek, P.; Karlický, M.; Klimushkin, D. Y.; Kolotkov, D. Y.; Lee, D. -H.; Nisticò, G.; Van Doorsselaere, T.; Verth, G.; Zimovets, I. V. Bibcode: 2016SSRv..200...75N Altcode: 2016SSRv..tmp....2N Magnetohydrodynamic (MHD) oscillatory processes in different plasma systems, such as the corona of the Sun and the Earth's magnetosphere, show interesting similarities and differences, which so far received little attention and remain under-exploited. The successful commissioning within the past ten years of THEMIS, Hinode, STEREO and SDO spacecraft, in combination with matured analysis of data from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI) makes it very timely to survey the breadth of observations giving evidence for MHD oscillatory processes in solar and space plasmas, and state-of-the-art theoretical modelling. The paper reviews several important topics, such as Alfvénic resonances and mode conversion; MHD waveguides, such as the magnetotail, coronal loops, coronal streamers; mechanisms for periodicities produced in energy releases during substorms and solar flares, possibility of Alfvénic resonators along open field lines; possible drivers of MHD waves; diagnostics of plasmas with MHD waves; interaction of MHD waves with partly-ionised boundaries (ionosphere and chromosphere). The review is mainly oriented to specialists in magnetospheric physics and solar physics, but not familiar with specifics of the adjacent research fields. Title: Ultra-High-Resolution Observations of MHD Waves in Photospheric Magnetic Structures Authors: Jess, D. B.; Verth, G. Bibcode: 2016GMS...216..449J Altcode: 2015arXiv150206960J This chapter reviews the recent observations of waves and oscillations manifesting in fine-scale magnetic structures in the solar photosphere, which are often interpreted as the "building blocks' of the magnetic Sun. The authors found, through phase relationships between the various waveforms, that small-scale magnetic bright points (MBPs) in the photosphere demonstrated signatures of specific magnetoacoustic waves, in particular the sausage and kink modes. Modern magnetohydrodynamic (MHD) simulations of the lower solar atmosphere clearly show how torsional motions can easily be induced in magnetic elements in the photosphere through the processes of vortical motions and/or buffeting by neighboring granules. The authors detected significant power associated with high-frequency horizontal motions, and suggested that these cases may be especially important in the creation of a turbulent environment that efficiently promotes Alfvén wave dissipation. Title: MHD Wave Modes Resolved in Fine-Scale Chromospheric Magnetic Structures Authors: Verth, G.; Jess, D. B. Bibcode: 2016GMS...216..431V Altcode: 2015arXiv150501155V Due to its complex and dynamic fine-scale structure, the chromosphere is a particularly challenging region of the Sun's atmosphere to understand. It is now widely accepted that to model chromospheric dynamics, even on a magnetohydrodynamic (MHD) scale, while also calculating spectral line emission, one must realistically include the effects of partial ionization and radiative transfer in a multi-fluid plasma under non-LTE conditions. Accurate quantification of MHD wave energetics must be founded on a precise identification of the actual wave mode being observed. This chapter focuses on MHD kink-mode identification, MHD sausage mode identification, and MHD torsional Alfvén wave identification. It then reviews progress in determining more accurate energy flux estimations of specific MHD wave modes observed in the chromosphere. The chapter finally examines how the discovery of these MHD wave modes has helped us advance the field of chromospheric magnetoseismology. Title: Vortex Identification in the Lower Solar Atmosphere Authors: Fedun, V.; Giagkiozis, I.; Verth, G.; Scullion, E. Bibcode: 2015AGUFMSH53B2484F Altcode: Vortices in the solar atmosphere present an ideal driving mechanism for Alfvenic waves that can efficiently carry energy in the upper layers of the chromosphere and corona. However, the identification and classification of vortical motions from observations and numerical simulations is a challenging task. In this work we leverage a number of methods conventionally employed in turbulence to identify for the fist time in the solar atmosphere vortices, in an automated fashion. We also present a statistical analysis of the properties of the identified motions and relate this with theoretical results for such magnetic structures. Title: Dissipationless Damping of Compressive MHD Modes in Twisted Flux Tubes Authors: Giagkiozis, I.; Fedun, V.; Verth, G.; Goossens, M. L.; Van Doorsselaere, T. Bibcode: 2015AGUFMSH53B2488G Altcode: Axisymmetric modes in straight magentic flux tubes exhibit a cutoff in the long wavelength limit and no damping is predicted. However, as soon as weak magnetic twist is introduced inside as well as outside the magnetic flux tube the cutoff recedes. Furthermore, when density variations are also incomporated within the modelresonant absorption appears. In this work we explore analytically the expected damping times for waves within the Alfven continuum for different solar atmospheric conditions. Based on the results in this work we offer insight on recent observations of sausage wave damping in the chromosphere. Title: Axisymmetric Modes in Magnetic Flux Tubes with Internal and External Magnetic Twist Authors: Giagkiozis, I.; Fedun, V.; Erdélyi, R.; Verth, G. Bibcode: 2015ApJ...810...53G Altcode: 2017arXiv170609669G Observations suggest that twisted magnetic flux tubes are ubiquitous in the Sun's atmosphere. The main aim of this work is to advance the study of axisymmetric modes of magnetic flux tubes by modeling both twisted internal and external magnetic fields, when the magnetic twist is weak. In this work, we solve the derived wave equations numerically assuming that the twist outside the tube is inversely proportional to the distance from its boundary. We also study the case of a constant magnetic twist outside the tube and solve these equations analytically. We show that the solution for a constant twist outside the tube is a good approximation for the case where the magnetic twist is proportional to 1/r, namely, the error is in all cases less than 5.4%. The solution is in excellent agreement with solutions to simpler models of twisted magnetic flux tubes, i.e., without external magnetic twist. It is shown that axisymmetric Alfvén waves are naturally coupled with magnetic twist as the azimuthal component of the velocity perturbation is nonzero. We compared our theoretical results with observations and comment on what the Doppler signature of these modes is expected to be. Lastly, we argue that the character of axisymmetric waves in twisted magnetic flux tubes can lead to false positives in identifying observations with axisymmetric Alfvén waves. Title: Observations and mode identification of sausage waves in a magnetic pore Authors: Moreels, M. G.; Freij, N.; Erdélyi, R.; Van Doorsselaere, T.; Verth, G. Bibcode: 2015A&A...579A..73M Altcode:
Aims: We aim to determine the phase speed of an oscillation in a magnetic pore using only intensity images at one height. The observations were obtained using the CRisp Imaging SpectroPolarimeter at the Swedisch 1-m Solar Telescope and show variations in both cross-sectional area and intensity in a magnetic pore.
Methods: We have designed and tested an observational method to extract the wave parameters that are important for seismology. We modelled the magnetic pore as a straight cylinder with a uniform plasma both inside and outside the flux tube and identify different wave modes. Using analytic expressions, we are able to distinguish between fast and slow modes and obtain the phase speed of the oscillations.
Results: We found that the observed oscillations are slow modes with a phase speed around 5 km s-1. We also have strong evidence that the oscillations are standing harmonics.

Appendix A is available in electronic form at http://www.aanda.org Title: Multiwavelength Studies of MHD Waves in the Solar Chromosphere. An Overview of Recent Results Authors: Jess, D. B.; Morton, R. J.; Verth, G.; Fedun, V.; Grant, S. D. T.; Giagkiozis, I. Bibcode: 2015SSRv..190..103J Altcode: 2015arXiv150301769J; 2015SSRv..tmp...14J The chromosphere is a thin layer of the solar atmosphere that bridges the relatively cool photosphere and the intensely heated transition region and corona. Compressible and incompressible waves propagating through the chromosphere can supply significant amounts of energy to the interface region and corona. In recent years an abundance of high-resolution observations from state-of-the-art facilities have provided new and exciting ways of disentangling the characteristics of oscillatory phenomena propagating through the dynamic chromosphere. Coupled with rapid advancements in magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly investigate the role waves play in supplying energy to sustain chromospheric and coronal heating. Here, we review the recent progress made in characterising, categorising and interpreting oscillations manifesting in the solar chromosphere, with an impetus placed on their intrinsic energetics. Title: Wave Damping Observed in Upwardly Propagating Sausage-mode Oscillations Contained within a Magnetic Pore Authors: Grant, S. D. T.; Jess, D. B.; Moreels, M. G.; Morton, R. J.; Christian, D. J.; Giagkiozis, I.; Verth, G.; Fedun, V.; Keys, P. H.; Van Doorsselaere, T.; Erdélyi, R. Bibcode: 2015ApJ...806..132G Altcode: 2015arXiv150501484G We present observational evidence of compressible MHD wave modes propagating from the solar photosphere through to the base of the transition region in a solar magnetic pore. High cadence images were obtained simultaneously across four wavelength bands using the Dunn Solar Telescope. Employing Fourier and wavelet techniques, sausage-mode oscillations displaying significant power were detected in both intensity and area fluctuations. The intensity and area fluctuations exhibit a range of periods from 181 to 412 s, with an average period ∼290 s, consistent with the global p-mode spectrum. Intensity and area oscillations present in adjacent bandpasses were found to be out of phase with one another, displaying phase angles of 6.°12, 5.°82, and 15.°97 between the 4170 Å continuum-G-band, G-band-Na i D1, and Na i D1-Ca ii K heights, respectively, reiterating the presence of upwardly propagating sausage-mode waves. A phase relationship of ∼0° between same-bandpass emission and area perturbations of the pore best categorizes the waves as belonging to the “slow” regime of a dispersion diagram. Theoretical calculations reveal that the waves are surface modes, with initial photospheric energies in excess of 35,000 W m-2. The wave energetics indicate a substantial decrease in energy with atmospheric height, confirming that magnetic pores are able to transport waves that exhibit appreciable energy damping, which may release considerable energy into the local chromospheric plasma. Title: Energy Propagation by Transverse Waves in Multiple Flux Tube Systems Using Filling Factors Authors: Van Doorsselaere, T.; Gijsen, S. E.; Andries, J.; Verth, G. Bibcode: 2014ApJ...795...18V Altcode: In the last few years, it has been found that transverse waves are present at all times in coronal loops or spicules. Their energy has been estimated with an expression derived for bulk Alfvén waves in homogeneous media, with correspondingly uniform wave energy density and flux. The kink mode, however, is localized in space with the energy density and flux dependent on the position in the cross-sectional plane. The more relevant quantities for the kink mode are the integrals of the energy density and flux over the cross-sectional plane. The present paper provides an approximation to the energy propagated by kink modes in an ensemble of flux tubes by means of combining the analysis of single flux tube kink oscillations with a filling factor for the tube cross-sectional area. This finally allows one to compare the expressions for energy flux of Alfvén waves with an ensemble of kink waves. We find that the correction factor for the energy in kink waves, compared to the bulk Alfvén waves, is between f and 2f, where f is the density filling factor of the ensemble of flux tubes. Title: The Transverse and Rotational Motions of Magnetohydrodynamic Kink Waves in the Solar Atmosphere Authors: Goossens, M.; Soler, R.; Terradas, J.; Van Doorsselaere, T.; Verth, G. Bibcode: 2014ApJ...788....9G Altcode: Magnetohydrodynamic (MHD) kink waves have now been observed to be ubiquitous throughout the solar atmosphere. With modern instruments, they have now been detected in the chromosphere, interface region, and corona. The key purpose of this paper is to show that kink waves do not only involve purely transverse motions of solar magnetic flux tubes, but the velocity field is a spatially and temporally varying sum of both transverse and rotational motion. Taking this fact into account is particularly important for the accurate interpretation of varying Doppler velocity profiles across oscillating structures such as spicules. It has now been shown that, as well as bulk transverse motions, spicules have omnipresent rotational motions. Here we emphasize that caution should be used before interpreting the particular MHD wave mode/s responsible for these rotational motions. The rotational motions are not necessarily signatures of the classic axisymmetric torsional Alfvén wave alone, because kink motion itself can also contribute substantially to varying Doppler velocity profiles observed across these structures. In this paper, the displacement field of the kink wave is demonstrated to be a sum of its transverse and rotational components, both for a flux tube with a discontinuous density profile at its boundary, and one with a more realistic density continuum between the internal and external plasma. Furthermore, the Doppler velocity profile of the kink wave is forward modeled to demonstrate that, depending on the line of sight, it can either be quite distinct or very similar to that expected from a torsional Alfvén wave. Title: The Generation and Damping of Propagating MHD Kink Waves in the Solar Atmosphere Authors: Morton, R. J.; Verth, G.; Hillier, A.; Erdélyi, R. Bibcode: 2014ApJ...784...29M Altcode: 2013arXiv1310.4650M The source of the non-thermal energy required for the heating of the upper solar atmosphere to temperatures in excess of a million degrees and the acceleration of the solar wind to hundreds of kilometers per second is still unclear. One such mechanism for providing the required energy flux is incompressible torsional Alfvén and kink magnetohydrodynamic (MHD) waves, which are magnetically dominated waves supported by the Sun's pervasive and complex magnetic field. In particular, propagating MHD kink waves have recently been observed to be ubiquitous throughout the solar atmosphere, but, until now, critical details of the transport of the kink wave energy throughout the Sun's atmosphere were lacking. Here, the ubiquity of the waves is exploited for statistical studies in the highly dynamic solar chromosphere. This large-scale investigation allows for the determination of the chromospheric kink wave velocity power spectra, a missing link necessary for determining the energy transport between the photosphere and corona. Crucially, the power spectra contain evidence for horizontal photospheric motions being an important mechanism for kink wave generation in the quiescent Sun. In addition, a comparison with measured coronal power spectra is provided for the first time, revealing frequency-dependent transmission profiles, suggesting that there is enhanced damping of kink waves in the lower corona. Title: Wave Energy Deposition in the Solar Corona Authors: Van Doorsselaere, Tom; Goossens, Marcel; Verth, Gary; Soler, Roberto; Gijsen, Stief; Andries, Jesse Bibcode: 2014cosp...40E3464V Altcode: Recently, a significant amount of transverse wave energy has been estimated propagating along solar atmospheric magnetic fields. However, these estimates have been made with the classic bulk Alfven wave model which assumes a homogeneous plasma. In this talk, the kinetic, magnetic, and total energy densities and the flux of energy are first computed for transverse MHD waves in one-dimensional cylindrical flux tube models with a piecewise constant density profile. There are fundamental deviations from the properties for classic bulk Alfven waves. (1) There is no local equipartition between kinetic and magnetic energy. (2) The flux of energy and the velocity of energy transfer have, in addition to a component parallel to the magnetic field, components in the planes normal to the magnetic field. (3) The energy densities and the flux of energy vary spatially, contrary to the case of classic bulk Alfven waves. This last property is then used to connect the energy flux in such a simple model to the energy flux in multiple flux tube systems. We use the plasma filling factor f to derive an ad-hoc formula for estimating the energy that is propagated in bundles of loops. We find that the energy flux in kink waves is lower than the energy computed from a bulk Alfven wave interpretation, by a factor that is (approximately) between f and 2f. We consider some geometric models to quantify this correction factor. Title: Doppler displacements in kink MHD waves in solar flux tubes Authors: Goossens, Marcel; Van Doorsselaere, Tom; Terradas, Jaume; Verth, Gary; Soler, Roberto Bibcode: 2014cosp...40E1045G Altcode: Doppler displacements in kink MHD waves in solar flux tubes Presenting author: M. Goossens Co-authors: R. Soler, J. Terradas, T. Van Doorsselaere, G. Verth The standard interpretation of the transverse MHD waves observed in the solar atmosphere is that they are non-axisymmetric kink m=1) waves on magnetic flux tubes. This interpretation is based on the fact that axisymmetric and non-axisymmetric fluting waves do not displace the axis of the loop and the loop as a whole while kink waves indeed do so. A uniform transverse motion produces a Doppler displacement that is constant across the magnetic flux tube. A recent development is the observation of Doppler displacements that vary across the loop. The aim of the present contribution is to show that spatial variations of the Doppler displacements across the loop can be caused by kink waves. The motion associated with a kink wave is purely transverse only when the flux tube is uniform and sufficiently thin. Only in that case do the radial and azimuthal components of displacement have the same amplitude and is the azimuthal component a quarter of a period ahead of the radial component. This results in a unidirectional or transverse displacement. When the flux tube is non-uniform and has a non-zero radius the conditions for the generation of a purely transverse motion are not any longer met. In that case the motion in a kink wave is the sum of a transverse motion and a non-axisymmetric rotational motion that depends on the azimuthal angle. It can produce complicated variations of the Doppler displacement across the loop. I shall discuss the various cases of possible Doppler displacenents that can occur depending on the relative sizes of the amplitudes of the radial and azimuthal components of the displacement in the kink wave and on the orientation of the line of sight. Title: Characteristics of Transverse Waves in Chromospheric Mottles Authors: Kuridze, D.; Verth, G.; Mathioudakis, M.; Erdélyi, R.; Jess, D. B.; Morton, R. J.; Christian, D. J.; Keenan, F. P. Bibcode: 2013ApJ...779...82K Altcode: 2013arXiv1310.3628K Using data obtained by the high temporal and spatial resolution Rapid Oscillations in the Solar Atmosphere instrument on the Dunn Solar Telescope, we investigate at an unprecedented level of detail transverse oscillations in chromospheric fine structures near the solar disk center. The oscillations are interpreted in terms of propagating and standing magnetohydrodynamic kink waves. Wave characteristics including the maximum transverse velocity amplitude and the phase speed are measured as a function of distance along the structure's length. Solar magnetoseismology is applied to these measured parameters to obtain diagnostic information on key plasma parameters (e.g., magnetic field, density, temperature, flow speed) of these localized waveguides. The magnetic field strength of the mottle along the ~2 Mm length is found to decrease by a factor of 12, while the local plasma density scale height is ~280 ± 80 km. Title: Erratum: "Energy Content and Propagation in Transverse Solar Atmospheric Waves" (2013, ApJ, 768, 191) Authors: Goossens, M.; Van Doorsselaere, T.; Soler, R.; Verth, G. Bibcode: 2013ApJ...771...74G Altcode: No abstract at ADS Title: Evidence for the Photospheric Excitation of Incompressible Chromospheric Waves Authors: Morton, R. J.; Verth, G.; Fedun, V.; Shelyag, S.; Erdélyi, R. Bibcode: 2013ApJ...768...17M Altcode: 2013arXiv1303.2356M Observing the excitation mechanisms of incompressible transverse waves is vital for determining how energy propagates through the lower solar atmosphere. We aim to show the connection between convectively driven photospheric flows and incompressible chromospheric waves. The observations presented here show the propagation of incompressible motion through the quiet lower solar atmosphere, from the photosphere to the chromosphere. We determine photospheric flow vectors to search for signatures of vortex motion and compare results to photospheric flows present in convective simulations. Further, we search for the chromospheric response to vortex motions. Evidence is presented that suggests incompressible waves can be excited by the vortex motions of a strong magnetic flux concentration in the photosphere. A chromospheric counterpart to the photospheric vortex motion is also observed, presenting itself as a quasi-periodic torsional motion. Fine-scale, fibril structures that emanate from the chromospheric counterpart support transverse waves that are driven by the observed torsional motion. A new technique for obtaining details of transverse waves from time-distance diagrams is presented and the properties of transverse waves (e.g., amplitudes and periods) excited by the chromospheric torsional motion are measured. Title: Energy Content and Propagation in Transverse Solar Atmospheric Waves Authors: Goossens, M.; Van Doorsselaere, T.; Soler, R.; Verth, G. Bibcode: 2013ApJ...768..191G Altcode: Recently, a significant amount of transverse wave energy has been estimated propagating along solar atmospheric magnetic fields. However, these estimates have been made with the classic bulk Alfvén wave model which assumes a homogeneous plasma. In this paper, the kinetic, magnetic, and total energy densities and the flux of energy are computed for transverse MHD waves in one-dimensional cylindrical flux tube models with a piecewise constant or continuous radial density profile. There are fundamental deviations from the properties for classic bulk Alfvén waves. (1) There is no local equipartition between kinetic and magnetic energy. (2) The flux of energy and the velocity of energy transfer have, in addition to a component parallel to the magnetic field, components in the planes normal to the magnetic field. (3) The energy densities and the flux of energy vary spatially, contrary to the case of classic bulk Alfvén waves. This last property has the important consequence that the energy flux computed with the well known expression for bulk Alfvén waves could overestimate the real flux by a factor in the range 10-50, depending on the flux tube equilibrium properties. Title: SDO/AIA observations of periodic and quasi-periodic phenomenon associated with an EUV jet Authors: Morton, Richard; Verth, Gary; Erdelyi, Robertus; Srivastava, Abhi Bibcode: 2013EGUGA..15...52M Altcode: It has long been advocated that explosive magnetic activity is responsible for the mass-balance in the solar atmosphere, supplying the corona and the solar wind with heated plasma. The explosive events are thought to be the result of emerging bi-polar (EB) regions reconnecting with pre-existing, open fields, with the size of the EB's (i.e., granular, super-granular) being related to size of the resulting feature (i.e., spicules, EUV/X-ray jets). Recent evidence has suggested a deeper relationship between spicules and EUV jets (Sterling et al., 2010). We present here observations of a EUV jet observed with SDO/AIA close to a southern coronal hole. The jet can be considered as a 'Blowout jet' (using the terminology of Moore et al., 2010), launching vast amounts of chromospheric plasma into the atmosphere along with hotter material. The hotter part of the jet appears to be composed of multiple, (quasi-)periodic ejections that individually resemble fast moving (>100 km/s) spicules. The multiple ejections appear crucial for distributing the hotter material high into the corona, possibly suggesting that larger EUV/X-ray are composed of many smaller spicule-like events. Although the event is close to the limb, evidence for reconnection at the chromospheric level is provided. Further, evidence for helicity (or torsional motion) and the presence of slow and fast Magnetohydrodynamic waves is given, with the wave mode excitation likely due to the reconnection process. Exploiting the observed wave motion, we also use magneto-seismological techniques to determine local plasma parameters with sub-resolution accuracy along one of the jets unique features. Title: Observations of ubiquitous compressive waves in the Sun's chromosphere Authors: Morton, Richard J.; Verth, Gary; Jess, David B.; Kuridze, David; Ruderman, Michael S.; Mathioudakis, Mihalis; Erdélyi, Robertus Bibcode: 2012NatCo...3.1315M Altcode: 2012NatCo...3E1315M; 2013arXiv1306.4124M The details of the mechanism(s) responsible for the observed heating and dynamics of the solar atmosphere still remain a mystery. Magnetohydrodynamic waves are thought to have a vital role in this process. Although it has been shown that incompressible waves are ubiquitous in off-limb solar atmospheric observations, their energy cannot be readily dissipated. Here we provide, for the first time, on-disk observation and identification of concurrent magnetohydrodynamic wave modes, both compressible and incompressible, in the solar chromosphere. The observed ubiquity and estimated energy flux associated with the detected magnetohydrodynamic waves suggest the chromosphere is a vast reservoir of wave energy with the potential to meet chromospheric and coronal heating requirements. We are also able to propose an upper bound on the flux of the observed wave energy that is able to reach the corona based on observational constraints, which has important implications for the suggested mechanism(s) for quiescent coronal heating. Title: Magneto-seismology of solar atmospheric loops by means of longitudinal oscillations Authors: Luna-Cardozo, M.; Verth, G.; Erdélyi, R. Bibcode: 2012IAUS..286..437L Altcode: 2012arXiv1204.4197L There is increasingly strong observational evidence that slow magnetoacoustic modes arise in the solar atmosphere. Solar magneto-seismology is a novel tool to derive otherwise directly un-measurable properties of the solar atmosphere when magnetohydrodynamic (MHD) wave theory is compared to wave observations. Here, MHD wave theory is further developed illustrating how information about the magnetic and density structure along coronal loops can be determined by measuring the frequencies of the slow MHD oscillations. The application to observations of slow magnetoacoustic waves in coronal loops is discussed. Title: Generation of Quasi-periodic Waves and Flows in the Solar Atmosphere by Oscillatory Reconnection Authors: McLaughlin, J. A.; Verth, G.; Fedun, V.; Erdélyi, R. Bibcode: 2012ApJ...749...30M Altcode: 2012arXiv1203.6846M We investigate the long-term evolution of an initially buoyant magnetic flux tube emerging into a gravitationally stratified coronal hole environment and report on the resulting oscillations and outflows. We perform 2.5-dimensional nonlinear numerical simulations, generalizing the models of McLaughlin et al. and Murray et al. We find that the physical mechanism of oscillatory reconnection naturally generates quasi-periodic vertical outflows, with a transverse/swaying aspect. The vertical outflows consist of both a periodic aspect and evidence of a positively directed flow. The speed of the vertical outflow (20-60 km s-1) is comparable to those reported in the observational literature. We also perform a parametric study varying the magnetic strength of the buoyant flux tube and find a range of associated periodicities: 1.75-3.5 minutes. Thus, the mechanism of oscillatory reconnection may provide a physical explanation to some of the high-speed, quasi-periodic, transverse outflows/jets recently reported by a multitude of authors and instruments. Title: Longitudinal Oscillations in Density Stratified and Expanding Solar Waveguides Authors: Luna-Cardozo, M.; Verth, G.; Erdélyi, R. Bibcode: 2012ApJ...748..110L Altcode: 2012arXiv1204.4201L Waves and oscillations can provide vital information about the internal structure of waveguides in which they propagate. Here, we analytically investigate the effects of density and magnetic stratification on linear longitudinal magnetohydrodynamic (MHD) waves. The focus of this paper is to study the eigenmodes of these oscillations. It is our specific aim to understand what happens to these MHD waves generated in flux tubes with non-constant (e.g., expanding or magnetic bottle) cross-sectional area and density variations. The governing equation of the longitudinal mode is derived and solved analytically and numerically. In particular, the limit of the thin flux tube approximation is examined. The general solution describing the slow longitudinal MHD waves in an expanding magnetic flux tube with constant density is found. Longitudinal MHD waves in density stratified loops with constant magnetic field are also analyzed. From analytical solutions, the frequency ratio of the first overtone and fundamental mode is investigated in stratified waveguides. For small expansion, a linear dependence between the frequency ratio and the expansion factor is found. From numerical calculations it was found that the frequency ratio strongly depends on the density profile chosen and, in general, the numerical results are in agreement with the analytical results. The relevance of these results for solar magneto-seismology is discussed. Title: Determination of Sub-resolution Structure of a Jet by Solar Magnetoseismology Authors: Morton, R. J.; Verth, G.; McLaughlin, J. A.; Erdélyi, R. Bibcode: 2012ApJ...744....5M Altcode: 2011arXiv1109.4851M A thin dark thread is observed in a UV/EUV solar jet in the 171 Å, 193 Å, and 211 Å, and partially in 304 Å. The dark thread appears to originate in the chromosphere but its temperature does not appear to lie within the passbands of the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. We therefore implement solar magnetoseismology to estimate the plasma parameters of the dark thread. A propagating kink (transverse) wave is observed to travel along the dark thread. The wave is tracked over a range of ~7000 km by placing multiple slits along the axis of the dark thread. The phase speed and amplitude of the wave are estimated and magnetoseismological theory is employed to determine the plasma parameters. We are able to estimate the plasma temperature, density gradient, magnetic field gradient, and sub-resolution expansion of the dark thread. The dark thread is found to be cool, T <~ 3 × 104, with both strong density and magnetic field gradients. The expansion of the flux tube along its length is ~300-400 km. Title: Frequency Filtering of Torsional Alfvén Waves by Chromospheric Magnetic Field Authors: Fedun, V.; Verth, G.; Jess, D. B.; Erdélyi, R. Bibcode: 2011ApJ...740L..46F Altcode: In this Letter, we demonstrate how the observation of broadband frequency propagating torsional Alfvén waves in chromospheric magnetic flux tubes can provide valuable insight into their magnetic field structure. By implementing a full nonlinear three-dimensional magnetohydrodynamic numerical simulation with a realistic vortex driver, we demonstrate how the plasma structure of chromospheric magnetic flux tubes can act as a spatially dependent frequency filter for torsional Alfvén waves. Importantly, for solar magnetoseismology applications, this frequency filtering is found to be strongly dependent on magnetic field structure. With reference to an observational case study of propagating torsional Alfvén waves using spectroscopic data from the Swedish Solar Telescope, we demonstrate how the observed two-dimensional spatial distribution of maximum power Fourier frequency shows a strong correlation with our forward model. This opens the possibility of beginning an era of chromospheric magnetoseismology, to complement the more traditional methods of mapping the magnetic field structure of the solar chromosphere. Title: Resonantly Damped Propagating Kink Waves in Longitudinally Stratified Solar Waveguides Authors: Soler, R.; Terradas, J.; Verth, G.; Goossens, M. Bibcode: 2011ApJ...736...10S Altcode: 2011arXiv1105.0067S It has been shown that resonant absorption is a robust physical mechanism for explaining the observed damping of magnetohydrodynamic kink waves in the solar atmosphere due to naturally occurring plasma inhomogeneity in the direction transverse to the direction of the magnetic field. Theoretical studies of this damping mechanism were greatly inspired by the first observations of post-flare standing kink modes in coronal loops using the Transition Region and Coronal Explorer. More recently, these studies have been extended to explain the attenuation of propagating coronal kink waves observed by the Coronal Multi-Channel Polarimeter. In the present study, for the first time we investigate the properties of propagating kink waves in solar waveguides including the effects of both longitudinal and transverse plasma inhomogeneity. Importantly, it is found that the wavelength is only dependent on the longitudinal stratification and the amplitude is simply a product of the two effects. In light of these results the advancement of solar atmospheric magnetoseismology by exploiting high spatial/temporal resolution observations of propagating kink waves in magnetic waveguides to determine the length scales of the plasma inhomogeneity along and transverse to the direction of the magnetic field is discussed. Title: Magnetoseismological Determination of Magnetic Field and Plasma Density Height Variation in a Solar Spicule Authors: Verth, G.; Goossens, M.; He, J. -S. Bibcode: 2011ApJ...733L..15V Altcode: The variation of magnetic field strength and plasma density along a solar spicule is determined by the use of magnetoseismology. From Solar Optical Telescope observations of a kink wave propagating along a spicule, by estimating the spatial change in phase speed and velocity amplitude, a novel approach is demonstrated to determine the chromospheric height variation of both magnetic field and plasma density. Furthermore, the magnetoseismological estimate of the plasma density gradient is combined with electron density estimates from spectroscopy to determine the changing degree of ionization of hydrogen along a spicule. Title: Propagating Kink Waves in Stratified Magnetic Waveguides of the Solar Corona Authors: Soler, Roberto; Terradas, J.; Verth, G.; Goossens, M. Bibcode: 2011SPD....42.1805S Altcode: 2011BAAS..43S.1805S Recent observations using the Coronal Multi-Channel Polarimeter (CoMP) show ubiquitous propagating waves of low amplitude in magnetic loops of the solar corona. These observations have been interpreted as magnetohydrodynamic (MHD) resonant kink waves. It has been shown that resonant absorption is a robust physical mechanism to explain the observed damping of MHD kink waves in the solar atmosphere due to naturally occurring plasma inhomogeneity in the direction transverse to the magnetic field. In the present study, for the first time we investigate the properties of propagating kink waves in solar magnetic waveguides including the effects of both longitudinal and transverse plasma inhomogeneity. Importantly, it is found that the wavelength is only dependent on the longitudinal stratification and the amplitude is simply a product of the two effects. In light of these results the advancement of solar atmospheric magnetoseismology by exploiting high spatial/temporal resolution observations of propagating kink waves in magnetic waveguides to determine the length scales of the plasma inhomogeneity along and transverse to the direction of the magnetic field is discussed. Title: Seismology of Transversely Oscillating Coronal Loops with Siphon Flows Authors: Terradas, J.; Arregui, I.; Verth, G.; Goossens, M. Bibcode: 2011ApJ...729L..22T Altcode: 2011arXiv1101.5238T There are ubiquitous flows observed in the solar atmosphere of sub-Alfvénic speeds; however, after flaring and coronal mass ejection events flows can become Alfvénic. In this Letter, we derive an expression for the standing kink mode frequency due to siphon flow in coronal loops, valid for both low and high speed regimes. It is found that siphon flow introduces a linear, spatially dependent phase shift along coronal loops and asymmetric eigenfunctions. We demonstrate how this theory can be used to determine the kink and flow speed of oscillating coronal loops with reference to an observational case study. It is shown that the presence of siphon flow can cause the underestimation of magnetic field strength in coronal loops using the traditional seismological methods. Title: Selective spatial damping of propagating kink waves due to resonant absorption Authors: Terradas, J.; Goossens, M.; Verth, G. Bibcode: 2010A&A...524A..23T Altcode: 2010arXiv1004.4468T Context. There is observational evidence of propagating kink waves driven by photospheric motions. These disturbances, interpreted as kink magnetohydrodynamic (MHD) waves are attenuated as they propagate upwards in the solar corona.
Aims: We show that resonant absorption provides a simple explanation to the spatial damping of these waves.
Methods: Kink MHD waves are studied using a cylindrical model of solar magnetic flux tubes, which includes a non-uniform layer at the tube boundary. Assuming that the frequency is real and the longitudinal wavenumber complex, the damping length and damping per wavelength produced by resonant absorption are analytically calculated in the thin tube (TT) approximation, valid for coronal waves. This assumption is relaxed in the case of chromospheric tube waves and filament thread waves.
Results: The damping length of propagating kink waves due to resonant absorption is a monotonically decreasing function of frequency. For kink waves with low frequencies, the damping length is exactly inversely proportional to frequency, and we denote this as the TGV relation. When moving to high frequencies, the TGV relation continues to be an exceptionally good approximation of the actual dependency of the damping length on frequency. This dependency means that resonant absorption is selective as it favours low-frequency waves and can efficiently remove high-frequency waves from a broad band spectrum of kink waves. The efficiency of the damping due to resonant absorption depends on the properties of the equilibrium model, in particular on the width of the non-uniform layer and the steepness of the variation in the local Alfvén speed.
Conclusions: Resonant absorption is an effective mechanism for the spatial damping of propagating kink waves. It is selective because the damping length is inversely proportional to frequency so that the damping becomes more severe with increasing frequency. This means that radial inhomogeneity can cause solar waveguides to be a natural low-pass filter for broadband disturbances. Kink wave trains travelling along, e.g., coronal loops, will therefore have a greater proportion of the high-frequency components dissipated lower down in the atmosphere. This could have important consequences for the spatial distribution of wave heating in the solar atmosphere. Title: Observational Evidence of Resonantly Damped Propagating Kink Waves in the Solar Corona Authors: Verth, G.; Terradas, J.; Goossens, M. Bibcode: 2010ApJ...718L.102V Altcode: 2010arXiv1007.1080V In this Letter, we establish clear evidence for the resonant absorption damping mechanism by analyzing observational data from the novel Coronal Multi-Channel Polarimeter. This instrument has established that in the solar corona there are ubiquitous propagating low-amplitude (≈1 km s-1) Alfvénic waves with a wide range of frequencies. Realistically interpreting these waves as the kink mode from magnetohydrodynamic wave theory, they should exhibit a frequency-dependent damping length due to resonant absorption, governed by the Terradas-Goossens-Verth relation showing that transverse plasma inhomogeneity in coronal magnetic flux tubes causes them to act as natural low-pass filters. It is found that the observed frequency dependence on damping length (up to about 8 mHz) can be explained by the kink wave interpretation; and furthermore, the spatially averaged equilibrium parameter describing the length scale of transverse plasma density inhomogeneity over a system of coronal loops is consistent with the range of values estimated from Transition Region and Coronal Explorer observations of standing kink modes. Title: Magnetoseismology: Eigenmodes of Torsional Alfvén Waves in Stratified Solar Waveguides Authors: Verth, G.; Erdélyi, R.; Goossens, M. Bibcode: 2010ApJ...714.1637V Altcode: There have recently been significant claims of Alfvén wave observation in the solar chromosphere and corona. We investigate how the radial and longitudinal plasma structuring affects the observational properties of torsional Alfvén waves in magnetic flux tubes for the purposes of solar magnetoseismology. The governing magnetohydrodynamic equations of these waves in axisymmetric flux tubes of arbitrary radial and axial plasma structuring are derived and we study their observable properties for various equilibria in both thin and finite-width magnetic flux tubes. For thin flux tubes, it is demonstrated that observation of the eigenmodes of torsional Alfvén waves can provide temperature diagnostics of both the internal and surrounding plasma. In the finite-width flux tube regime, it is shown that these waves are the ideal magnetoseismological tool for probing radial plasma inhomogeneity in solar waveguides. Title: Coronal Seismology by Means of Kink Oscillation Overtones Authors: Andries, J.; van Doorsselaere, T.; Roberts, B.; Verth, G.; Verwichte, E.; Erdélyi, R. Bibcode: 2009SSRv..149....3A Altcode: The detection of overtones of coronal loop kink oscillations has been an important advance in the development of coronal seismology. It has significantly increased the potential of coronal seismology and has thus initiated important theoretical and observational improvements. New detections of overtones have been made and a reduction of the error bars has been obtained. The efforts of theoreticians to extend eigenmode studies to more general coronal loop models is no longer a matter of checking the robustness of the model but now also allows for the estimation of certain equilibrium parameters. The frequencies of the detected (longitudinal) overtones are in particular sensitive to changes in the equilibrium properties along the loop, especially the density and the magnetic field expansion. Also, attempts have been made to use the limited longitudinal resolution in combination with the theoretical eigenmodes as an additional seismological tool. Title: Refined Magnetoseismological Technique for the Solar Corona Authors: Verth, G.; Erdélyi, R.; Jess, D. B. Bibcode: 2008ApJ...687L..45V Altcode: We present a step-by-step guide of a refined magnetoseismological technique for plasma diagnostics in the Sun's corona. Recently developed MHD wave theory which models a coronal loop as an expanding magnetic flux tube with an arbitrary longitudinal plasma density profile is applied to TRACE observations of fast kink oscillations. The theory predicts that using the observed ratio of the first overtone and fundamental mode to predict the plasma density scale height and not taking account of loop expansion will lead to an overestimation of scale height. For the first time, the size of this correction is quantified directly from observation and for the particular case study presented here, it is found that the overestimation is approximately by a factor of 2. Title: Transverse Oscillations of Longitudinally Stratified Coronal Loops with Variable Cross Section Authors: Ruderman, M. S.; Verth, G.; Erdélyi, R. Bibcode: 2008ApJ...686..694R Altcode: We consider transverse oscillations of coronal loops that have both variable circular cross-sectional area and plasma density in the longitudinal direction. The primary focus of this paper is to study the eigenmodes of these oscillations. Implementing the method of asymptotic expansions with the ratio of the loop radius to length as a small parameter, a second-order ordinary differential equation is derived describing the displacement of the loop axis. Together with the boundary conditions at the tube ends that follow from the frozen-in condition, this equation constitutes the Sturm-Liouville problem determining the eigenfrequencies and eigenmodes. Our results are relevant to the magnetoseismological method of estimating the coronal density scale height by using the observed ratio of the fundamental frequency and first overtone of loop kink oscillations. It is shown that this method is very sensitive to the tube expansion factor, which is the ratio of the tube radii at the apex and footpoints. The estimated scale height is a monotonically decreasing function of the expansion factor. Title: Effect of longitudinal magnetic and density inhomogeneity on transversal coronal loop oscillations Authors: Verth, G.; Erdélyi, R. Bibcode: 2008A&A...486.1015V Altcode: Context: Observations of post-flare transversal coronal loop oscillations by TRACE have given us an excellent opportunity to implement magneto-seismological techniques for probing the plasma fine structure of the Sun's upper atmosphere.
Aims: We investigate the combined effect of magnetic and density stratification on transversal coronal loop oscillations.
Methods: A coronal loop will be modelled as an expanding magnetic flux tube with arbitrary longitudinal plasma density. The governing equation of the fast kink body mode is derived and solved by analytical approximation and numerical methods.
Results: It is found that even a relatively small coronal loop expansion can have a significant and pronounced effect on the accuracy of the plasma density scale height measurements derived from observation of loop oscillations.
Conclusions: To conduct more accurate and realistic magneto-seismology of coronal loops, the magnetic field divergence should be taken into account. Title: Discovery of Spatial Periodicities in a Coronal Loop Using Automated Edge-Tracking Algorithms Authors: Jess, D. B.; Mathioudakis, M.; Erdélyi, R.; Verth, G.; McAteer, R. T. J.; Keenan, F. P. Bibcode: 2008ApJ...680.1523J Altcode: 2008arXiv0802.1971J A new method for automated coronal loop tracking, in both spatial and temporal domains, is presented. Applying this technique to TRACE data, obtained using the 171 Å filter on 1998 July 14, we detect a coronal loop undergoing a 270 s kink-mode oscillation, as previously found by Aschwanden et al. However, we also detect flare-induced, and previously unnoticed, spatial periodicities on a scale of 3500 km, which occur along the coronal loop edge. Furthermore, we establish a reduction in oscillatory power for these spatial periodicities of 45% over a 222 s interval. We relate the reduction in detected oscillatory power to the physical damping of these loop-top oscillations. Title: Magneto-seismology: effect of inhomogeneous magnetic field on transversal coronal loop oscillations Authors: Verth, G. Bibcode: 2008IAUS..247..123V Altcode: 2007IAUS..247..123V The extreme-ultraviolet (EUV) imagers onboard the planned Solar Dynamics Observatory (SDO) and Solar Orbiter (SO) will offer us the best chance yet of using observations of post-flare loop oscillations to probe the fine structure of the corona. Recently developed magnetohydrodynamic (MHD) wave theory has shown that the properties of loop oscillations depend on their plasma fine structure. Up to this point, many studies have concentrated solely on the effect of plasma density stratification on coronal loop oscillations. In this paper we develop MHD wave theory which models the effect of an inhomogeneous magnetic field on coronal loop oscillations. The results have the potential to be used in testing the efficacy of photospheric magnetic field extrapolations and have important implications regarding magneto-seismology of the corona. Title: Solar feature tracking in both spatial and temporal domains Authors: Jess, D. B.; Mathioudakis, M.; Erdélyi, R.; Verth, G.; McAteer, R. T. J.; Keenan, F. P. Bibcode: 2008IAUS..247..288J Altcode: 2007IAUS..247..288J A new method for automated coronal loop tracking, in both spatial and temporal domains, is presented. The reliability of this technique was tested with TRACE 171 Å observations. The application of this technique to a flare-induced kink-mode oscillation, revealed a 3500 km spatial periodicity which occur along the loop edge. We establish a reduction in oscillatory power, for these spatial periodicities, of 45% over a 322 s interval. We relate the reduction in oscillatory power to the physical damping of these loop-top oscillations. Title: Spatial magneto-seismology: effect of density stratification on the first harmonic amplitude profile of transversal coronal loop oscillations Authors: Verth, G.; Van Doorsselaere, T.; Erdélyi, R.; Goossens, M. Bibcode: 2007A&A...475..341V Altcode: Context: The new generation of extreme-ultraviolet (EUV) imagers onboard missions such as the Solar Dynamics Observatory (SDO) and Solar Orbiter (SO) will provide the most accurate spatial measurements of post-flare coronal loop oscillations yet. The amplitude profiles of these loop oscillations contain important information about plasma fine structure in the corona.
Aims: We show that the position of the anti-nodes of the amplitude profile of the first harmonic of the standing fast kink wave of a coronal loop relate to the plasma density stratification of that loop.
Methods: The MHD kink transversal waves of coronal loops are modelled both numerically and analytically. The numerical model implements the implicit finite element code pollux. Dispersion relations are derived and solved analytically. The results of the two methods are compared and verified.
Results: Density stratification causes the anti-nodes of the first harmonic to shift towards the loop footpoints. The greater the density stratification, the larger the shift. The anti-node shift of the first harmonic of a semi-circular coronal loop with a density scale height H=50 Mm and loop half length L=100 Mm is approximately 5.6 Mm. Shifts in the Mm range are measureable quantities providing valuable information about the subresolution structure of coronal loops.
Conclusions: The measurement of the anti-node shift of the first harmonic of the standing fast kink wave of coronal loops is potentially a new tool in the field of solar magneto-seismology, providing a novel complementary method of probing plasma fine structure in the corona. Title: Magneto-seismology of the solar atmosphere Authors: Verth, G. Bibcode: 2007AN....328..764V Altcode: Magnetohydrodynamic (MHD) waves in solar coronal loops, which were previously only predicted by theory have actually been detected with space-borne instruments. These observations have given an important and novel tool to measure fundamental parameters in the magnetically embedded solar corona. This paper will illustrate how information about the magnetic and density structure along coronal loops can be determined by measuring the frequency or amplitude profiles of standing fast kink mode oscillations Title: The effect of density stratification on the amplitude profile of transversal coronal loop oscillations Authors: Erdélyi, R.; Verth, G. Bibcode: 2007A&A...462..743E Altcode: Context: Measuring how the density of the plasma in the Sun's corona varies along fine structures such as coronal loops using emission measure is very difficult as the exact value of the line of sight filling factor and the correct amount of background subtraction are unknown.
Aims: To investigate if magnetohydrodynamic (MHD) wave theory can be used to complement existing observational methods to obtain diagnostic information about the density structure of coronal loops by measuring the amplitude profile of the fundamental standing mode of the fast kink wave.
Methods: Analytical and numerical approaches are used to show how the amplitude profile of the fundamental fast kink mode changes by varying the amount of density stratification in a coronal loop.
Results: Increasing the amount of density stratification in a coronal loop will increase the difference in amplitude profile from one of constant longitudinal density. For a semi-circular TRACE 171 Å temperature loop of length 200 Mm and density scale height, H=50 Mm, the maximum change in amplitude profile is of the order of 50 km. It is also found that the amplitudes of these oscillations are effectively negligible at altitudes lower than the transition region.
Conclusions: .The effect of density stratification on the amplitude profile of the fundamental standing kink mode of a TRACE 171 Å temperature loop may be very subtle. Unfortunately, the observational signatures of this effect are likely to be well below the resolution of the best currently available extreme-ultraviolet (EUV) imagers. Only loops with homogeneous magnetic fields have been addressed here, but inhomogeneous magnetic fields may well dominate over the effect of density stratification on the amplitude profile. Title: Solar Coronal Magneto- Seismology With Solar Orbiter Authors: Verth, G.; Erdéyi, R. Bibcode: 2007ESASP.641E..30V Altcode: 2006ESASP.641E..30V MHD waves in solar coronal loops, which were previously only predicted by theory have now actually been detected with instruments such as TRACE and SUMER on-board SOHO. These observations have given the solar community an important and novel tool to measure fundamental parameters in the magnetically embedded solar corona. Theory has been developed to derive detailed diagnostic information, e.g., density, magnetic field look structure, geometry, and stratifications. In this paper we demonstrate through examples of case studies how the EUV imager on Solar Orbiter can be used for solar atmospheric (coronal) magneto-seismology. Possible methods will be discussed to determine (i) if magnetic field divergence or plasma density stratification is the dominating factor in transversal loop oscillations (ii) important parameters such as the density scale heigh and magnetic dipole depth of a loop.