arXiv subjects

Jaume Terradas

Publications and source records attributed to Jaume Terradas.

At least 19 recordsLinked to original sources

arXiv2026

Automatic detection of solar filament oscillations I: Multi-scale spectral pipeline

Solar filament oscillations provide important diagnostics of prominence magnetic structure and stability, but their detection in long H\alpha archives has traditionally relied on visual inspection, manually placed slits, and time--distance diagrams. We present an automatic pipeline for detecting spatially coherent filament oscillations in GONG H\alpha image sequences. The method combines image preprocessing and coalignment, deep-learning-based filament detection and segmentation, multi-scale spatial averaging, Lomb--Scargle spectral analysis, convolutional-neural-network background estimation, empirical calibration of significance thresholds, and clustering of candidate detections in period and space. Only oscillations supported across at least four spatial scales are retained, reducing sensitivity to local pixel-scale intensity fluctuations. The pipeline recovers several events from the manual GONG catalog of Luna et al. (2018), including the 1 January 2014 oscillation with a period of approximately 76 min. Applied to the first two weeks of January 2014, it identifies 91 oscillatory events, compared with 22 non-duplicate events in the corresponding manual catalog, with detected periods ranging from about 20 to 126 min. It also detects previously unreported oscillations, including an event on 13 January 2014 with a period of approximately 86 min that is independently confirmed using a conventional time--distance diagram. These results demonstrate that automated filament segmentation, multi-scale spectral analysis, and calibrated significance testing can provide a reproducible and scalable alternative to manual slit-based searches. The pipeline substantially increases detection sensitivity while preferentially selecting coherent filament-scale oscillations, enabling future statistical studies over extended GONG intervals and across the solar cycle.

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arXiv2026

Reevaluating thermal instability in a uniform plasma: an extended analysis of instability domains

Thermal instability plays a crucial role in the dynamics of astrophysical plasmas. Building upon the foundational work of G. B. Field (1965) and the subsequent analysis by T. Waters & D. Proga (2019), this study revisits thermal instability in a uniform, non-magnetic medium. We aim to reevaluate and expand the understanding of instability domains, focusing on the classification and characteristics of thermal and acoustic modes in the presence of heating, radiative cooling, and thermal conduction. Except for Spitzer's expression for parallel thermal conductivity, heating and cooling processes are unspecified. Additionally, we investigate the existence of isobaric and isochoric thermal modes across the extreme limits of very short and very long wavelengths, as well as at intermediate wavelengths; we address a common misconception about the existence of purely adiabatic perturbations. We also perform an in-depth analysis of the dispersion relation for an infinite, uniform hydrodynamic medium, as derived by G. B. Field (1965). This enables the generation of growth rate and dispersion diagrams, providing insight into thermal instability across different wavelength ranges. With the inclusion of thermal conduction, our study refines the classification of the instability regions previously outlined by T. Waters & D. Proga (2019). Our findings confirm that their classification holds when the Field length is smaller than or comparable to the thermal wavelength. For larger Field lengths, a simplified classification becomes impractical. Furthermore, we discuss the potential implications of the catastrophic cooling instability (T. Waters & A. Stricklan 2025) in coronal rain formation.

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arXiv2025

MHD waves with mixed properties / Alfvén waves with pressure variations: a review

Non-uniformity plays an important role for MHD waves. For a uniform plasma of infinite extent the MHD waves can be subdivided in two classes with distinct properties. The first class contains the Alfvén waves. The Alfvén waves are incompressible and propagate parallel vorticity. They do not have a parallel component of displacement, they do not cause variations in pressure and are driven by magnetic tension only. The second class contains the magneto-sonic waves. They are compressible and have a parallel component of displacement. They do not propagate parallel vorticity and are driven by pressure and magnetic tension. In non-uniform plasmas the situation can be very different. The clear division between Alfvén waves and magneto-sonic waves is no longer present. In a given part of the equilibrium an MHD wave can strongly resemble a magneto-sonic wave with little or no resemblance to Alfvén waves; while in another part of the equilibrium the MHD wave is practically an Alfvén wave, which has the amazing property of being accompanied by variations in pressure.

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arXiv2024

Study of the excitation of large amplitude oscillations in a prominence by nearby flares

Large amplitude oscillations commonly occur in solar prominences, triggered by energetic phenomena such as jets and flares. On March 14-15, 2015, a filament partially erupted in two stages, leading to oscillations in different parts. This study explores longitudinal oscillations from the eruption, focusing on the mechanisms behind their initiation, with special attention to the large oscillation on March 15. The oscillations and jets are analyzed using the time-distance technique. For flares and their interaction with the filament, we analyze AIA channels and use the DEM technique. Initially, a jet fragments the filament, splitting it into two segments. One remains in place, while the other detaches and moves. This causes oscillations in both segments: (a) the position change causes the detached segment to oscillate with a period of $69 \pm 3$ minutes; (b) the jet flows cause the remaining filament to oscillate with a period of $62 \pm 2$ minutes. In the second phase, on March 15, another jet seemingly activates the detached filament eruption, followed by a flare. A large longitudinal oscillation occurs in the remnant segment with a period of $72 \pm 2$ minutes and velocity amplitude $73 \pm 1 \, \mathrm{km s^{-1}}$. During the oscillation trigger, bright field lines connect the flare with the filament, appearing only in the AIA 131$Å$ and 94$Å$ channels, indicating the presence of hot plasma. DEM analysis confirms this, showing plasma around 10 MK pushing the prominence from its southeastern side, displacing it along the field lines and starting the oscillation. From this, the flare -- not the preceding jet-triggers the oscillation. The hot plasma flows into the filament channel. We explain how flares trigger large oscillations in filaments by proposing that post-flare loops reconnect with the filament channel's magnetic field.

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arXiv2023

Self-consistent equilibrium models of prominence thin threads heated by Alfvén waves propagating from the photosphere

The fine structure of solar prominences is made by thin threads that outline the magnetic field lines. Observations show that transverse waves of Alfvénic nature are ubiquitous in prominence threads. These waves are driven at the photosphere and propagate to prominences suspended in the corona. Heating due to Alfvén wave dissipation could be a relevant mechanism in the cool and partially ionized prominence plasma. We explore the construction of 1D equilibrium models of prominence thin threads that satisfy energy balance between radiative losses, thermal conduction, and Alfvén wave heating. We assume the presence of a broadband driver at the photosphere that launches Alfvén waves towards the prominence. An iterative method is implemented, in which the energy balance equation and the Alfvén wave equation are consecutively solved. From the energy balance equation and considering no wave heating initially, we compute the equilibrium profiles along the thread of the temperature, density, ionisation fraction. We use the Alfvén wave equation to compute the wave heating rate, which is then put back in the energy balance equation to obtain new equilibrium profiles. The process is repeated until convergence to a self-consistent thread model heated by Alfvén waves is achieved. We have obtained equilibrium models composed of a cold and dense thread, a extremely thin PCTR, and an extended coronal region. The length of the cold thread decreases with the temperature at the prominence core and increases with the Alfvén wave energy flux. Equilibrium models are not possible for sufficiently large wave energy fluxes when the wave heating rate inside the cold thread becomes larger than radiative losses. The maximum value of the wave energy flux that allows an equilibrium depends on the prominence core temperature. This constrains the existence of equilibria in realistic conditions.

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arXiv2022

Three-dimensional solar active region magnetohydrostatic models and their stability using Euler potentials

Active regions (ARs) are typical magnetic structures found in the solar atmosphere. We calculate several magnetohydrostatic (MHS) equilibrium models that include the effect of a finite plasma-$β$ and gravity and that are representative of these structures in three dimensions. The construction of the models is based on the use of two Euler potentials, $α$ and $β$, that represent the magnetic field as ${\bf B}=\nabla α\times \nabla β$. The ideal MHS nonlinear partial differential equations are solved numerically using finite elements in a fixed 3D rectangular domain. The boundary conditions are initially chosen to correspond to a potential magnetic field (current-free) with known analytical expressions for the corresponding Euler potentials. The distinctive feature is that we incorporate the effect of shear by progressively deforming the initial potential magnetic field. This procedure is quite generic and allows us to generate a vast variety of MHS models. The thermal structure of the ARs is incorporated through the dependence of gas pressure and temperature on the Euler potentials. Using this method we achieve the characteristic hot and over-dense plasma found in ARs, but we demonstrate that the method can also be applied to study configurations with open magnetic field lines. Furthermore, we investigate basic topologies that include neutral lines. Our focus is on the force balance of the structures and we do not consider the energy balance in the constructed models. In addition, we address the difficult question of the stability of the calculated 3D models. We find that if the plasma is convectively stable, then the system is not prone in general to develop magnetic Rayleigh-Taylor instabilities.

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arXiv2021

Excitation of ion-acoustic waves by non-linear finite-amplitude standing Alfvén waves

We investigate, using a multi-fluid approach, the main properties of standing ion-acoustic modes driven by nonlinear standing Alfvén waves. The standing character of the Alfvénic pump is because we study the superposition of two identical circularly polarised counter-propagating waves. We consider parallel propagation along the constant magnetic field and we find that left and right-handed modes generate via ponderomotive forces the second harmonic of standing ion-acoustic waves. We demonstrate that parametric instabilities are not relevant in the present problem and the secondary ion-acoustic waves attenuate by Landau damping in the absence of any other dissipative process. Kinetic effects are included in our model where ions are considered as particles and electrons as a massless fluid, and hybrid simulations are used to complement the theoretical results. Analytical expressions are obtained for the time evolution of the different physical variables in the absence of Landau damping. From the hybrid simulations we find that the attenuation of the generated ion-acoustic waves follows the theoretical predictions even under the presence of a driver Alfvénic pump. Due to the nonlinear induced ion-acoustic waves the system develops density cavities and an electric field parallel to the magnetic field. Theoretical expressions for this density and electric field fluctuations are derived. The implications of these results in the context of standing slow mode oscillations in coronal loops is discussed.

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arXiv2021

Transverse kink oscillations of inhomogeneous prominence threads: numerical analysis and H$α$ forward modelling

Prominence threads are very long and thin flux tubes which are partially filled with cold plasma. Observations have shown that transverse oscillations are frequent in these solar structures. The observations are usually interpreted as the fundamental kink mode, while the detection of the first harmonic remains elusive. Here, we aim to study how the density inhomogeneity in the longitudinal and radial directions modify the periods and damping times of kink oscillations, and how this effect would be reflected in observations. We solve the ideal magnetohydrodynamics equations through two different methods: a) performing 3D numerical simulations, and b) solving a 2D generalised eigenvalue problem. We study the dependence of the periods, damping times and amplitudes of transverse kink oscillations on the ratio between the densities at the centre and at the ends of the tube, and on the average density. We apply forward modelling on our 3D simulations to compute synthetic H$α$ profiles. We confirm that the ratio of the period of the fundamental oscillation mode to the period of the first harmonic increases as the ratio of the central density to the footpoint density is increased or as the averaged density of the tube is decreased. We find that the damping times due to resonant absorption decrease as the central to footpoint density ratio increases. Contrary to the case of longitudinally homogeneous tubes, we find that the damping time to period ratio also increases as the density ratio is increased or the average density is reduced. We present snapshots and time-distance diagrams of the emission in the H$α$ line. The results presented here have implications for the field of prominence seismology. While the H$α$ emission can be used to detect the fundamental mode, the first harmonic is barely detectable in H$α$. This may explain the lack of detections of the first harmonic.

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arXiv2018

Energy transport and heating by torsional Alfvén waves propagating from the photosphere to the corona in the quiet Sun

In the solar atmosphere, Alfvén waves are believed to play an important role in the transfer of energy from the photosphere to the corona and solar wind, and in the heating of the chromosphere. We perform numerical computations to investigate energy transport and dissipation associated with torsional Alfvén waves propagating in magnetic flux tubes that expand from the photosphere to the corona in quiet-Sun conditions. We place a broadband driver at the photosphere that injects a wave energy flux of $10^7$ erg cm$^{-2}$ s$^{-1}$ and consider Ohm's magnetic diffusion and ion-neutral collisions as dissipation mechanisms. We find that only a small fraction of the driven flux, $\sim 10^5$ erg cm$^{-2}$ s$^{-1}$, is able to reach coronal heights, but it may be sufficient to partly compensate the total coronal energy loss. The frequency of maximal transmittance is $\sim 5$ mHz for a photospheric field strength of 1 kG and is shifted to smaller/larger frequencies for weaker/stronger fields. Lower frequencies are reflected at the transition region, while higher frequencies are dissipated producing enough heat to balance chromospheric radiative losses. Heating in the low and middle chromosphere is due to Ohmic dissipation, while ion-neutral friction dominates in the high chromosphere. Ohmic diffusion is enhanced by phase mixing because of the expansion of the magnetic field. This effect has the important consequence of increasing the chromospheric dissipation and, therefore, reducing the energy flux that reaches the corona. We provide empirical fits of the transmission coefficient that could be used as input for coronal models.

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arXiv2018

GONG Catalog of Solar Filament Oscillations Near Solar Maximum

We have catalogued 196 filament oscillations from the GONG $Hα$ network data during several months near the maximum of solar cycle 24 (January - June 2014). Selected examples from the catalog are described in detail, along with our statistical analyses of all events. Oscillations were classified according to their velocity amplitude: 106 small-amplitude oscillations (SAOs), with velocities $<10\mathrm{\, km \; s^{-1}}$, and 90 large-amplitude oscillations (LAOs), with velocities $>10\mathrm{\, km \; s^{-1}}$. Both SAOs and LAOs are common, with one event of each class every two days on the visible side of the Sun. For nearly half of the events we identified their apparent trigger. The period distribution has a mean value of 58$\pm$15 min for both types of oscillations. The distribution of the damping time per period peaks at $τ/P=1.75$ and $1.25$ for SAOs and LAOs respectively. We confirmed that LAO damping rates depend nonlinearly on the oscillation velocity. The angle between the direction of motion and the filament spine has a distribution centered at $27^\circ$ for all filament types. This angle agrees with the observed direction of filament-channel magnetic fields, indicating that most of the catalogued events are longitudinal (i.e., undergo field-aligned motions). We applied seismology to determine the average radius of curvature in the magnetic dips, $R\approx89$ Mm, and the average minimum magnetic-field strength, $B\approx16$ G. The catalog is available to the community online, and is intended to be expanded to cover at least 1 solar cycle.

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arXiv2018

Multi-fluid approach to high-frequency waves in plasmas. III. Nonlinear regime and plasma heating

The multi-fluid modelling of high-frequency waves in partially ionized plasmas has shown that the behavior of magnetohydrodynamics waves in the linear regime is heavily influenced by the collisional interaction between the different species that form the plasma. Here, we go beyond linear theory and study large-amplitude waves in partially ionized plasmas using a nonlinear multi-fluid code. It is known that in fully ionized plasmas, nonlinear Alfvén waves generate density and pressure perturbations. Those nonlinear effects are more pronounced for standing oscillations than for propagating waves. By means of numerical simulations and analytical approximations, we examine how the collisional interaction between ions and neutrals affects the nonlinear evolution. The friction due to collisions dissipates a fraction of the wave energy, which is transformed into heat and consequently rises the temperature of the plasma. As an application, we investigate frictional heating in a plasma with physical conditions akin to those in a solar quiescent prominence.

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arXiv2017

Multi-fluid Approach to High-frequency Waves in Plasmas. II. Small-amplitude Regime in Partially Ionized Media

The presence of neutral species in a plasma has been shown to greatly affect the properties of magnetohydrodynamic waves. For instance, the interaction between ions and neutrals through momentum transfer collisions causes the damping of Alfvén waves and alters their oscillation frequency and phase speed. When the collision frequencies are larger than the frequency of the waves, single-fluid magnetohydrodynamic approximations can accurately describe the effects of partial ionization, since there is a strong coupling between the various species. However, at higher frequencies, the single-fluid models are not applicable and more complex approaches are required. Here, we use a five-fluid model with three ionized and two neutral components, which takes into consideration Hall's current and Ohm's diffusion in addition to the friction due to collisions between different species. We apply our model to plasmas composed of hydrogen and helium, and allow the ionization degree to be arbitrary. By means of the analysis of the corresponding dispersion relation and numerical simulations, we study the properties of small-amplitude perturbations. We discuss the effect of momentum transfer collisions on the ion-cyclotron resonances and compare the importance of magnetic resistivity, ion-neutral and ion-ion collisions on the wave damping at various frequency ranges. Application to partially ionized plasmas of the solar atmosphere are performed.

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arXiv2016

Multi-fluid approach to high-frequency waves in plasmas: I. Small-amplitude regime in fully ionized medium

Ideal MHD provides an accurate description of low-frequency Alfvén waves in fully ionized plasmas. However, higher frequency waves in many plasmas of the solar atmosphere cannot be correctly described by ideal MHD and a more accurate model is required. Here, we study the properties of small-amplitude incompressible perturbations in both the low and the high frequency ranges in plasmas composed of several ionized species. We use a multi-fluid approach and take into account the effects of collisions between ions and the inclusion of Hall's term in the induction equation. Through the analysis of the corresponding dispersion relations and numerical simulations we check that at high frequencies ions of different species are not as strongly coupled as in the low frequency limit. Hence, they cannot be treated as a single fluid. In addition, elastic collisions between the distinct ionized species are not negligible for high frequency waves since an appreciable damping is obtained. Furthermore, Coulomb collisions between ions remove the cyclotron resonances and the strict cut-off regions that are present when collisions are not taken into account. The implications of these results for the modelling of high-frequency waves in solar plasmas are discussed.

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arXiv2016

The role of Alfvén wave heating in solar prominences

Observations have shown that magnetohydrodynamic waves over a large frequency range are ubiquitous in solar prominences. The waves are probably driven by photospheric motions and may transport energy up to prominences suspended in the corona. Dissipation of wave energy can lead to heating of the cool prominence plasma, so contributing to the local energy balance within the prominence. Here we discuss the role of Alfvén wave dissipation as a heating mechanism for the prominence plasma. We consider a slab-like quiescent prominence model with a transverse magnetic field embedded in the solar corona. The prominence medium is modelled as a partially ionized plasma composed of a charged ion-electron single fluid and two separate neutral fluids corresponding to neutral hydrogen and neutral helium. Friction between the three fluids acts as a dissipative mechanism for the waves. The heating caused by externally-driven Alfvén waves incident on the prominence slab is analytically explored. We find that the dense prominence slab acts as a resonant cavity for the waves. The fraction of incident wave energy that is channelled into the slab strongly depends upon the wave period, $P$. Using typical prominence conditions, we obtain that wave energy trapping and associated heating are negligible when $P \gtrsim 100$ s, so that it is unlikely that those waves have a relevant influence on prominence energetics. When $1$ s $\lesssim P \lesssim 100$ s the energy absorption into the slab shows several sharp and narrow peaks, that can reach up to 100%, when the incident wave frequency matches a cavity resonance of the slab. Wave heating is enhanced at those resonant frequencies. Conversely, when $P \lesssim 1$ s cavity resonances are absent, but the waves are heavily damped by the strong dissipation. We estimate that wave heating may compensate for about 10% of radiative losses of the prominence plasma.

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arXiv2015

Onset of the Kelvin-Helmholtz instability in partially ionized magnetic flux tubes

Context. Recent observations of solar prominences show the presence of turbulent flows that may be caused by Kelvin-Helmholtz instabilites (KHI). However, the observed flow velocities are below the classical threshold for the onset of KHI in fully ionized plasmas. Aims. We investigate the effect of partial ionization on the onset of KHI in dense and cool cylindrical magnetic flux tubes surrounded by a hotter and lighter environment. Methods. The linearized governing equations of a partially ionized two-fluid plasma are used to describe the behavior of small-amplitude perturbations superimposed on a magnetic tube with longitudinal mass flow. A normal mode analysis is performed to obtain the dispersion relation for linear incompressible waves. We focus on the appearance of unstable solutions and study the dependence of their growth rates on various physical parameters. An analytical approximation of the KHI linear growth rate for slow flows and strong ion-neutral coupling is obtained. An application to solar prominence threads is given. Results. The presence of a neutral component in a plasma may contribute to the onset of the KHI even for sub-Alfvénic longitudinal shear flows. Collisions between ions and neutrals reduce the growth rates of the unstable perturbations but cannot completely suppress the instability. Conclusions. Turbulent flows in solar prominences with sub-Alfvénic flow velocities may be interpreted as consequences of KHI in partially ionized plasmas.

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arXiv2015

Magnetohydrodynamic kink waves in nonuniform solar flux tubes: phase mixing and energy cascade to small scales

Magnetohydrodynamic (MHD) kink waves are ubiquitously observed in the solar atmosphere. The propagation and damping of these waves may play relevant roles for the transport and dissipation of energy in the solar atmospheric medium. However, in the atmospheric plasma dissipation of transverse MHD wave energy by viscosity or resistivity needs very small spatial scales to be efficient. Here, we theoretically investigate the generation of small scales in nonuniform solar magnetic flux tubes due to phase mixing of MHD kink waves. We go beyond the usual approach based on the existence of a global quasi-mode that is damped in time due to resonant absorption. Instead, we use a modal expansion to express the MHD kink wave as a superposition of Alfvén continuum modes that are phase mixed as time evolves. The comparison of the two techniques evidences that the modal analysis is more physically transparent and describes both the damping of global kink motions and the building up of small scales due to phase mixing. In addition, we discuss that the processes of resonant absorption and phase mixing are intimately linked. They represent two aspects of the same underlying physical mechanism: the energy cascade from large scales to small scales due to naturally occurring plasma and/or magnetic field inhomogeneities. This process may provide the necessary scenario for efficient dissipation of transverse MHD wave energy in the solar atmospheric plasma.

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arXiv2013

The behavior of transverse waves in nonuniform solar flux tubes. II. Implications for coronal loop seismology

Seismology of coronal loops using observations of damped transverse oscillations in combination with results from theoretical models is a tool to indirectly infer physical parameters in the solar atmospheric plasma. Existing seismology schemes based on approximations to the period and damping time of kink oscillations are often used beyond their theoretical range of applicability. These approximations assume that the variation of density across the loop is confined to a nonuniform layer much thinner than the radius of the loop, but the results of the inversion problem often do not satisfy this preliminary hypothesis. Here, we determine the accuracy of the analytic approximations to the period and damping time, and its impact on seismology estimates, when largely nonuniform loops are considered. We find that the accuracy of the approximations when used beyond their range of applicability is strongly affected by the form of the density profile across the loop, that is observationally unknown and so must be arbitrarily imposed as part of the theoretical model. The error associated with the analytic approximations can be larger than 50% even for relatively thin nonuniform layers. This error directly affects the accuracy of approximate seismology estimates compared to actual numerical inversions. In addition, assuming different density profiles can produce noncoincident intervals of the seismic variables in inversions of the same event. The ignorance about the true shape of density variation across the loop is an important source of error that may dispute the reliability of parameters seismically inferred assuming an ad hoc density profile.

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arXiv2013

The behavior of transverse waves in nonuniform solar flux tubes. I. Comparison of ideal and resistive results

Magnetohydrodynamic (MHD) waves are ubiquitously observed in the solar atmosphere. Kink waves are a type of transverse MHD waves in magnetic flux tubes that are damped due to resonant absorption. The theoretical study of kink MHD waves in solar flux tubes is usually based on the simplification that the transverse variation of density is confined to a nonuniform layer much thinner than the radius of the tube, i.e., the so-called thin boundary approximation. Here, we develop a general analytic method to compute the dispersion relation and the eigenfunctions of ideal MHD waves in pressureless flux tubes with transversely nonuniform layers of arbitrary thickness. Results for kink waves are produced and are compared with fully numerical resistive MHD eigenvalue computations in the limit of small resistivity. We find that the frequency and resonant damping rate are the same in both ideal and resistive cases. The actual results for thick nonuniform layers deviate from the behavior predicted in the thin boundary approximation and strongly depend on the shape of the nonuniform layer. The eigenfunctions in ideal MHD are very different from those in resistive MHD. The ideal eigenfunctions display a global character regardless of the thickness of the nonuniform layer, while the resistive eigenfunctions are localized around the resonance and are indistinguishable from those of ordinary resistive Alfvén modes. Consequently, the spatial distribution of wave energy in the ideal and resistive cases is dramatically different. This poses a fundamental theoretical problem with clear observational consequences.

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