We investigate, using a multi-fluid approach, the main properties of standing ion-acoustic modes ... more 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 dampin...
Context. Recent observations of the corona reveal ubiquitous transverse velocity perturbations th... more Context. Recent observations of the corona reveal ubiquitous transverse velocity perturbations that undergo strong damping as they propagate. These can be understood in terms of propagating kink waves that undergo mode coupling in inhomogeneous regions. Aims. The use of these propagating waves as a seismological tool for the investigation of the solar corona depends upon an accurate understanding of how the mode coupling behaviour is determined by local plasma parameters. Our previous work suggests the exponential spatial damping profile provides a poor description of the behaviour of strongly damped kink waves. We aim to investigate the spatial damping profile in detail and provide a guide to the approximations most suitable for performing seismological inversions. Methods. We propose a general spatial damping profile based on analytical results that accounts for the initial Gaussian stage of damped kink waves as well as the asymptotic exponential stage considered by previous authors. The applicability of this profile is demonstrated by a full parametric study of the relevant physical parameters. The implication of this profile for seismological inversions is investigated. Results. The Gaussian damping profile is found to be most suitable for application as a seismological tool for observations of oscillations in loops with a low density contrast. This profile also provides accurate estimates for data in which only a few wavelengths or periods are observed.
Coronal holes and active regions are typical magnetic structures found in the solar atmosphere. W... more Coronal holes and active regions are typical magnetic structures found in the solar atmosphere. We propose several magnetohydrostatic equilibrium solutions that are representative of these structures in two dimensions. Our models include the effect of a finite plasma-β and gravity, but the distinctive feature is that we incorporate a thermal structure with properties similar to those reported by observations. We developed a semi-analytical method to compute the equilibrium configuration. Using this method, we obtain cold and under-dense plasma structures in open magnetic fields representing coronal holes, while in closed magnetic configurations, we achieve the characteristic hot and over-dense plasma arrangements of active regions. Although coronal holes and active regions seem to be antagonistic structures, we find that they can be described using a common thermal structure that depends on the flux function. In addition to the force balance, the energy balance is included in the co...
Active regions (ARs) are magnetic structures typically found in the solar atmosphere. We calculat... more Active regions (ARs) are magnetic structures typically found in the solar atmosphere. We calculated several magnetohydrostatic (MHS) equilibrium models that include the effect of a finite plasma-β and gravity and that are representative of AR 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 B = ∇α × ∇β. The ideal MHS nonlinear partial differential equations were solved numerically using finite elements in a fixed 3D rectangular domain. The boundary conditions were initially chosen to correspond to a potential magnetic field (current-free) with known analytical expressions for the corresponding Euler potentials. The distinctive feature of our model is that we incorporated the effect of shear by progressively deforming the initial potential magnetic field. This procedure is quite generic and allowed us to generate a vast variety of MHS models. The thermal structure of the ARs was i...
The excitation and damping of transverse coronal loop oscillations is studied using a one-dimensi... more The excitation and damping of transverse coronal loop oscillations is studied using a one-dimensional model of a line-tied cylindrical loop. By solving the time-dependent magnetohydrodynamic (MHD) equations, we show how an initial disturbance produced in the solar corona induces kink-mode oscillations. We analyze the effect of the disturbance on a loop with a nonuniform boundary layer and investigate the damping of such a disturbance due to resonant absorption. We find that the period and attenuation time of the time-dependent results agree with the calculations of the corresponding quasi-mode (i.e., the kink mode resonantly coupled to Alfvén modes) and that the resonant absorption mechanism is capable of damping the oscillations almost immediately after the excitation. We study in detail the behavior of solutions in the inhomogeneous layer and show how the energy of the global oscillation is converted into torsional oscillations in the inhomogeneous layer. In addition, we estimate that the amplitude of the torsional oscillations is, for large magnetic Reynolds numbers and for thick layers, between 4 and 6 times the amplitude of the initial transverse motions. The implications of these results and their relationship with the observations are discussed.
Observations of coronal waves (CWs) interacting with coronal holes (CHs) show the formation of ty... more Observations of coronal waves (CWs) interacting with coronal holes (CHs) show the formation of typical wave-like features, such as reflected, refracted and transmitted waves (collectively, secondary waves). In accordance with these observations, numerical evidence for the wave characteristics of CWs is given by simulations, which demonstrate effects of deflection and reflection when a CW interacts with regions exhibiting a sudden density drop, such as CHs. However, secondary waves are usually weak in their signal and simulations are limited in the way the according idealisations have to be chosen. Hence, several properties of the secondary waves during a CW–CH interaction are unclear or ambiguous and might lead to misinterpretations. In this study we follow a theoretical approach and focus in particular on the geometrical properties of secondary waves caused by the interaction between oblique incoming CWs and CHs. Based on a linear theory, we derive analytical expressions for reflec...
Context. Threads are the building blocks of solar prominences and very often show longitudinal os... more Context. Threads are the building blocks of solar prominences and very often show longitudinal oscillatory motions that are strongly attenuated with time. The damping mechanism responsible for the reported oscillations is not fully understood yet. Aims. To understand the oscillations and damping of prominence threads we must first investigate the nature of the equilibrium solutions that arise under static conditions and under the presence of radiative losses, thermal conduction, and background heating. This provides the basis to calculate the eigenmodes of the thread models. Methods. The non-linear ordinary differential equations for hydrostatic and thermal equilibrium under the presence of gravity are solved using standard numerical techniques and simple analytical expressions are derived under certain approximations. The solutions to the equations represent a prominence thread, a dense and cold plasma region of a certain length that connects with the corona through a prominence co...
Large-amplitude longitudinal oscillations (LALOs) in prominences are spectacular manifestations o... more Large-amplitude longitudinal oscillations (LALOs) in prominences are spectacular manifestations of the solar activity. In such events nearby energetic disturbances induce periodic motions on filaments with displacements comparable to the size of the filaments themselves and with velocities larger than 20 km s −1. The pendulum model, in which the gravity projected along a rigid magnetic field is the restoring force, was proposed to explain these events. However, it can be objected that in a realistic situation where the magnetic field reacts to the mass motion of the heavy prominence, the simplified pendulum model could be no longer valid. We have performed non-linear time-dependent numerical simulations of LALOs considering a dipped magnetic field line structure. In this work we demonstrate that for even relatively weak magnetic fields the pendulum model works very well. We therefore validate the pendulum model and show its robustness, with important implications for prominence seismology purposes. With this model it is possible to infer the geometry of the dipped field lines that support the prominence.
During last years, an intense research about adiabatic MagnetoHydroDynamic (MHD) waves in the sol... more During last years, an intense research about adiabatic MagnetoHydroDynamic (MHD) waves in the solar corona has been performed. To make further progress, it is necessary to remove the usual adiabaticity assumption. Here, we have used an isothermal model with Newtonian cooling, to study the damping and propagation of magneto-acoustic-gravity waves in solar prominences. The inclusion of radiative dissipation in MHD waves is of great importance in many applications.
Unfortunately, the credit to the author of Figs. 9, 10, 11 and 12a was not given in the main text... more Unfortunately, the credit to the author of Figs. 9, 10, 11 and 12a was not given in the main text of the review. The author of these plots, who also performed the corresponding numerical calculations, was Prof. Ramon Oliver.
We discuss the possibility that disturbances generated in flares can produce the emission measure... more We discuss the possibility that disturbances generated in flares can produce the emission measure enhancement at the top of coronal loops observed by the Transition Region and Coronal Explorer ({TRACE}). The mechanism involved is the ponderomotive force of standing waves in the loops. We study the effect of large amplitude waves in loops using first a simple one-dimensional model for which analytical expressions are available, and second by solving the time dependent nonlinear magnetohydrodynamic (MHD) equations in a more realistic three-dimensional configuration. Waves are launched by an initial transverse velocity profile in the loop with footpoints fixed in the photosphere. We find that large initial disturbances can provide an imbalance along the loop, which results in an upflow from the legs of the loop. The accumulation of mass at the top of the loop produces a strong density enhancement. In a later stage, the pressure gradient becomes dominant and inhibits the concentration o...
We study the possibility that waves in coronal loops can produce density enhancements at loop top... more We study the possibility that waves in coronal loops can produce density enhancements at loop tops. The mechanism involved is the ponderomotive force of standing magnetohydrodynamic (MHD) waves. We study the effect of large-amplitude waves in loops by first using a simple second-order one-dimensional MHD model for which analytical expressions are available. This simple model shows how Alfvén waves can excite density fluctuations on slow timescales. Next, we solve the time-dependent nonlinear 2.5-dimensional MHD equations in an arcade model, in order to study the effect of nonuniform Alfvén speed along the loop on largeamplitude waves. Finally, we solve the nonlinear three-dimensional MHD equations in a flux-tube configuration. Waves are launched by an initial transverse velocity profile in the loop, with footpoints fixed in the photosphere. We find that large initial disturbances produce a pressure imbalance along the loop, which results in an upflow from its legs. The accumulation of mass at the top of the oscillating loop can produce significant density enhancements for typical coronal conditions. In a later stage, the pressure gradient becomes dominant and inhibits the concentration of mass at the loop top. We find that oscillating loops observed by the EUV telescope on board TRACE on 1998 July 14 exhibit emission measure enhancements consistent with this mechanism.
Observations of active regions and limb prominences often show cold, dense blobs descending with ... more Observations of active regions and limb prominences often show cold, dense blobs descending with an acceleration smaller than that of free fall. The dynamics of these condensations falling in the solar corona is investigated in this paper using a simple fully ionised plasma model. We find that the presence of a heavy condensation gives rise to a dynamical rearrangement of the coronal pressure that results in the formation of a large pressure gradient that opposes gravity. Eventually this pressure gradient becomes so large that the blob acceleration vanishes or even points upwards. Then, the blob descent is characterised by an initial acceleration phase followed by an essentially constant velocity phase. These two stages can be identified in published time-distance diagrams of coronal rain events. Both the duration of the first stage and the velocity attained by the blob increase for larger values of the ratio of blob to coronal density, for larger blob mass, and for smaller coronal temperature. Dense blobs are characterised by a detectable density growth (up to 60% in our calculations) and by a steepening of the density in their lower part, that could lead to the formation of a shock. They also emit sound waves that could be detected as small intensity changes with periods of the order of 100 s and lasting between a few and about ten periods. Finally, the curvature of the falling path is only relevant when a very dense blob falls along inclined magnetic field lines.
The solar corona is structured by the magnetic Ðeld, which has the shape of loops and arcades. Th... more The solar corona is structured by the magnetic Ðeld, which has the shape of loops and arcades. These structures remain stable from days to weeks, and there is some evidence, such as sporadic or periodic brightenings detected in UV and soft X-rays, pointing to the existence of periodic oscillations or waves in the corona. Recently Oliver et al. studied, both analytically and numerically, the modes of oscillation of potential and nonpotential magnetic arcades ; here we investigate the fast mode properties in a fully magnetostatic arcade (Zweibel & Hundhausen) in which the gravity force is present. The results show that in this equilibrium the fast and slow modes are essentially decoupled and that the spatial structure of the perturbed velocity Ðeld is determined by the behavior of the speed with height. An arcade Alfve n in which the speed increases with height is characterized by vertically evanescent fast mode solu-Alfve n tions. These are the kind of modes that could be more easily detected since their energy is conÐned within the solar corona. On the other hand, an equilibrium in which the speed decreases with Alfve n height has solutions whose energy tends to escape toward large heights, making difficult the detection of these modes. Also, it is found that the frequencies of fast modes in a nonpotential equilibrium are considerably reduced with respect to the potential case.
The solar corona is a complex magnetic environment where several kinds of waves can propagate. In... more The solar corona is a complex magnetic environment where several kinds of waves can propagate. In this work, the properties of fast, Alfvén, and slow magnetohydrodynamic waves in a simple curved structure are investigated. We consider the linear regime, i.e., small-amplitude waves. We study the time evolution of impulsively generated waves in a coronal arcade by solving the ideal magnetohydrodynamic equations. We use a numerical code specially designed to solve these equations in the low-regime. The results of the simulations are compared with the eigenmodes of the arcade model. Fast modes propagate nearly isotropically through the whole arcade and are reflected at the photosphere, where line-tying conditions are imposed. On the other hand, Alfvén and slow perturbations are very anisotropic and propagate along the magnetic field lines. Because of the different physical properties in different field lines, there is a continuous spectrum of Alfvén and slow modes. Curvature can have a significant effect on the properties of the waves. Among other effects, it considerably changes the frequency of oscillation of the slow modes and enhances the possible dissipation of the Alfvén modes due to phase mixing.
Here we study the low-frequency variability of the tropical Indian and Pacific basins with a new ... more Here we study the low-frequency variability of the tropical Indian and Pacific basins with a new statistical technique, Bayesian oscillation patterns (BOP). To describe the climatic system in this region, zonal wind and sea surface temperature (SST) are the selected variables. Their variability can be explained in terms of a reduced number of frequencies and spatial patterns. These are identified for each field by a statistical procedure. With the help of the patterns and the frequencies a predictive scheme is devised and applied in two forecast experiments. In the first, zonal wind anomalies are predicted using patterns and frequencies identified in the wind field. A more sophisticated scheme, a linear model which includes non-harmonic oscillations and interactions between patterns, is used when forecasting SST seasonal anomalies in the Nifio3 region. In this case, the predictors include the values of the frequencies identified in the BOP analysis of both wind and SST fields, and the corresponding patterns.
We investigate the influence of the geometry of the solar filament magnetic structure on the larg... more We investigate the influence of the geometry of the solar filament magnetic structure on the largeamplitude longitudinal oscillations. A representative filament flux tube is modeled as composed of a cool thread centered in a dipped part with hot coronal regions on either side. We have found the normal modes of the system, and establish that the observed longitudinal oscillations are well described with the fundamental mode. For small and intermediate curvature radii and moderate to large density contrast between the prominence and the corona, the main restoring force is the solar gravity. In this full wave description of the oscillation a simple expression for the oscillation frequencies is derived in which the pressure-driven term introduces a small correction. We have also found that the normal modes are almost independent of the geometry of the hot regions of the tube. We conclude that observed large-amplitude longitudinal oscillations are driven by the projected gravity along the flux tubes, and are strongly influenced by the curvature of the dips of the magnetic field in which the threads reside.
We investigate, using a multi-fluid approach, the main properties of standing ion-acoustic modes ... more 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 dampin...
Context. Recent observations of the corona reveal ubiquitous transverse velocity perturbations th... more Context. Recent observations of the corona reveal ubiquitous transverse velocity perturbations that undergo strong damping as they propagate. These can be understood in terms of propagating kink waves that undergo mode coupling in inhomogeneous regions. Aims. The use of these propagating waves as a seismological tool for the investigation of the solar corona depends upon an accurate understanding of how the mode coupling behaviour is determined by local plasma parameters. Our previous work suggests the exponential spatial damping profile provides a poor description of the behaviour of strongly damped kink waves. We aim to investigate the spatial damping profile in detail and provide a guide to the approximations most suitable for performing seismological inversions. Methods. We propose a general spatial damping profile based on analytical results that accounts for the initial Gaussian stage of damped kink waves as well as the asymptotic exponential stage considered by previous authors. The applicability of this profile is demonstrated by a full parametric study of the relevant physical parameters. The implication of this profile for seismological inversions is investigated. Results. The Gaussian damping profile is found to be most suitable for application as a seismological tool for observations of oscillations in loops with a low density contrast. This profile also provides accurate estimates for data in which only a few wavelengths or periods are observed.
Coronal holes and active regions are typical magnetic structures found in the solar atmosphere. W... more Coronal holes and active regions are typical magnetic structures found in the solar atmosphere. We propose several magnetohydrostatic equilibrium solutions that are representative of these structures in two dimensions. Our models include the effect of a finite plasma-β and gravity, but the distinctive feature is that we incorporate a thermal structure with properties similar to those reported by observations. We developed a semi-analytical method to compute the equilibrium configuration. Using this method, we obtain cold and under-dense plasma structures in open magnetic fields representing coronal holes, while in closed magnetic configurations, we achieve the characteristic hot and over-dense plasma arrangements of active regions. Although coronal holes and active regions seem to be antagonistic structures, we find that they can be described using a common thermal structure that depends on the flux function. In addition to the force balance, the energy balance is included in the co...
Active regions (ARs) are magnetic structures typically found in the solar atmosphere. We calculat... more Active regions (ARs) are magnetic structures typically found in the solar atmosphere. We calculated several magnetohydrostatic (MHS) equilibrium models that include the effect of a finite plasma-β and gravity and that are representative of AR 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 B = ∇α × ∇β. The ideal MHS nonlinear partial differential equations were solved numerically using finite elements in a fixed 3D rectangular domain. The boundary conditions were initially chosen to correspond to a potential magnetic field (current-free) with known analytical expressions for the corresponding Euler potentials. The distinctive feature of our model is that we incorporated the effect of shear by progressively deforming the initial potential magnetic field. This procedure is quite generic and allowed us to generate a vast variety of MHS models. The thermal structure of the ARs was i...
The excitation and damping of transverse coronal loop oscillations is studied using a one-dimensi... more The excitation and damping of transverse coronal loop oscillations is studied using a one-dimensional model of a line-tied cylindrical loop. By solving the time-dependent magnetohydrodynamic (MHD) equations, we show how an initial disturbance produced in the solar corona induces kink-mode oscillations. We analyze the effect of the disturbance on a loop with a nonuniform boundary layer and investigate the damping of such a disturbance due to resonant absorption. We find that the period and attenuation time of the time-dependent results agree with the calculations of the corresponding quasi-mode (i.e., the kink mode resonantly coupled to Alfvén modes) and that the resonant absorption mechanism is capable of damping the oscillations almost immediately after the excitation. We study in detail the behavior of solutions in the inhomogeneous layer and show how the energy of the global oscillation is converted into torsional oscillations in the inhomogeneous layer. In addition, we estimate that the amplitude of the torsional oscillations is, for large magnetic Reynolds numbers and for thick layers, between 4 and 6 times the amplitude of the initial transverse motions. The implications of these results and their relationship with the observations are discussed.
Observations of coronal waves (CWs) interacting with coronal holes (CHs) show the formation of ty... more Observations of coronal waves (CWs) interacting with coronal holes (CHs) show the formation of typical wave-like features, such as reflected, refracted and transmitted waves (collectively, secondary waves). In accordance with these observations, numerical evidence for the wave characteristics of CWs is given by simulations, which demonstrate effects of deflection and reflection when a CW interacts with regions exhibiting a sudden density drop, such as CHs. However, secondary waves are usually weak in their signal and simulations are limited in the way the according idealisations have to be chosen. Hence, several properties of the secondary waves during a CW–CH interaction are unclear or ambiguous and might lead to misinterpretations. In this study we follow a theoretical approach and focus in particular on the geometrical properties of secondary waves caused by the interaction between oblique incoming CWs and CHs. Based on a linear theory, we derive analytical expressions for reflec...
Context. Threads are the building blocks of solar prominences and very often show longitudinal os... more Context. Threads are the building blocks of solar prominences and very often show longitudinal oscillatory motions that are strongly attenuated with time. The damping mechanism responsible for the reported oscillations is not fully understood yet. Aims. To understand the oscillations and damping of prominence threads we must first investigate the nature of the equilibrium solutions that arise under static conditions and under the presence of radiative losses, thermal conduction, and background heating. This provides the basis to calculate the eigenmodes of the thread models. Methods. The non-linear ordinary differential equations for hydrostatic and thermal equilibrium under the presence of gravity are solved using standard numerical techniques and simple analytical expressions are derived under certain approximations. The solutions to the equations represent a prominence thread, a dense and cold plasma region of a certain length that connects with the corona through a prominence co...
Large-amplitude longitudinal oscillations (LALOs) in prominences are spectacular manifestations o... more Large-amplitude longitudinal oscillations (LALOs) in prominences are spectacular manifestations of the solar activity. In such events nearby energetic disturbances induce periodic motions on filaments with displacements comparable to the size of the filaments themselves and with velocities larger than 20 km s −1. The pendulum model, in which the gravity projected along a rigid magnetic field is the restoring force, was proposed to explain these events. However, it can be objected that in a realistic situation where the magnetic field reacts to the mass motion of the heavy prominence, the simplified pendulum model could be no longer valid. We have performed non-linear time-dependent numerical simulations of LALOs considering a dipped magnetic field line structure. In this work we demonstrate that for even relatively weak magnetic fields the pendulum model works very well. We therefore validate the pendulum model and show its robustness, with important implications for prominence seismology purposes. With this model it is possible to infer the geometry of the dipped field lines that support the prominence.
During last years, an intense research about adiabatic MagnetoHydroDynamic (MHD) waves in the sol... more During last years, an intense research about adiabatic MagnetoHydroDynamic (MHD) waves in the solar corona has been performed. To make further progress, it is necessary to remove the usual adiabaticity assumption. Here, we have used an isothermal model with Newtonian cooling, to study the damping and propagation of magneto-acoustic-gravity waves in solar prominences. The inclusion of radiative dissipation in MHD waves is of great importance in many applications.
Unfortunately, the credit to the author of Figs. 9, 10, 11 and 12a was not given in the main text... more Unfortunately, the credit to the author of Figs. 9, 10, 11 and 12a was not given in the main text of the review. The author of these plots, who also performed the corresponding numerical calculations, was Prof. Ramon Oliver.
We discuss the possibility that disturbances generated in flares can produce the emission measure... more We discuss the possibility that disturbances generated in flares can produce the emission measure enhancement at the top of coronal loops observed by the Transition Region and Coronal Explorer ({TRACE}). The mechanism involved is the ponderomotive force of standing waves in the loops. We study the effect of large amplitude waves in loops using first a simple one-dimensional model for which analytical expressions are available, and second by solving the time dependent nonlinear magnetohydrodynamic (MHD) equations in a more realistic three-dimensional configuration. Waves are launched by an initial transverse velocity profile in the loop with footpoints fixed in the photosphere. We find that large initial disturbances can provide an imbalance along the loop, which results in an upflow from the legs of the loop. The accumulation of mass at the top of the loop produces a strong density enhancement. In a later stage, the pressure gradient becomes dominant and inhibits the concentration o...
We study the possibility that waves in coronal loops can produce density enhancements at loop top... more We study the possibility that waves in coronal loops can produce density enhancements at loop tops. The mechanism involved is the ponderomotive force of standing magnetohydrodynamic (MHD) waves. We study the effect of large-amplitude waves in loops by first using a simple second-order one-dimensional MHD model for which analytical expressions are available. This simple model shows how Alfvén waves can excite density fluctuations on slow timescales. Next, we solve the time-dependent nonlinear 2.5-dimensional MHD equations in an arcade model, in order to study the effect of nonuniform Alfvén speed along the loop on largeamplitude waves. Finally, we solve the nonlinear three-dimensional MHD equations in a flux-tube configuration. Waves are launched by an initial transverse velocity profile in the loop, with footpoints fixed in the photosphere. We find that large initial disturbances produce a pressure imbalance along the loop, which results in an upflow from its legs. The accumulation of mass at the top of the oscillating loop can produce significant density enhancements for typical coronal conditions. In a later stage, the pressure gradient becomes dominant and inhibits the concentration of mass at the loop top. We find that oscillating loops observed by the EUV telescope on board TRACE on 1998 July 14 exhibit emission measure enhancements consistent with this mechanism.
Observations of active regions and limb prominences often show cold, dense blobs descending with ... more Observations of active regions and limb prominences often show cold, dense blobs descending with an acceleration smaller than that of free fall. The dynamics of these condensations falling in the solar corona is investigated in this paper using a simple fully ionised plasma model. We find that the presence of a heavy condensation gives rise to a dynamical rearrangement of the coronal pressure that results in the formation of a large pressure gradient that opposes gravity. Eventually this pressure gradient becomes so large that the blob acceleration vanishes or even points upwards. Then, the blob descent is characterised by an initial acceleration phase followed by an essentially constant velocity phase. These two stages can be identified in published time-distance diagrams of coronal rain events. Both the duration of the first stage and the velocity attained by the blob increase for larger values of the ratio of blob to coronal density, for larger blob mass, and for smaller coronal temperature. Dense blobs are characterised by a detectable density growth (up to 60% in our calculations) and by a steepening of the density in their lower part, that could lead to the formation of a shock. They also emit sound waves that could be detected as small intensity changes with periods of the order of 100 s and lasting between a few and about ten periods. Finally, the curvature of the falling path is only relevant when a very dense blob falls along inclined magnetic field lines.
The solar corona is structured by the magnetic Ðeld, which has the shape of loops and arcades. Th... more The solar corona is structured by the magnetic Ðeld, which has the shape of loops and arcades. These structures remain stable from days to weeks, and there is some evidence, such as sporadic or periodic brightenings detected in UV and soft X-rays, pointing to the existence of periodic oscillations or waves in the corona. Recently Oliver et al. studied, both analytically and numerically, the modes of oscillation of potential and nonpotential magnetic arcades ; here we investigate the fast mode properties in a fully magnetostatic arcade (Zweibel & Hundhausen) in which the gravity force is present. The results show that in this equilibrium the fast and slow modes are essentially decoupled and that the spatial structure of the perturbed velocity Ðeld is determined by the behavior of the speed with height. An arcade Alfve n in which the speed increases with height is characterized by vertically evanescent fast mode solu-Alfve n tions. These are the kind of modes that could be more easily detected since their energy is conÐned within the solar corona. On the other hand, an equilibrium in which the speed decreases with Alfve n height has solutions whose energy tends to escape toward large heights, making difficult the detection of these modes. Also, it is found that the frequencies of fast modes in a nonpotential equilibrium are considerably reduced with respect to the potential case.
The solar corona is a complex magnetic environment where several kinds of waves can propagate. In... more The solar corona is a complex magnetic environment where several kinds of waves can propagate. In this work, the properties of fast, Alfvén, and slow magnetohydrodynamic waves in a simple curved structure are investigated. We consider the linear regime, i.e., small-amplitude waves. We study the time evolution of impulsively generated waves in a coronal arcade by solving the ideal magnetohydrodynamic equations. We use a numerical code specially designed to solve these equations in the low-regime. The results of the simulations are compared with the eigenmodes of the arcade model. Fast modes propagate nearly isotropically through the whole arcade and are reflected at the photosphere, where line-tying conditions are imposed. On the other hand, Alfvén and slow perturbations are very anisotropic and propagate along the magnetic field lines. Because of the different physical properties in different field lines, there is a continuous spectrum of Alfvén and slow modes. Curvature can have a significant effect on the properties of the waves. Among other effects, it considerably changes the frequency of oscillation of the slow modes and enhances the possible dissipation of the Alfvén modes due to phase mixing.
Here we study the low-frequency variability of the tropical Indian and Pacific basins with a new ... more Here we study the low-frequency variability of the tropical Indian and Pacific basins with a new statistical technique, Bayesian oscillation patterns (BOP). To describe the climatic system in this region, zonal wind and sea surface temperature (SST) are the selected variables. Their variability can be explained in terms of a reduced number of frequencies and spatial patterns. These are identified for each field by a statistical procedure. With the help of the patterns and the frequencies a predictive scheme is devised and applied in two forecast experiments. In the first, zonal wind anomalies are predicted using patterns and frequencies identified in the wind field. A more sophisticated scheme, a linear model which includes non-harmonic oscillations and interactions between patterns, is used when forecasting SST seasonal anomalies in the Nifio3 region. In this case, the predictors include the values of the frequencies identified in the BOP analysis of both wind and SST fields, and the corresponding patterns.
We investigate the influence of the geometry of the solar filament magnetic structure on the larg... more We investigate the influence of the geometry of the solar filament magnetic structure on the largeamplitude longitudinal oscillations. A representative filament flux tube is modeled as composed of a cool thread centered in a dipped part with hot coronal regions on either side. We have found the normal modes of the system, and establish that the observed longitudinal oscillations are well described with the fundamental mode. For small and intermediate curvature radii and moderate to large density contrast between the prominence and the corona, the main restoring force is the solar gravity. In this full wave description of the oscillation a simple expression for the oscillation frequencies is derived in which the pressure-driven term introduces a small correction. We have also found that the normal modes are almost independent of the geometry of the hot regions of the tube. We conclude that observed large-amplitude longitudinal oscillations are driven by the projected gravity along the flux tubes, and are strongly influenced by the curvature of the dips of the magnetic field in which the threads reside.
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Papers by Jaume Terradas