Coronal Arcade Research Articles

Excitation of vertical kink waves in a solar coronal arcade loop by a periodic driver

Aims. We study an oscillatory driver as a possible excitation mechanism of vertical kink loop oscillations in the ideal MHD regime.Methods. We consider a solar coronal magnetic arcade with a dense photospheric layer. The two-dimensional numerical model that we implement includes the effects of nonlinearity and line curvature on the excitation and attenuation of fast magnetosonic kink waves. We investigate the effects of a driven sinusoidal pressure pulse and compare it with the impulsive excitation by a pressure pulse that impacts the overlying loop.Results. Our numerical simulations reveal wave signatures that are reminiscent of vertical loop oscillations seen in TRACE observational data.Conclusions. We conclude that attenuation of vertical kink oscillations can be reduced to the value observed by adopting an oscillatory instead of an impulsive excitation. An oscillatory driver also naturally explains why only a small subset of all loops is excited to oscillate transversally in an active region.

Impacts of torus model on studies of geometrical relationships between interplanetary magnetic clouds and their solar origins

Abstract Our recent analysis of interplanetary magnetic clouds (MCs) showed that the orientations of MC axes determined by a model fitting with curvature of MCs taken into account (referred to as a torus model, hereafter) can be significantly different from those obtained from fittings with a straight cylinder model. Motivated by this finding, we re-examined geometrical relationships between magnetic field structures of MCs and their solar origins. This paper describes the results of the re-examination with special attention paid to two MC events, for which different orientations of MC axes were obtained from a torus model and a cylinder model. For both cases, it is shown that the torus models give the MC geometries of magnetic field structures in good agreement with those of coronal arcade structures which were formed in association with the launch of MCs along the magnetic field inversion lines. Summarizing the analysis results for 12 MCs investigated here, we conclude that: (1) the formation of coronal arcade structure is a good indication of MC formation; (2) MC geometries can be obtained that are consistent with the coronal arcades with respect to the axis orientation and the magnetic field structure including chirality, indicating that no significant direction changes occurred during the propagation of MCs through the interplanetary medium.

Slow magnetoacoustic standing waves in a curved solar coronal slab

<i>Aims. <i/>We consider a model of a two-dimensional solar coronal arcade to explore the effects of a curved magnetic field topology on excitation and attenuation of slow magnetoacoustic standing waves.<i>Methods. <i/>The time-dependent ideal magnetohydrodynamic equations are solved numerically to find the spatial and temporal signatures of these waves.<i>Results. <i/>A pulse in gas pressure initially launched at a loop footpoint excites the fundamental mode of slow magnetoacoustic standing waves. The typical excitation time of such a wave mode is 2.5 wave periods, with a similar attenuation timescale. These values are remarkably similar to those recovered from observations by SOHO/SUMER in an Fe XIX line.<i>Conclusions. <i/>Slow magnetoacoustic standing waves are excited and attenuated more efficiently in curved magnetic field lines than in a straight magnetic slab topology. The waves supported by the magnetic arcade are in far closer agreement with observations.

Processes and mechanisms governing the initiation and propagation of CMEs

Abstract. The most important observational characteristics of coronal mass ejections (CMEs) are summarized, emphasizing those aspects which are relevant for testing physical concepts employed to explain the CME take-off and propagation. In particular, the kinematics, scalings, and the CME-flare relationship are stressed. Special attention is paid to 3-dimensional (3-D) topology of the magnetic field structures, particularly to aspects related to the concept of semi-toroidal flux-rope anchored at both ends in the dense photosphere and embedded in the coronal magnetic arcade. Observations are compared with physical principles and concepts employed in explaining the CME phenomenon, and implications are discussed. A simple flux-rope model is used to explain various stages of the eruption. The model is able to reproduce all basic observational requirements: stable equilibrium and possible oscillations around equilibrium, metastable state and possible destabilization by an external disturbance, pre-eruptive gradual-rise until loss of equilibrium, possibility of fallback events and failed eruptions, relationship between impulsiveness of the CME acceleration and the source-region size, etc. However, it is shown that the purely ideal MHD process cannot account for highest observed accelerations which can attain values up to 10 km s−2. Such accelerations can be achieved if the process of reconnection beneath the erupting flux-rope is included into the model. Essentially, the role of reconnection is in changing the magnetic flux associated with the flux-rope current and supplying "fresh" poloidal magnetic flux to the rope. These effects help sustain the electric current flowing along the flux-rope, and consequently, reinforce and prolong the CME acceleration. The model straightforwardly explains the observed synchronization of the flare impulsive phase and the CME main-acceleration stage, as well as the correlations between various CME and flare parameters.

On the Properties of Low‐β Magnetohydrodynamic Waves in Curved Coronal Fields

The solar corona is a complex magnetic environment where several kinds of waves can propagate. In this work, the properties of fast, Alfven, 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-beta 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, Alfven 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 Alfven 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 Alfven modes due to phase mixing.

A quantitative MHD study of the relation among arcade shearing, flux rope formation, and eruption due to the tearing instability

The quantitative relationship between the magnetohydrodynamic (MHD) activity of solar coronal arcade and the magnetic helicity injection, which is caused by shearing motion, has been investigated, using azimuthally symmetric model of MHD simulation. We have calculated several cases in which the width of the shearing region is varied and examined the relationship between the magnetic arcade dynamics and magnetic helicity evolution. As a result, it is found that as the shearing motion is imposed on narrower regions along each side of the magnetic inversion line, the magnetic arcade can be easily destabilized by the resistive tearing mode. However, in this case, even though reconnection driven by the tearing mode produces plasmoids, the plasmoid elevation is almost in proportion to the total amount of magnetic helicity contained in the arcade, and it is too slow to explain the trigger process of coronal mass ejections (CMEs). On the other hand, in the case where the shearing motion is imposed on the entire region, much larger magnetic helicity injection is required to injected arcade in order to destabilize the system, compared to practical helicity injection measured in the solar corona. The results suggest that it may be difficult to trigger a CME just by the axisymmetric shearing motion and that some other mechanisms should be involved in the triggering process of a CME. The results also imply that the relation between the magnetic helicity and the overlying magnetic flux can be a key parameter for the CME occurrence.

A Possible Structure of the Magnetic Field in Solar Filaments Obtained by Flux Emergence

Abstract We report a result, that the emergence of a subsurface magnetic field naturally reproduces the feature of the global magnetic configuration observed in solar filaments. This was obtained by performing a three-dimensional simulation of a twisted flux tube that emerges in the shape of a multi-$\Omega$-loop in a highly stratified solar atmosphere, extending from the subsurface layer to the corona. One of the key findings is that a kinking of the twisted flux tube occurs at the site of a linkage between adjacent $\Omega$-loops. This develops a magnetic structure that has remarkable similarities to the structure of a filament, in that the inner part of the twisted flux tube can be applied to the main body of a filament, while the outer part is reminiscent of observations of filament feet and a coronal arcade overlying the main body of a filament. Based on the result, we concluded that the orientation of filaments observed on the Sun may have its origin in the handedness of a twisted magnetic field below the solar surface.

Onset of Coronal Mass Ejections Due to Loss of Confinement of Coronal Flux Ropes

Using MHD numerical simulations in a three-dimensional spherical geometry, we model the loss of confinement and eruption of a flux rope emerging quasi-statically into a preexisting coronal arcade field. Our numerical experiments investigated two distinct mechanisms that led to the eruption of the flux rope. In one case, the overlying arcade field declines with height slowly such that the emerging flux rope remains confined until its self-relative magnetic helicity normalized by the square of the rope's flux reaches -1.4 and the flux rope becomes significantly kinked. The kinking motion causes rotation of the tube to an orientation that makes it easier for it to rupture through the arcade field, leading to an eruption. In the second case, the overlying field declines more rapidly with height, and the emerging flux rope is found to lose equilibrium and erupt via the torus instability when its self-relative magnetic helicity normalized by the square of its flux is only approximately -0.63, before it becomes kinked. The values of the total relative magnetic helicity of the entire coronal magnetic field (including both the flux rope and the arcade field) normalized by the square of the total magnetic flux are, on the other hand, of similar magnitudes for the two cases when the eruption takes place. We compare and contrast the eruptive properties and the posteruption states resulting from the two cases.

Effect of longitudinal density structure on a straight magnetic field modelling coronal arcade oscillations

Aims. Motivated by recent observations of oscillations in coronal arcades, we investigate analytically the influence of longitudinal structuring on the modes of oscillation of a straight coronal loop arcade. As a first step towards more complicated models, we use a simple structure to obtain analytical solutions. Methods. A partial differential equation is derived for the total pressure perturbation of the fast modes in a zero beta plasma and it is solved analytically. We first recover the results for a homogeneous structure, and then study an equilibrium with an exponentially structured density profile, solving it in terms of Bessel functions of non-integer order and exponential argument, thus obtaining a dispersion relation. The properties of this dispersion relation are discussed and some limits studied, leading to analytical approximations to the eigenfrequencies. Results. The introduction of longitudinal structuring results in a modification to the oscillatory frequencies of the modes of oscillation in such structures when compared with the uniform case. Regarding the oscillatory periods Pn, n = 1, 2 , . .. , the period ratios P1/2P2 and P1/3P3 are both shifted from unity. Other properties described in structured coronal loops are also found in an arcade: the occurrence of avoided crossings in the dispersion diagram and the displacement of the extrema towards the footpoints in the spatial structure of the eigenmodes. Conclusions. We show analytically for simple arcade modes that the shift in the fundamental period proves to be small, but the ratio P1/2P2 depends strongly on the density structure. Moreover, transversal propagation also shifts the ratio P1/2P2 from unity, so it can be used in the coronal seismology of arcades in which transversal propagation is present. We use the currently available observational data to illustrate this application.

Helioseismically Determined Near‐Surface Flows Underlying a Quiescent Filament

The extended filaments seen in Hα images of the solar disk, and the corresponding prominences when viewed at the solar limb, are one of the great hallmarks of solar magnetism. Such arches of magnetic field and the coronal plasma structures they support are both beautiful and enigmatic. Many models of filament formation and maintenance invoke the existence of surface plasma flows, which are used to drive the magnetic reconnection needed to form twisted loops of flux held down by a coronal arcade. These flows are typically composed of a converging flow, which brings flux elements of opposite polarity together, combined with a tangential shear that stresses the coronal arcade. In this paper we present helioseismic measurements of near-surface flows underlying a single quiescent filament lying within a decayed active region. Newly devised high-resolution ring analyses (HRRA) with both 2° and 4° spatial resolution were applied to Doppler imaging data provided by the Michelson Doppler Imager (MDI) instrument on the SOHO spacecraft. A long-lived filament appearing in 2002 May and April was studied. We find that the filament channel is a region of vigorous subphotospheric convection. The largest observed scales of such convection span the region of weak magnetic field separating the active region's two polarities. Thus, the magnetic neutral line that forms the spine of the filament channel tends to lie along the centers of large convection cells. In temporal and spatial averages of the flow field, we do not find a systematic converging flow. However, we do detect a significant shearing flow parallel to the neutral line. This shear takes the form of two oppositely directed jets, one to either side of the neutral line and within 20 Mm of the line. The jets produce a net shear in the flow speed of 30 m s-1 occurring over a distance of 20 Mm.

Numerical simulations of vertical oscillations of a multi-stranded coronal loop

Aims. We consider impulsively generated oscillations in a 2D model of a curved solar coronal arcade loop that consists of up to 5 strands of dense plasma. Methods. First we do a simulation for a loop which consists of two curved strands. We evaluate by means of numerical simulations the influence of the distance between the strands and their number on wave period, attenuation time, and amplitudes of standing kink waves. Results. The results of the numerical simulations reveal that only strands which are very close to each other (distance comparable to the strand width) considerably change the collective behavior of kink oscillations. More distant strands also exhibit weak coupling of the oscillations. However, their behavior can essentially be explained in terms of separate oscillating loops. We compare the numerical results with recent TRACE observational findings, and find qualitative agreement.

MHD waves in coronal magnetic arcades

We analyze the eigenmodes of the solar coronal magnetic arcade that describes the magnetic field of a bipolar active region using the eikonal method for ideal magnetohydrodynamic equations. We write out the eikonal equations for Alfven and magnetoacoustic waves and derive the equations for the amplitudes of the zeroth approximation. We construct the wave fields for Alfven and fast magnetoacoustic modes and derive the expressions for the eigenfrequencies. We show that Alfven modes of a given frequency are near a number of magnetic surfaces, while fast magnetoacoustic eigenmodes are near nonmagnetic surfaces. A discrete set of eigenfrequencies that continuously change from one surface to another corresponds to each such surface.

Fast magnetohydrodynamic oscillations in an elliptical coronal arcade

Aims. A model of a elliptically shaped coronal arcade with piecewise constant density is discussed to explore the effects of curvature on radially polarised fast modes. It is important to test whether the main results in the straight and cylindrical geometries can be extrapolated to these more complex equilibria. Methods. An equilibrium model for a force-free, line-tied elliptical arcade is introduced and a partial differential equation is derived for the velocity perturbation of the fast modes, which is solved analytically. The properties of the modes are studied in terms of the dispersion relation, which depends on the eccentricity, the arcade width, and the density contrast. Results. Modes mainly contained in the cavity below the arcade are also present, and have avoided crossings with the modes of the arcade. Even the fundamental mode becomes leaky due to curvature. Approximated relations are deduced for the frequency of the modes and the spatial structure is discussed, focusing on the different families through which a rich mode spectrum can be classified. Conclusions. The different types of modes of the spectrum are described and its relevance to observations is discussed. The periods obtained in Cartesian geometry provide a reasonable approximation, but this geometry lacks some other key ingredients: the damping rates are different and some types of modes present in the elliptical geometry are not sustained in the straight slab.

Fast magnetohydrodynamic oscillations in a force-free line-tied coronal arcade

Aims. We discuss a simple model of a line-tied coronal arcade with piecewise constant density to explore the effects of curvature on radially polarised fast modes. Methods. A partial differential equation is derived for the velocity perturbation of the fast modes and it is solved analytically in terms of Bessel functions of half integer order, obtaining a dispersion relation. Results. The properties of the modes are studied in terms of the parameters. All the modes are leaky under these conditions. Besides the usual kink and sausage modes, new families are described: the vertical , swaying (longitudinal), and rocking modes arise. Conclusions. The damping rates are similar to observed rates. For thin arcades the modes are markedly different from those of a straight slab and resemble more the modes of a circular membrane.

Numerical simulations of impulsively generated vertical oscillations in a solar coronal arcade loop

Aims. The main aims of the paper are to carry out numerical simulations of the vertical oscillations in a coronal loop in order to determine their dependence on various parameters and to compare them with recent TRACE observations. Methods. We consider impulsively generated oscillations in a solar coronal arcade loop. The two-dimensional numerical model we implement in the ideal MHD regime includes the effects of nonlinearity and line curvature. We perform parametric studies by varying both the position and the width/strength of the pulse. Results. A pulse launched below a loop is in general found to excite multiple wave modes, in particular a vertical oscillation with many properties of a kink mode, fast mode oscillations and a slow mode pulse (or two slow mode pulses, depending on the location of the original pulse). From our parametric studies we deduce that wave periods and attenuation times of the excited waves depend on the position below the loop summit, as well as on the width of the pulse. Wider pulses launched closer to a foot-point and to the loop's apex trigger wave packets of longer period waves which are more strongly attenuated. A perturbed loop does not return to its initial state but is instead stretched, with its apex shifted upwards. As a result the perturbations propagate along the stretched loop and consequently stronger and wider pulses which stretch a loop more lead to longer period oscillations. A pulse located near one of the foot-points is found to excite a distortion mode leading to asymmetric oscillations which are distinct from the vertical or horizontal kink modes that have been identified in TRACE data.

Models of the Large‐Scale Corona. II. Magnetic Connectivity and Open Flux Variation

In this paper the changing connectivity of the coronal magnetic field during the formation and ejection of magnetic flux ropes is considered. Using recent simulations of the coronal field, it is shown that reconnection may occur both above and below the flux ropes. Those occurring above slowly strip away coronal arcades overlying the flux ropes and allow the flux ropes to be ejected. In contrast, those below help to push the flux ropes out. It is found that the reconnection occurring below each flux rope may result in significant skew being maintained within the coronal field above the PIL after the flux rope is ejected. In addition, after the eruption, as the coronal field closes down, the large-scale transport of open flux across the bipoles takes place through the process of interchange reconnection. As a result, new photospheric domains of open flux are created within the centers of the bipoles, where field lines were previously closed. The net open flux in the simulation may be split into two distinct contributions. The first contribution is due to the nonpotential equilibrium coronal fields of the bipoles. The second contribution is a temporary enhancement to this during the ejection of the flux ropes, where previously closed field lines become open. It is shown that the nonpotential equilibrium contribution to the open flux is significantly higher than that due to a potential field deduced from the same photospheric boundary conditions. These results suggest that the nonpotential nature of coronal magnetic fields may affect the variation of the Sun's open flux during periods of high solar activity and should be considered in future simulations.

On the origin and configuration of the 20 March 2003 interplanetary shock and magnetic cloud at 1 AU

On 20 March 2003, a forward shock was observed in the near‐Earth solar wind, followed 8 hours later by an interplanetary magnetic cloud (IMC), in a configuration having several uncommon features: Both were parts of a 38‐hour interval containing transient solar outflows that occurred in an extended high‐speed stream from a Y‐shaped extension of the south polar coronal hole. (In contrast, IMCs, and ejecta in general, were rarely observed within high‐speed streams at low heliolatitudes during cycle 23.) The most likely solar source for the IMC is AR 10314, located at S15°, just above the “fork” of the Y‐shaped coronal hole. Several solar flares occurred in this active region on 17–18 March, as well as a succession of four coronal mass ejections (CMEs). Velocity considerations narrow the most likely source of the 38‐hour interval of activity to two CMEs on 17 March 2003 associated with solar flares at W33° and W39°. The IMC axis had a north‐south orientation, which is unusual for IMCs during this solar cycle. Its left‐handedness implies an association with a left‐skewed coronal arcade, which is less common in the Southern Hemisphere. Considering the shock observed ahead of the IMC, we conclude based on orientation and ram pressure arguments that this shock was not driven by the IMC, as might be presumed, but was the flank of an unrelated shock that overtook the IMC approximately halfway between Sun and Earth, heating the plasma and accelerating particles within the IMC. The CME associated with the X‐class flare, at 1208 UT on 18 March in AR 10314 appears to be the solar source for this shock.

Transverse oscillations in solar coronal loops induced by propagating Alfvénic pulses

The propagation and the evolution of Alfvenic pulses in the solar coronal arcades is investigated by means of MHD numerical simulations. Significant transverse oscillations in coronal loops, triggered by nearby flare events, are often measured in EUV lines and are generally interpreted as standing kink modes. However, the damping times of these oscillations are typically very short (from one to a few periods) and the physical mechanism responsible for the decay is still a matter of debate. Moreover, the majority of the observed cases actually appears to be better modeled by propagating, rather than standing, modes. Here we perform 2.5D compressible MHD simulations of impulsively generated Alfven waves propagating in a potential magnetic arcade (assumed as a simplified 2D loop model), taking into account the stratification of the solar atmosphere with height from the photosphere to the corona. The results show a strong spreading of the initially localized pulses along the loop, due to the variations in the Alfven velocity with height, and correspondingly an efficient damping of the amplitude of the oscillations. We believe that simple explanations based on the effects of wave propagation in highly inhomogeneous media may apply to the majority of the reported cases, and that variations of the background density and Alfven speed along the loop should be considered as key ingredients in future models.

Relating near-Earth observations of an interplanetary coronal mass ejection to the conditions at its site of origin in the solar corona

[1] A halo coronal mass ejection (CME) was detected on January 20, 2004. We use solar remote sensing data (SOHO, Culgoora) and near-Earth in situ data (Cluster) to identify the CME source event and show that it was a long duration flare in which a magnetic flux rope was ejected, carrying overlying coronal arcade material along with it. We demonstrate that signatures of both the arcade material and the flux rope material are clearly identifiable in the Cluster and ACE data, indicating that the magnetic field orientations changed little as the material traveled to the Earth, and that the methods we used to infer coronal magnetic field configurations are effective.

Observations of CME-related phenomena in a wide spectral range

We study pre-eruptive, eruptive, and post-eruptive phenomena related to a CME that occurred on November 23, 2000 by means of joint analyses of data from various spectral ranges. Almost all known CME-associated phenomena were observed during this event, i.e., a filament eruption, solar flare, dimmings, and a post-eruptive arcade formation. Following a chain of events observed in various spectral ranges, we find that the event occurred in an activity complex consisting of active regions 9231 and 9238, and that it was triggered by a magnetic flux emergence, which caused a flare in AR 9231. In turn, the flare triggered activation and eruption of the filament followed by the CME and the flare in AR 9238 in which the post-eruptive arcade was observed. We discuss some characteristics of the flare and CME and also estimate the magnetic field strength in the coronal arcade to be about 200 G from spatially resolved polarization measurements in microwaves with radio telescopes. In this particular case, the only significant emission mechanism is optically thin free-free emission, and the possible contribution of nonthermal emissions cannot change our estimate of the magnetic field strength in the corona. However, generally one should make sure that the nonthermal contribution cannot be important in similar cases; otherwise, the magnetic field can be well overestimated. Here, we specifically address the identification technique of the radio emission mechanism.