Magnetohydrodynamic Waves in Coronal Magnetostatic Arcades (original) (raw)

Three-Dimensional Propagation of Magnetohydrodynamic Waves in Solar Coronal Arcades

Astrophysical Journal, 2010

We numerically investigate the excitation and temporal evolution of oscillations in a two-dimensional coronal arcade by including the three-dimensional propagation of perturbations. The time evolution of impulsively generated perturbations is studied by solving the linear, ideal magnetohydrodynamic (MHD) equations in the zero-β approximation. As we neglect gas pressure the slow mode is absent and therefore only coupled MHD fast and Alfvén modes remain. Two types of numerical experiments are performed. First, the resonant wave energy transfer between a fast normal mode of the system and local Alfvén waves is analyzed. It is seen how, because of resonant coupling, the fast wave with global character transfers its energy to Alfvénic oscillations localized around a particular magnetic surface within the arcade, thus producing the damping of the initial fast MHD mode. Second, the time evolution of a localized impulsive excitation, trying to mimic a nearby coronal disturbance, is considered. In this case, the generated fast wavefront leaves its energy on several magnetic surfaces within the arcade. The system is therefore able to trap energy in the form of Alfvénic oscillations, even in the absence of a density enhancement such as that of a coronal loop. These local oscillations are subsequently phase-mixed to smaller spatial scales. The amount of wave energy trapped by the system via wave energy conversion strongly depends on the wavelength of perturbations in the perpendicular direction, but is almost independent from the ratio of the magnetic to density scale heights.

Magnetohydrodynamic waves in a sheared potential coronal arcade

Astronomy and Astrophysics, 2004

We study the effects of magnetic field shear (B y 0) and longitudinal propagation of perturbations (k y 0) on the linear and adiabatic magnetohydrodynamic (MHD) normal modes of oscillation of a potential coronal arcade. In a cold plasma, the inclusion of these two effects produces the linear coupling of discrete fast modes, characterised by a discrete spectrum of frequencies and a global velocity structure, and Alfvén continuum modes, characterised by a continuous spectrum of frequencies and with a velocity perturbation confined to given magnetic surfaces in such a way that modes with mixed properties arise (Arregui et al. 2004). The wave equations governing the velocity perturbations have been solved numerically and our results show that the couplings between fast and Alfvén modes are governed by some parity rules for the symmetry of the eigenfunctions of fast and Alfvén modes in the direction along the equilibrium magnetic field. The nature of the coupling between fast and Alfvén modes can be resonant or non-resonant depending on the location of the fast mode frequency within the different Alfvén continua. Also, an important result is that in this kind of configurations coupled modes could be difficult to observe since when both magnetic field shear and longitudinal propagation are present the spatial distribution of the velocity may not be confined to low heights in the solar corona.

Vertical oscillations of an arcade loop in a gravitationally stratified solar corona

Astronomy and Astrophysics, 2008

Aims. We consider impulsively-generated vertical oscillations of an arcade loop that is embedded in the gravitationally-stratified solar corona. Methods. Two-dimensional magnetohydrodynamic equations are solved numerically in the limit of an ideal plasma. Results. The numerical results indicate that the effect of gravity produces a decrease in the wave period and an increase in the attenuation time. This decrease is a consequence of a higher Alfvén speed within the gravitationally-stratified arcade loop, while the weaker attenuation reflects a smaller amount of wave tunneling than in the gravity-free case. These results are reminiscent of those for TRACE data.

Magnetohydrodynamic Waves in Two-Dimensional Prominences Embedded in Coronal Arcades

The Astrophysical Journal, 2013

Solar prominence models used so far in the analysis of MHD waves in such structures are quite elementary. In this work, we calculate numerically magnetohydrostatic models in two-dimensional configurations under the presence of gravity. Our interest is in models that connect the magnetic field to the photosphere and include an overlying arcade. The method used here is based on a relaxation process and requires solving the time-dependent nonlinear ideal MHD equations. Once a prominence model is obtained, we investigate the properties of MHD waves superimposed on the structure. We concentrate on motions purely two-dimensional neglecting propagation in the ignorable direction. We demonstrate how by using different numerical tools we can determine the period of oscillation of stable waves. We find that vertical oscillations, linked to fast MHD waves, are always stable and have periods in the 4-10 min range. Longitudinal oscillations, related to slow magnetoacoustic-gravity waves, have longer periods in the range of 28-40 min. These longitudinal oscillations are strongly influenced by the gravity force and become unstable for short magnetic arcades.

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

Astronomy & Astrophysics, 2006

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.

Fundamental-Mode Oscillations of Two Coronal Loops Within a Solar Magnetic Arcade

The Astrophysical Journal, 2015

We analyse intensity variations, as measured by the Atmospheric Imaging Assembly (AIA) in the 171Å passband, in two coronal loops embedded within a single coronal magnetic arcade. We detect oscillations in the fundamental mode with periods of roughly 2 minutes and decay times of 5 minutes. The oscillations were initiated by interaction of the arcade with a large wavefront issuing from a flare site. Further, the power spectra of the oscillations evince signatures consistent with oblique propagation to the field lines and for the existence of a 2-D waveguide instead of a 1-D one.

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

The Astrophysical Journal, 2008

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.

The Temporal Evolution of Linear Fast and Alfven MHD Waves in Solar Coronal Arcades

Highlights of Spanish Astrophysics V, 2010

The excitation and temporal evolution of fast and Alfvén magnetohydrodynamic oscillations in a two-dimensional coronal arcade are investigated. The approach is to consider an equilibrium magnetic and plasma structure and then to introduce a perturbation trying to mimic a nearby disturbance, such as a flare or filament eruption. By numerically solving the time-dependent linearised MHD wave equations the properties of the solutions have been studied. First, the properties of uncoupled fast and Alfvén waves are described. Then, longitudinal propagation of perturbations is included, and the properties of coupled waves are determined.

Fast magnetohydrodynamic waves in a two-slab coronal structure: collective behaviour

Astronomy & Astrophysics, 2006

Aims. We study fast magnetohydrodynamic waves in a system of two coronal loops modeled as smoothed, dense plasma slabs in a uniform magnetic field. This allows us to analyse in a simple configuration the collective behaviour of the structure due to the interaction between the slabs. Methods. We first calculate the normal modes of the system and find analytical expressions for the dispersion relation of the two-slab configuration. Next, we study the time-dependent problem of the excitation of slab oscillations by numerically solving the initial value problem. We investigate the behaviour of the system for several shapes of the initial disturbances.