Numerical simulations of sheared magnetic lines at the solar null line

Abstract

Aims. We perform numerical simulations of sheared magnetic lines at the magnetic null line configuration of two magnetic arcades that are settled in a gravitationally stratified and magnetically confined solar corona. Methods. We developed a general analytical model of a 2.5D solar atmospheric structure. As a particular application of this model, we adopted it for the curved magnetic field lines with an inverted Y shape that compose the null line above two magnetic arcades, which are embedded in the solar atmosphere that is specified by the realistic temperature distribution. The physical system is described by 2.5D magnetohydrodynamic equations that are numerically solved by the FLASH code. Results. The magnetic field line shearing, implemented about 200 km below the transition region, results in Alfvén and magnetoacoustic waves that are able to penetrate solar coronal regions above the magnetic null line. As a result of the coupling of these waves, partial reflection from the transition region and scattering from inhomogeneous regions the Alfvén waves experience fast attenuation on time scales comparable to their wave periods, and the physical system relaxes in time. The attenuation time grows with the large amplitude and characteristic growing time of the shearing. Conclusions. By having chosen a different magnetic flux function, the analytical model we devised can be adopted to derive equilibrium conditions for a diversity of 2.5D magnetic structures in the solar atmosphere.

New analytical and numerical models of a solar coronal loop

“…Therefore, from Eqs. (12) and (13), we find the following expressions for and p (Solov'ev 2010; Kraśkiewicz et al 2015;Kuźma et al 2015):…”

“…These models are based on the analytical models of Solov'ev (2010). The Kraśkiewicz et al (2015) and Kuźma et al (2015) reports can also be referred for detailed mathematical formulation of the analytical and numerical models. Using these models, we performed the 2D numerical simulations of the vertical kink oscillations of this loop excited by the forced periodic driver that acts at the top of the photosphere, centrally below the apex of the loop.…”

3D WKB solution for fast magnetoacoustic wave behaviour around an X-line

“…see Régnier 2013) and such extrapolations show the existence of important features of the topology: null points -specific points where the magnetic field is zero, separatrices -topological features that separate regions of different magnetic flux connectivity, and X-lines or null lines -extended locations where the magnetic field, and thus the Alfvén speed, is zero. Investigations of the coronal magnetic field using such potential field calculations can be found in, e.g., Brown & Priest (2001), Beveridge et al (2002), Régnier et al (2008) and in a comprehensive review by Longcope (2005). These two areas of scientific study, namely ubiquitous MHD waves and magnetic topology, will naturally encounter each other in the solar atmosphere, e.g.…”

“…All these investigations were carried out using cylindrical models in which the generated waves encircled the X-point and so the cylindrical symmetry meant that the disturbances can only propagate either towards or away from the X-point. The behaviour of MHD waves around twodimensional X-points in a Cartesian geometry has been investigated by McLaughlin & Hood (2004, 2005, 2006b, McLaughlin et al (2009) and more recently by Kuźma et al (2015). Of note is also McLaughlin & Hood (2006a) who investigated fast MHD wave propagation in the neighbourhood of two dipoles.…”

Behaviour of Magnetoacoustic Waves in the Neighbourhood of a Two-Dimensional Null Point: Initially Cylindrically Symmetric Perturbations

“…In the limit of B → , the two modes are decoupled and the results of 2D work are recovered. More recently, Kuźma et al (2015) investigated similar coupling for a X-line formed above two magnetic arcades, but now embedded in a model solar atmosphere with a realistic temperature distribution. They found that the formation of the Alfvén waves at the initial phase of temporal evolution is followed by linear coupling between Alfvén and magnetoacoustic waves at a later time.…”