Magnetohydrodynamic wave propagation from the subphotosphere to the corona in an arcade-shaped magnetic field with a null point^⋆

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife Spain
e-mail: irantzu@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38205, La Laguna, Tenerife, Spain
³ Main Astronomical Observatory, NAS, 03680 Kyiv, Ukraine

Received: 29 July 2014
Accepted: 26 February 2015

Abstract

Aims. The aim of this work is to study the energy transport by means of Magnetohydrodynamic (MHD) waves propagating in quiet-Sun magnetic topology from layers below the surface to the corona. Upwardly propagating waves find obstacles, such as the equipartition layer with plasma β = 1, the transition region, and null points, and they get transmitted, converted, reflected, and refracted. Understanding the mechanisms by which MHD waves can reach the corona can give us information about the solar atmosphere and the magnetic structures.

Methods. We carried out two-dimensional numerical simulations of wave propagation in a magnetic field structure that consists of two vertical flux tubes with the same polarity separated by an arcade-shaped magnetic field. This configuration contains a null point in the corona, which significantly modifies the behavior of the waves as they pass near it.

Results. We describe in detail the wave propagation through the atmosphere under different driving conditions. We also present the spatial distribution of the mean acoustic and magnetic energy fluxes for the cases where these calculations are possible, as well as the spatial distribution of the dominant frequencies in the whole domain.

Conclusions. We conclude that the energy reaches the corona preferably along almost vertical magnetic fields, that is, inside the vertical flux tubes. This energy is acoustic in nature. Most of the magnetic energy stays concentrated below the transition region owing to the refraction of the magnetic waves and the continuous conversion of acoustic-like waves into fast magnetic waves in the equipartition layer located in the photosphere where plasma β = 1. However, part of the magnetic energy reaches the low corona when propagating in the region where the arcades are located, but waves are sent back downward into the lower atmosphere at the null-point surroundings. This phenomenon, together with the reflection and refraction of waves in the TR and the lower turning point, act as a re-feeding of the atmosphere, which keeps oscillating during all the simulation time even if a driver with a single pulse was used as initial perturbation. In the frequency distribution, we find that high frequency waves can reach the corona outside the vertical flux tubes.