Simulations of emerging flux in a coronal hole: oscillatory reconnection

¹ University College London, Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK e-mail: mjm@mssl.ucl.ac.uk
² Observatoire de Paris, LESIA, UMR 8109 (CNRS), Meudon-Principal Cedex, France
³ Konkoly Observatory, Budapest, Hungary

Received: 17 June 2008
Accepted: 27 November 2008

Abstract

Context. Observations and simulations show that reconnection will take place when a flux tube emerges into a coronal hole, which is characterised by magnetic fieldlines “open” towards interplanetary space. Although the mechanism by which reconnection is initiated has been thoroughly studied, the long-term evolution of this reconnecting magnetic system remains unreported.

Aims. We aim to understand the long-term evolution of the reconnecting flux tube and coronal hole system and, in particular, to ascertain whether it can reach an equilibrium state in which all reconnection has ceased. By determining the evolution in this particular scenario, we aim to be able to select a subset from the broad spectrum of reconnecting systems, which will undergo the same progression to equilibrium.

Methods. Using a 2.5-dimensional numerical magnetohydrodynamic (MHD) code, we evolve a simple stratified atmospheric domain, which is endowed with a vertical magnetic field, representing the interior of a coronal hole, and a horizontal buoyant flux tube that is placed near the bottom of the domain. To investigate the long-term evolution of the system, we continue to study the domain long after the flux tube has emerged and reconnection has commenced between the magnetic fields of the flux tube and coronal hole.

Results. We find that a series of reconnection reversals (or oscillatory reconnection) takes place, whereby reconnection occurs in distinct bursts and the inflow and outflow magnetic fields of one burst of reconnection become the outflow and inflow fields in the following burst of reconnection, respectively. During each burst of reconnection the gas pressure in the bounded outflow regions increases above the level of that in the inflow regions and, eventually, gives rise to a reconnection reversal. In consecutive bursts of reconnection, the contrast in the gas pressure across the boundaries of the inflow and outflow regions decreases and, over time, the system settles towards equilibrium. Once the equilibrium state is reached, all reconnection ceases. This is the first reported instance of oscillatory reconnection initiated in a self-consistent manner, and the signatures of the mechanism compare favourably with observations of select flux emergence events and with solar and stellar flares.

Conclusions. Across the broader spectrum of reconnecting systems, oscillatory reconnection will only occur if the outflow regions are quasi-bounded during each burst of reconnection. The swaying outflow jet and periodic heating signatures of oscillatory reconnection are exceedingly similar to those exhibited by MHD modes and, in many observations, distinction between the two mechanisms may be impossible.