Magnetic reconnection in 2D stratified atmospheres

I. Dynamical consequences

¹ University of St. Andrews, School of Mathematics and Statistics, North Haugh, St. Andrews, Fife, KY16 9SS, Scotland, UK e-mail: klaus@mcs.st-and.ac.uk
² High Altitude Observatory, NCAR, 3450 Mitchell Lane, Boulder, CO 80301, USA
³ Armagh Observatory, College Hill, Armagh, BT61 9DG, N. Ireland

Corresponding author: I. Roussev, ilr@ucar.edu

Received: 1 August 2001
Accepted: 4 December 2001

Abstract

We explore the dynamical consequences of magnetic reconnection in a 2D stratified physical configuration representing a “quiet” solar environment. By including gravity, an initial magneto-hydrostatic solution is found that allows the magnetic field to expand with height. The change in kinetic gas pressure with height leads to the formation of a cold current “sheet” in the case of strong stratification, in contrast to a hot current “sheet” in the case of negligible stratification. Here the “sheet” temperature is measured relative to the temperature in the ambient background plasma. The dynamics of magnetic reconnection in a stratified atmosphere evolves through a new initial stage, with a more complex velocity structure than the quadrupolar velocity pattern present in traditional 2D X-point reconnection. As time progresses, the new initial phase is suppressed and the driven reconnection evolves into the traditional 2D reconnection pattern. The transition time between the two regimes is found to depend on the imposed stratification, and through this, on the degree of expansion of the initial magnetic field with height. The new reconnection regime undergoes a more complicated physical evolution and seems to have a lower reconnection rate than the classical 2D X-point reconnection. The faster the magnetic field expands with height, the slower and more complex are the dynamics of the magnetic reconnection at the early stages of its evolution.