Spontaneous reconnection at a separator current layer: 2. Nature of the waves and flows

Abstract

Sudden destabilizations of the magnetic field, such as those caused by spontaneous reconnection, will produce waves and/or flows. Here we investigate the nature of the plasma motions resulting from spontaneous reconnection at a 3‐D separator. In order to clearly see these perturbations, we start from a magnetohydrostatic equilibrium containing two oppositely signed null points joined by a generic separator along which lies a twisted current layer. The nature of the magnetic reconnection initiated in this equilibrium as a result of an anomalous diffusivity is discussed in detail in Stevenson and Parnell (2015). The resulting sudden loss of force balance inevitably generates waves that propagate away from the diffusion region carrying the dissipated current. In their wake a twisting stagnation flow, in planes perpendicular to the separator, feeds flux back into the original diffusion site (the separator) in order to try to regain equilibrium. This flow drives a phase of slow weak impulsive bursty reconnection that follows on after the initial fast‐reconnection phase.

Spontaneous reconnection at a separator current layer: 1. Nature of the reconnection

“…The rapid loss of equilibrium caused as a result of the reconnection in the separator current layer naturally launches waves that generate flows within the system. These waves and flows are discussed in detail in the second paper of this series [ Stevenson and Parnell , ].…”

“…The reconnection in this experiment, therefore, occurs in two phases: the first phase is a highly dynamic fast‐reconnection phase with low velocities which is dominated by Ohmic heating (0 t f ≤ t ≤0.09 t f ) and the second phase is a slow impulsive bursty reconnection phase which has a comparatively low level of Ohmic heating, but with higher velocities than in phase I providing greater amounts of viscous heating (0.09 t f < t ≤0.76 t f where t = 0.76 t f is where we stop the experiment since at this time the waves, launched by the reconnection, approach the boundaries of the domain; see Stevenson and Parnell [] for more details).…”

“…As the first phase (in which there is fast Ohmic dissipation) comes to an end at t = 0.09 t f , the gradients of the magnetic and internal energies change and the Ohmic heating rate decreases: most of the current above j crit in the separator current layer has been dissipated. In phase II, the kinetic energy, which until now has been slowly building, increases sharply associated with the presence of flows in the system following the rapid adiabatic cooling (see Stevenson and Parnell [] for further details of the waves and flows created by this reconnection). This increase in kinetic energy is reflected by an increase in viscous heating to 8 × 10 −5 ; however, the amount of Ohmic heating is always greater than the viscous heating throughout both phases due to the high value of the plasma beta (the mean value of the plasma beta is at t = 0 t f ) which makes it hard for large waves to be produced.…”

“…Due to the lack of colocation of the counterrotation point and the outflow point of flow along the separator, in the region of peak reconnection the local flows are more complicated. Note that details of the nature of the flows farther out from the separator are discussed in Stevenson and Parnell [].…”

“…Here we study the nature of nondriven 3‐D separator reconnection at a generic single‐separator current layer. Specifically, we choose an idealistic setup (a straight single separator) to allow us to relatively easily analyze (i) the nature of the reconnection found at all points along the separator and (ii) the nature of the waves and flows [ Stevenson and Parnell , ] resulting from the reconnection. In particular, we do not attempt to simulate one specific space physics event but rather produce a model whose general results may be applicable in many events involving separators.…”