Two-fluid reconnection jets in a gravitationally stratified atmosphere

Abstract

Context. Density decreases exponentially with height in the gravitationally stratified solar atmosphere, and therefore collisional coupling between the ionized plasma and the neutrals also decreases. Reconnection is a process observed at all heights in the solar atmosphere. Aims. Here, we investigate the role of collisions between ions and neutrals in the reconnection process occurring at various heights in the atmosphere. Methods. We performed simulations of magnetic reconnection induced by a localized resistivity in a gravitationally stratified atmosphere, in which we varied the height of the initial reconnection X-point. We compared a magnetohydrodynamic (MHD) model and two two-fluid configurations: one in which the collisional coupling was calculated from local plasma parameters, and another in which the coupling was decreased so that collisional effects would be enhanced. The latter setup has a more representative solar collisionality regime. Results. Simulations in a stratified atmosphere show similar structures in MHD and two-fluid simulations, with strong coupling. However, when collisional effects are increased to attain representative parameter regimes, we find a nonlinear runaway instability, which separates the plasma-neutral densities across the current sheet (CS). With increased collisional effects, the initial decoupling in velocity heats the neutrals and this sets up a nonlinear feedback loop, according to which neutrals migrate outside the CS, replacing charged particles that accumulate toward the center of the CS. Conclusions. The reconnection rate has a maximum value of around 0.1 for both reconnection heights, and is consistent with the locally enhanced resistivity used in all three models. The early-stage plasmoid formation observed near the end of our simulations is influenced by the outflow from the primary reconnection point, rather than by collisions. We synthesized optically thin emission for both MHD and two-fluid models, which can show a very different evolution when the charged-particle density is used instead of the total density. Our simulations have relevance for observed plasmoid features associated with chromospheric to low-coronal flare events.