How Gas-Dynamic Flare Models Powered by Petschek Reconnection Differ From Those With Ad Hoc Energy Sources

Abstract

Aspects of solar flare dynamics, such as chromospheric evaporation and flare lightcurves, have long been studied using one-dimensional models of plasma dynamics inside a static flare loop, subjected to some energy input. While extremely successful at explaining the observed characteristics of flares, all such models so far have specified energy input ad hoc, rather than deriving it self-consistently. There is broad consensus that flares are powered by magnetic energy released through reconnection. Recent work has generalized Petschek's basic reconnection scenario, topological change followed by field line retraction and shock heating, to permit its inclusion ina onedimensional flare loop model. Here we compare the gas dynamics driven by retraction and shocking to those from more conventional static loop models energized by ad hoc source terms. We find significant differences during the first minute, when retraction leads to larger kinetic energies and produces higher densities at the loop top, while ad hoc heating tends to rarify the loop top. The loop-top density concentration is related to the slow magnetosonic shock, characteristic of Petschek's model, but persists beyond the retraction phase occurring in the outflow jet. This offers an explanation for observed loop-top sources of X-ray and EUV emission, with advantages over that provided by ad hoc heating scenarios. The cooling phases of the two models are, however, notably similar to one another, suggesting that observations at that stage will yield little information on the nature of energy input.

Long Duration Flare Emission: Impulsive Heating or Gradual Heating?

“…These include almost eight full scale heights of gravitational stratification, over which the electron density rises from 4×10 10 cm −3 , at the top, to 10 14 cm −3 at the lower boundary. The chromospheres are intended only as massreservoirs, and are treated using coronal dynamics, assuming full ionization (Longcope 2014;Longcope & Klimchuk 2015).…”

“…The dynamical evolution of the one-dimensional loop is solved using the PREFT code (standing for Post-Reconnection Evolution of a Flux Tube, Longcope & Klimchuk 2015). While it is designed to model retracting loops, we run it here with a loop fixed and straight.…”

A Reconnection-Driven Model of the Hard X-Ray Loop-Top Source From Flare 2004 February 26

“…In a two-dimensional, steady-state model, this outflow takes the form of a narrow, steady jet originating at the reconnection site (Petschek 1964;Vasyliunas 1975;Soward 1982;Priest & Forbes 1986). In more recent variations (Longcope et al 2009;Guidoni & Longcope 2010Longcope & Klimchuk 2015), reconnection can occur more sporadically, and the outflow can be less coherent, perhaps forming a disordered collection of flux tubes (or "plasmoids") retracting though the current sheet. In either event, a portion of the flow's kinetic energy is thermalized in SMSs.…”

“…In unsteady versions the SMSs travel with the retracting tube, bounding a moving plug of hot compressed plasma, rather than a standing wedge. Moreover, the compressive flows initiated during retraction are not necessarily stopped when the retraction stops, and can continue driving SMSs while the tube rests in the post-flare arcade Longcope & Klimchuk 2015). This offers the possibility that Petschek-like SMSs have sufficient temperature and emission measure, and the correct location, to account for the observed loop-top sources.…”

“…The flare observations were modeled using an unsteady version of Petschek reconnection, called the thin flux tube model (TFT, Linton & Longcope 2006;Longcope et al 2009;Guidoni & Longcope 2010Longcope & Klimchuk 2015). Longcope et al (2010) used analytic expressions from this model, based on Rankine-Hugoniot (RH) conditions of the shocks, to estimate the temperature and density of the outflow.…”