Modeling Observable Differences in Flare Loop Evolution due to Reconnection Location and Current Sheet Structure

Abstract

Flare reconnection is expected to occur at some point within a large-scale coronal current sheet. The structure of the magnetic field outside this sheet is almost certain to affect the flare, especially its energy release. Different models for reconnection have invoked different structures for the current sheet’s magnetic field and different locations for the reconnection electric field within it. Models invoking Petschek-type reconnection often use a uniform field. Others invoke a field bounded by two Y-points with a field strength maximum between them and propose this maximum as the site of the reconnection electric field. Still other models, such as the collapsing trap model, require that the field strength peak at or near the edge of the current sheet and propose that reconnection occurs above this peak. At present there is no agreement as to where reconnection might occur within a global current sheet. We study the post-reconnection dynamics under all these scenarios, seeking potentially observable differences between them. We find that reconnection occurring above the point of strongest field leads to the highest density and the highest emission measure of the hottest material. This scenario offers a possible explanation of superhot coronal sources seen in some flares.

A Possible Mechanism for the “Late Phase” in Stellar White-light Flares

“…In this study, we use loop-top and footpoint heating sources to mimic the flare energy injection. Physically, the loop-top heating is directly related to the outflow of the magnetic field reconnection, either from the bow shock on the loop top or the collision of energetic particles from the reconnection (Masuda et al 1994;Shibata et al 1995;Forbes & Acton 1996;Guidoni & Longcope 2010;Unverferth & Longcope 2020Fleishman et al 2022). The footpoint heating, on the other hand, can be attributed to turbulence dissipation of Alfvénic waves following loop retraction (e.g., Ashfield & Longcope 2023), the thermalization of energetic particles, which is nonthermal electrons or protons, (e.g., Brown 1971;Emslie 1978;Holman et al 2011), Alfvén wave dissipation (e.g., Emslie & Sturrock 1982;Fletcher & Hudson 2008;Kerr et al 2016;Reep et al 2016), thermal conduction, and other wave-particle processes (Kowalski 2023).…”

A Model for Gradual-phase Heating Driven by MHD Turbulence in Solar Flares

“…The TFT model describes the evolution of flare loops retracting under a tension force brought about by a change in magnetic topology. Previous studies assumed the retraction occurs without any interaction with the background current sheet, and therefore saw typical outflow velocities reaching the local Alfvén speed on the order of 3 Mm s −1 (Longcope et al 2018;Unverferth & Longcope 2020). Alfvénic outflow speeds are common to all idealized models of magnetic reconnection, but are not supported by much observational evidence.…”

Interrogating solar flare loop models with IRIS observations 2: Plasma properties, energy transport, and future directions

“…Radiative losses are optically thin, and normally an isothermal, but gravitationally stratified, chromosphere is included mostly as a mass reservoir. Solutions of the TFT equations show that thermal conduction carries heat away from the shocks, drastically altering the temperature and density of the post-flare plasma (Longcope and Guidoni, 2011;Longcope and Klimchuk, 2015;Longcope et al, 2016;Unverferth and Longcope, 2020).…”