Enhanced Peak and Extended Cooling of the Extreme-ultraviolet Late Phase in a Confined Solar Flare

TL;DR

This study analyzes a non-eruptive solar flare with an unusually large late-phase EUV peak, revealing that loop expansion and thermodynamic evolution can prolong cooling and enhance late-phase emissions without additional heating.

Contribution

It introduces a novel explanation for prolonged late-phase cooling in solar flares based on loop cross-sectional expansion and thermodynamic evolution, supported by observations and analytical modeling.

Findings

Late-phase peak exceeds main flare peak by 1.4 times.

Loop expansion prolongs cooling and sustains high density.

Thermodynamic evolution explains late-phase enhancement without extra heating.

Abstract

We present observations and analysis of an X1.8 non-eruptive solar flare on 2012 October 23, which is characterized by an extremely large late-phase peak seen in the warm coronal extreme-ultraviolet (EUV) emissions ($\sim$ 3 MK), with the peak intensity over 1.4 times that of main flare peak. The flare is driven by a failed eruption of a magnetic flux rope (MFR), whose strong squeeze force acting on the overlying magnetic structures gives rise to an intense early heating of the late-phase loops. Based on differential emission measure (DEM) analysis, it is found that the late-phase loops experience a "longer-than-expected" cooling without the presence of any obvious additional heating, and meanwhile, their volume emission measure (EM) maintains a plateau level for a long time before turning into an evident decay. Without the need for an additional heating, we propose that the special thermodynamic evolution of the late-phase loops revealed in this flare might arise from loop cross-sectional expansions with height, which are evidenced by both direct measurements from EUV images and by magnetic field extrapolation. By blocking the losses of both heat flux and mass from the corona, such an upward cross-sectional expansion not only elongates the loop cooling time, but also more effectively sustains the loop density, therefore leading to a later-than-expected occurrence of the warm coronal late phase in combination with a sufficiently high late-phase peak. We further verify such a scenario by analytically solving the cooling process of a late-phase loop characterized by a variable cross section.