On the origin of the Extreme-Ultraviolet late phase of solar flares

TL;DR

This paper investigates the origin of the EUV late phase in solar flares, revealing it results from a larger, higher magnetic loop system with distinct thermal evolution, connected magnetically to the main flare loops.

Contribution

It provides detailed analysis of two solar flares showing the EUV late phase originates from a separate, larger magnetic loop system with unique thermal and magnetic properties.

Findings

Late phase emission comes from a larger, higher magnetic loop system.

The late phase loop arcade peaks later and cools gradually over more than an hour.

The two loop systems are magnetically connected, forming an asymmetric quadruple configuration.

Abstract

Solar flares typically have an impulsive phase that followed by a gradual phase as best seen in soft X-ray emissions. A recent discovery based on the EUV Variability Experiment (EVE) observations onboard the Solar Dynamics Observatory (SDO) reveals that some flares exhibit a second large peak separated from the first main phase peak by tens of minutes to hours, which is coined as the flare's EUV late phase. In this paper, we address the origin of the EUV late phase by analyzing in detail two late phase flares, an M2.9 flare on 2010 October 16 and an M1.4 flare on 2011 February 18, using multi-passband imaging observations from the Atmospheric Imaing Assembly (AIA) onboard SDO. We find that: (1) the late phase emission originates from a different magnetic loop system, which is much larger and higher than the main phase loop system. (2) The two loop systems have different thermal evolution. While the late phase loop arcade reaches its peak brightness progressively at a later time spanning for more than one hour from high to low temperatures, the main phase loop arcade reaches its peak brightness at almost the same time (within several minutes) in all temperatures. (3) Nevertheless, the two loop systems seem to be connected magnetically, forming an asymmetric magnetic quadruple configuration. (4) Further, the footpoint brightenings in UV wavelengths show a systematic delay of about one minute from the main flare region to the remote footpoint of the late phase arcade system. We argue that the EUV late phase is the result of a long-lasting cooling process in the larger magnetic arcade system.