Determine the physical origin of EUV late-phase emission

Determine whether the EUV late-phase emission in eruptive solar flares is primarily produced by additional energy injection and heating through magnetic reconnection or by prolonged cooling of post-flare coronal loops, using coronal green-line line-width diagnostics.

Background

The paper discusses competing explanations for the EUV late phase (ELP), a delayed enhancement of warm coronal EUV emission following the main flare phase. One interpretation attributes the ELP to additional heating or energy injection from later magnetic reconnection, whereas another explains it as the result of prolonged cooling of plasma heated during the main reconnection episode.

In the reported prominence-eruption event, the Fe XIV 5303 Å line width decreases during the ELP, which the authors interpret as evidence favoring cooling. However, they explicitly state that the origin of the ELP remains unsettled, leaving the relative contributions of delayed heating and cooling as an open problem requiring observations of additional events.

References

More importantly, owing to greeline correspondence with warm ELP emission, the $lw$ information becomes crucial for understanding the origin of the ELP emission, which remains under debate. Some studies suggest that the ELP occurs from additional energy injection and heating, whereas others favor a prolonged cooling process following the main-phase reconnection heating.

— Coronal Green-Line Emission During a Solar Prominence Eruption: Spectroscopic Evidence of Its Correspondence with the EUV Late Phase  (2609.30164 - Vemareddy et al., 24 Sep 2026) in Section 3, “Sit and stare Spectroscopic observation by VELC”

However, this intriguing possibility needs to be confirmed by analyzing additional events, then only the green line emission can be regarded as a proxy for EUV emission during the gradual phase of flares.

— Coronal Green-Line Emission During a Solar Prominence Eruption: Spectroscopic Evidence of Its Correspondence with the EUV Late Phase  (2609.30164 - Vemareddy et al., 24 Sep 2026) in Section 5, “Summary and Discussion”