Evidence of Impulsive Heating in Active Region Core Loops

TL;DR

This study uses spectral data to analyze emission measure distributions in active region moss, finding evidence that impulsive heating and strong condensation dominate the transition region dynamics, supporting nanoflare heating models.

Contribution

It provides observational evidence linking impulsive heating and strong condensation to active region core loops, supporting nanoflare heating theories.

Findings

EM distributions suggest strong condensation dominates

Downflows indicate cooling and draining of plasma

Supports nanoflare heating in hot coronal loops

Abstract

Using a full spectral scan of an active region from the Extreme-Ultraviolet Imaging Spectrometer (EIS) we have obtained Emission Measure EM$(T)$ distributions in two different moss regions within the same active region. We have compared these with theoretical transition region EMs derived for three limiting cases, namely \textit{static equilibrium}, \textit{strong condensation} and \textit{strong evaporation} from \cite{ebtel}. The EM distributions in both the moss regions are strikingly similar and show a monotonically increasing trend from $\log T[\mathrm{K}]=5.15 -6.3$. Using photospheric abundances we obtain a consistent EM distribution for all ions. Comparing the observed and theoretical EM distributions, we find that the observed EM distribution is best explained by the \textit{strong condensation} case (EM$_{con}$), suggesting that a downward enthalpy flux plays an important and possibly dominant role in powering the transition region moss emission. The downflows could be due to unresolved coronal plasma that is cooling and draining after having been impulsively heated. This supports the idea that the hot loops (with temperatures of 3{-}5 MK) seen in the core of active regions are heated by nanoflares.