Diagnosing the time-dependence of active region core heating from the emission measure: I. Low-frequency nanoflares

TL;DR

This study investigates whether low-frequency nanoflares can explain the heating of active region cores by analyzing emission measure slopes through hydrodynamic modeling, finding it plausible for a significant fraction of observed regions.

Contribution

It provides a comprehensive analysis of low-frequency nanoflare heating as a viable mechanism for active region core heating using emission measure slope modeling.

Findings

Low-frequency nanoflare heating explains up to 77% of observed active regions.

Proper atomic data uncertainties significantly affect the interpretation.

Heating cannot account for slopes greater than 3.

Abstract

Observational measurements of active region emission measures contain clues to the time-dependence of the underlying heating mechanism. A strongly non-linear scaling of the emission measure with temperature indicates a large amount of hot plasma relative to warm plasma. A weakly non-linear (or linear) scaling of the emission measure indicates a relatively large amount of warm plasma, suggesting that the hot active region plasma is allowed to cool and so the heating is impulsive with a long repeat time. This case is called {\it low-frequency} nanoflare heating and we investigate its feasibility as an active region heating scenario here. We explore a parameter space of heating and coronal loop properties with a hydrodynamic model. For each model run, we calculate the slope $\alpha$ of the emission measure distribution $EM(T) \propto T^\alpha$. Our conclusions are: (1) low-frequency nanoflare heating is consistent with about 36% of observed active region cores when uncertainties in the atomic data are not accounted for; (2) proper consideration of uncertainties yields a range in which as many as 77% of observed active regions are consistent with low-frequency nanoflare heating and as few as zero; (3) low-frequency nanoflare heating cannot explain observed slopes greater than 3; (4) the upper limit to the volumetric energy release is in the region of 50 erg cm$^{-3}$ to avoid unphysical magnetic field strengths; (5) the heating timescale may be short for loops of total length less than 40 Mm to be consistent with the observed range of slopes; (6) predicted slopes are consistently steeper for longer loops.