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

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.

References in corpus (5)

• Highly Efficient Modeling of Dynamic Coronal Loops
• Enthalpy-based Thermal Evolution of Loops: II. Improvements to the Model
• Non-equilibrium of Ionization and the Detection of Hot Plasma in Nanoflare-heated Coronal Loops
• The role of radiative losses in the late evolution of pulse-heated coronal loops/strands
• Can a Long Nanoflare Storm Explain the Observed Emission Measure Distributions in Active Region Cores?

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