Hard X-Ray Constraints on Small-Scale Coronal Heating Events

Abstract

Much evidence suggests that the solar corona is heated impulsively, meaning that nanoflares may be ubiquitous in quiet and active regions (ARs). Hard X-ray (HXR) observations with unprecedented sensitivity $>$3~keV are now enabled by focusing instruments. We analyzed data from the \textit{Focusing Optics X-ray Solar Imager (FOXSI)} rocket and the \textit{Nuclear Spectroscopic Telescope Array (NuSTAR)} spacecraft to constrain properties of AR nanoflares simulated by the EBTEL field-line-averaged hydrodynamics code. We generated model X-ray spectra by computing differential emission measures for homogeneous nanoflare sequences with heating amplitudes $H_0$, durations $\tau$, delay times between events $t_N$, and filling factors $f$. The single quiescent AR observed by \textit{FOXSI-2} on 2014 December 11 is well fit by nanoflare sequences with heating amplitudes 0.02 erg cm${}^{-3}$ s${}^{-1}$ $<$ $H_0$ $<$ 13 erg cm${}^{-3}$ s${}^{-1}$ and a wide range of delay times and durations. We exclude delays between events shorter than $\sim$900 s at the 90\% confidence level for this region. Three of five regions observed by {\nustar} on 2014 November 1 are well fit by homogeneous nanoflare models, while two regions with higher fluxes are not. Generally, the {\nustar} count spectra are well fit by nanoflare sequences with smaller heating amplitudes, shorter delays, and shorter durations than the allowed \textit{FOXSI-2} models. These apparent discrepancies are likely due to differences in spectral coverage between the two instruments and intrinsic differences among the regions. Steady heating ($t_N$ = $\tau$) was ruled out with $>$99\% confidence for all regions observed by either instrument.

17 pages, 21 figures. Accepted for publication in The Astrophysical Journal

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Cited by in corpus (11)

• The effects of numerical resolution, heating timescales and background heating on thermal non-equilibrium in coronal loops
• Statistical Signatures of Nanoflare Activity. I. Monte Carlo Simulations and Parameter-space Exploration
• Transient brightenings in the quiet Sun detected by ALMA at 3 mm
• Impulsive coronal heating from large-scale magnetic rearrangements: from IRIS to SDO/AIA
• Coronal Loop Scaling Laws for Various Forms of Parallel Heat Conduction
• Magnetic filed dynamics and varying plasma emission in large coronal loops
• Detection of weak ubiquitous impulsive nonthermal emissions from the solar corona
• Hot plasma in a quiescent solar active region as measured by RHESSI, XRT, and AIA
• Transition region contribution to AIA observations in the context of coronal heating
• The need for focused, hard X-ray investigations of the Sun
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