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 Focusing Optics X-ray Solar Imager (FOXSI) rocket and the 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 τ, delay times between events t N, and filling factors f. The single quiescent AR observed by 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 ∼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 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 = τ) was ruled out with >99% confidence for all regions observed by either instrument.
Bibliographical noteFunding Information:
This paper made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. This research also made use of the NuSTAR Data Analysis Software (NUSTARDAS) jointly developed by the ASI Science Data Center (ASDC; Italy) and the California Institute of Technology (USA). We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. The FOXSI-2 sounding rocket was funded by NASA LCAS grant NNX11AB75G. The FOXSI team would like to acknowledge the contributions of each member of the FOXSI experiment team to the project, particularly our team members at ISAS for the provision of Si and CdTe detectors and at MSFC for the fabrication of the focusing optics. The authors would like to thank P. S. Athiray for providing AIA data co-temporal with the FOXSI-2 observations. Additional thanks goes to Will Barnes for helpful discussions about ebtel++ and the physics of small heating events. A.J.M. was supported by NASA Earth and Space Science Fellowship award NNX13AM41H. L.G. was supported by NSF grant AGS-1429512. S.J.B. was supported in this effort by NSF CAREER award AGS-1450230. I.G.H. was supported by a Royal Society University Research Fellowship. This work was supported by NASA grants NNX12AJ36G and NNX14AG07G. Facility: NuSTAR.
A.J.M. was supported by NASA Earth and Space Science Fellowship award NNX13AM41H. L.G. was supported by NSF grant AGS-1429512. S.J.B. was supported in this effort by NSF CAREER award AGS-1450230. I.G.H. was supported by a Royal Society University Research Fellowship. This work was supported by NASA grants NNX12AJ36G and NNX14AG07G.
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- Sun: X-rays, gamma rays
- Sun: corona
- Sun: flares