FOXSI-2 Solar Microflares II: Hard X-ray Imaging Spectroscopy and Flare Energetics

Abstract

We study the nature of energy release and transfer for two sub-A class solar microflares observed during the second flight of the Focusing Optics X-ray Solar Imager (FOXSI-2) sounding rocket experiment on 2014 December 11. FOXSI is the first solar-dedicated instrument to utilize focusing optics to image the Sun in the hard X-ray (HXR) regime, sensitive to the energy range 4-20 keV. Through spectral analysis of the two microflares using an optically thin isothermal plasma model, we find evidence for plasma heated to temperatures of ~10 MK and emissions measures down to ~$10^{44}$cm${}^{-3}$. Though nonthermal emission was not detected for the FOXSI-2 microflares, a study of the parameter space for possible hidden nonthermal components shows that there could be enough energy in nonthermal electrons to account for the thermal energy in microflare 1, indicating that this flare is plausibly consistent with the standard thick-target model. With a solar-optimized design and improvements in HXR focusing optics, FOXSI-2 offers approximately five times greater sensitivity at 10 keV than the Nuclear Spectroscopic Telescope Array (NuSTAR) for typical microflare observations and allows for the first direct imaging spectroscopy of solar HXRs with an angular resolution at scales relevant for microflares. Harnessing these improved capabilities to study the evolution of small-scale events, we find evidence for spatial and temporal complexity during a sub-A class flare. These studies in combination with contemporanous observations by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory (SDO/AIA) indicate that the evolution of these small microflares is more similar to that of large flares than to the single burst of energy expected for a nanoflare.

arXiv comment20 pages, 16 figures, 2 tables; Submitted to ApJ

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Cited by in corpus (10)

• STIX X-ray microflare observations during the Solar Orbiter commissioning phase
• Multiple electron acceleration instances during a series of solar microflares observed simultaneously at X-rays and microwaves
• On the faintest solar coronal hard X-rays observed with FOXSI
• First joint X-ray solar microflare observations with NuSTAR and Solar Orbiter/STIX
• Ionizing Radiation Escape from Low-Redshift Galaxies and Its Connection to Cosmic Reionization
• Signatures of Type III Solar Radio Bursts from Nanoflares: Modeling
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• The need for focused, hard X-ray investigations of the Sun
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• X-ray fine structure of a limb solar flare revealed by Insight-HXMT, RHESSI and Fermi