NuSTAR Observation of a Minuscule Microflare in a Solar Active Region

Abstract

We present X-ray imaging spectroscopy of one of the weakest active region (AR) microflares ever studied. The microflare occurred at $\sim$11:04 UT on 2018 September 9 and we studied it using the Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic Observatory's Atmospheric Imaging Assembly (SDO/AIA). The microflare is observed clearly in 2.5-7 keV with NuSTAR and in Fe XVIII emission derived from the hotter component of the 94 $\unicode{x212B}$ SDO/AIA channel. We estimate the event to be three orders of magnitude lower than a GOES A class microflare with an energy of 1.1$\times$10${}^{26}$ erg. It reaches temperatures of 6.7 MK with an emission measure of 8.0$\times$10${}^{43}$ cm${}^{-3}$. Non-thermal emission is not detected but we instead determine upper limits to such emission. We present the lowest thermal energy estimate for an AR microflare in literature, which is at the lower limits of what is still considered an X-ray microflare.

Accepted for publication in ApJL

References in corpus (5)

• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• Evidence of Non-Thermal Particles in Coronal Loops Heated Impulsively by Nanoflares
• Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
• NuSTAR hard X-ray observation of a sub-A class solar flare
• Evidence of Significant Energy Input in the Late Phase of a Solar Flare from NuSTAR X-Ray Observations

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• Plasma heating and nanoflare caused by slow-mode wave in a coronal loop
• First joint X-ray solar microflare observations with NuSTAR and Solar Orbiter/STIX
• An in-depth analysis of quiet-Sun IRIS Brightenings
• Detecting non-thermal emission in a solar microflare using nested sampling
• Thermal Evolution of an Active Region through Quiet and Flaring Phases as Observed by NuSTAR XRT, and AIA
• Quiet Sun impulsive events observed with NuSTAR during solar minimum
• X-ray fine structure of a limb solar flare revealed by Insight-HXMT, RHESSI and Fermi
• Evolution of Elemental Abundances During B-Class Solar Flares: Soft X-ray Spectral Measurements with Chandrayaan-2 XSM