NuSTAR observations of a repeatedly microflaring active region

Abstract

We investigate the spatial, temporal, and spectral properties of 10 microflares from AR12721 on 2018 September 9 and 10 observed in X-rays using the Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic Observatory's Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager (SDO/AIA and HMI). We find GOES sub-A class equivalent microflare energies of 10${}^{26}$-10${}^{28}$ erg reaching temperatures up to 10 MK with consistent quiescent or hot active region core plasma temperatures of 3-4 MK. One microflare (SOL2018-09-09T10:33), with an equivalent GOES class of A0.1, has non-thermal HXR emission during its impulsive phase (of non-thermal power $\sim$7$\times$10${}^{24}$ erg s${}^{-1}$) making it one of the faintest X-ray microflares to have direct evidence for accelerated electrons. In 4 of the 10 microflares, we find that the X-ray time profile matches fainter and more transient sources in the EUV, highlighting the need for observations sensitive to only the hottest material that reaches temperatures higher than those of the active region core ($>$5 MK). Evidence for corresponding photospheric magnetic flux cancellation/emergence present at the footpoints of 8 microflares is also observed.

arXiv commentAccepted for published in MNRAS

References in corpus (28)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission
• An Observational Overview of Solar Flares
• Calibration of the NuSTAR High Energy Focusing X-ray Telescope
• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• A Systematic Survey of High Temperature Emission in Solar Active Regions
• Microflares and the Statistics of X-ray Flares
• Solar Coronal Loops Associated with Small-scale Mixed Polarity Surface Magnetic Fields
• Global Energetics of Solar Flares: II. Thermal Energies
• Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
• Accelerated electrons observed down to <7 keV in a NuSTAR solar microflare
• Compact solar UV burst triggered in a magnetic field with a fan-spine topology
• The First Focused Hard X-ray Images of the Sun with NuSTAR
• The first X-ray imaging spectroscopy of quiescent solar active regions with NuSTAR
• IRIS observations of short-term variability in moss associated with transient hot coronal loops
• NuSTAR hard X-ray observation of a sub-A class solar flare
• NuSTAR Detection of X-Ray Heating Events in the Quiet Sun
• Impulsive coronal heating during the interaction of surface magnetic fields in the lower solar atmosphere
• Observations of the Quiet Sun During the Deepest Solar Minimum of the Past Century with Chandrayaan-2 XSM -- Sub-A Class Microflares Outside Active Regions
• Sphinx measurements of the 2009 solar minimum x-ray emission
• Joint X-ray, EUV and UV Observations of a Small Microflare
• NuSTAR Observation of a Minuscule Microflare in a Solar Active Region
• NuSTAR Observation of Energy Release in Eleven Solar Microflares
• Solar microflares: a case study on temperatures and the Fe XVIII emission
• Observations of the Quiet Sun During the Deepest Solar Minimum of the Past Century with Chandrayaan-2 XSM -- Elemental Abundances in the Quiescent Corona
• Energetics of magnetic transients in a solar active region plage

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• Coronal energy release by MHD avalanches III. Identification of a reconnection outflow from a nanoflare
• First joint X-ray solar microflare observations with NuSTAR and Solar Orbiter/STIX
• Multifrequency microwave imaging of weak transients from the quiet solar corona
• High Resolution Observations of the Low Atmospheric Response to Small Coronal Heating Events in an Active Region Core
• An in-depth analysis of quiet-Sun IRIS Brightenings
• The need for focused, hard X-ray investigations of the Sun
• Thermal Evolution of an Active Region through Quiet and Flaring Phases as Observed by NuSTAR XRT, and AIA
• Detecting non-thermal emission in a solar microflare using nested sampling
• Accelerated particle beams in a 3D simulation of the quiet Sun. Lower atmospheric spectral diagnostics
• Quiet Sun impulsive events observed with NuSTAR during solar minimum