NuSTAR Observation of a Minuscule Microflare in a Solar Active Region

Cooper, Kristopher; Hannah, I. G.; Grefenstette, Brian W.; Glesener, Lindsay; Krucker, S.; Hudson, H. S.; White, S. M.; Smith, David M.

doi:10.3847/2041-8213/ab873e

Cited by 28 publications

(35 citation statements)

References 31 publications

(54 reference statements)

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“…NuSTAR observations have allowed multiple detailed studies of sub-A-class events in active regions, as well as one paper concerned with three even smaller (GOES∼A0.01) quiet Sun brightenings (Glesener et al 2017;Wright et al 2017;Kuhar et al 2018;Cooper et al 2020). The spectra of events in Glesener et al (2017), Kuhar et al (2018), and Cooper et al (2020) were best fit by isothermal spectral models throughout their evolution, though the Glesener et al (2017) microflare displayed some high-energy excess over this fit during the impulsive phase.…”

Section: Nustar Solar Observationmentioning

confidence: 99%

NuSTAR Observation of Energy Release in 11 Solar Microflares

Duncan

Glesener

Grefenstette

et al. 2021

ApJ

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Solar flares are explosive releases of magnetic energy. Hard X-ray (HXR) flare emission originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, giving insight into flare energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR) utilizes direct-focusing optics to attain much higher sensitivity in the HXR range than that of previous indirect imagers. This paper presents 11 NuSTAR microflares from two active regions (AR 12671 on 2017 August 21 and AR 12712 on 2018 May 29). The temporal, spatial, and energetic properties of each are discussed in context with previously published HXR brightenings. They are seen to display several "large flare" properties, such as impulsive time profiles and earlier peak times in higher-energy HXRs. For two events where the active region background could be removed, microflare emission did not display spatial complexity; differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high-energy excess over a single thermal model in all events. This excess was consistent with additional higher-temperature plasma volumes in 10/11 microflares and only with an accelerated particle distribution in the last. Previous NuSTAR studies focused on one or a few microflares at a time, making this the first to collectively examine a sizable number of events. Additionally, this paper introduces an observed variation in the NuSTAR gain unique to the extremely low livetime (<1%) regime and establishes a correction method to be used in future NuSTAR solar spectral analysis.

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Section: Nustar Solar Observationmentioning

confidence: 99%

NuSTAR Observation of Energy Release in 11 Solar Microflares

Duncan

Glesener

Grefenstette

et al. 2021

ApJ

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“…Some information has been provided with imaging spectroscopy of Bremsstrahlung emission with e.g., NuSTAR and FOXSI-2. A recent NuSTAR observation of a microflare was published by Cooper et al (2020). The microflare was estimated to be approximately equivalent to a GOES 0.005 A-class flare, i.e., much weaker than the 0.1 A class microflares recently observed by FOXSI-2 (Athiray et al, 2020).…”

Section: From Flares To Microflaresmentioning

confidence: 90%

“…An understanding of the physics of microflares remains elusive, as key spatially-resolved spectroscopic observations have been lacking, and given that they have peak temperatures in the 4-8 MK range (see e.g., Feldman et al, 1996;Hannah et al, 2019;Mitra-Kraev and Del Zanna, 2019;Cooper et al, 2020;Vadawale et al, 2021 1 ) which have largely been unexplored by previous and current imaging spectrometers. Consequently, only a few models of microflare loops and associated events have been developed (see e.g., Testa and Reale, 2020;Joshi et al, 2021).…”

Section: From Flares To Microflaresmentioning

confidence: 99%

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High Resolution Soft X-ray Spectroscopy and the Quest for the Hot (5–10 MK) Plasma in Solar Active Regions

Zanna

Andretta

Cargill

et al. 2021

Front. Astron. Space Sci.

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We discuss the diagnostics available to study the 5–10 MK plasma in the solar corona, which is key to understanding the heating in the cores of solar active regions. We present several simulated spectra, and show that excellent diagnostics are available in the soft X-rays, around 100 Å, as six ionization stages of Fe can simultaneously be observed, and electron densities derived, within a narrow spectral region. As this spectral range is almost unexplored, we present an analysis of available and simulated spectra, to compare the hot emission with the cooler component. We adopt recently designed multilayers to present estimates of count rates in the hot lines, with a baseline spectrometer design. Excellent count rates are found, opening up the exciting opportunity to obtain high-resolution spectroscopy of hot plasma.

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“…NuSTAR has observed several B, A, and equivalent sub-A class AR microflares of energies from 10 28 erg down to 10 26 erg (Glesener et al 2017;Wright et al 2017;Hannah et al 2019;Cooper et al 2020;Duncan et al 2021). Quiet Sun brightenings outside ARs have also been observed with thermal energies of 10 26 erg (Kuhar et al 2018).…”

Section: Introductionmentioning

confidence: 96%

NuSTAR observations of a repeatedly microflaring active region

Cooper,

Hannah,

Grefenstette

et al. 2021

Preprint

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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 nonthermal HXR emission during its impulsive phase (of non-thermal power ∼7×10 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.

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NuSTAR Observation of a Minuscule Microflare in a Solar Active Region

Cited by 28 publications

References 31 publications

NuSTAR Observation of Energy Release in 11 Solar Microflares

NuSTAR Observation of Energy Release in 11 Solar Microflares

High Resolution Soft X-ray Spectroscopy and the Quest for the Hot (5–10 MK) Plasma in Solar Active Regions

NuSTAR observations of a repeatedly microflaring active region

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