Joint X-Ray, EUV, and UV Observations of a Small Microflare

Abstract

We present the first joint observation of a small microflare in X-rays with the Nuclear Spectroscopic Telescope ARray (NuSTAR), UV with the Interface Region Imaging Spectrograph (IRIS) and EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). These combined observations allow us to study the hot coronal and cooler chromospheric/transition region emission from the microflare. This small microflare peaks from 2016 Jul 26 23:35 to 23:36UT, in both NuSTAR, SDO/AIA and IRIS. Spatially this corresponds to a small loop visible in the SDO/AIA Fe XVIII emission, which matches a similar structure lower in the solar atmosphere seen by IRIS in SJI1330Å and 1400Å. The NuSTAR emission in both 2.5-4 keV and 4-6 keV, is located in a source at this loop location. The IRIS slit was over the microflaring loop, and fits show little change in Mg II but do show intensity increases, slight width enhancements and redshifts in Si IV and O IV, indicating that this microflare had most significance in and above the upper chromosphere. The NuSTAR microflare spectrum is well fitted by a thermal component of 5.1MK and 6.2 × 10 44 cm −3 , which corresponds to a thermal energy of 1.5 × 10 26 erg, making it considerably smaller than previously studied active region microflares. No nonthermal emission was detected but this could be due to the limited effective exposure time of the observation. This observation shows that even ordinary features seen in UV can remarkably have a higher energy component that is clear in X-rays.

NuSTAR Observation of a Minuscule Microflare in a Solar Active Region

“…The event excess was fitted with a temperature of 6.7MK and an emission measure of 8.0×10 43 cm −3 . The temperature is similar, or slightly hotter, to those found from other weak microflaring events, whereas the emission measure is an order of magnitude smaller (Glesener et al 2017;Hannah et al 2019).…”

“…In addition, this volume estimate does not consider a loop-filling factor, making the thermal energy estimate an upper limit. This thermal energy value is lower than the previous smallest observed NuSTAR microflare (Hannah et al 2019), which was cooler but with a higher emission measure and had a GOES class of A0.02. EUV observations of magnetically braided loops observed heating with thermal energy of about 10 26 erg (Cirtain et al 2013); however, this was for material up to 4MK.…”

“…The fit gives a temperature of 3.2MK and an emission measure of 1.7×10 46 cm −3 . These are typical values of quiescent/nonflaring ARs measured by NuSTAR (Glesener et al 2017;Wright et al 2017;Hannah et al 2019). However, the livetime recorded throughout this event is significantly larger than those previously studied (∼15% compared to 1-5%), resulting in a better sensitivity, and hence constraint, on hotter material.…”

“…Since the first solar NuSTAR observations in 2014 (see Grefenstette et al 2016;Hannah et al 2016;Kuhar et al 2017), solar activity has decreased allowing sub-A class microflares to be observed regularly within ARs (Glesener et al 2017(Glesener et al , 2020Wright et al 2017;Hannah et al 2019) and small brightenings outside of ARs (Kuhar et al 2018). 8 The AR microflares observed by NuSTAR have been found to have thermal energy releases down to 10 27 erg with quiet Sun brightenings having energies down to 10 26 erg.…”

Accelerated Electrons Observed Down to <7 keV in a NuSTAR Solar Microflare

“…However, the most advanced solar HXR spacecraft instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft, was limited in its sensitivity to small flares due to its indirect imaging method, although GOES class A microflares could be observed Hannah et al 2008). The recent advent of directly focusing HXR instruments in the form of the Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft and the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket have enabled the observation of small microflares orders of magnitude fainter than those observed by RHESSI (Krucker et al 2014;Glesener et al 2016Glesener et al , 2017Wright et al 2017;Kuhar et al 2018;Hannah et al 2019;Athiray et al 2020;J. T. Vievering et al 2020, in preparation).…”