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

Iain G Hannah,Säm Krucker,Lucia Kleint,Hugh S Hudson,David M Smith,Lindsay Glesener,Stephen M White,Brian W Grefenstette

doi:10.3847/1538-4357/ab2dfa

Abstract

Abstract We present the first joint observation of a small microflare in X-rays with the Nuclear Spectroscopic Telescope ARray (NuSTAR), in UV with the Interface Region Imaging Spectrograph (IRIS), and in 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 July 26 23:35 to 23:36 UT, 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 SJI1330 and 1400 Å. The NuSTAR emission in both 2.5–4 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 5.1 MK and 6.2 × 1044 cm−3, which corresponds to a thermal energy of 1.5 × 1026 erg, making it considerably smaller than previously studied active region microflares. No non-thermal 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.

Highlights

Microflares are small releases of stored magnetic energy in the solar atmosphere that heat material and accelerate particles
We presented the smallest microflare seen yet with Nuclear Spectroscopic Telescope ARray (NuSTAR), about an order of magnitude weaker than those previously observed with NuSTAR (Glesener et al 2017; Wright et al 2017) and well beyond the microflares observed with RHESSI (Hannah et al 2008a)
This event is similar in thermal energy to quiet-Sun flares seen with NuSTAR (Kuhar et al 2018); the microflare presented in this paper demonstrates higher-temperature emission and is from an active region

Summary

Introduction

Microflares are small releases of stored magnetic energy in the solar atmosphere that heat material and accelerate particles. It has targeted the Sun several times since the first pointing in late 2014 (Grefenstette et al 2016), and has observed active region microflares as well as quiet-Sun brightenings These microflares are about an order of magnitude weaker than RHESSI could observe, down to an estimated GOES level of ∼A0.1, and showed heating up to about 10 MK, with thermal energies of 1027erg (Glesener et al 2017; Wright et al 2017). The observed blueshifts (upflows) of the Si IV 1403 Å line in these “moss” brightenings (Berger et al 1999) are consistent with RADYN numerical simulations of chromosphere/transition region heating by a beam of accelerated electrons (a power law of non-thermal energy 6 × 1024 erg, with spectral index δ = 7 above a cutoff of EC = 10 keV). The thermal properties found from the NuSTAR spectra are compared to the emission observed by SDO/AIA and GOES/XRS

Observation Overview

IRIS Spectra

NuSTAR Spectrum

Discussions and Conclusions