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

## TL;DR

This study presents the first simultaneous X-ray, UV, and EUV observations of a small solar microflare, revealing its thermal properties and chromospheric impact, and demonstrating the multi-wavelength complexity of microflare phenomena.

## Contribution

It provides the first joint multi-wavelength observation of a small microflare, combining X-ray, UV, and EUV data to analyze its thermal and chromospheric characteristics.

## Key findings

- Microflare peaks observed in X-ray, UV, and EUV wavelengths.
- Thermal spectrum fitted with 5.8 MK temperature and 10^26 erg energy.
- No non-thermal emission detected, possibly due to limited exposure.

## 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 allows us to study the microflare's hot coronal and cooler chromospheric/transition region emission. This small microflare peaks from SOL2016-07-26T23:35 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 SJI1330{\AA} and 1400\AA. The NuSTAR emission in both 2.5-4 keV and 4-6 keV, is located in a small 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 andO 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 5.8MK and $2.3\times10^{44}$ cm$^{-3}$, which corresponds to a thermal energy of $10^{26}$ erg, making it considerably smaller than previously studied X-ray 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 and EUV, can remarkably have a higher energy component that is clear in X-rays.