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

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.