Solar microflares: a case study on temperatures and the Fe XVIII emission

In this paper, we discuss the temperature distribution and evolution of a microflare, simultaneously observed by Hinode's X-Ray Telescope (XRT), its Extreme Ultraviolet Imaging Spectrometer (EIS), as well as the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO). Using EIS lines, we find that during peak emission the distribution is nearly isothermal and peaked around 4.5 MK. This temperature is in good agreement with that obtained from the XRT filter ratio, validating the use of XRT to study these small events, invisible to full-Sun X-ray monitors such as the Geostationary Operational Environmental Satellite (GOES). The increase in the estimated Fe XVIII emission in the AIA 94 Å band can mostly be explained with the small temperature increase from the background temperatures. The presence of Fe XVIII emission does not guarantee that temperatures of 7 MK are reached, as is often assumed. With the help of new atomic data, we also revisit the temperatures measured by a Solar and Heliospheric Observatory (SoHO) Solar Ultraviolet Measurements of Emitted Radiation (SUMER) observation of an active region that produced microflares, also finding low temperatures (3-4 MK) from an Fe XVIII/Ca XIV ratio.