Can Emission Measure Distributions Derived from Extreme-ultraviolet Images Accurately Constrain High-temperature Plasma?

Measuring the relative amount of high-temperature, low emission measure (EM) plasma is considered to be a smoking-gun observation to constrain the frequency of plasma heating in coronal structures. Often, narrowband, extreme-ultraviolet images, such as those obtained by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), are used to determine the EM distribution, though the sensitivity to high-temperature plasma is limited. Conversely, the soft X-ray wavelength range offers multiple high-temperature diagnostics, including emission lines of N VII, O VII, O VIII, Fe XVII, Ne IX, and Mg XI, which can provide tight constraints to the high-temperature plasma in the log T = 6.1–6.7 (∼1–5+ MK) range. The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), a slitless spectrograph launched on a NASA sounding rocket on 2021 July 30, resolved an X-ray-bright point in multiple emission lines in the soft X-ray wavelength range. Using coordinated observations of the same X-ray-bright point from SDO/AIA, we compare and contrast the EM distributions from the EUV image data, the X-ray spectra, and the combined EUV and X-ray data set. In this paper, we demonstrate that EM distributions from SDO/AIA data alone can overestimate the amount of high-temperature (log T > 6.4) plasma in the solar corona by a factor of 3–15. Furthermore, we present our effort to cross-calibrate Hinode/X-ray Telescope (XRT) response functions by comparing the observed XRT fluxes with the predicted ones from combined MaGIXS-1 + AIA EM analysis.