In-flight cross-calibration of HRI_EUV/EUI and AIA/SDO

Abstract

Context. The extreme ultraviolet High-Resolution Imager (HRI_EUV) of the Extreme Ultraviolet Imager (EUI) telescope on board Solar Orbiter observes the solar corona in an ∼5 Å wide passband near 174 Å with unprecedented high spatial resolution. Aims. We aim to perform radiometric cross-calibration of the HRI_EUV and the Atmospheric Imaging Assembly (AIA) telescope in order to allow further mutual analyses of the observational data. Methods. We applied a differential emission measure analysis using quasi-simultaneous images taken in seven spectral channels -HRI_EUV and six channels of AIA -and compared the real and the simulated images. Results. The comparison suggests that the real HRI_EUV images have ~40% larger signal than the simulated images predicted by the differential emission measure analysis. Conclusions. With our method we cannot conclude which instrument has errors in the absolute calibration, as it can be the case for either of them or both of them simultaneously, but to a lesser degree. However, in order to improve the accuracy of simultaneous data analysis, one needs to take this discrepancy into account. We see that introduction of the HRI_EUV signal into the DEM analysis modifies the warm plasma with 1 MK. The ability of the method to reproduce HRI_EUV images using only the AIA data further validates the underlying assumptions and our approach. Lastly, we believe the approach can be used as a strategy to establish a golden reference of contemporary EUV imagers.