Authors: Sijie Yu (NJIT), Bin Chen (NJIT), Meiqi Wang (NJIT), Surajit Mondal (NJIT), Dale E Gary (NJIT) and The EOVSA team

The solar extreme ultraviolet (EUV) flux, a dominant driver of heating and ionization in the ionosphere-thermosphere-mesosphere (ITM) system, cannot be observed directly from the ground due to atmospheric absorption. Historically, the radio flux at a wavelength of 10.7 cm (or frequency of 2.8 GHz), known as the F10.7 index, has served as a reliable proxy for solar EUV irradiance, boasting an excellent correlation with EUV since its consistent record began in 1947. While the correlation between the F10.7 flux and solar EUV irradiance remains strong at lower levels of solar activity, it becomes highly nonlinear with increased solar activity. This nonlinearity is partly attributed to the gyroresonance emissions from active regions with strong magnetic flelds, which are included in the F10.7 flux but do not correlate directly with EUV flux. Utilizing advanced solar imaging spectroscopy data from the Expanded Owens Valley Solar Array, our study quantifies the gyroresonance contribution across the solar disk, enabling us to isolate these components from the F10.7 index. With the aid of concurrent multi-band EUV imaging data from SDO/AIA, we have developed a new, image-based daily F10.7 index, termed “F10.7im,” which serves as a more accurate proxy for solar EUV irradiance.