Nowcasting Solar EUV Irradiance With Photospheric Magnetic Fields and the Mg II Index

A new method to nowcast spectral irradiance in extreme ultraviolet (EUV) and far ultraviolet (FUV) bands is presented here, utilizing only solar photospheric magnetograms and the Mg II index (i.e., the core-to-wing ratio). The EUV and FUV modeling outlined here is a direct extension of the SIFT (Solar Indices Forecasting Tool) model, based on Henney et al. (2015, ). SIFT estimates solar activity indices using the earth-side solar photospheric magnetic field sums from global magnetic maps generated by the ADAPT (Air Force Data Assimilative Photospheric Flux Transport) model. Utilizing strong and weak magnetic field sums from ADAPT maps, Henney et al. (2015, ) showed that EUV & FUV observations can also be well modeled using this technique. However, the original forecasting method required a recent observation of each SIFT model output to determine and apply a 0-day offset. The new method described here expands the SIFT and ADAPT modeling to nowcast the observed Mg II index with a Pearson correlation coefficient of 0.982. By correlating the Mg II model-observation difference with the model-observation difference in the EUV & FUV channels, Mg II can be used to apply the 0-day offset correction yielding improvements in modeling each of the 37 studied EUV & FUV bands. With daily global photospheric magnetic maps and Mg II index observations, this study provides an improved method of nowcasting EUV & FUV bands used to drive thermospheric and ionospheric modeling. Ultraviolet irradiance from the Sun can create variability in Earth's atmosphere and cause problems, for example, with satellite communication and their orbital paths. However, we are limited in measuring solar ultraviolet irradiance since it must be observed from space and therefore models of the irradiance are important. In this paper, we present an improved way to model ultraviolet irradiance using solar magnetic fields and a well-calibrated solar activity proxy. We find that models of the irradiance improve when the proxy is used to correct daily variations compared to models driven using just the magnetic fields. Improved nowcast models for commonly used extreme ultraviolet (EUV) and far ultraviolet (FUV) solar irradiance bands Utilization of well-calibrated MgII observations decreases the error between the modeled and observed EUV values Real-time EUV observations are not required to correct and improve the EUV irradiance models