Characterizing Far-side Magnetogram EUV Irradiance Forecasts with EUV Irradiance Measurements Made from Mars

Abstract:

Solar extreme ultraviolet (EUV) irradiance is the primary energy input into the Earth's thermosphere and ionosphere. Its highly variable nature drives corresponding variability in the upper atmosphere, which can adversely affect low earth orbiting satellites, as well as communication and navigation signals. As a result, there is much interest in improving forecasting methods for EUV irradiance. One recent and promising technique forecasts EUV irradiance using magnetic flux transport models (MFTMs), which predict the time evolution of solar magnetic flux from some state determined from magnetograph measurements. Solar irradiances are derived from the magnetic flux predictions using pre-determined empirical relationships between magnetic flux and the solar irradiances of interest. A magnetograph located off of the Earth-Sun line-of-sight (LOS) with view of the Sun's far-side would provide advanced knowledge, which would, in principle, improve EUV irradiance forecasts. This study leverages calibrated EUV irradiance measurements made by the EUVM instrument onboard the MAVEN probe, currently orbiting Mars, to validate EUV irradiance forecasts for Mars that are derived from MFTM-based EUV irradiance predictions initialized by Earth-based magnetograms. This method directly characterizes the performance of potential far-side magnetogram measurements for forecasting EUV irradiance at Earth.

In this study, the three MAVEN EUVM channels, which measure ~5-nm wide bands near 121.6 nm, 19.5 nm and 3.5 nm, are first calibrated against magnetograms from the GONG network using two established magnetic indices, representing the disk-integrated strong and weak flux components. This is achieved by proxy, using analog measurements made from Earth orbit made by the SDO EVE and SORCE SOLSTICE instruments that the MAVEN bands are calibrated against. The Air Force Data Assimilative Photospheric Flux Transport (ADAPT) synchronic maps are then used to predict the full-disk magnetic flux as viewed from Mars for a given day, which are then used to predict the irradiances of MAVEN EUVM bands at Mars. We compare these predictions with EUVM measurements to provide an upper bound on the error associated with using a far-side magnetograph for irradiance forecasts at Earth.