Blending of Space-Based and Ground-Based Magnetograms: Application to Combining L5 and Ground-Based Observations for Solar Wind Forecasting

Accurate maps of the solar surface longitudinal magnetic flux (aka “magnetograms”) are key inputs to models of the solar wind and CME transport in the heliosphere. For example, the primary inputs to the WSA/Enlil model used by NOAA’s Space Weather Prediction Center (SWPC) to forecast solar wind conditions and CME arrival time at Earth currently are "synoptic" (full-Sun) magnetic field maps assempled from Global Oscillations Network Group (GONG) magnetograms. But current magnetograms capture only about two-thirds of the Earth-facing hemisphere - the rest of the sphere is invisible, and is thus either extrapolated or modeled, at times resulting in highly inaccurate inputs to solar wind models. Missions to the L5 Lagrangian point are planned in order to increase the observed coverage to about one-half of the solar sphere, but there are currently no magnetogram instruments on planned L1 missions. Is it possible to merge L5 space-based magnetogram data with ground-based SEL data to create accurate increased-coverage synoptic maps of the solar magnetic field?

We address this question by attempting to cross-calibrate and merge space-based magnetogram data with ground-based data to create a "hybrid synoptic map" for input into the WSA/Enlil model. The space- and ground-based magnetogram data are from SDO/HMI and GONG, respectively. Both full-disk magnetograms and merged synoptic maps are analyzed. WSA/Enlil runs for Carrington Rotation 2182 (October 2016) are performed with HMI, GONG, and merged synoptic maps and the predictions compared with ACE in-situ solar wind data. Our results show that instrumental and spatial resolution differences prevent merging L5 and ground-based SEL data to create an accurate synoptic field map unless the spatial resolution is severely degraded. In addition, we show that both space- and ground-based magnetograms taken from the ecliptic plane significantly underestimate the polar flux, in the case of CR2182 causing a blown forecast of the G3 geomagnetic storm on 25 October 2016. We conclude that the only way to provide accurate full-Sun magnetic field data to solar wind models is to measure the entire Sun with multiple, identical, space-based instruments, including polar orbiting missions to ensure that polar flux levels are captured.