System forcing parameters over two of the five simulation days. “B” is the magnitude of the solar wind magnetic field in the YZ plane, “angle” is the angle of the solar wind magnetic field in the YZ plane, “SW speed” is the solar wind speed, “SW density” is the number density of the solar wind, activity level, and hemispheric power are descriptors of the intensity and spatial extent of the statistical particle precipitation pattern. The observed system forcing parameters used in the reference run is shown in blue. During large gaps of missing data, the previous valid value is repeated. The orange line shows the inferred forcing obtained from the assimilation of GRACE-A, GRACE-B, and CHAMP neutral density measurements into the special member. The estimated forcing from one assimilation time step is used over the next. The inferred drivers are not necessarily physically realistic but instead represent a set of drivers that produce the best agreement with observations.