A Calibration and Data Assimilation Approach to Use GRACE, GRACE-FO and Swarm Accelerometer Measurements for Forecasting Global and Multi-level Thermospheric Neutral Density Fields

An accurate simulation global thermospheric neutral density (TND) on various altitudes is important for geodetic and space weather applications. In addition, this is essential for designing the low-Earth-orbit (LEO) missions, predict their missions’ lifetime and performing a reliable attitude control. Although empirical and physics-based models typically simulate TND variations, the quality of these models is limited due to various structural simplifications and the uncertainty of inputs. Here, we present an ensemble Kalman filter (EnKF)-based calibration and data assimilation (C/DA) technique that updates the model's states and simultaneously calibrates its key parameters. The proposed approach provides the opportunity to improve the now-cast and forecast skills of the NRLMISISE-00 and NRMSIS-2.0 models through re-calibrating the model’s key parameters including those controlling the influence of solar radiation and geomagnetic activity as well as those related to the calculation of exospheric temperature.

In this research, TND estimates from on-board accelerometer measurements of GRACE, GRACE-FO and Swarm are ingested as observations into the NRLMSISE-00 and NRLMSIS-2.0 models based on the C/DA. The newly calibrated model, called here ‘C/DA-NRLMSISE’, is then used to simulate global maps of TND as well as individual neutral mass densities covering the altitudes of 300-600 km. Various investigations are performed to test the temporal and vertical consistency of the TND outputs from C/DA-NRLMSISE.