First Results of a New Inversion Tool for Thermospheric Neutral Mass Density Computations During Severe Geomagnetic Storms

In Low Earth Orbit (LEO), atmospheric drag is the dominant source of trajectory prediction error below approximately 700 km altitude, primarily due to inaccuracies in thermospheric density models. This work presents a method for extracting localized density estimates from Energy Dissipation Rates (EDRs), which can then be utilized to study the storm-time density response and improve thermospheric density modeling. An inversion tool algorithm estimates corrections to the Ap index using the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar Extended model (NRLMSISE-00, commonly referred to as MSIS) as the background density model within an Ensemble Kalman Filter (EnKF) framework. EDRs from calibration satellites, satellites with known and relatively constant ballistic coefficients, are processed individually to invert a time series of density values localized to each satellite's perigee. The performance of the inversion tool is evaluated through comparisons with MSIS, the High Accuracy Satellite Drag Model (HASDM), and accelerometer-derived densities during two periods: 1) two consecutive G4 storms in May 2005 during the declining phase of solar cycle 23 near solar minimum, and 2) the May 2024 Gannon G5 storm near the solar maximum of cycle 25. The inversion tool estimated effective densities show strong agreement with observed effective densities at perigee altitudes ranging from ~200 km to ~750 km. However, discrepancies between the inversion tool perigee densities and those from MSIS and HASDM may reflect limitations in existing models.