Vector Spherical Harmonics for Data‐Assimilative Neutral Wind Estimation

In this work we enable the data assimilation algorithm Estimating Model Parameters from Ionospheric Reverse Engineering (EMPIRE) to estimate global neutral winds. EMPIRE corrects the ion drifts and neutral winds from background models using electron density in the F region derived mainly from total electron content measurements. The new EMPIRE basis functions for the neutral winds are vector spherical harmonics, enforcing the field to be smooth and continuous globally. Global basis functions allow us to estimate the horizontal wind vector from time‐varying plasma densities. The geomagnetic storm on 25 October 2011 is studied to investigate the global implementation of neutral wind estimation. During this storm, the estimates are also compared to those of the old method of estimation using power series and to Fabry‐Perot interferometer (FPI) measurements for validation at three different sites: Pisgah, Cariri, and Nasca. The new global estimation is 10%–20% closer to the FPI measurements than the model and than the old method for most of the viewing directions of the sites, in terms of root‐mean‐square residuals. In the northward direction at Pisgah the estimates using either the old or new implementation disagree with the model, and might be related to an insufficiently small temporal scale used in EMPIRE. Plain Language Summary The study of the ionosphere dynamics during geomagnetic storms is a challenge. The charged particles in this region of the upper atmosphere drift in response to electric and magnetic fields. The ions can also interact with neutral particles, which are driven by pressure and temperature variations. This study investigates the estimation of the ionospheric drivers using measurements of charged particle density during a geomagnetic storm. We analyze how the estimation of global neutral winds compares to measurements of neutral velocity, to model values and to regional estimation speeds at three locations. We find that the global estimation, besides imposing continuity over the globe, usually agrees comparably or better with the measurements than either the regional estimation or the model values of the neutral winds do. Key Points We use vector spherical harmonics to estimate global neutral winds from total electron content‐derived density rates We find results usually agree better with Fabry‐Perot interferometer measurements than the climate model HWM14 during a geomagnetic storm Root‐mean‐square errors are comparable to or lower tha