Thermospheric Density Disturbances During the 2015 St. Patrick's Day Geomagnetic Storm: Model–Observation Comparisons and Analysis of Intense Polar Density Spikes

Abstract Drag affecting satellites in low Earth orbit varies significantly during geomagnetic storms due to changes in thermospheric density. While neutral density can be inferred from satellite measurements, such data sets are limited and restrict neutral density/orbit prediction capabilities. Neutral density predictions can be produced by upper atmospheric models, however the assortment of models have varying accuracies under varying conditions. It is therefore important that models are tested and validated. In this study, Swarm accelerometer‐derived density measurements for the 2015 St. Patrick's Day geomagnetic storm are compared to four models; three empirical and one physics‐based. Overall, JB2008 reproduced Swarm neutral densities better than other models. Additionally, observations showed localized enhancements/spikes in neutral density at high latitudes, similar to those observed by previous satellite missions, that were not reproduced by any of the models. Further investigation into these enhancements revealed a clear separation of auroral and polar density enhancements/spikes. All auroral spikes occurred directly above the electrojet regions, based on coincident field‐aligned current data, and the polar spikes exhibited a close association with enhancements in the Polar Cap indices/current data. It is concluded that, while both localized auroral and polar density enhancements are caused by Joule heating, the source of the associated electric fields is different; the polar spikes are the result of electric fields directly from the solar wind, whereas the auroral spikes are the result of magnetospheric electric fields. Improved understanding of these storm‐time thermospheric density variations will help modeling efforts that are becoming increasingly necessary for low‐Earth orbit activities.