Observed and modelled neutral densities during the 2015 St. Patrick’s Day geomagnetic storm

Countless processes are occurring in Earth’s upper atmosphere constantly, many of which are not fully understood. An increase in solar and geomagnetic activity can add to, and even change, these processes, which can exacerbate their effects. The neutral density of the atmosphere is one factor that varies according to these processes and can, in turn, have its own impact on satellites in low Earth orbit. In situ measurements of neutral density in the upper atmosphere are scarce, however a small number of satellites have been used for this. Another option is to use upper atmospheric models to estimate neutral density, which has the advantage of predicting on a global scale, but with the drawback of an unknown level of accuracy. To quantify models’ neutral density accuracy, validation against satellite data is commonly performed. In this study, Swarm A and Swarm C satellite neutral densities are used to validate outputs from the physical TIE-GCM and the empirical NRLMSISE-00, JB06, and JB08 for the 2015 St. Patrick’s Day geomagnetic storm. Reliability of Swarm C density data is discussed. One result of the analysis is that both satellites measure intense spikes in neutral density over polar regions during the early stages of the storm, which are not reproduced within the models. The physics responsible for these intense high-latitude density spikes is explored in the context of the variable high-latitude plasma convection pattern, and its relationship to Joule heating during geomagnetic storms.