Response of Ionosphere and Thermosphere to the 2015 St. Patrick's Day Storm using high-resolution WAM-IPE

Being a highly coupled system, the response of the Earths ionosphere to the solar geomagnetic disturbance is affected by the variability and the prior state of the thermosphere. The thermosphere is in turn significantly influenced by the mesoscale and day-to-day variability from the lower atmosphere. In this study, our goal is to understand how the mesoscale internal variability from the Earths lower atmosphere affects the response of the ionosphere thermosphere (IT) system to a geomagnetic storm. We use the Whole Atmosphere Model (WAM) to investigate the IT response to the St. Patricks day geomagnetic event, 17 March 2015. Whole Atmosphere Modeling provides a unique way to study the Earths lower and upper atmosphere together as an integrated system, which is not possible through empirical or the numerical IT models. WAM is built upon the Global Forecast System model, extending from the surface till 600 km in the thermosphere. We compare the storm time responses for different resolutions of WAM, T62 vs. T254. The differences between the runs are larger in the lower thermosphere owing to the natural variability in the T254 run, and decrease with altitude because of the dominance of external drivers. The T254 run because of more mesoscale mixing has lower thermospheric O/N2 before the storm onset which affects its response. A high latitude depletion and low latitude accumulation of O/N2 is observed in the T62 run, whereas the T254 run shows depletion in both the regions, along with a larger increase in the global temperature at storm onset. Moreover, storm time wave perturbations propagating from high latitudes due to Joule heating are also observed in the thermosphere of the T254 run. Ionospheric responses during this event are also investigated using the high-resolution Ionosphere Plasmasphere Electrodynamics (IPE). These results are then compared with different observational datasets. Our study emphasizes the importance of including lower atmospheric variability for better predicting the Earths space weather.