Thermospheric damping response to sheath-enhanced geospace storms

We show evidence that solar wind density enhancements and pressure pulses can lead to intense low-energy particle precipitation and an associated, but unexpected, damping of thermospheric density response. Ground-based indices, used as proxies for thermospheric energy deposition, fail to capture these interactions in forecasting algorithms. Superposed epoch comparison of a group of poorly specified neutral density storms suggests an event-chain of multi-hour, pre-storm solar wind density enhancement, followed by solar wind dynamic pressure pulses that trigger excess low-energy particle flux to the upper atmosphere; enhanced production of thermospheric Nitric Oxide (NO) by precipitating particles and storm heating; NO infrared cooling and damping of the thermosphere; and mis-forecast of neutral density. In the control storms, these features are absent or muted. We discuss the roles of solar wind pre-conditioning and solar cycle dependency in the problem storms. These problem neutral-density storms reveal an element of "geo-effectiveness" that highlights competition between hydrodynamic aspects of the solar wind and other interplanetary drivers.