Global thermospheric density and composition response to geomagnetic storms: current understanding and future measurement needs

We note that future applications will require a much greater knowledge of thermospheric density changes — including the ability to predict those changes days ahead of time. It is well recognized that intense geomagnetic storms cause the largest thermospheric density enhancements and changes in composition (e.g. O/N2 ratio decrease/increase and NO density increase). The similarity and difference between O/N2 and NO variations could be explained by the storm-time neutral circulation. Rapid neutral density recovery during geomagnetic storms is attributed to enhanced IR radiation, especially the NO 5.3 μm emission (the NO ‘thermostat’). The storm-time thermospheric response also depends on thermospheric pre-conditioning; whether the NO IR emissions can cause thermospheric overcooling remains to be resolved. Under similar geomagnetic conditions, the thermospheric O/N2 ratio measurements show a larger variation during solar minimum years than that in solar maximum years. In addition to in situ neutral density measurements, optical remote sensing (such as FUV) can provide information on neutral density profiles. The DMSP/SSUSI FUV limb data provide information on neutral density variations at 210 km during the February 3-4, 2022 storm which was connected to the loss of ~40 Starlink satellites, indicating the space weather could significantly increase the operational risk in LEO orbits. However, there are few in situ measurements around 210 km and below. We point out that to fully understand and predict the state of the thermosphere, combined in situ and remote sensing observations are required for global volumetric measurements between 100 and 1000 km. We suggest disposable CubeSats launched on demand from motherships as one of the options for in situ measurements below 250 km.