Capturing the Storm-Time F-Region Ionospheric Response in an Empirical Model

Understanding the response of the thermosphere and ionosphere to geomagnetic disturbances has reached a point where some of the F-region ionospheric characteristics can be captured in an empirical model. The integrated effect of magnetospheric energy injection during a storm disturbs the thermospheric temperature, global circulation, and neutral composition. The composition modifications accumulate over tens of hours, have a particular seasonal/latitude structure, and are very slow to recover. The long-lived reaction of the neutral upper atmosphere produces an ionospheric response that is consistent from storm to storm, and so enables the characteristics to be captured by an empirical model. The goal of this first empirical storm-time model is to establish a correction to the F-region peak density, or critical frequency, as a function of season and latitude for any given a(p) time history of a storm. Guided by the knowledge gained from previous data analysis, and from simulations with a physically-based model, observations of the F-region peak density from available sites and from many storms have been sorted by latitude and season, and by the magnitude of the storm. A coherent picture begins to emerge particularly in the summer and equinox mid-latitudes. Several features are still unable to be included in the empirical model, although they are clearly important, and can be simulated in physical models. These include the local-time dependence, and the dynamic response to transient large-scale gravity waves. The latter in particular requires accurate knowledge of the spatial and temporal variation of the geomagnetic sources.