Duration of an ionospheric data assimilation of a coupled thermosphere-ionosphere model

Initial conditions provide a critical input for accurate numerical forecast models in meteorology and oceanography. In this paper, we address this problem in space weather forecast models of the thermosphere-ionosphere system by using the electron densities from the Global Assimilation of Ionospheric Measurements (GAIM) model to initialize the ionospheric part of the Thermosphere Ionosphere Nested Grid (TING) model. The electron densities from the GAIM-initialized TING model (G-TING) are compared with the output from the stand-alone TING model (S-TING) for geomagnetically quiet and disturbed times in the early April 2004 period in order to observe how long the effects of the initialization would last. Our study shows that the e-folding time of the initialization is about 2-3 hours for most conditions, although this result would probably be different if the initialization for the thermosphere is also included. However, this relaxation time displays significant variations with latitude, local time, and height, and it may also depend on the initial electron density differences between G-TING and S-TING. Furthermore, positive (G-TING > S-TING) and negative (G-TING < S-TING) density differences have different time durations of the initialization effects. Our study also indicates that there is little variation of the relaxation time with the geomagnetic activity despite the impact of geomagnetic storms on the thermosphere-ionosphere system.