The Global Analysis of the Ionospheric Correlation Time and its Implications for Ionospheric Data Assimilation

Ionospheric correlation time is an important parameter that contains information about the temporal variability, structures and dynamics of the ionosphere. This parameter is also important in forecasting of the ionospheric state. Ionospheric data assimilation algorithms that employ empirical background models, such as Ionospheric Data Assimilation Four-Dimensional (IDA4D), apply Gauss-Markov approximation for the propagation of temporal updates from a particular time stamp to the next one. In this process the relaxation parameter, or the correlation time, determines to what degree the projected state depends on the background model and analysis density. An ad hoc approach is usually applied to choose this user-adjustable parameter. This presentation focuses on the estimation of the ionospheric correlation time using high-temporal resolution Global Ionospheric Maps (GIM). It is found that the correlation time changes significantly with latitude. The longest correlation time is observed at equatorial region, whereas the shortest correlation time is observed at high-latitude and polar cap regions. The global distribution of the correlation time exhibits seasonal variation, and depends on the solar flux conditions. The correlation time at the equatorial and mid-latitude regions increases with increasing solar and geomagnetic activity, whereas the correlation time at high-latitude regions decreases. The results of this study can be directly applied to the improvement of the ionospheric data assimilation models.