Assessment of GPS global ionosphere maps (GIM) by comparison between CODE GIM and TOPEX/Jason TEC data: Ionospheric perspective

We performed a comprehensive comparison between GPS global ionosphere map (GIM) and TOPEX/Jason (T‐J) total electron content (TEC) data for the periods of 1998–2009 in order to assess the performance of GIM over the global ocean where the GPS ground stations are very sparse. Using the GIM model constructed by the Center for Orbit Determination in Europe at the University of Bern, the GIM TEC values were obtained along the T‐J satellite orbit at specific locations and times of measurements and then binned into various geophysical conditions for direct comparison with the T‐J TEC. On the whole, the GIM model was able to reproduce the spatial and temporal variations of the global ionosphere as well as the seasonal variations. However, the GIM model was not accurate enough to represent the well‐known ionospheric structures such as the equatorial anomaly, the Weddell Sea Anomaly, and the longitudinal wave structure. Furthermore, a fundamental limitation of the model seems to be evident in the unexpected negative differences (i.e., GPS < T‐J) in the northern high‐latitude and the southern middle‐ and high‐latitude regions in comparison with the T‐J TECS. The positive relative differences (i.e., GIM > T‐J) at night represent the plasmaspheric contribution to GPS TEC, which is maximized, reaching up to 100% of the corresponding T‐J TEC values in the early morning sector. In particular, the relative differences decreased with increasing solar activity, and this may indicate that the plasmaspheric contribution to the maintenance of the nighttime ionosphere does not increase with solar activity, which is different from what we normally anticipate.