Longitudinal structure in the CHAMP electron densities and their implications for global ionospheric modeling

Detailed examination of the electron densities in the equatorial ionosphere observed by the CHAMP satellite at ∼400 km and mapped for a fixed local time has revealed a 90°‐wide longitudinal structure with a maximum to minimum ratio that can exceed two or more. To date, the role of these large density variations has not been considered in either the development or validation of current global assimilative models. The best defined structure has nodes near 60°, 150°, 240°, and 330°, and is variously called the 4‐node or wave number 4 structure. Unless a global model includes the physics that leads to the 4‐node structure, or has the structure imposed upon it by the assimilation of data that includes the structure, a very significant part of the physics of the equatorial ionosphere will be missing. The version of the Utah State University (USU) Global Assimilation of Ionospheric Measurements (GAIM) model currently used by the U.S. Air Force takes the Ionospheric Forecast Model (IFM) as its background ionosphere. The IFM does not produce a 4‐node structure. However, USU‐GAIM is driven mainly by the assimilation of slant GPS total electron content observations, which could impose a 4‐node structure, given sufficient equatorial GPS sites. The UV radiances from the Special Sensor Ultraviolet Spectrographic Imager instrument on DMSP also have a 4‐node structure, and this is maintained in the GAIM global specifications. In the absence of sufficient assimilated data to impose a 4‐node structure, such a structure could be imposed on the IFM by using a model of the equatorial E × B drift that contains a 4‐node structure.