Global electron density distribution specified by GAIM and observed by COSMIC 
satellites 

Clayton Coker, Patrick B. Dandenault, Kenneth F. Dymond, Scott A. Budzien and 
Damien Chua 
Space Science Division, Naval Research Laboratory, Washington, DC, USA; 
clayton.coker@nrl.navy.mil 
 
Abstract 
 
Global patterns of ionospheric electron density distribution in the equatorial 
anomaly have been observed by low-earth orbiting satellites. The observed 
4-cell, longitudinal variation of the equatorial anomaly has been attributed to 
non-migrating tides with a source mechanism originating from deep convection in 
the troposphere.  The Global Assimilation of Ionospheric Measurements (GAIM) 
Model ingests ground-based GPS and ionosonde data, and a limited set of 
space-based data in situ electron density data from Defense Meteorological 
Satellite Program (DMSP) satellites.  Projection of GAIM results in a fixed 
local time similar to the low-earth orbiting satellites also reveals the 4-cell, 
non-migrating tidal pattern in the equatorial anomaly.  These patterns are 
reproduced primarily by the data coverage of ground-based GPS receivers and the 
spatial/temporal correlations employed by the model.  Data from the low-earth 
orbiting Constellation Observing System for Meteorology, Ionosphere, and Climate
(COSMIC) satellites are used to validate the GAIM ionospheric specification 
of the equatorial anomaly.  COSMIC Tiny Ionospheric Photometer (TIP) data 
are used to examine the longitudinal and latitudinal distribution of electron 
density.  COSMIC GPS radio occultation (RO) data are used to examine the 
distribution of electron density with altitude at low and midlatitudes.  
These comparisons reveal the strengths and limitations of GAIM, in its current 
implementation, for specifying the ionosphere and global patterns in the 
equatorial anomaly.