Click here for the latest version of the above picture (but note that the hole may have disappeared or moved in later updates).
Here is some information from the NASA-JPL site about these maps:
Global Ionospheric (TEC) Maps
GPS users with single-frequency receivers need ionospheric electron content information in order to achieve positioning accuracy similar to dual-frequency receivers. The GDGPS System provides a global real-time maps of ionospheric electron content (currently producing a map every 5 minutes). These maps are also of value in monitoring the effect of the ionosphere on radio signals, power grids, and on space weather.
The maps are derived using data from the ~100 real-time GDGPS tracking sites, ougmented with additional sites that are available on an hourly basis, probing the ionosphere with high spatial and temporal resolution. The integrated electron density data along each receiver-GPS satellite link is processed through a Kalman filter in a sun-fixed frame to produce the global gridded maps of TEC.
The maps are available redundantly from multiple GDGPS Operations Centers (GOCs) as images, as text files containing the gridded TEC values, or as a binary data stream containing the gridded TEC values.
TEC values from these maps are used in correcting the ionospheric delay in arbitrary geographic localtions on Earth for the Virtual Reference Site (VRS) data product of the GDGPS System.