From the JPL website: “Global maps of ionospheric total electron content (TEC) are produced in real-time (RT) by mapping GPS observables collected from ground stations. These maps are produced to test real-time data acquisition, monitoring facilities, and mapping techniques. The RT TEC mapping can provide accurate ionospheric calibrations to navigation systems.

These maps are also used to monitor ionospheric weather, and to nowcast ionospheric storms that often occur responding to activities in solar wind and Earth's magnetosphere as well as thermosphere."