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TEC Regional Map	TEC Disturbance Map	TEC Disturbance Index	GPS L1 Iono Error

Total Electron Content (TEC) is a measure of the total number of electrons in a vertical column of the ionosphere. It is indicative of the average electron density of the ionosphere and is proportional to the delay in transmission of radio frequency signals (such as GPS) through the ionosphere. When there is increased ionisation in the ionosphere caused by enhanced solar radiation or geomagnetic storm conditions, particularly during times of enhanced solar activity, TEC may increase significantly, often in an spatially non-uniform way. This has implications for GPS navigation and satellite communications as well as HF radio communications. For more information on TEC mapping, please refer to the About TEC Mapping page.

S4 Index	S4 Regional Map

The S4 Index is a measure of ionospheric scintillation on trans-ionospheric radio frequency signals. Ionospheric scintillation occurs when a radio frequency signal in the form of a plane wave traverses a region of small scale irregularities in electron density in the ionosphere. Signals from GPS satellites are an example of trans-ionospheric signals affected by ionospheric scintillation, with a loss of tracking of GPS satellites by ground based receivers possible under strong scintillation conditions. Satellite communication, at a range of radio frequencies is also affected. For more information on ionospheric scintillation, please refer to the About Ionospheric Scintillation page.