The T index is an indicator of the highest frequencies able to be refracted from regions in the ionosphere. The higher the T index, the higher the frequencies able to be refracated from an ionospheric region. The index is based on the measurement of ionospheric foF2 obtained from ionograms. Following geomagnetic activity the typical ionospheric response at mid latitudes is to become depressed, this will result in a drop in the value of the T index produced from ionospheric stations within the depressed ionospheric region. The T Index is also forecast by IPS and is used in the prediction of point to point frequencies in HF communication.

T Index Product	Application
T Index Map	Shows the variation in ionospheric support level in the Australian region for the current hour. Moderate level geomagnetic activity can depress southern region (roughly below 30 degrees) ionospheric support levels, stronger geomagnetic activity can depress the entire region.
T Index Plot	A plot of the daily T Index for the Australian Region. This plot can be used to identify past disturbed days, where the index can be much lower than the average level. This can be useful if you had trouble communicating on a given day and you are investigating why you had difficulty.
Real Time T Indices	These T Indices apply when you are making a prediction using ASAPS or the on line tools and are using it immediately. These indices will fluctuate as they are based on real time foF2 measurements and will account for ionospheric storms.
Monthly T Indices	These T Indices apply when you are making a prediction using ASAPS or the on line tools and are planning to use the prediction later on (ie on a monthly basis). These predictions will not account for ionospheric storms.
T Index FAQ