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.
HF Systems