The IPS T index is used by ASAPS computer program for predicting HF propagation frequencies. It also appears on many of the formats of IPS predictions and is tabulated each month in the IPS Solar and Geophysical Summary. But many people are confused by the index and a very commonly asked question is "What is the T Index?". We will attempt to answer this question on this page.

For any HF communication circuit, the value of the maximum frequency (called the MUF) reflected back to earth by the ionosphere varies with the time of day, month of the year, and with the solar cycle. At the peak of the cycle (such as in 1988-1992) higher frequencies were reflected by the ionosphere and so HF communications were able to make use of this wider bandwidth. At the low points of the solar cycle (such in 1995) only a small bandwidth is available.

The solar cycle varies over a period of typically 10 to 11 years from trough to peak and then back to another low point. The conventional index of the cycle is known as the sunspot number which is defined from observations of the sun by optical telescopes. This index can be defined on a daily basis but is more normally averaged over a month or a year. The yearly average is mostly used as an indicator of the progress of the solar cycle.

Maximum frequencies for ionospheric reflection are measured by an instrument called an ionosonde. An ionosonde transmits a vertical signal and records all reflections - it is basically a radar for scanning the ionosphere. The highest frequency reflected back from the ionosphere is usually written as foF2, and is essentially the MUF for the circuit.

The sunspot number can be compared with these measurements of ionospheric frequencies and can be used as a predictor of these (and the MUFs for any other circuit). But conditions in the ionosphere are affected by more than just the factors giving rise to the sunspot number - for example, geomagnetic storms can change the ability of the ionosphere to reflect signals. Also, the solar EUV radiation, which actually produces the ionosphere, does not always vary in exact accord with the sunspot number.

The solution is to use an ionospheric index such as the T index. Such an index is derived from observed values of maximum ionospheric frequencies and has the same scale as sunspot number.

To derive an ionospheric index system such as the T index, it is necessary to make extensive observations of the ionosphere over several solar cycles and to then plot the maximum ionospheric frequency against the sunspot number. From this, it is possible to obtain a relationship between frequency and sunspot number. Then, given some recent observations of maximum ionospheric frequencies, the relationship can be used to derive a value of the equivalent sunspot number - this we call the T index and it will generally be slightly different to the real sunspot number because of the effects mentioned earlier.

That is essentially the way that T indices are derived, although there are complications because it is necessary to repeat the process for each time of the day and for each month of the year. Also, each point on earth is different and you need to produce maps.

The T index can be best regarded as an "equivalent sunspot number" - the sunspot number which would best match the observations made by ionosondes. IPS has a large network of ionosondes and can derive the most appropriate T index. This can then be used by IPS customers to get best results for their HF communications.

Material Prepared by Richard Thompson. © Copyright IPS - Radio and Space Services.