Ionospheric T Index
The values of the maximum frequency reflected by the ionosphere varies with the solar cycle. At the peak of the cycle higher frequencies are reflected and so HF communications can make use of this wider bandwidth. At the low points of the solar cycle only a small bandwidth is available.
The solar cycle varies over a period of typically 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.
The sunspot number can be compared with ionospheric maximum frequencies and can be used as a predictor of these. But the ionosphere responds to more than just the sunspot number - geomagnetic storms can affect the ionosphere. Also, the EUV flux from the sun (which actually produces the ionosphere) does not necessarily 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 frequencies and is scaled onto the sunspot number - maximum frequency relationship.
A typical process would be: Observe the ionosphere over several solar cycles and plot the maximum frequency versus sunspot number. This gives basically a linear relationship although there is sometimes evidence of saturation at higher sunspot numbers. Fit a linear relationship to this data.
Then, given some recent observations of maximum frequencies, use the relationship to derive a value of the equivalent sunspot number - this we call the T index and it will in general be slightly different to the real sunspot number because of the effects mentioned above.
That is essentially the process - there are complications because it is necessary to do this for each time of the day and for each month of the year. Also, each point on earth is different and a mapping process is needed.