NOTE: If you are looking for the observed sunspot number derived from optical observations of the sun, you've been miss-routed.
This link provides a comparison of the effective SSN shown on this page to the observed solar sunspot number.

The effective sunspot number (SSNe) index is defined as that SSN which will return a zero average-error between foF2 values generated from a model (to which the SSNe is input) and a set of foF2 observations. The NWRA SSNe (solid lines on the plot) is generated by Northwest Research Associates, Inc., using mid-latitude foF2 observations from a single day as input (provided by the NOAA NCEI) and is based on the URSI-88 model.

The T index (dashed lines) is similar to the SSNe index in concept, although it is calculated using a different algorithm. The index plotted here is the daily value valid for the Australian sector calculated by the Australian Space Weather Services (SWS) organization, Sydney, AU, as reported in their Daily Solar and Geophysical Report. (See references below)

The green dashed line is an SSNe value calculated as described above using foF2 data from stations in the southern hemisphere, defined as in the geomagnetic latitude range of -20 to -55 degrees. This is a more direct comparison with the T index, which is also derived from only southern hemisphere stations. Only the current date range is plotted to cut down on clutter.

References for the T index are as follows:

• Turner, J.F., The development of the ionospheric index T, IPS Series R Report, R11, June, 1968.
• Caruana, J., The IPS monthly T index, Solar-Terrestrial Predictions, Proceedings of a workshop at Leura, Australia October 16-20, 1989, Vol 2, pp 257-263, 1990.
• Wilkinson, P.J., Predictability of ionospheric variations for quiet and disturbed conditions, J. Atmosph. Terr. Phys., 57, 1469-1481, 1995.