The quality of propagation on a HF circuit depends on many factors including the electron content and distribution in the ionosphere, the presence of ionospheric disturbances and irregularities, and the occurrence of short-wave fadeouts. HF propagation conditions may vary with the latitude of the circuit, being generally poorer for higher latitude circuits. Spread F Irregularities in the F region of the ionosphere which scatter radio signals causing a degradation in communications. The MUF is the highest frequency that can be used on an HF circuit and depends on the electron distribution in the ionosphere. Many factors determine the value of MUFs for HF circuits.
Higher indices correspond to higher MUFs.
Indices are often defined over periods of a day, a month or a year and may be defined for a region, a hemisphere, or globally. The IPS index given in the report has the same general scale as the sunspot number and is for the Australian region.