An improved phenomenological model of ionospheric density.

A global representation of the large scale variations of ionospheric electron density with the annual, diurnal and solar activity cycles is described. A phenomenological model requiring minimal computer storage space is constructed from monthly-averaged hourly ionospheric sounding data from some 50 stations during the epoch 1957-1970 provided by World Data Center (A). The model describes noontime F2-layer critical frequencies to better than 2 MHz average maximum error and to better than 0.5 MHz root mean squared error and is particularly suitable for applications in space communication and ionosphere-atmosphere coupling studies. Magnetic dip angle and north-south asymmetry effects are incorporated in the model. It is shown that a 'secular' variation of the ionospheric density exists in the epoch 1960-1971. The basis of comparison with data is broadened to include the 1958-1968 chronologies of 18 stations in addition to the original samplings of 50 stations. In addition, a modeling of the polar ionospheres is included.