Spatial correlations of foF2 deviations and their implications for global ionospheric models: 2. Digisondes in the United States, Europe, and South Africa

The deviations of foF2 from the monthly median, Delta foF2, have been calculated for each UT for a limited selection of months and times at solar minimum using ionograms recorded by Digisondes in the United States, Europe and South Africa. The linear correlation coefficients between simultaneous (same UT) values of Delta foF2 have then been calculated, with a view to determining the correlation lengths that are important for global ionospheric models. The results presented here are for station separations ranging from 66 km to several thousand kilometers, and complement those given in a companion paper that used values of foF2 from Australian ionosondes, the closest of which were 857 km apart. The correlation lengths derived from the two sets of data are used to specify midlatitude correlation lengths that can be used with global ionospheric models for different levels of solar activity.