The deviations of foF2 from the monthly median, Delta foF2, have been calculated for each UT hour of each month for the years 2002, 2003, 2004 and 2006, for 10 ionosondes in Australia and Papua New Guinea. The linear correlation coefficients between simultaneous (same UT) values of DfoF2 for 45 station pairs have then been calculated for each hour for the 48 months. Since mixing up magnetically quiet and disturbed days can lead to deceptive correlations that correspond to neither quiet nor disturbed days, most of the analysis has been restricted to magnetically quiet days, defined as Ap < 25. On a monthly basis, it is found that the correlation coefficients are significantly lower for months with no magnetically disturbed days. In general, the correlation coefficients decrease with decreasing solar activity, from 2002/2003 to 2006. It is inferred that the deviations that lead to the observed correlation coefficients are due to changes in the solar wind, and the consequent effects on neutral winds, electric fields, temperature, and composition. The magnitudes of the measured correlation coefficients have implications for the F2 region correlation lengths used in global ionospheric models. For the Australian ionosondes and for quiet days, the inferred north-south correlation length would be similar to 1000 km, while the east-west correlation length would be similar to 1500 km, with somewhat lower values for winter and low solar activity. The lower limit is not well defined because the shortest station separation is 857 km. Shorter station separations are considered in a companion paper.
