2. General Information on the Experiment and Database

[8]  To study the dependence of the absolute and relative amplitude of TEC variations on various geophysical factors for the period from 1999 to 2005, 12 quiet days ( Kp < 3 ) and 40 disturbed days ( Kp > 3 ) were chosen. The dates included in this study, the corresponding maximum and minimum values of Dst and Kp indices, and also the daily mean values of the solar activity index F_10.7 are given in Table 1. The selected data cover mostly fall and spring equinox and a few winter days.

[9]  To obtain statistically significant results, we (in some sense arbitrarily) chose three latitudinal zones with the maximum number of GPS stations presented in the Web. They are high-latitude zone of North America (50-80^oN, 200-300^oE; 59 stations), midlatitude zone of North America (20-50^oN, 200-300^oE; 817 stations), and equatorial zone (20^oS-20^oN, 0-360^oE; 76 stations).

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