Ionospheric total electron content: Global and hemispheric climatology

A new general linear model of the climatology of the ionosphere's total electron content (TEC) is described, accounting simultaneously for the influences of solar and geomagnetic activity, oscillations at four frequencies and a secular trend. The model captures more than 98% of the variance in the daily averaged, global TEC derived from GPS observations during the 16 years from 1995 to 2010, and enables the reconstruction of TEC variations since 1950. Solar EUV irradiance variations, the dominant ionospheric influence, directly increase TEC by as much as 40 TECU from solar activity minimum to maximum and produce additional 27-day fluctuations of as much as 15 TECU (in October 2003). Semiannual and annual oscillations in TEC are comparable in magnitude to the 27-day fluctuations, with (peak to valley) amplitudes that increase from a few TECU at low solar activity to similar to 17 TECU during solar activity maximum. The phase and amplitude of the semiannual oscillation are identical in the northern and southern geographic hemispheres (and hence globally). In contrast, the annual oscillation is twice as large in the southern hemisphere (where it peaks in December-January) than in the northern hemisphere (where it peaks in April-May). Seasonal, semiannual and annual anomalies in TEC are direct effects of semiannual and annual oscillations produced by orbitally driven photoionization and thermospheric composition changes, not of corresponding oscillations in solar or geomagnetic activity. Geomagnetic influences on daily averaged global TEC are relatively modest, with the maximum effect a reduction of 11 TECU (in October 2003) and only 11 episodes in excess of 5 TECU depletions during the past 16 years.