Climatology of the mean total electron content derived from GPS global ionospheric maps

Abstract

We analyzed 11 years' (1998–2008) worth of the total electron content (TEC) data derived at the Jet Propulsion Laboratory (JPL) from Global Positioning System (GPS) observations to investigate the overall climatological features of the ionosphere in a new way. The global ionospheric maps (GIM) of JPL TEC are averaged globally and over low‐, middle‐, and high‐latitude ranges in the southern (northern) hemisphere and both hemispheres to identify their capability of capturing the overall features of the ionosphere. These mean TEC data show strong annual/semiannual, solar cycle, and 27‐day variations. The mean TEC presents stronger solar activity sensitivity at lower‐latitude bands. Moreover, the saturation effect exists in these mean TEC versus solar index F10.7, more pronounced at low latitudes, while the mean TEC increases faster with higher solar EUV fluxes, being evident at high latitudes. The annual asymmetry (differences in June and December solstices) can be detected in the mean TEC averaged globally and at low latitudes under all solar epochs as well as at middle and high latitudes under most solar activities. The hemispheric asymmetry of the TEC in conjugate hemispheres follows the control of solar declination. Both the hemispheric differences and annual asymmetry are more marked with increasing solar activity. The annual components of the mean TEC are stronger in the southern hemisphere, and the semiannual components are of similar phases and comparable amplitudes in conjugate hemispheres, which suggest close couplings of the ionosphere in both hemispheres. Further, the mean TEC averaged in one hemisphere can reliably be used as ionospheric indices to monitor the solar activity variabilities and to capture the overall climatological features of the ionosphere over specified regions, while it should be cautioned that the mitigation of the dominant annual components with opposite phases in conjugate hemispheres leaves a significant semiannual component in the mean TEC averaged in both hemispheres, especially under low solar activity.