Climatology of the mean total electron content derived from GPSglobal ionospheric maps

摘要

We analyzed 11 years' (1998-2008) worth of the total electron content (TEC) dataderived at the Jet Propulsion Laboratory (JPL) from Global Positioning System (GPS)observations to investigate the overall climatological features of the ionosphere in a newway. The global ionospheric maps (GIM) of JPL TEC are averaged globally and overlow-, middle-, and high-latitude ranges in the southern (northern) hemisphere and bothhemispheres 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, morepronounced at low latitudes, while the mean TEC increases faster with higher solar EUVfluxes, being evident at high latitudes. The annual asymmetry (differences in June andDecember solstices) can be detected in the mean TEC averaged globally and at lowlatitudes under all solar epochs as well as at middle and high latitudes under most solaractivities. The hemispheric asymmetry of the TEC in conjugate hemispheres follows the control of solar declination. Both the hemispheric differences and annual asymmetryare more marked with increasing solar activity. The annual components of the mean TECare stronger in the southern hemisphere, and the semiannual components are of similarphases and comparable amplitudes in conjugate hemispheres, which suggest closecouplings of the ionosphere in both hemispheres. Further, the mean TEC averaged in onehemisphere can reliably be used as ionospheric indices to monitor the solar activityvariabilities and to capture the overall climatological features of the ionosphere overspecified regions, while it should be cautioned that the mitigation of the dominant annualcomponents with opposite phases in conjugate hemispheres leaves a significantsemiannual component in the mean TEC averaged in both hemispheres, especially underlow solar activity.