GPS observations at quasi-conjugate points during solar minimum

This paper aims at investigating the climatology of ionospheric TEC at quasi-conjugate points located at different latitude ranges during low solar activity in 2007 with focus on the annual/hemispheric and semiannual asymmetry. Data from four pairing GPS stations are used for investigating the symmetry/or asymmetry between the conjugate hemispheres. The stations are selected based on data availability and manipulation possibility, and based on location of the conjugate point that was obtained by IGRF/DGRF model parameters. Our observations were in good agreement with plenty of studies conducted in this area. The results show evident annual asymmetry, hemispheric differences, and also weak semiannual asymmetry in GPS-TEC at all GPS stations. The annual asymmetry is occurring at all latitudes from equatorial to polar cap regions. The asymmetry was quantified based on differencing of the annual mean-TEC between the conjugate hemispheres (i.e., TECN°-TECS°) which show differences in the range between 10 and 30%. The annual asymmetry is mainly driven by several factors, mainly: the geomagnetic field configuration, the Sun-Earth distance, and lower atmosphere tidal forcing. The hemispheric asymmetry of the TEC in conjugate hemispheres follows the control of solar declination and is a manifestation of the seasonal variations. The semiannual anomaly (also equinoctial asymmetry) was seen at SBA/RESO and also SUWN/KARR with more activity seen in March equinox compared with September. The occurrence of semiannual anomaly suggests close couplings of the ionosphere in both hemispheres.