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.
If you can't find a tool you're looking for, please click the link at the top of the page to "Go to old article view". Alternatively, view our Knowledge Base articles for additional help. Your feedback is important to us, so please let us know if you have comments or ideas for improvement.
 The F2-layer ionosphere behaves abnormally in its global structures and temporal anomalies. The anomalies include the winter/seasonal anomaly, annual/non-seasonal anomaly, semiannual and equatorial anomalies [Rishbeth, 1998]. The winter anomaly (or seasonal anomaly) is that the daytime values of midlatitude peak Ne of the F2 layer, NmF2 values are greater in winter than in summer [Liu et al., 2009a; Rishbeth and Muller-Wodarg, 2006]. Liu et al. [2009a]showed that the winter/seasonal anomaly is widely exists in the northern hemisphere and southern low latitudes and in Indian Ocean region at low altitudes but gradually disappears at higher altitudes. Their measurement based on the ionospheric electron density (Ne) profiles obtained from the FORMOSAT-3/COSMIC (F3/C) radio occultation measurements during the interval from day of year 194, 2006, to day of year 279, 2008, and emphasis on investigation the seasonal behaviors of daytime Ne in the altitude range of 200–560 km. The annual asymmetry implies that the peak electron densityNmF2 of the ionospheric F2-layer, or the critical frequencyfoF2, is greater in December/January than in June/July particularly during high solar flux. It was first reported by Berkner and Wells  and Seaton and Berkner . The semi-annual anomaly implies that TEC is greater at equinox than at solstice [Zhao et al., 2007]. Liu et al. [2009a] showed that the semiannual variation peaks in equinoctial months in most regions, while it has maxima in solstice months, first in the South Pacific region (around 30°S, 120°W) at 250 km altitude and expanding over the South Pacific and South Atlantic oceans at higher altitudes. Moreover, there is a region around 45°S, 30°W with a dominant semiannual component, moving toward east–north with increasing altitude in the range of 200–270 km. The NmF2 is obviously higher in March Equinox than in September Equinox, which is known as equinoctial asymmetry. The equinoctial asymmetry is strongest over equatorial anomaly crest regions [Liu et al., 2011]. The Equatorial Ionization Anomaly (EIA) is characterized by double-humped latitudinal distribution of ionization centered at the dip equator. It is approximately within ±20 degrees of the magnetic equator.
 Earlier studies made by Yonezawa and Arima  suggested that the annual asymmetry might be due to interplanetary corpuscular radiation, but had no evidence to support this idea. Nelson  presented accurate diurnal variation curves of electron content over Sydney using the 20 Mc/s transmissions Faraday rotations measurements during the years of very high solar flux 1958–1960. The results were compared with results at conjugate points in the northern hemisphere. Nelson showed that in both hemispheres the values of electron content are greater in winter than in summer at high solar fluxes and the reverse is true at low solar fluxes. He concluded that during low solar fluxes periods, the seasonal (winter) anomaly is virtually absent but the non-seasonal (annual) anomaly may still exist.Yonezawa  realized the desirability of pairing individual northern and southern stations according to both geographic and magnetic latitude, and gave comprehensive results based on ionosonde data. Titheridge and Buonsanto  measured TEC using beacon satellites, at four stations that form two good pairs, Stanford (United States)–Auckland (New Zealand) and Honolulu (United States)–Rarotonga (Cook Island), in years around the low solar maximum of 1969/1970, and related their findings to changes in neutral composition. The results at the two pairs of stations show annual asymmetry. A very similar result at the two pairs of stations was also observed using the Fourier series. Buonsanto suggested that the neutral O/O2 concentration ratio also varies annually, because the varying Sun-Earth distance modulates the radiation that dissociates molecular oxygen. He also suggested that the varying O/O2 ratio modulates the electron loss coefficient in the F2-layer, thus enhancing the effect of varying Sun-Earth distance where this is called “Buonsanto's hypothesis.”
 Further studies were made later using different techniques: NmF2 data from pairing Ionosonde measurements [Yonezawa, 1971; Rishbeth and Muller-Wodarg, 2006], total electron content (TEC) data from a worldwide network of GPS observations [Mendillo et al., 2005], and topside ionospheric observations [e.g., Su et al., 1998; Liu et al., 2007]. Zeng et al. show that the annual asymmetry is mainly driven by three factors: the geomagnetic field configuration, the Sun-Earth distance, and lower atmosphere tidal forcing.Zeng et al. employed COSMIC data and “Asymmetry Index” (AI-index) that was proposed byRishbeth and Muller-Wodarg  to quantify the annual asymmetry between December and June and found that the asymmetry has significant dependence on latitude, longitude and local time: strong peak occurs in the about local noon and another one around midnight. The TIEGCM simulations made by Zeng et al. show that the offset of geomagnetic center from geographic center is the important cause of the F2-layer annual asymmetry, the tides from lower altitudes also contributes to the asymmetry; and finally change in solar radiations between December and June solstices that is caused by differences in the Sun-Earth distance are another factors.Rishbeth and Muller-Wodarg show that the annual asymmetry between northern and southern hemispheres does not have exactly the same amplitude everywhere, so the annual variation in the flux of solar ionizing radiation cannot be the only factor. They show that a possible cause of the annual asymmetry is the January/July variation of 3.5% in Sun-Earth distance and the consequent 7% variation in the flux of ionizing radiation. However, the phase of the variation of Sun-Earth distance is the main reason for comparing January and July instead of the actual solstice months December and June. Their results show that the asymmetry is strong at noon and at midnight, occurring at all latitudes from equatorial to polar cap regions, and tends to be greater at solar minimum than solar maximum.Zhao et al.  mentioned that an annual anomaly is also shown to prevail during both daytime and nighttime and is least evident at sunrise and sunset. The magnitude of various anomalies is shown to be clearly modulated by the solar activity. Liu et al. [2009b] analyzed the 11 years' (1998–2008) 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. The differences in the daily mean TEC which was averaged globally and over low, middle, and high latitudes show obvious annual asymmetry, hemispheric differences and also semiannual anomaly. They declared that both the hemispheric differences and annual asymmetry are more marked with increasing solar activity. The annual asymmetry show stronger component in the southern hemisphere obtained from the mean TEC averaged globally and at low, middle and high latitudes under most solar activities. The hemispheric asymmetry over conjugate hemispheres follows the control of solar declination and is a manifestation of the seasonal variations, mainly due to the annual components with different phases in conjugate hemispheres. The semiannual components are of similar phases and comparable amplitudes in conjugate hemispheres, which suggest close couplings of the ionosphere in both hemispheres.
 Earlier beacon satellite measurements as well as predominantly ionosondes measurements, radio occultation measurements COSMIC, modeling, and satellite observations are used to study the ionospheric climatology. The availability of GPS signals gives a good opportunity for the study of spatial and temporal variation of the ionosphere at a lower cost. GPS-TEC measurement is now becoming a widely used method by many researchers for the study of the ionospheric variations caused by solar activity [i.e.,Astafyeva et al., 2008]. Several studies have been conducted to quantify and understand the similarities and differences of the ionospheric response between northern and southern upper atmospheres. Most of the previous studies in this area focused on the ionospheric response during storm periods, lack of research present the response during solar minimum [Liu et al., 2009a, 2009b]. Although of these studies, still several ionospheric phenomena are not completely studied such as the interhemispheric conjugacy effects of the polar total electron content TEC and scintillation activities. Therefore, further investigations in these areas are required.
 This paper aims at investigation the climatology of ionospheric TEC at quasi-conjugate points locating at different latitude ranges during low solar activity with focus on the annual/hemispheric and semiannual asymmetry. Data from four pairing GPS stations are used in analysis; these stations are summarized inTable 1. The geocentric coordinates for these stations are presented in Figure 1. 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 (http://omniweb.gsfc.nasa.gov/). As shown in Table 1, the stations are not exactly conjugate but they are approximately conjugate. For example the conjugate point for KARR (20.98°N) is 37.27°S, which is exactly the location of SUWN (37.28°S). It is also noticed that the CGM for SUWN and KARR stations are also same (Suwn: ∼31.05°N, Karr: ∼31.53°S). Although CHPI (GEO: 22.68°S) and CRO1 (GEO: 17.76°N) are not exactly conjugate to each other (the conjugate point for CHPI is 13.07°N), the behavior at both sites was compared for better understanding the ionospheric response at low magnetic latitudes (both locates in the equatorial ionization anomaly region).
Table 1. Geocentric (GEO) and Corrected Geomagnetic Coordinate (CGM) for Eight Quasi-Conjugate GPS Stations
UTC + 8
UTC + 9
Scott Base (SBA)
UTC + 12
Resolute Bay (RESO)
UTC + 3
UTC + 0
Cachoeira Paulista (CHPI)
St. Croix VLBA (CRO1)
2. Magnetic/Solar Condition During 2007
 The year 2007 was relatively quiet solar flux activity year with maximum recorded monthly disturbance storm time index (Dst) between −21 nT and −69 nT, interplanetary 3-h Kp index between 4 and 7, sunspot number (SSN) between 15 and 63, and F10.7 cm flux between 69 and 94. The data are obtained from NOAA Space Environment Center (SEC)–Space Weather Data and products and WDC for Geomagnetism, Kyoto University. Only few moderate storm events were took place during this year 2007 particularly: the 24 March storm (Dst of −69 nT), the 23 May storm (Dst of −63 nT) and 20 November storm (Dst of −69 nT). The maximum monthly readings of the solar and magnetic indices are presented inTable 2.
Table 2. Maximum Monthly Solar and Magnetic Indices During the Year 2007
3. Data Processing
 The dual-frequency GPS observables are biased on instrumental delays; therefore it is necessary to remove the differential instrumental biases for accurate estimation of TEC [Sardon et al., 1994]. The absolute TEC using GPS observables may be obtained from differential time delay (P1-P2) or from differential phase advance (L1-L2). The TEC obtained from differential time delay gives the level of absolute TEC but it is highly exposed to multipath effect, while the TEC obtained from differential phase advance gives high precision TEC but the level is unknown due to the initial offset called the ambiguity. Therefore, the level of TEC is adjusted to the TEC derived from the corresponding code difference for each satellite-receiver pair [Otsuka et al., 2002]. In this work, the time delay measurements at each GPS station were used to remove the ambiguity term, and by combining the phase and code measurements for the same satellite pass, the absolute TEC are obtained with high precision [Warnant and Pottiaux, 2000; Klobuchar, 1996] The TEC values were corrected from the satellite and receiver bias using the data obtained from AIUB Data Center of Bern University in Switzerland (ftp://ftp.unibe.ch/aiub/CODE/). The GPS data are obtained from Scripps Orbit and Permanent Array Center (SOPAC) through Internet (http://sopac.ucsd.edu/). The GPS data in compact Rinex files (xxx.07d) were converted into observation Rinex (xxx.07o) by using crx2rnx utility under the file RNXCMP_4.0.3_window.tar. The observation file parameters (L1, L2, C1, P1 and P2) were extracted to individual text files using rinex2mat MATLAB software. In the analysis, the equivalent vertical TEC (VTEC) for each satellite path is determined by using standard formulas [e.g., Mannucci et al., 1993]. The equivalent vertical TEC (VTEC) for each satellite-receiver pair is obtained by usingequation (1):
where the subscripts r and superscripts s are represent the receiver and the satellite, respectively, TEC is slant TEC measurements, χis the zenith angle of the line of sight at the sub-ionospheric point. The zenith angle of the line of sight at the sub-ionospheric point is calculated usingequation (2):
where θis the elevation angle, R is the mean radius of the Earth (6378.137 km) and hm is the height of the sub-ionospheric point that assumed at 400 km. The 30 s GPS TEC data was further processed to obtain average TEC over 15 min based on moving average technique. The geocentric coordinate of each GPS station is transformed to the corrected geomagnetic coordinate by using the online IGRF/DGRF model parameters (http://omniweb.gsfc.nasa.gov/).
 The paper investigates the ionospheric GPS TEC observations at approximately conjugate points in northern and southern hemispheres. Section 4.1presents the observations in middle-latitude region represented by SUWAN-KARR;sections 4.2 and 4.3present the observations in high-latitude region represented by RESO-SBA and SCOR-SYOG. Finally,section 4.4analyzes the observations at St. Croix VLBA and Cachoeira Paulista stations. The paper compares the annual, monthly and daily mean/maximum-TEC variations at the quasi-conjugate points in northern and southern hemispheres. Moreover, the annual, hemispheric and also semiannual anomalies are investigated from the average and maximum daily TEC.
4.1. Observations at Swan-shi and Karratha Stations
Figure 2presents the daily Max-TEC (maximum daily reading) at KARR and SUWN quasi-conjugate points during year 2007. The readings present the maximum daily TEC readings at around 14:00 LT.Figures 2a and 2b present the maximum daily TEC readings, while Figure 2c presents the TEC difference between two stations (i.e., TECkarr-TECsuwan). As shown in the figure, almost similar TEC response was seen at both stations; with higher TEC amplitudes are observed during local summer season. The elevated electron density at Suwan-ahi was observed during May, June, July and August while at Karratha station it is observed during January, February, November and December. The annual mean-TEC at the conjugate stations is about 11.7 TECU at SUWN while it is about 8.2 TECU at KARR. The difference between the two stations signified stronger TEC densities occurring in the northern hemisphere comparing with southern hemisphere (value of 3.5 TECU) which represents a percentage of 30%. This difference in the annual mean-TEC at both stations indicates occurrence of annual/hemispheric anomaly. Semiannual anomaly was also seen in the TEC data especially during the March and September equinox as shown inFigure 2a. To confirm the asymmetry in both hemispheres, Figure 3presents the monthly mean-and Max-TEC at both stations. As shown in the figure, the monthly mean was in the range between 8 and 15 TECU at Suwan-shi and 5–12 TECU at Karratha whereas the monthly maximum TEC was in the range between 13 and 22 TECU at Suwan-shi and 10–25 TECU at Karratha.
Figure 4also presents the average monthly diurnal-TEC obtained from the average hourly TEC over the month at SUWN (north) and KARR (south). As shown in the figure, the peak and minimum daily TEC at both stations occur at around 14:00 LT and 04:00 LT local time. Lower electron content was seen during winter months, the response during January and December is almost similar. During February and November the response is close to each other while in March and September months the TEC is slightly higher at SUWN. Starting from March until October 2007 the TEC started to elevate in the northern hemisphere, but it decreases in the southern hemisphere. Maximum deviation was observed in the June/July months. As a result the TEC response obvious asymmetry in the mean diurnal-TEC was clearly seen around equinox and vernal with stronger TEC amplitudes were seen in the northern hemisphere.
 The results show obvious asymmetry in the average TEC at both hemispheres (i.e., annual, hemispheric and weak semiannual anomalies). The annual asymmetry between SUWN and KARR is probably due to the January/July variation in Sun-Earth distance and the consequent variation in the flux of ionizing radiation. According toRishbeth and Muller-Wodarg the possible cause of annual asymmetry between SUWN in northern hemisphere and KARR in southern hemisphere is the annual variation in the flux of solar ionizing radiation. The January/July variation of 3.5% in Sun-Earth distance and the consequent 7% variation in the flux of ionizing radiation is the possible cause of asymmetry. The phase of the variation of Sun-Earth distance is the main reason for comparing January and July instead of the actual solstice months December and June.Rishbeth and Muller-Wodarg  show that the asymmetry is strong at noon and at midnight, occurring at all latitudes from equatorial to polar cap regions, and tends to be greater at solar minimum than solar maximum. Zeng et al. show that the annual asymmetry is mainly driven by three factors: the geomagnetic field configuration, the Sun-Earth distance, and lower atmosphere tidal forcing. According toLiu et al. [2009b], the hemispheric asymmetry over conjugate hemispheres follows the control of solar declination and is a manifestation of the seasonal variations. The obvious semiannual anomaly components (peaks in equinoctial months) between at conjugate points suggest close couplings of the ionosphere in both hemispheres. He also declared that the annual asymmetry and hemispheric difference was more distinct with increasing solar activity.
4.2. Observations at RESO/SBA Stations
Figure 5presents the daily Max-daily TEC variations at RESO (GEO: 74.69°N) and SBA (GEO: 77.85°S) quasi-conjugate stations during 2007. As shown in the figure, almost similar TEC profile was observed at both stations, with the higher TEC occurred in summer and lower TEC in winter. At RESO station the TEC began to increase between March and August (∼6 months). The maximum and minimum TEC at this station was observed in June and December respectively. On contrary, in the southern polar region, stronger electron density was observed during summer season particularly between September and March, while lower TEC is observed during winter between April and August. The maximum and minimum TEC at SBA was seen in November and June respectively. The GPS- TEC observation shows occurrence of TEC peaks during equinox (mainly in March and September at SBA in the southern hemisphere) as clearly seen inFigure 5. The semi-annual anomaly implies that NmF2 is greater at equinox than at solstice. The observed semiannual anomaly took place in both hemisphere but was stronger in the southern hemisphere. The concurrent occurrence of TEC peaks at conjugate hemispheres suggests close couplings of the ionosphere in both hemispheres. According toZhao et al.  a semiannual anomaly exists at all the latitudes during the daytime but most pronounced in the equatorial anomaly region and persists to midnight.
 To confirm the symmetry/asymmetry between the two hemispheres, the monthly mean, maximum and total TEC at both stations was investigated as shown in Figure 6. As shown in the figure, the monthly mean-TEC (Figure 6a) was in the range between 2 and 8 TECU at RESO and 2–8 TECU at SBA. The monthly mean Max-TEC (Figure 6b) was in the range between 5 and 11 TECU at RESO and 6–14 TECU at SBA. The figure shows an elevated electron density during summer season at both stations. The annual mean-TEC at the conjugate stations was 4.9 TECU at RESO and 5.4 TECU at SBA. The mean-TEC difference between the two stations indicates slightly stronger TEC at SBA station in the south hemisphere (∼0.5 TECU). This value is obtained by differencing the annual mean TEC that are shown inFigures 5a and 5b: 4.9 TECU at RESO and 5.4 TECU at SBA, using the formula: [(TECSBA-TECRESO)]. This value signifies a percentage of 9.3%. The percentage change in TEC value between SBA and RESO stations is obtained by the formula [TECSBA-TECRESO]/TECOSBA× 100%]. The annual asymmetry between SBA and RESO is probably due to the January/July variation in Sun-Earth distance and the consequent variation in the flux of ionizing radiation as suggested byRishbeth and Muller-Wodarg . Zeng et al. show that the annual asymmetry is mainly driven by three factors: the geomagnetic field configuration, the Sun-Earth distance, and lower atmosphere tidal forcing.Zeng et al. show that the offset of geomagnetic center from geographic center is the important cause of the F2-layer annual asymmetry, the tides from lower altitudes also contributes to the asymmetry; and finally change in solar radiations between December and June solstices that is caused by differences in the Sun-Earth distance are another factors.
 The mean monthly diurnal TEC at RESO and SBA conjugate points is shown in Figure 7. Data during the January month is missing. Comparing with Figure 6, Figure 7presents the monthly diurnal-TEC (the mean diurnal -TEC over the month) at both RESO and SBA. The effect of local time is clearly seen in the figure, where the maximum TEC at both stations occurs during post midday time (∼14–16 LT). The figure shows obvious asymmetry in the monthly mean diurnal-TEC at both stations during all months, the hemispheric asymmetry at conjugate hemispheres follows the control of solar declination and is a manifestation of the seasonal variations. Observations made byPrikryl et al. over Resolute Bay during 2007 using CHAIN GPS receivers (GSV-4004B) shows an elevated electron density during summer months due to the increased solar illumination and a deep minimum during the winter months. The deep minimum in plasma density during winter months corresponds to times when the cusp is located in darkness at magnetic longitudes around 100°E [Prikryl et al., 2010]. The S4 index computed at Resolute Bay remained very low, enhancements of phase scintillation are observed even during solar minimum. The phase scintillation is low when the patch plasma density is expected to be low, especially during winter. The measurements show that during summer solstice months (June–July) there is a deep minimum around 18:00 UT and a moderate increase in echoes before 12:00 UT. The diurnal variation reverses phase near the winter solstice (late autumn and early winter) with a minimum in the echo occurrence between about 06:00 and 12:00 UT and a maximum between 18:00 and 23:00 UT. Near the equinoxes the number of echoes maximizes before 12:00 UT (particularly for the spring equinox months) and is reduced after 12:00 UT.
4.3. Observations at SCOR and SYOG Stations
Figure 8presents the daily mean-TEC variations at SCOR station in Greenland (GEO: 70.48°N) with SYOG station in Antarctica (GEO: 69.00°S). As shown in the figure, the ionospheric response at SCOR station implies lower TEC amplitudes during winter (September to March) and higher TEC amplitudes in summer (April to August). The strongest TEC was observed between May and June. The figure shows that mean-TEC at both stations was less than 15 TECU.Figure 9presents the monthly mean, maximum and total TEC at both stations. The figure shows an elevated electron density during summer months comparing with winter month. The figure also shows that the monthly mean was in the range between 1.5 and 12.1 TECU at SCOR and 2.5–8.4 TECU at SYOG whereas the monthly mean Max-TEC is between 5 and 15 TECU at SCOR and 5–18 at SYOG. The annual mean-TEC at both stations was 6.8 TECU at SCOR and 5.5 TECU at SOYG. This result shows obvious asymmetry in conjugate hemisphere with stronger activity at SCOR station by ∼20% more. The percentage change in TEC value between SCOR and SYOG stations is obtained by the formula [TECSCOR-TECSYOG]/TECOSCOR× 100%]. Although the annual mean shows more TEC in the northern hemisphere, the average Max-TEC in the year (Figure 9b) shows pronounced activity at SYOG station (∼10 TECU at SCOR and ∼12 TECU at SOYG). This difference in maximum TEC is probably due to significant variability of daily TEC measurements as seen in Figure 8. The difference in the annual mean-TEC at both stations confirm occurrence of annul/hemispheric anomalies whereas the semiannual anomaly was not obvious. The annual asymmetry is mainly driven by three factors: the geomagnetic field configuration, the Sun-Earth distance, and lower atmosphere tidal forcing as suggested byZeng et al. . According to Nelson the annual anomaly may exist during low solar fluxes periods, but the winter anomaly is virtually absent. Semiannual anomaly was not pronounced SYOG and SCOR high-latitudes stations. According toZhao et al.  the semiannual anomaly exists at all the latitudes during the daytime and is most pronounced in the equatorial anomaly region and persists to midnight.
4.4. Observations at CHIPI and CRO1 Stations
Figure 10presents the daily Max-TEC variations at CHPI and CRO1 quasi-conjugate points during year 2007.Figures 10a and 10b present the maximum daily TEC readings at CHPI and CRO1 respectively, while Figure 10cindicates the TEC-difference (i.e., TECCHPI-TECCRO1). Although the two stations are not exactly conjugate to each other, the TEC response shows higher TEC during summer and minimum TEC during winter. The semiannual/equinoctial anomaly was not obviously seen in the figure but more activity was seen during March at CHPI. Figure 11presents the monthly mean-TEC, and monthly mean Max-TEC at both stations. As shown in the figure, the monthly mean-TEC was in the range between 5 and 15 TECU at CHPI and 10–19 TECU at CRO1. The monthly mean Max-TEC was in the range between 16 and 38 TECU at CHPI and 22–56 TECU at CRO1. The average annual mean-TEC indicates a value of ∼10 TECU at CHPI and ∼15 TECU at CRO1. The stronger TEC activity at CRO1 station by ∼5 TECU indicates a percentage of 34%. This value was obtained by the formula [TECCRO1-TECCHPI/TECOCRO1× 100%]. The observations at CHPI/CRO1 show obvious annual and hemispheric asymmetry in the conjugate hemisphere which is probably due to the January/July variation in Sun-Earth distance and variation in the flux of ionizing radiation. As mentioned earlier, the hemispheric asymmetry over conjugate hemispheres follows the control of solar declination and is a manifestation of the seasonal variations. The annual asymmetry is mainly driven by three factors: the geomagnetic field configuration, the Sun-Earth distance, and lower atmosphere tidal forcing as suggested byZeng et al. . Figure 12 shows more TEC during summer and low TEC during winter with obvious asymmetry (annual and hemispheric asymmetry) between two stations. The response during equinox (mainly March and October) was close to each other, which suggest close couplings of the ionosphere in both hemispheres.
5. Discussion and Conclusion
 The paper investigates the annual/hemispheric and semiannual asymmetry at quasi-conjugate points in northern and southern hemispheres during low solar flux periods using GPS data. Data Obtained from four pairs of GPS receivers are employed in the analysis; Suwan-Shi (SUWN) station in South Korea with Karratha in Australia, Cachoeira Paulista (CHPI) in Brazil with St. Croix VLBA (CRO1) in the United States, Scott Base station (SBA), Antarctica with Resolute Cornwallis bay in Arctic, and finally Scoresbysund/Ittoqqoormiit station in Greenland with Syowa station in East Ongle Island Antarctica. Experiment was conducted during solar minimum in 2007 where the maximum recorded Dst index was −69 nT. For each conjugate station, the annual, monthly mean, and maximum GPS-TEC as well as the monthly diurnal-TEC responses are compared and quantified.
 The study shows evident annual and hemispheric asymmetry and weak semiannual anomaly at all stations data. The annual asymmetry was in the range between ∼10–30%. The results were in good agreement with the plenty of studies that have been conducted in this area of research. The results also confirm that during solar minimum, the annual asymmetry is occurring at all latitudes from equatorial to polar cap regions. Titheridge and Buonsanto , and by Buonsanto suggested that annual asymmetry is mainly due to changes of neutral composition or to annual variations of neutral O/O2 concentration ratio as the varying Sun-Earth distance modulates the radiation that dissociates molecular oxygen.Zeng et al. explained that the annual asymmetry is mainly driven by three factors: the geomagnetic field configuration, the Sun-Earth distance, and lower atmosphere tidal forcing.Zhao et al.  mentioned that an annual anomaly exist during both daytime and nighttime and is least evident at sunrise and sunset. The magnitude of various anomalies is shown to be clearly modulated by the solar activity. The reason for the asymmetry at middle latitude region is possibly due to the different driving forces (e.g., neutral winds and zonal electric fields) that mainly control the plasma distribution in these two regions. The changes in the solar illumination in mid latitude lead to a correlated response, but these correlations would be observed over the entire illuminated disk and would not be constrained to the conjugate points. The interhemispheric transport, driven by neutral winds would lead to a flow of ionization along magnetic field line from one hemisphere to its conjugate point in the other hemisphere. Electric fields on the other hand, can be locally generated through neutral wind dynamo action in one hemisphere and then map along magnetic field lines to the conjugate point located in the opposite hemisphere [Schunk and Nagy, 2000]. The asymmetry of GPS TEC population between Cro1 and Chipi (high TEC in the northern hemisphere by 40%) took place since the two stations are not exactly conjugate. Furthermore, the “seasonal” or “winter” anomaly which is often found at midlatitudes, in which midday NmF2 is greater in winter than in summer as suggested by Rishbeth and Muller-Wodarg . Rishbeth and Muller-Wodarg mentioned that winter anomaly is always present in the northern hemisphere but is usually absent in the southern hemisphere during periods of low solar activity. In the polar region, elevated electron density was observed during summer months and deep minimum TEC is observed during winter. The elevated TEC is due to the increased solar illumination while deep minimum during winter is corresponding to times when the cusp is located in darkness at magnetic longitudes around 100°E. Further research in this area is required since the annual asymmetry is not completely confirmed; some studies relate the asymmetry to geomagnetic field configuration, or to the variation in the Sun-Earth distance, lower atmosphere tidal forcing, and some relate it to interplanetary corpuscular radiation, Buonsanto's hypothesis states that the varying O/O2 ratio modulates the electron loss coefficient in the F2-layer, thus enhancing the effect of varying Sun-Earth distance. Our observations agree with the observations made byLiu et al. [2009b] who observed obvious annual, hemispheric and semiannual anomalies. 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 was not obvious although it is seen at SBA/RESO and SUWN/KARR with different amplitudes. The occurrence of semiannual anomaly suggests close couplings of the ionosphere in both hemispheres.
 The author would like to thank Jerash University for supporting this work, SOPAC data archive for providing GPS data, NOAA Space Environment Center (SEC)–Space Weather Data, and WDC for Geomagnetism at Kyoto University for providing the solar and magnetic data.