Solar Epoch Research Articles

Observation of the north-south asymmetry in the equatorial spread F intensity at the Brazilian longitude

This study presents an analysis of the observed north-south asymmetry of the range spread F (RSF) intensity at the low latitude region during an equinoctial month of different solar epochs (2002, 2015 and 2017). The ionospheric parameters were obtained during geomagnetic quiet days from four digisonde stations located along the Brazilian longitude, which include a dip equator station (Sao Luiz (SL: 2.33 S, 44.2 W)), conjugate stations (Campo Grande (CG: 20.5°S, 55°W) and Boa Vista (BV: 2.8°N, 60.7°W)) and another low latitude station (Cachoeira Paulista (CP: 22.7°S, 45°W)). The results highlight the competing effect of the post-sunset electric field strength and the trans-equatorial wind on the latitudinal distribution of the irregularity intensity at both hemispheres under varying background ionospheric condition. The RSF intensity was seen to reduce as the solar flux index decreased and the latitudinal peak shifted closer to the dip equator. This was dependent on the variation of the field line mapped irregularity spectrum and the density gradient. Likewise, the north-south asymmetry in the irregularity occurrence was seen to become more significant as a denser ionosphere was observed at the hemisphere with the equatorward meridional wind. This has further proven that the non-linear cascading of the plasma irregularity across the low latitude region is strongly influenced by the local electric field.

Observation of seasonal asymmetry in the range spread F occurrence at different longitudes during low and moderate solar activity

Abstract. A comparative study of the equatorial spread F occurrence was conducted at different longitudes during 2010 and 2013 representing the low (LSA) and moderate (MSA) solar activity periods respectively. The ionogram data were recorded at low-latitude stations including Jicamarca (JIC;75.76∘ W, 8.17∘ S), Fortaleza (FZA; 38.52∘ W, 3.73∘ S), Ilorin (ILR; 7.55∘ E, 9.93∘ N), Chumphon (CPN; 88.46∘ E, 11∘ N) and Kwajalein (KWA; 167.73∘ E, 8.72∘ N). The range type spread F (RSF) occurrence was manually recorded at an hourly interval between 18:00 and 06:00 LT, and a monthly average of the RSF occurrence was estimated for each season. The longitudinal distribution of the RSF occurrence features included the observed difference in the onset time, the duration and the seasonal occurrence peak. The seasonal asymmetry in the RSF occurrence distribution was analysed in relation to the zonal drift reversal’s effect on the plasma irregularity initiation. We believe that the inconsistent equinoctial asymmetry pattern in the RSF occurrence is modulated by the seasonal/longitudinal variation of the zonal drift reversal delay during both solar epochs. Likewise, the seeding effect and the background ionospheric condition were also considered as major factors influencing the frequency of irregularity generation in these regions.

On the Nature of the Intraseasonal Variability of Nighttime Ionospheric Irregularities Over Taiwan

AbstractThe nature of intraseasonal (within a season) variability of the nighttime ionospheric irregularities has been studied utilizing the observation of the recently installed VHF scintillation receiver from Pingtung, Taiwan. Investigation is based on observations made in 2015 (high‐moderate solar epoch) and 2017 (low solar epoch). Detailed investigation has revealed coexistence of planetary‐scale variability (3–7 days), 10‐ to 16‐day variability, and > 25‐day variability in 2015. Interestingly, the Lomb‐Scargle frequency domain analysis indicated the variability of larger timescales to display somewhat greater amplitudes. The virtual height of the F layer, h'F, over Sanya, also indicated intraseasonal periodicities similar to S4 periodicities observed at Pingtung. Observations have also revealed that, equatorial plasma bubbles (EPBs) over Pingtung tended to occur on contiguous nights as compared to isolated occurrence, and > 60% of the EPB nights were part of two or more (up to 6) nights of contiguous occurrence. Spectral analysis revealed a quasi‐27‐day variability in the ap and AE geomagnetic indices presumably associated with the solar rotation period. The time period of geomagnetic variability corresponded with the period of dominant S4 variability, indicating that the effect of geomagnetic activity on the low‐latitude ionosphere can be periodic in nature. In 2017 (low solar flux), even with a very low occurrence of ionospheric irregularities, planetary‐scale variability was observed. The dominant period of S4 variability in the autumnal equinox was highly correlated with the periodic geomagnetic activity. Results indicate that a deeper understanding of the periodic intraseasonal variability can be imperative for extended range forecast/prediction of ionospheric irregularities.

Climatology of GW-TIDs in the magnetic equatorial upper thermosphere over India

An analysis of Gravity wave induced travelling ionospheric disturbances (GW-TIDs) in the thermosphere during high and low solar epochs is undertaken using ionosonde data at Trivandrum (8.50N, 770E). Wavelet analysis is performed on the temporal variations of foF2 and the amplitudes of waves present in two period bands of (0.5–1.5) h and (2–4) h are extracted. The real height profiles are generated at 15 min internal for the whole day (for sample days) during high and low solar activity years. The study reveals that the GW-TID activity is significantly greater for solar minimum compared to solar maximum for the period 8.5–17.5 h. Diurnally the GW-TID activity in the (2–4) h period band peaks in the post sunset hours for both high and low solar epochs. For the 0.5–1.5 h period band, the diurnal maximum in GW-TID is occurring in the post sunset hours for high solar epoch while it occurs in the morning hours around 10 h LT for low solar epoch. Seasonally the day time GW-TID activity maximizes (minimizes) for winter (vernal equinox). The post sunset time GW-TID maximizes (minimizes) either for summer/winter (vernal equinox). The other interesting observation is the anti correlation of GW-TID in upper thermosphere with solar activity for day time and the correlation of the same with solar activity in the post sunset hours. The present results for daytime are in agreement with the equatorial daytime GW-TID behaviour reported from CHAMP satellite observations. The GW-TID activity during post sunset time for equatorial region upper thermosphere has not been reported so far.

Gravity wave control on ESF day‐to‐day variability: An empirical approach

AbstractThe gravity wave control on the daily variation in nighttime ionization irregularity occurrence is studied using ionosonde data for the period 2002–2007 at magnetic equatorial location Trivandrum. Recent studies during low solar activity period have revealed that the seed perturbations should have the threshold amplitude required to trigger equatorial spread F (ESF), at a particular altitude and that this threshold amplitude undergoes seasonal and solar cycle changes. In the present study, the altitude variation of the threshold seed perturbations is examined for autumnal equinox of different years. Thereafter, a unique empirical model, incorporating the electrodynamical effects and the gravity wave modulation, is developed. Using the model the threshold curve for autumnal equinox season of any year may be delineated if the solar flux index (F10.7) is known. The empirical model is validated using the data for high, moderate, and low solar epochs in 2001, 2004, and 1995, respectively. This model has the potential to be developed further, to forecast ESF incidence, if the base height of ionosphere is in the altitude region where electrodynamics controls the occurrence of ESF. ESF irregularities are harmful for communication and navigation systems, and therefore, research is ongoing globally to predict them. In this context, this study is crucial for evolving a methodology to predict communication as well as navigation outages.

Comparison of quiet time vertical plasma drifts with global empirical models over the Indian sector: Some insights

The threshold vertical plasma drifts and their polarities are inferred from E-region irregularities reported earlier using different experiments conducted from India during geomagnetically quiet time. In addition, the hourly variations of magnetometer data sets are used in conjunction with an equatorial electrojet model (Anandarao, 1976) to deduce the vertical drifts around noon time (10:00–140:00 LT). These results are then compared with the vertical drifts presented by empirical models (Scherliess and Fejer, 1999; Fejer et al., 2008a) corresponding to the 60°E longitude sector. In general, the vertical drifts presented by empirical models are consistent with those inferred from snapshot measurements of E-region irregularities at different local times of the day except around sunrise hours. Further, the vertical drifts presented by Fejer et al. (2008a) model match fairly well with seasonally averaged vertical drifts deduced (within 1σ variation) using magnetometer data. A time difference is noticed between occurrence of pre-reversal enhancement in vertical drifts over India reported earlier using different techniques and Scherliess and Fejer (1999) model. Probable reason for the time difference is discussed. The occurrence characteristic of afternoon equatorial counter electrojet in June solstice during low solar epoch is consistent with the drifts obtained from empirical models. The seasonally averaged vertical drifts during nighttime reported earlier using ionosonde/HF radar experiments are not consistent with the presence of streaming plasma waves on a few occasions. Further, nocturnal vertical drifts are systematically under-estimated and probable reason for this is discussed.

The Relationship of Stratospheric QBO with the Difference of Measured and Calculated NmF2

The relationship between stratospheric QBO and the difference (ΔNmF2) between NmF2 calculated with IRI-2012 and measured from ionosondes at the Singapore and Ascension stations in the equatorial region was statistically investigated. As statistical analysis, the regression analysis was used on variables. As a result, the relationship between QBO and ΔNmF2 was higher for 24:00 LT (local time) than 12:00 LT. This relationship is positive in the solar maximum epoch for both stations. In the solar minimum epoch, it is negative at 24:00 LT for Ascension and at 12:00 LT for Singapore. Furthermore, it was seen that the relationship of the ΔNmF2 with both the easterly and westerly QBO was negative for all solar epochs and every LT, at Ascension station. This relationship was only positive for solar maximum epoch and 12:00 LT, at Singapore station.

Ionospheric vertical plasma drift and electron density response during total solar eclipses at equatorial/low latitude

AbstractThe response of the vertical plasma drift (Vz) and the electron density (NmF2) during different solar eclipses was investigated. The diurnal values of the direct scaled measurement of F2 peak height and the one derived from M(3000) F2 data, acquired over an equatorial/low‐latitude stations, have been used to determine the vertical plasma drift. The ionosphere during a solar eclipse is significantly affected by the E × B vertical drift; the large depletion of electron density at low altitudes can be transported to high altitudes through the plasma vertical drift. The loss in ionization density during the eclipse phase decreases the electron density, which was accompanied by rapid increase in hmF2. This deviation in the NmF2 during eclipse compared to control days can be related to the increase in the loss rate due to recombination, as a result of reduction in thermal energy. However, the maximum reduction in NmF2 is not synchronous with the time of maximum totality but some minutes later. The differences in the solar epochs may contribute to the observed relative changes in the ionospheric F2 region behavior during the eclipse window. Lastly, it is very difficult to separate the influence of magnetic disturbances from solar eclipse. The deviation in NmF2 is higher during magnetic disturbed days than the quiet day. The reverse is the case for hmF2 observation. However, the NmF2 variation increases with an increase in solar activity.

Hilbert–Huang transform analysis of periodicities in the last two solar activity cycles

We investigated periodicities associated with the last two-and-a-half solar activity cycles with the novel Hilbert–Huang transform (HHT) method. Raw data signals of five different observational proxies: the 10.7 cm radio flux intensity, the helioseismic frequency shift, and the sunspot area signals recorded from the whole solar disc, and separately from the Northern and Southern hemispheres, were expanded into a set of intrinsic modes with the ensemble empirical mode decomposition technique. Instant and mean periods of each empirical mode were obtained with the use of the Hilbert transform applied independently to each separate mode. The periodicities were allocated to three distinct groups: short-term variations (with periods shorter than 0.5 yr), quasi-biennial oscillations (with typical periods from 0.5 yr to 3.9 yr), and longer periodicities, e.g. such as the 11 yr cycle. All periodicities detected in the examined solar cycles 22–24 are consistent with the well-known results found in the earlier solar epochs. We have demonstrated that the HHT method is a good tool for characterizing periodicities in the helioseismic data, which are necessarily relatively limited in terms of their time resolution. Periodicities obtained using the helioseismic data are, nevertheless, consistent with those found in other proxies. Since helioseismic oscillations are sensitive to the solar interior, this indicates that the behaviour of surface and atmospheric magnetic activity reflects that of the Sun's internal magnetic field. All identified intrinsic modes are seen to have clear amplitude modulation highly correlated with the 11 yr cycle. This amplitude modulation is most pronounced in the short-period modes. The short-term periodicities were found to be multiples of the shortest period, of 25 d. This ordering of the short-term periodicities is consistent with the previous findings. Signatures of the north–south asymmetry were detected in the individual hemisphere sunspot area indices. Furthermore, evidence of the last ‘extended’ solar minimum was detected too.

On the solar activity variations of nocturnal F region vertical drifts covering two solar cycles in the Indian longitude sector

AbstractA comprehensive analysis of the seasonal and solar cycle variabilities of nighttime vertical drift over the Indian longitude sector is accomplished using ionosonde data located at the magnetic equatorial location, Trivandrum (8.5°N, 76.5°E). The analysis extends over a span of two decades (1988–2008). The representative seasonal variations based on the extensive data of nocturnal vertical drift during three different solar activity epochs is arrived at, for the first time. Seasonally, it is seen that maximum post sunset Vd is obtained in vernal equinox (VE), followed by autumnal equinox (AE), winter solstice (WS), and summer solstice (SS) for high and moderate solar epochs, while for low solar epoch, maximum Vd occurs in WS followed by VE, AE, and SS. Further, the role of sunset times at the magnetic conjugate points in modulating the time and magnitude of peak drifts during different solar epochs is ascertained. The equinoctial asymmetry in peak Vd during high and moderate solar epochs is another significant outcome of this study. The solar activity dependence of vertical drift for a wide range of solar fluxes has been quantified for all the seasons. In the present era of GPS‐based communication and navigation, these are important results that give a better handle in understanding essential factors that impact equatorial ionospheric phenomena.

The performance of ionospheric correction based on NeQuick 2 model adaptation to Global Ionospheric Maps

In this paper, the Global Ionospheric Maps (GIMs) during 1998–2012 have been adopted as the primary source of data to generate the Galileo-like ionospheric correction parameters. Taking the previous day GIMs as ingested dataset and the current day GIMs as reference, we have computed the cumulative distribution function (CDF) of TEC (Total Electron Content) error less than 5 TECU (TEC Unit, 1TECU=1016electron/m2) or 20% (denoted as zi20) to study the performance of NeQuick 2 in providing global ionospheric correction. The statistical analysis is carried out to study how the performance changes with seasons, solar activities and geographic latitudes. Our studies showed that: (1) in general, the NeQuick 2 model driven by the previous day GIMs agrees well with the current day GIMs, with monthly averaged zi20 exceeds 60%. (2) The performance of GIMs-driven-NeQuick model varies with geographic latitudes. The model behaves better in mid-latitudes than in low-latitudes, which is more prominent in northern hemisphere. (3) There is a nonlinear relationship between zi20 and F10.7. When the solar activity level is low or medium, zi20 tends to decrease with increasing solar activities. However, when F10.7 increases further to exceed a certain threshold, zi20 generally shows either saturation or an increase for high solar epoch. The zi20 decreases with F10.7 more sharply in mid-latitudes than in low-latitudes, in the southern hemisphere than in the northern hemisphere. (4) The seasonal behavior of zi20 in different latitudinal belts are quite different. In the southern hemisphere, zi20 mainly presents annual variation, and it is largest in winter and smallest in summer. In the northern hemisphere, the semi-annual variation is dominant, and zi20 is larger in solstice than in equinox. During the High Solar Activity years, the zi20 is much larger in summer than in winter in the low-latitude regions.

Ionospheric foF2 morphology and response of F2 layer height over Jicamarca during different solar epochs and comparison with IRI-2012 model

Diurnal, seasonal and annual foF2 variability and the response of the F2-layer height over Jicamarca (11.9 °S, 76.8 °W, 1 °N dip) during periods of low (LSA), moderate (MSA) and high (HSA) solar activities was investigated. The relative standard deviation (V R ) was used for the analysis. The F2-layer critical frequency pre-noon peak increases by a factor of 2 more than the post-noon peak as the solar activity increases. The variability coefficient (V R ) is lowest during the day (7–16%) for the three solar epochs; increases during nighttime (20–26%, 14–26%, and 10–20%, respectively for the LSA, MSA and HSA years); and attained highest magnitude during sunrise (21–27%, 24–27%, and 19–30%, respectively in similar order). Two major peaks were observed in V R – the pre-sunrise peak, which is higher, and the post-sunset peak. Generally, the variability increases as the solar activity decreases. Annually, V R peaks within 23–24%, 19–24% and 15–24% for the LSA, MSA, and HSA periods, respectively. The ionospheric F2-layer height rises to the higher level with increasing solar activity. The foF2 comparison results revealed that Jicamarca is well represented on the IRI-2012 model, with an improvement on the URSI option. The importance of vertical plasma drift and photochemistry in the F2-layer was emphasized.

Geometric properties of the coronal neutral surface

The interrelation between the density inhomogeneity and the magnetic field configuration in the solar corona has been studied. The situation of consistency with the regular cellular magnetic field organization, established by the authors based on polarization data, and helmet-shaped coronal structures has been analyzed. Consistency was found when coronal helmets appeared as projections on the plane of the sky of dense layer folds. It turned out that a dense layer on a regularly deformed neutral surface on a scale of 60° generates coronal shapes typical of solar activity minimum epochs. Threby, a weighty argument for the Molodenskii hypothesis that a dense layer is formed on the neutral surface has been obtained and it has been determined that one can find the 3D geometry of the coronal neutral surface based on modeling the visible shape of coronal helmets. The study of this geometry based on the observed coronal structures in the epoch of minimum and low solar activity indicated that the neutral surface longitudinal deformation scale remains stable and the surface latitudinal amplitude at the corona base reaches 50°–70° latitudes and tends to increase with increasing solar activity. This amplitude decreases as a power function with a variable index close to −1 with increasing distance from the Sun. In addition, a 10° small-scale deformation of the studied surface with a predominant latitudinal flexure direction has been revealed.

Signatures of moderate (M-class) and low (C and B class) intensity solar flares on the equatorial electrojet current: Case studies

The present investigation brings out, in contrast to the earlier works, the changes in the equatorial electrojet (EEJ) current in response to a few moderate (M-class) and low (C and B class) intensity solar flares during 2005–2010. Special care is taken to pick these flare events in the absence of prompt electric field perturbations associated with geomagnetic storms and substorms that also affect the electrojet current. Interestingly, only the normalized (with respect to the pre-flare level) deviations of daytime EEJ (and not the deviations alone) change linearly with the increases in the EUV and X-ray fluxes. These linear relationships break down during local morning hours when the E-region electric field approaches zero before reversal of polarity. This elicits that the response of EEJ strength corresponding to less-intense flares can be appropriately gauged only when the local time variation of the quiet time E-region zonal electric field is taken into account. The flare events enhanced the EEJ strength irrespective of normal or counter electrojet (CEJ) conditions that shows that solar flares change the E-region ionization density and not the electric field. In addition, the enhancements in the X-ray and EUV fluxes, for these flares occurring during this solar minimum period, are found to be significantly correlated as opposed to the solar maximum period, indicating the differences in the solar processes in different solar epochs.

North-south asymmetry in solar, interplanetary, and geomagnetic indices

Data of hourly interplanetary plasma (field magnitude, solar wind speed, and ion density), solar (sunspot number, solar radio flux), and geomagnetic indices (Kp, Ap) over the period 1970-2010, have been used to examine the asymmetry between the solar field north and south of the heliospheric current sheet (HCS). A persistent yearly north-south asymmetry of the field magnitude is clear over the considered period, and there is no magnetic solar cycle dependence. There is a weak N-S asymmetry in the averaged solar wind speed, exhibited well at times of maximum solar activities. The solar plasma is more dense north of the current sheet than south of it during the second negative solar polarity epoch (qA < 0). Moreover, the N - S asymmetry in solar activity (Rz) can be statistically highly significant. The sign of the average N - S asymmetry depends upon the solar magnetic polarity. The annual magnitudes of N - S asymmetry depend positively on the solar magnetic cycle. Most of the solar radio flux asymmetries occurred during the period of positive IMF polarity.

Modeling of the Near-Earth Low-Energy Antiproton Fluxes

The local interstellar antiproton spectrum is simulated taking into account antineutron decay, (He,p) interaction, secondary and tertiary antiproton production, and the solar modulation in the “force field” approximation. Inclusive invariant cross-sections were obtained through a Monte Carlo procedure using the Multistage Dynamical Model code simulating various processes of the particle production. The results of the simulations provided flux values of to and to antiprotons/( s sr GeV) at energies of 0.2 and 1 GeV, respectively, for the solar maximum and minimum epochs. Simulated flux of the trapped antiprotons in the inner magnetosphere due to galactic cosmic ray (GCR) interactions with the atmospheric constituents exceeds the galactic antiproton flux up to several orders. These simulation results considering the assumptions with the attendant limitations are in comprehensive agreement with the experimental data including the PAMELA ones.

Occurrence and onset conditions of postsunset equatorial spread F at Jicamarca during solar minimum and maximum

The data of digisonde and the NRLMSISE‐00 model on the occurrence of postsunset equatorial spread F (ESF) at Jicamarca during solar minimum and maximum years are used in a detailed statistical analysis. The onset conditions chosen in this study are the prereversal enhancement velocity (dhF80/dt), minimum virtual height of F‐layer (h′F), gradient density scale length (L), and local linear growth rate (γ) of collisional Rayleigh‐Taylor instability. The monthly variation in ESF occurrence during solar minimum generally agrees with that during maximum. This agreement indicates that the ESF occurrence does not depend on solar activity. For the onset conditions, the differences between solar minimum and maximum exists in the monthly variations in dhF80/dt, h′F, and L, but not in γ. This consistence in monthly γ variation between two solar epochs mainly account for the solar cycle independence of ESF occurrence. Furthermore, the variations in collision frequency and recombination rate are principally responsible to the consistence in γ.

Prediction of solar minimum and maximum epochs on the basis of spectral characteristics for the next millennium

A prevision of solar activity minimum and maximum epochs for the next 1000 years was obtained using a sum of sine waves derived from spectral analysis of the time series of 10 years average sunspot number for the period 1016–2006. The predicted 1000 year solar activity evidence the possibility of forthcoming four minima and one maximum. The 10 years average sunspot number activity in each anomalous period was used in linear equations to obtain estimates of the solar irradiance, solar wind velocity, and the southward component of the interplanetary magnetic field.

Variation in the ionospheric propagating factor M(3000)F2 at Ouagadougou, Burkina Faso

We use hourly monthly median values of propagation factor M(3000)F2 data observed at Ouagadougou Ionospheric Observatory (geographic12.4°N, 1.5°W; 5.9 o dip), Burkina Faso (West Africa) during the years Januar1987–December1988 (average F10.7 < 130 × 10 −22 W/m 2 /Hz, representative of low solar flux conditions) and for January 1989–December1990 (average F10.7 ⩾ 130 × 10 −22 W/m 2 /Hz, representative of high solar epoch) for magnetically quiet conditions to describe local time, seasonal and solar cycle variations of equatorial ionospheric propagation factor M(3000)F2 in the African region. We show that that seasonal trend between solar maximum and solar minimum curves display simple patterns for all seasons and exhibits reasonable disparity with root mean square error (RMSE) of about 0.31, 0.29 and 0.26 for December solstice, June solstice and equinox, respectively. Variability Σ defined by the percentage ratio of the absolute standard deviation to the mean indicates significant dissimilarity for the two solar flux levels. Solar maximum day (10–14 LT) and night (22–02 LT) values show considerable variations than the solar minimum day and night values. We compare our observations with those of the IRI 2007 to validate the prediction capacity of the empirical model. We find that the IRI model tends to underestimate and overestimate the observed values of M(3000)F2, in particular, during June solstice season. There are large discrepancies, mainly during high solar flux equinox and December solstice between dawn and local midnight. On the other hand, IRI provides a slightly better predictions for M(3000)F2 between 0900 and 1500 LT during equinox low and high solar activity and equinox high sunspot number. Our data are of great importance in the area of short-wave telecommunication and ionospheric modeling.

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

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.