Periods Of High Solar Activity Research Articles

Quiet and Disturbed Time Characteristics of Blanketing Es (Esb) During Solar Cycle 23

AbstractA sporadic E layer with dense ionization blocks the upper ionospheric layers in ionosonde observations and it is called, blanketing sporadic E (Esb). Although earlier studies have demonstrated that Esb occurrence is dependent on solar activity, seasons, local time, and equatorial electrojet/counter equatorial electrojet (EEJ/CEEJ) strength, the physics behind this dependence is not well established particularly at the dip equatorial stations. Moreover, fewer comprehensive studies are available on Esb. Present work is a detailed statistical study of Esb occurrence and its characteristics during solar cycle 23 (1996–2006) at Indian dip equatorial station Trivandrum (dip latitude 0.5°N). In present study, solar flux dependence of Esb occurrence is clearly evident not only for magnetically quiet but also for the disturbed periods with maximum during low solar activity. Known seasonal peak Esb occurrence during summer (May–August) is found to be dominated by larger percentage of total blanketing events (fbEs ≥ 8 MHz) on magnetically disturbed days as compared to quiet days. We noticed prevalent Esb occurrence in the postmidnight (00–06 India Standard Time) periods during low solar activity. Besides Esb characteristics, the present study aims to investigate effect of solar flux on association of Esb and CEEJ. Coexistence of Esb and CEEJ is emphasized in earlier observational studies. However, any dependence of their association on solar flux is not yet examined. We find that their association is weaker during low solar activity as compared to high solar activity in summer, indicating that Esb occurrence is highly likely on CEEJ days during high solar activity periods.

Modeling and analysis of GPS-TEC low latitude climatology during the 24th solar cycle using empirical orthogonal functions

The Total Electron Content (TEC) is an essential component describing the temporal and spatial characteristics of the ionosphere. In this paper, an empirical orthogonal function (EOF) model is constructed by using ground based Global Navigational Satellite System (GNSS) TEC observation data at the Bangalore International GNSS Service (IGS) station (geographic – 13.02° N, 77.57° E; geomagnetic latitude 4.4° N) during an extended period (2009–2016) in the 24th solar cycle. EOF model can be decomposed into base functions and its corresponding coefficients. These decomposed modes well represented the influence of solar and geomagnetic activity towards TEC. The first three EOFs modes constitute about 98% of the total variance of the observed data sets. The Fourier Series Analysis (FSA) is carried out to characterize the solar-cycle, annual and semi-annual dependences by modulating the first three EOF coefficients with solar (F10.7) and geomagnetic (Ap and Dst) indices. The TEC model is validated during daytime and nighttime conditions as well as under different solar activity and geomagnetic conditions. A positive correlation (0.85) of averaged daily GPS-TEC with averaged daily F10.7 strongly supports those time-varying characteristics of the ionosphere features depends on the solar activity. Further, the validity and reliability of EOF model is verified by comparing with the GPS-TEC data, and standard global ionospheric models (International Reference Ionosphere, IRI2016 and Standard Plasmasphere-Ionosphere Model, SPIM). The performances of the standard ionosphericmodelsare marked to be relatively better during High Solar Activity (HSA) periods as compared to the Low Solar Activity (LSA) periods.

Comparison and Time Evolution of the Geomagnetic Cutoff at the ISS Position: Internal vs External Earth’s Magnetic Field Models

AbstractOur back-tracing code (GeoMagSphere) reconstructs the cosmic ray trajectories inside the Earth’s magnetosphere. GeoMagSphere gets the incoming directions of particles entering the magnetopause and disentangles primary from secondary particles (produced in atmosphere) or even particles trapped inside the Earth’s magnetic field. The separation of these particle families allows us to evaluate the geomagnetic rigidity cutoff. The model can be used considering the internal symmetric (IGRF-12) magnetic field only, or adding the asymmetric external one (Tsyganenko models: T89, T96 or TS05). A quantitative comparison among these models is presented for quiet (solar pressure Pdyn < 4 nPa) and disturbed (Pdyn > 4 nPa) periods of solar activity, as well as during solar events like flares, CMEs. In this analysis we focused our attention on magnetic field data in magnetosphere, from Cluster, and simulated cosmic rays for a generic detector on the ISS as for example AMS-02. We found that high solar activity periods, like a large fraction of the period covering years 2011-2015, are better described using IGRF+TS05 model. Results, i.e. the average vertical rigidity cutoff at the ISS orbit, are shown in geographic maps of 2° × 2° cells.

Relationship between solar activity and Δ14C peaks in AD 775, AD 994, and 660 BC

ABSTRACTSince the AD 775 and AD 994 Δ14C peak (henceforth M12) was first measured by Miyake et al. (2012, 2013), several possible production mechanisms for these spike have been suggested, but the work of Mekhaldi et al. (2015) shows that a very soft energy spectrum was involved, implying that a strong solar energetic particle (SEP) event (or series of events) was responsible. Here we present Δ14C values from AD 721–820 Sequoiadendron giganteum annual tree-ring samples from Sequoia National Park in California, USA, together with Δ14C in German oak from 650–670 BC. The AD 721–820 measurements confirm that a sharp Δ14C peak exists at AD 775, with a peak height of approximately 15‰ and show that this spike was preceded by several decades of rapidly decreasing Δ14C. A sharp peak is also present at 660 BC, with a peak height of about 10‰, and published data (Reimer et al. 2013) indicate that it too was preceded by a multi-decadal Δ14C decrease, suggesting that solar activity was very strong just prior to both Δ14C peaks and may be causally related. During periods of strong solar activity there is increased probability for coronal mass ejection (CME) events that can subject the Earth’s atmosphere to high fluencies of solar energetic particles (SEPs). Periods of high solar activity (such as one in October–November 2003) can also often include many large, fast CMEs increasing the probability of geomagnetic storms. In this paper we suggest that the combination of large SEP events and elevated geomagnetic activity can lead to enhanced production of 14C and other cosmogenic isotopes by increasing the area of the atmosphere that is irradiated by high solar energetic particles.

Locally adapted NeQuick 2 model performance in European middle latitude ionosphere under different solar, geomagnetic and seasonal conditions

The ionosphere introduces positioning error in Global Navigation Satellite Systems (GNSS). There are several approaches for minimizing the error, with various levels of accuracy and different extents of coverage area. To model the state of the ionosphere in a region containing low number of reference GNSS stations, a locally adapted NeQuick 2 model can be used. Data ingestion updates the model with local level of ionization, enabling it to follow the observed changes of ionization levels. The NeQuick 2 model was adapted to local reference Total Electron Content (TEC) data using single station approach and evaluated using calibrated TEC data derived from 41 testing GNSS stations distributed around the data ingestion point. Its performance was observed in European middle latitudes in different ionospheric conditions of the period between 2011 and 2015. The modelling accuracy was evaluated in four azimuthal quadrants, with coverage radii calculated for three error thresholds: 12, 6 and 3 TEC Units (TECU). Diurnal radii change was observed for groups of days within periods of low and high solar activity and different seasons of the year. The statistical analysis was conducted on those groups of days, revealing trends in each of the groups, similarities between days within groups and the 95th percentile radii as a practically applicable measure of model performance.In almost all cases the modelling accuracy was better than 12 TECU, having the biggest radius from the data ingestion point. Modelling accuracy better than 6 TECU was achieved within reduced radius in all observed periods, while accuracy better than 3 TECU was reached only in summer. The calculated radii and interpolated error levels were presented on maps. That was especially useful in analyzing the model performance during the strongest geomagnetic storms of the observed period, with each of them having unique development and influence on model accuracy. Although some of the storms severely degraded the model accuracy, during most of the disturbed periods the model could be used, but with lower accuracy than in the quiet geomagnetic conditions. The comprehensive analysis of locally adapted NeQuick 2 model performance highlighted the challenges of using the single point data ingestion applied to a large region in middle latitudes and determined the achievable radii for different error thresholds in various ionospheric conditions.

A two-step ionospheric modeling algorithm considering the impact of GLONASS pseudo-range inter-channel biases

The Global Navigation Satellite System presents a plausible and cost-effective way of computing the total electron content (TEC). But TEC estimated value could be seriously affected by the differential code biases (DCB) of frequency-dependent satellites and receivers. Unlike GPS and other satellite systems, GLONASS adopts a frequency-division multiplexing access mode to distinguish different satellites. This strategy leads to different wavelengths and inter-frequency biases (IFBs) for both pseudo-range and carrier phase observations, whose impacts are rarely considered in ionospheric modeling. We obtained observations from four groups of co-stations to analyze the characteristics of the GLONASS receiver P1P2 pseudo-range IFB with a double-difference method. The results showed that the GLONASS P1P2 pseudo-range IFB remained stable for a period of time and could catch up to several meters, which cannot be absorbed by the receiver DCB during ionospheric modeling. Given the characteristics of the GLONASS P1P2 pseudo-range IFB, we proposed a two-step ionosphere modeling method with the priori IFB information. The experimental analysis showed that the new algorithm can effectively eliminate the adverse effects on ionospheric model and hardware delay parameters estimation in different space environments. During high solar activity period, compared to the traditional GPS + GLONASS modeling algorithm, the absolute average deviation of TEC decreased from 2.17 to 2.07 TECu (TEC unit); simultaneously, the average RMS of GPS satellite DCB decreased from 0.225 to 0.219 ns, and the average deviation of GLONASS satellite DCB decreased from 0.253 to 0.113 ns with a great improvement in over 55%.

Ionospheric correction using NTCM driven by GPS Klobuchar coefficients for GNSS applications

Global Navigation Satellite Systems (GNSS) require mitigation of ionospheric propagation errors because the ionospheric range errors might be larger than tens of meters at the zenith direction. Taking advantage of the frequency-dispersive property of ionospheric refractivity, the ionospheric range errors can be mitigated in dual-frequency applications to a great extent by a linear combination of carrier phases or pseudoranges. However, single-frequency GNSS operations require additional ionospheric information to apply signal delay or range error corrections. To aid single-frequency operations, the global positioning system (GPS) broadcasts 8 coefficients as part of the navigation message to drive the ionospheric correction algorithm (ICA) also known as Klobuchar model. We presented here an ionospheric correction algorithm called Neustrelitz TEC model (NTCM) which can be used as complementary to the GPS ICA. Our investigation shows that the NTCM can be driven by Klobuchar model parameters to achieve a significantly better performance than obtained by the mother ICA algorithm. Our research, using post-processed reference total electron content (TEC) data from more than one solar cycle, shows that on average the RMS modeled TEC errors are up to 40% less for the proposed NTCM model compared to the Klobuchar model during high solar activity period, and about 10% less during low solar activity period. Such an approach does not require major technology changes for GPS users rather requires only introducing the NTCM approach a complement to the existing ICA algorithm while maintaining the simplicity of ionospheric range error mitigation with an improved model performance.

High solar activity predictions through an artificial neural network

The effects of high-energy particles coming from the Sun on human health as well as in the integrity of outer space electronics make the prediction of periods of high solar activity (HSA) a task of significant importance. Since periodicities in solar indexes have been identified, long-term predictions can be achieved. In this paper, we present a method based on an artificial neural network to find a pattern in some harmonics which represent such periodicities. We used data from 1973 to 2010 to train the neural network, and different historical data for its validation. We also used the neural network along with a statistical analysis of its performance with known data to predict periods of HSA with different confidence intervals according to the three-sigma rule associated with solar cycles 24–26, which we found to occur before 2040.

Determination of the ionospheric foF2 using a stand-alone GPS receiver

The critical frequency of ionospheric F2 layer (foF2) is a measure of the highest frequency of radio signal that may be reflected back by the F2 layer, and it is associated with ionospheric peak electron density in the F2 layer. Accurate long-term foF2 variations are usually derived from ionosonde observations. In this paper, we propose a new method to observe foF2 using a stand-alone global positioning system (GPS) receiver. The proposed method relies on the mathematical equation that relates foF2 to GPS observations. The equation is then implemented in the Kalman filter algorithm to estimate foF2 at every epoch of the observation (30-s rate). Unlike existing methods, the proposed method does not require any additional information from ionosonde observations and does not require any network of GPS receivers. It only requires as inputs the ionospheric scale height and the modeled plasmaspheric electron content, which practically can be derived from any existing ionospheric/plasmaspheric model. We applied the proposed method to estimate long-term variations of foF2 at three GPS stations located at the northern hemisphere (NICO, Cyprus), the southern hemisphere (STR1, Australia) and the south pole (SYOG, Antarctic). To assess the performance of the proposed method, we then compared the results against those derived by ionosonde observations and the International Reference Ionosphere (IRI) 2012 model. We found that, during the period of high solar activity (2011–2012), the values of absolute mean bias between foF2 derived by the proposed method and ionosonde observations are in the range of 0.2–0.5 MHz, while those during the period of low solar activity (2009–2010) are in the range of 0.05–0.15 MHz. Furthermore, the root-mean-square-error (RMSE) values during high and low solar activities are in the range of 0.8–0.9 MHz and of 0.6–0.7 MHz, respectively. We also noticed that the values of absolute mean bias and RMSE between foF2 derived by the proposed method and the IRI-2012 model are slightly larger than those between the proposed method and ionosonde observations. These results demonstrate that the proposed method can estimate foF2 with a comparable accuracy. Since the proposed method can estimate foF2 at every epoch of the observation, it therefore has promising applications for investigating various scales (from small to large) of foF2 irregularities.

Ionospheric winter anomaly and annual anomaly observed from Formosat-3/COSMIC Radio Occultation observations during the ascending phase of solar cycle 24

Ionospheric winter and annual anomalies have been investigated during the ascending phase of solar cycle 24 using high-resolution global 3D – data of the FORMOSAT – 3/COSMIC (Formosa satellite – 3/Constellation Observing System for Meterology, Ionosphere and Climate) radio occultation observations. Our detailed analysis shows that the occurrence of winter anomaly at low-latitudes is confined only to the early morning to afternoon hours, whereas, the winter anomaly at mid-latitudes is almost absent at all local times during the ascending phase of solar cycle 24. Further, in the topside ionosphere (altitudes of 400km and above), the winter anomaly is completely absent at all local times. In contrast, the ionospheric annual anomaly is consistently observed at all local times and altitudes during this ascending phase of solar cycle 24. The annual anomaly exhibits strong enhancements over southern EIA crest latitudes during day time and around Weddle Sea Anomaly (WSA) region during night times. The global mean annual asymmetry index is also computed to understand the altitudinal variation. The global mean AI maximizes around 300–500km altitudes during the low solar active periods (2008–10), whereas it extends up to 600km during moderate to high (2011) solar activity period. These findings from our study provide new insights to the current understanding of the annual anomaly.

Investigation of geoeffective and non-geoeffective CMEs according to data from the URAGAN muon hodoscope

Coronal mass ejections are the brightest manifestations of solar activity. Dozens of coronal mass ejections are observed daily during periods of higher solar activity. They directly affect cosmic ray fluxes that carry information on plasma clouds, including clouds moving toward the Earth. Several aspects of geoeffective and non-geoeffective coronal mass ejections, observed with the ground-based URAGAN muon hodoscope operated as part of the NEVOD experimental complex at MEPhI, are discussed. The anisotropy of cosmic ray muon fluxes recorded during coronal mass ejections in 2014 and 2015 is investigated.

Solar radio proxies for improved satellite orbit prediction

Specification and forecasting of solar drivers to thermosphere density models is critical for satellite orbit prediction and debris avoidance. Satellite operators routinely forecast orbits up to 30 days into the future. This requires forecasts of the drivers to these orbit prediction models such as the solar Extreme-UV (EUV) flux and geomagnetic activity. Most density models use the 10.7 cm radio flux (F10.7 index) as a proxy for solar EUV. However, daily measurements at other centimetric wavelengths have also been performed by the Nobeyama Radio Observatory (Japan) since the 1950's, thereby offering prospects for improving orbit modeling. Here we present a pre-operational service at the Collecte Localisation Satellites company that collects these different observations in one single homogeneous dataset and provides a 30 days forecast on a daily basis. Interpolation and preprocessing algorithms were developed to fill in missing data and remove anomalous values. We compared various empirical time series prediction techniques and selected a multi-wavelength non-recursive analogue neural network. The prediction of the 30 cm flux, and to a lesser extent that of the 10.7 cm flux, performs better than NOAA's present prediction of the 10.7 cm flux, especially during periods of high solar activity. In addition, we find that the DTM-2013 density model (Drag Temperature Model) performs better with (past and predicted) values of the 30 cm radio flux than with the 10.7 flux.

GPS-TEC Variation during Low to High Solar Activity Period (2010-2014) under the Northern Crest of Indian Equatorial Ionization Anomaly Region

The measurements of ionospheric TEC (total electron content) are conducted at a low latitude Indian station Surat (21.16°N, 72.78°E Geog.), which lies under the northern crest of the equatorial anomaly in Indian region. The data obtained are for a period of five years from low to high solar activity (2010- 2014) using GPS (Global Positioning System) receiver. In this study, we report the diurnal and seasonal variation of GPS-TEC, dependence of GPS-TEC with solar activity, geomagnetic condition and EEJ strength. From the seasonal analysis, it is found that greater values of the GPS-TEC are observed during equinox season followed by winter and summer. The appearance (in the year 2011 and 2014) and disappearance (in the year 2010 and 2012) of “winter anomaly” have been observed at the station. From the correlation of GPS-TEC with different solar indices, i.e. solar EUV flux, F10.7 cm solar radio flux and Zurich sunspot number (SSN), it is concluded that the solar index EUV flux is a better controller of GPS-TEC, compared to F10.7 cm and SSN. Further, it is observed that there is no effect of rising solar activity on correlation. Moreover, the percentage variability of GPS-TEC and the standard deviation of GPS-TEC obtained for quiet and disturbed days show that dependence of GPS-TEC on geomagnetic condition is seasonal. Also, there is a positive correlation observed between GPS-TEC and EEJ strength.

Solar cycle variation of ionospheric parameters over the low latitude station Hainan, China, during 2002–2012 and its comparison with IRI-2012 model

The low latitude ionospheric data observed by digisonde at Hainan station (19.5°N, 109.1°E) in a whole solar activity cycle period from 2002 to 2012 within Ap<20 have been analyzed to explore the diurnal, seasonal, annual variations and solar activity dependences of the ionospheric peak parameters (foF2, hmF2, and Chapman scale height Hm), as well as some quantitative comparison with IRI-2012 modeling predictions. The results show that the winter anomaly in the daytime foF2 appears at different levels of solar activity. The semiannual anomaly in the daytime and nighttime foF2 with two maxima in equinox seasons is present. The foF2 have a close correlation with a solar activity factor F107P=(F107+F107A)/2 and the correlation coefficients (r) in their diurnal variation are around 0.7. The slope of foF2 varying with F107P in daytime is usually smaller than in nighttime. The afternoon and evening hmF2 show good correlation with F107P (their r values exceed 0.6), but hmF2 at other time are low or poor related to F107P. The prominent character of hmF2 in equinox and summer seasons is its strong increase at sunset in high solar activity period, which may be due to pre-reversal enhancement (PRE) of local electric field. We also note that hmF2 values around midnight slightly decrease with increasing F107P index in equinox seasons. The diurnal variation of Hm usually has two peaks around noontime and pre-sunrise. The daytime Hm has an annual variation with maximum in summer and minimum in winter. Moreover, the dependence of the daytime Hm on solar activity is not strong due to meridional wind and other factors. The above results over Hainan are considerably different from those reported over Millstone Hill, which is attributed to their different geomagnetic locations. The quantitative results compared between IRI-2012 model predictions and observations show that the predicted foF2 values are basically underestimated and the magnitude of their deviations obviously increases with increasing solar activity. The predicted hmF2 obtained with measured M(3000)F2 inputs in low and moderate solar activity agree well with the observed ones. However, their deviations in high solar activity are significantly magnified.

Variability of ionospheric parameters during solar minimum and maximum activity and assessment of IRI model

The ionospheric parameters (electron and ion plasma temperatures (Te and Ti) and total ion density, Ni) as obtained by the Indian SROSS-C2 satellite (altitude ∼500km) have been investigated during low (year 1995, F10.7∼77 sfu) and high (year 2000, F10.7∼177 sfu) solar activity periods. The region under study spans over 5°S-30°N geomag. latitude and 60–100°E geog. longitude over the Indian sector. The observations are compared with the modelled values using IRI-2007 and IRI-2012 versions to assess model predictability. We found that minimum plasma temperatures (Te and Ti) in nighttime gets twice hotter whereas maximum temperatures in early morning gets reduced by half (cooler) when the solar flux gets doubled indicating a direct relation of Te and Ti with solar flux, F10.7 in nighttime but inverse in the morning hours. The ion density (Ni) exhibits solar activity dependence throughout the day and increases by one order when solar activity gets doubled. The modelled Te and Ti are found in agreement to the observed values for high solar activity over both the regions. Whereas the discrepancy exists during low solar activity period over both the regions with over-/under-estimated values in nighttime/morning and noontime respectively. The latest IRI-2012 model improves the nighttime Te and Ti whereas the modelled Ni is found in complete agreement to the observations.

Electrodynamic influence on the diurnal behaviour of neutral daytime airglow emissions

Abstract. The diurnal variations in daytime airglow emission intensity measurements at three wavelengths OI 777.4 nm, OI 630.0 nm, and OI 557.7 nm made from a low-latitude location, Hyderabad (17.5° N, 78.4° E; 8.9° N MLAT) in India have been investigated. The intensity patterns showed both symmetric and asymmetric behaviour in their respective diurnal emission variability with respect to local noon. The asymmetric diurnal behaviour is not expected considering the photochemical nature of the production mechanisms. The reason for this observed asymmetric diurnal behaviour has been found to be predominantly the temporal variation in the equatorial electrodynamics. The plasma that is transported across latitudes due to the action of varying electric field strengths over the magnetic equator in the daytime contributes to the asymmetric diurnal behaviour in the neutral daytime airglow emissions. Independent magnetic and radio measurements support this finding. It is also noted that this asymmetric diurnal behaviour in the neutral emission intensities has a solar cycle dependence with a greater number of days during high solar activity period showing asymmetric diurnal behaviour compared to those during a low solar activity epoch. These intensity variations over a long timescale demonstrate that the daytime neutral optical emissions are extremely sensitive to the changes in the eastward electric field over low and equatorial latitudes.

Solar Transients Disturbing the Mid Latitude Ionosphere during the High Solar Activity

We investigate the effect of solar transients on the mid latitude ionosphere during the high solar activity period of solar cycle 23 i.e 2003 and 2004. A mid latitude station, Guangzhou (23.1N, 113.4E) was selected to carry out the investigation. The ionospheric behaviour at the selected station is characterized by considering the critical frequency of F2 layer (foF2) obtained by using the ground based Ionosonde observations. Then we selected two types of solar transients viz. solar flares and Coronal Mass Ejections (CMEs). To quantify the effect of solar flares we have considered the X-ray flux (1-8 Å) and EUV flux (26-34nm). Similarly to quantify the effect of CMEs, we have considered the geomagnetic storms, because during high solar activity the geomagnetic storms are caused by CMEs. From our analysis we conclude that during the geomagnetic storms the value of foF2 decreases as compared to quiet days thereby showing a negative effect. On the contrary we found that during solar flares there is sudden and intense increase in foF2. We also performed a correlation analysis to access the magnitude of association between changes in flux values and peak values of Dst during flares and storms with the corresponding changes and peak values of foF2. We found that a strong correlation exists between the enhancements/decrements in foF2 and enhancements in flux values and Dst. We conclude, while geomagnetic activity suppresses ionospheric activity the flares enhance the same.

Quiet-time foF2 characteristics at an equatorial station in the African sector

A study on the morphology of the critical frequency of the F2 layer (foF2) has been done using data obtained with Ionospheric Prediction Sounder (IPS-42) located at Ouagadougou, Burkina Faso (geo. Lat. 12.40N, Long. 358.50, magnetic dip latitude 5.90N) an equatorial station in the West African sector. Data covering a whole solar cycle period from 1985 – 1995 were used to characterize the quiet time morphology of the F2-layer for that region. Results show that foF2 exhibit diurnal, seasonal and solar activity dependence. Diurnal morphology of foF2 is characterized by two peaks; pre-noon and post-noon peaks with the presence of noon-time “bite-out”. While the pre-noon peak is more pronounced during high solar activity periods, the post-noon peak is dominant at low solar activity. Also, while the post noon peaks are more pronounced at equinoctial months during low solar activity, the pre-noon peak is more pronounced at the solstices during high solar activity periods.

Performance of IRI‐2012 model during a deep solar minimum and a maximum year over global equatorial regions

AbstractPresent paper inspects the prediction capability of the latest version of the International Reference Ionosphere (IRI‐2012) model in predicting the total electron content (TEC) over seven different equatorial regions across the globe during a very low solar activity phase 2009 and a high solar activity phase 2012. This has been carried out by comparing the ground‐based Global Positioning System (GPS)‐derived VTEC with those from the IRI‐2012 model. The observed GPS‐TEC shows the presence of winter anomaly which is prominent during the solar maximum year 2012 and disappeared during solar minimum year 2009. The monthly and seasonal mean of the IRI‐2012 model TEC with IRI‐NeQ topside has been compared with the GPS‐TEC, and our results showed that the monthly and seasonal mean value of the IRI‐2012 model overestimates the observed GPS‐TEC at all the equatorial stations. The discrepancy (or over estimation) in the IRI‐2012 model is found larger during solar maximum year 2012 than that during solar minimum year 2009. This is a contradiction to the results recently presented by Tariku (2015) over equatorial regions of Uganda. The discrepancy is found maximum during the December solstice and a minimum during the March equinox. The magnitude of discrepancy in the IRI‐2012 model showed longitudinal dependent which maximized in western longitude sector during both the years 2009 and 2012. The significant discrepancy in the IRI‐2012 model observed during the solar minimum year 2009 could be attributed to larger difference between F10.7 flux and EUV flux (26–34 nm) during low solar activity period 2007–2009 than that during high solar activity period 2010–2012. This suggests that to represent the solar activity impact in the IRI model, implementation of new solar activity indices is further required for its better performance.

VARIATIONS IN TOTAL ELECTRON CONTENT DURING THE PHASES OF LOW AND HIGH SOLAR ACTIVITIES

Variations in the Total Electron Content of the ionosphere were studied by utilizing data from the GISTM receiver installed at Universiti Tun Hussein Onn Malaysia. The study was conducted during periods of low solar activity (July 2007–July 2008) and high solar activity (July 2013–July 2014). Results show that the TEC are dependent on the solar activity.The values during high solar activity were significantly higher than that obtained during the solar minimum phase. The minimum TEC values for both phases varied between 89% and 97%, and the maximum TEC values varied between 70% and 81%. The pattern of daily TEC value changes was constant, and TEC peaked in the afternoon at ~14 LT. The highest TEC recorded during the solar maximum phase was 144.5 TEC Unit (TECU) in April 2014, whereas the highest TEC recorded during the solar minimum phase was 36.3 TECU in April 2008. TEC was maximized from March to May under both solar maximum and minimum phases.