Diurnal Variation Of Total Electron Content Research Articles

Responses of the Nigerian low-latitude ionosphere to geomagnetic storms of the ascending and maximum phases of solar cycle 24

This study investigates the responses of the Nigerian ionosphere to thirteen geomagnetic storms that occurred during the ascending and maximum phases of the solar cycle 24. The Total Electron Content (TEC) data obtained from the Nigerian Global Navigation Satellite System (GNSS) network (NIGNET) were used.The ionosphere over Nigeria recorded marked diurnal TEC variations around 1100–1700 LT, with the most significant variations between 1400 and 1600 LT time period, and minimum variation at 0600 LT. Another cogent piece of information from this study is that the values of TEC and ionospheric irregularities at all the five GNSS stations under investigation consistently matched one another. The implication of this is that the values of TEC and irregularities, at both quiet time and storm time, at any location in Nigeria could be used as the representative values at any other locations in the country, particularly in areas where there are no GNSS systems. The equinoctial maxima and June solstice minima effects were clearly observed in our data. Furthermore, ionospheric irregularities occurrences also followed semiannual patterns with two peaks in April and October, and the least occurrences in June, and generally localized around 2000–0100 LT. At daytime during geomagnetic storms in the equatorial/low-latitude regions, Prompt Penetration Electric Field (PPEF) intensified forward plasma fountain effect, while at nighttime, the reversed plasma fountain effect returns plasma backward to cause the Equatorial Ionization Anomaly (EIA) crests to contract, or totally coalesce the EIA structure to form a single strip of plasma density around the equator.

A Comparison of a GNSS‐GIM and the IRI‐2020 Model Over China Under Different Ionospheric Conditions

AbstractThe ionosphere is a crucial factor affecting Global Navigation Satellite System positioning. The Global Ionosphere Map (GIM) or the International Reference Ionosphere (IRI) model can be used for regional ionospheric correction. Since southern China is located near the electron density equatorial anomaly, this study evaluates the performance of the Wuhan University GIM (WHU‐GIM) and the IRI‐2020 from 2008 to 2020 over the China region. The comparison indicates that the Total Electron Content (TEC) from IRI‐2020 is lower than that from WHU‐GIM overall, the discrepancy is more obvious in high solar conditions and low‐latitude regions. The differential Slant TEC (dSTEC) during a phase‐arc with about 0.1 TECU accuracy derived from Global Positioning System (GPS) observations is used for model validation, the results show that the accuracies of WHU‐GIM and IRI‐2020 are 3.14 and 4.57 TECU, respectively. The dSTEC error is larger at low latitudes and decreases with increasing latitude. GPS‐derived TEC is taken for reference to evaluate the model reliability. Results show that both models can reproduce the diurnal TEC variations, but IRI‐2020 is more influenced by geomagnetic activities. The TEC correction percentage for IRI‐2020 is about 60%–80% under different ionospheric conditions, while for WHU‐GIM is 80%–90%. The Single‐Frequency Precise Point Positioning is performed with the ionosphere delay corrected by the two models, respectively. The positioning errors show that using IRI‐2020 has a lower accuracy, and the TEC discrepancy of the IRI‐2020 can cause a large bias in the up direction, especially at low‐latitude regions.

A comparative study of ionospheric TEC diurnal variations at two stations near cusp latitudes in the Southern Hemisphere

Based on ionospheric total electron content (TEC) data for Zhongshan station (ZHS) and Scott Base station (SBA) in Antarctica, acquired during 2010–2020, high-latitude ionospheric TEC diurnal variations of two near cusp latitude stations were studied. The magnetic latitude and longitude differences between the two stations were approximately 5° and 135°, respectively. It was found that during the 11-year solar activity cycle, the maximum diurnal variation of the ionospheric TEC at ZHS occurred mainly between local noon and magnetic noon. Statistically, the TEC peaks occurred closer to local (magnetic) noon under low (high) solar activity conditions. The maximum diurnal variation of ionospheric TEC at SBA occurred mainly around local noon under low solar activity conditions but before magnetic midnight under high solar activity conditions. The effects of solar radiation, particle precipitation, and polar ionospheric convection pattern on the diurnal variation of TEC were investigated. At both stations, photoionization caused by solar radiation was the main reason for the maximum diurnal variation of ionospheric TEC around local noon. During high solar activity with high concentration plasma in the dayside polar ionosphere, the convection pattern was the dominant influence on the maximum diurnal variation of TEC before magnetic midnight (noon) at SBA (ZHS).

Transient Variations of Total Electron Content (TEC) Over a Terrestrial Point within Magnetic Anomaly Region

This paper investigate the variability of Total Electron Content TEC over a terrestrial point within equatorial anomaly region using the NovAtel GSV 4000B GPS-SCINDA system at Akure (7.3°N, 5.2°E), Nigeria. This system is capable of tracking up to 14 GPS satellites simultaneously. Total Electron Content (TEC) over equatorial region using a real time data collected via a GPS-SCINDA facility were analyzed to study the ionospheric variations in terms of Total Electron Content (TEC) for the period of three years. Diurnal variations and Monthly mean variations of Total Electron Content within the equatorial anomaly region were examined. The diurnal variation of TEC showed pre-dawn minimum for a short period of time, followed by a steep early morning increase and then reached maximum value between 14:00 UT and 16:00 UT. The influence of solar activity on VTEC was investigated by taking the correlation coefficients between VTEC, F10.7cm radio flux index and sunspot numbers. The range of solar flux variation during the period of observation is very limited; there is high positive correlation (Correlation Coefficient 0.61) between daytime peak TEC and the solar F10.7 flux.

GPS derived ionospheric TEC variability with different solar indices over Saudi Arab region

In this paper, we investigate the variation of ionospheric total electron content (TEC) derived from global positioning system (GPS) observations at a low latitude station in Magna (Station code: RASH, Geographic coordinate: 28°17′N, 34°47′E), Saudi Arab. The study includes a linear regression method for validating the relationship between regional TEC and solar indices during the period 2015–2017 representing the descending phase of 24th solar cycle. The results show a relatively better agreement of diurnal variation of TEC with solar extreme ultraviolet (EUV) flux (0.1–50 nm wavelength) as compared to the Sunspot number (SSN) and 10.7 cm radio flux (F10.7). Interestingly, the seasonal variation shows a higher magnitude of TEC during the winter season as compared to the summer and fall equinox seasons, which is reported as the winter anomaly over the low latitude region. The correlation coefficient between VTEC and solar indices is also higher during the winter as compared to summer season. Concerning diurnal variability, the results show a higher correlation coefficient during the local noon time (1200SAST) that gradually descends towards evening time (1800SAST) and after that decays during the night time (2100SAST) to attend the diurnal minima. This manifests the diurnal solar irradiance dependence over the low latitude region. Moreover, the lesser agreement in the linear relationship between the TEC and solar indices during the year 2015 than the year 2017 substantiates the dependence of TEC magnitude and vitiations on solar activity. The outcomes from this strengthens the regional understanding and modeling of ionospheric parameters and would complement towards the global efforts for modeling and mitigating the ionospheric effects on transionospheric communication, timing and navigation services. Above all, the intents of this study are in line with the augmentation of aviation services performances in Saudi Arab that plays a vital role in the international civil aviation and logistic trade due to its central geographic location.

SCINDA-GPS derived TEC depletions and amplitude scintillations over Kisumu, Kenya during selected quiet and storm days of 2013 and 2014

Total Electron Content (TEC) depletion and amplitude scintillation (S4) can be derived from, SCINDA-GPS receivers situated in various parts of the equatorial region. In this paper we present results of characterization of TEC depletions and amplitude scintillations over Kisumu, Kenya (Geomagnetic coordinates: 9.64o S, 108.59o E; Geographic coordinates: 0.02o S, 34.6o E) for both selected geomagnetically quiet and geomagnetically disturbed conditions between 1st January 2013 and 31st December 2014 using data derived from the Kisumu NovAtel GSV4004B SCINDA-GPS receiver situated at Maseno University. TEC depletions and amplitude scintillations affect Global Positioning System (GPS) signals in the ionosphere as they propagate from the satellite to the receiver. This study aims to investigate day to day variability of TEC depletions and amplitude scintillations over Kisumu, Kenya during both geomagnetically quiet and geomagnetically disturbed days of 2013 and 2014 which was a high solar activity period for Solar Cycle 24. Seasonal variability of TEC depletions and S4 index is also presented. The Receiver Independent Exchange (RINEX) data for the years 2013 and 2014 was retrieved from the Kisumu SCINDA-GPS receiver, processed to obtain Vertical Total Electron Content (VTEC), S4 and Universal Time (UT) and fed into MATLAB to generate VTEC and S4 plots against UT for each selected quiet and storm day within the 2013 and 2014 period. The obtained results showed a diurnal variation of TEC where TEC was minimum at pre-sunrise, maximum during daytime and minimum during nighttime. The minimum TEC during pre-sunrise and nighttime was attributed to reduced solar intensity while maximum TEC during daytime is attributed to increased solar intensity. Most of the selected quiet and storm days of the years 2013 and 2014 showed TEC depletions and TEC enhancements corresponding with enhanced amplitude scintillations between 1800UT and 20:00UT. This might be attributed to the rapid rise of the F-layer and the increase in the vertical E x B plasma drift due to the Pre-reversal Enhancement (PRE) of the eastward electric field. Post-midnight TEC depletions and amplitude scintillations were observed for some days and this was attributed to the effect of zonal winds which brought post-midnight enhancement of the E x B drift. The percentage occurrence of amplitude scintillations for the selected quiet and storm days exhibited a seasonal dependence with equinoctial months having higher occurrences than the solstitial months. The higher average S4 index during equinoctial months might be attributed to increased solar intensity resulting from the close alignment of the solar terminator and the geomagnetic meridian. Â

Effects of the Solar Eclipse of March 20, 2015 on the Total Electron Content over Europe

The effect of the solar eclipse of March 20, 2015 on variations of the total electron content (TEC) in the ionosphere over Europe is analyzed based on GPS/GLONASS measurements at EUREF network stations. The specific feature of this eclipse is that it occurred under conditions of geomagnetic disturbance remaining after a strong geomagnetic storm on March 17, 2015. This made it more difficult to identify the effects of the solar eclipse. To detect these effects, diurnal TEC variations at individual stations were analyzed. The effect manifested itself as a TEC depression with the minimum close to the time of the maximum phase of the eclipse over the observation station. The effect was more pronounced in TEC variations along the individual passes of the satellites, appearing as a trough-like TEC depression. The diurnal variation of the TEC on the day of the eclipse was significantly different from the nearest control days, which made it difficult to determine the absolute magnitude of the eclipse effect. The space–time behavior of the TEC was represented in the form of TEC maps for the European region. For this purpose, concurrent GPS/GLONASS observations at more than 150 stations were used. A high space–time resolution of the maps made it possible to reveal the dynamics of TEC behavior over Europe during the event under consideration. The eclipse led to a clear change in the structure of the latitudinal and longitudinal TEC gradients. TEC maps generated with a 5-min resolution demonstrate the dynamics of the spatiotemporal behavior of the TEC during the eclipse.

ANALYSES OF TOTAL ELECTRON CONTENT VARIATIONS OVER NORTHERN AND SOUTHERN NIGERIA

Total electron content (TEC) is a parameter of the ionosphere that produces great effect on radio signals. We present the diurnal and seasonal variations of vertical total electron content (vTEC) during the ascending phase of solar cycle 24. A moderate solar activity year (2011) with sunspot number, Rz = 55.7 is used in this study. Total electron content (TEC) deduced from the dual frequency GPS measurements obtained at two ground stations namely: ABUZ (Zaria) with longitude 7.39oE in the north and UNEC (Enugu) with longitude 7.30oE in the south are considered. Both stations are located within the same longitude and has a latitudinal difference of 4.74o in the Nigerian equatorial ionosphere (NEI). Comparison of diurnal and seasonal variations of TEC is carried out for both stations. The diurnal variation of TEC shows a steep increase starting from sunrise, reaching daytime maximum between 13 – 15 LT at UNEC and 14 – 16 LT at ABUZ, then falls to a minimum at sunset. Dawn depression occurred at the same local time of 04 LT at both stations. On a seasonal scale, Pre- and post-midnight values were highest during the Equinoxes, followed by December solstice and least in June Solstice season at ABUZ. Pre- and post-midnight values were also higher during the Equinoxes than the Solstice season at UNEC, although they are about the same range. Also, TEC values are observed to be slightly higher for all hours and seasons at Enugu in the south than Zaria in the north except during March equinox at Zaria where TEC values were higher during the daytime. This implies that there could be little variations in TEC even within the same latitudinal zone.

A single‐station empirical model for TEC over the Antarctic Peninsula using GPS‐TEC data

AbstractCompared with regional or global total electron content (TEC) empirical models, single‐station TEC empirical models may exhibit higher accuracy in describing TEC spatial and temporal variations for a single station. In this paper, a new single‐station empirical total electron content (TEC) model, called SSM‐month, for the O'Higgins Station in the Antarctic Peninsula is proposed by using Global Positioning System (GPS)‐TEC data from 01 January 2004 to 30 June 2015. The diurnal variation of TEC in the O'Higgins Station may have changing features in different months, sometimes even in opposite forms, because of ionospheric phenomena, such as the Mid‐latitude Summer Nighttime Anomaly (MSNA). To avoid the influence of different diurnal variations, the concept of monthly modeling is proposed in this study. The SSM‐month model, which is established by month (including 12 submodels that correspond to the 12 months), can effectively describe the diurnal variation of TEC in different months. Each submodel of the SSM‐month model exhibits good agreement with GPS‐TEC input data. Overall, the SSM‐month model fits the input data with a bias of 0.03 TECU (total electron content unit, 1 TECU = 1016 el m−2) and a standard deviation of 2.78 TECU. This model, which benefits from the modeling method, can effectively describe the MSNA phenomenon without implementing any modeling correction. TEC data derived from Center for Orbit Determination in Europe global ionosphere maps (CODE GIMs), International Reference Ionosphere 2012 (IRI2012), and NeQuick are compared with the SSM‐month model in the years of 2001 and 2015–2016. Result shows that the SSM‐month model exhibits good consistency with CODE GIMs, which is better than that of IRI2012 and NeQuick, in the O'Higgins Station on the test days.

Role of neutral wind in the performance of artificial neural‐network based TEC models at diverse longitudes in the low latitudes

AbstractThe equatorial ionosphere is characterized by (i) large values of total electron content (TEC) and sharp latitudinal gradients of TEC, (ii) steep temporal variation of TEC, (iii) large diurnal variation of TEC, and (iv) postsunset secondary enhancement of TEC. These features cause major limitations in the accuracy of standard ionospheric TEC models in this region. Three artificial neural‐network (ANN) based models have been developed based on real‐time low‐latitude TEC data along 77°E, 88°E, and 121°E longitudes in the region between the magnetic equator and locations beyond the northern crest of equatorial ionization anomaly to predict the vertical TEC values. ANN models have shown more accurate predictions than other standard ionospheric TEC models like International Reference Ionosphere, Parameterized Ionospheric Model, and NeQuick. The effects of the neutral wind in the variation of TEC are significant and have been incorporated as inputs to these ANN models. The outputs with neutral wind incorporated shows better correspondence with measured TEC than the models without neutral wind inputs. The longitudinally separated models have been used to find any longitudinal differences in TEC along equatorial regions. The causes behind the longitudinal differences in TEC and its diurnal variations in these regions have been explained in terms of the geomagnetic declination and inclination angles along with the role of zonal wind.

Magnetic storm effects on the variation of TEC over Ilorin an equatorial station

We have used total electron content (TEC) derived from dual-frequency GPS receivers to study magnetically quiet and storm time variations of the ionosphere at Ilorin(8.47°N, 4.68°E), an equatorial station in the African sector. Four years (2009–2012) data were used for the study. The result on the quiet time variation of the ionosphere showed that the diurnal variation of TEC is not symmetrical about noon. This is a departure from a typical Chapman variation. Daytime maximum occurred after local noon (13–16 LT) for all the seasons and at all solar activity levels considered. A significant effect of solar activity variation was observed on the seasonal trend in 2011. The tendency for magnetic storms to cause increases in TEC is much greater than those of decreases. Daytime maximum TEC usually occurred closer to the noon time during storm periods when compared to those of quiet periods. Maximum percent change in TEC on storm days varied from about 25 to 131%.

Ionospheric effects of sudden stratospheric warmings in eastern Siberia region

Ionospheric effects observed in Russia's Asia region during sudden stratospheric warmings (SSWs) in the winters 2008/2009 and 2012/2013 corresponding to both extreme solar minimum and moderate solar maximum conditions have been examined. To detect the ionospheric effects which must have been induced by the SSWs, we have carried out a joint analysis of total electron content (TEC) global ionospheric maps (GIM), MLS (Microwave Limb Sounder, EOS Aura) measurements of vertical temperature profiles, as well as NCEP/NCAR and UKMO Reanalysis data. It has been revealed for the first time that during strong SSWs the amplitude of diurnal variation of TEC decreases nearly by half in the mid-latitude ionosphere. Besides, the intensity of TEC deviations from the background level increases during SSWs. It has also revealed that during SSW peak the midday TEC maximum considerably decreases, and the night/morning TEC increases compared to quiet days. The pattern of TEC response to SSW is shown to be identical for both quiet and disturbed geophysical conditions.

Diurnal Variation of TEC and S 4 Index During the Period of Low Geomagnetic Activity at Ile-Ife, Nigeria

Ile-Ife lies on the equatorial anomaly region where the ionospheric current is greatly influenced by the existence of the equatorial electrojet. The dual frequency SCINDA NovAtel GSV 4004B GPS receiver recently installed at Ile-Ife [on geographical latitude 7°33′N and longitude 4°33′E and geomagnetic dipole (coordinate) of latitude 9.84°N and longitude 77.25°E] is currently operational and recording data from the available global positioning system satellites. The receiver provides the data on total electron content (TEC) and the scintillation index (S 4). This paper presents the first sets of results from this station. Data records for the month of February 2010 were analyzed using the WinTec-P software program and these were interpreted to discuss the diurnal variation of the TEC and S 4 index during the period considered, as having low geomagnetic activity. The vertical TEC in this study showed that the values vary widely from as low as 0 TECu about sunrise to about 35 TECu during the day. Depletion in TEC was also noticed about sunset and marked by the occurrence of scintillations with a maximum index value of 0.3. Results of the IRI models and the observed TEC differ considerably; hence, there is the need to improve IRI models for its adaptability to the Africa ionospheric conditions.

Study of total electron content variations over equatorial and low latitude ionosphere during extreme solar minimum

In recent years with the advancement in satellite based navigational applications, study of Total Electron Content (TEC) has gained significant importance. It is well known that due to dynamical behaviour of equatorial and low latitude ionosphere, the levels of ionization is relatively high herein. The sustained decrease in solar extreme ultraviolet radiations during the current minimum is greater than any in recent history. This gives us the opportunity to study the observations of global positioning system total electron content (GPS-TEC) dual frequency signals from the GPS satellites continuously recorded at Trivandrum (an equatorial station) and Delhi (a low latitude station) during the extremely low solar activity period from January 2007 to June 2009. This study illustrates the diurnal, seasonal and annual variations of TEC during the extended solar minimum period. This study also investigates the behaviour of daytime ionosphere around spring and autumn equinoxes at low solar activity period. The results clearly reveal the presence of equinoctial asymmetry which is more pronounced at equatorial station Trivandrum. The diurnal variation of TEC shows a short-lived day minimum which occurs between 0500 to 0600 LT at both the stations. Delhi TEC values show its steep increase and reach at its peak value between 1200 and 1400 LT, while at the equator the peak is broad and occurs around 1600 LT. Further, the daily maximum TEC ranges from about 5 to 40 TEC units at Trivandrum and about 10 to 40 TEC units at Delhi, which correspond to range delay variations of about 1 to 8 m at the GPS L1 frequency of 1.575 GHz. The Maximum values of TEC were observed during spring equinox rather than autumn equinox, showing presence of semi annual variation at both the locations. The minimum values of TEC were observed during the summer solstice at Trivandrum indicating the presence of winter anomaly at equatorial region while Delhi TEC values were minimum during winter solstice showing absence of winter anomaly. Also the TEC values at both the locations have been decreasing since 2007 onwards exhibit good positive correlation with solar activity.

Modeling of Low-Latitude Ionosphere Using GPS Data With SHF Model

The Indian ionosphere is characterized by large variations, irregularities, and equatorial anomaly conditions. Therefore, an appropriate ionospheric model is necessary for satellite-based navigation systems. An ionospheric model based on spherical harmonic function (SHF) is used for modeling. Using the basis functions and measured vertical total electron content (TEC) data, spherical harmonic coefficients are computed by a weighted least square method. Seventeen stations of GPS Aided Geo Augmented Navigation TEC data are considered in the analysis. The results of the SHF model are presented in terms of accuracy of the residual error and compared with other standard grid models. The diurnal TEC variations are well depicted using the SHF model. The summer- and autumn-equinox-season ionospheric delay variations are also presented. The results indicate that the SHF model is capable of estimating ionospheric delays well.

Study of TEC, slab-thickness and neutral temperature of the thermosphere in the Indian low latitude sector

Abstract. The ionospheric equivalent slab-thickness is an important parameter which measures the skewness of the electron density profile of the ionosphere. In this paper, the diurnal, seasonal, day-to-day and latitudinal variations of ionospheric parameters namely total electron content (TEC), the peak ionization density of F-layer (NmF2), equivalent slab-thickness (τ) and neutral temperature (Tn) are presented. The simultaneous data of GPS-TEC and NmF2 from Trivandrum (8.47° N, 76.91° E), Waltair (17.7° N, 83.3° E) and Delhi (28.58° N, 77.21° E) are used to compute the slab-thickness (τ = TEC/NmF2) of the low sunspot period, 2004–2005. The day-time TEC values at Waltair are found to be greater than those at Trivandrum, while at Delhi the day-time TEC values are much lower compared to those at Trivandrum and Waltair. The trends of variation in the monthly mean diurnal variation of TEC and NmF2 are similar at Delhi, while they are different at Trivandrum and Waltair during the day-time. The slab-thickness (τ) has shown a pre-sunrise peak around 05:00 LT at all the three stations, except during the summer months over Delhi. A consistent secondary peak in slab-thickness around noon hours has also been observed at Trivandrum and Waltair. During equinox and winter months a large night-time enhancement in the slab-thickness (comparable to the early morning peak in slab-thickness) is observed at Delhi. The latitudinal variation of slab-thickness has shown a decrease from the equatorial station, Trivandrum to the low-mid latitude station, Delhi. The neutral temperatures (Tn) computed from the slab-thickness (τ) has shown a sharp increase around 05:00 LT over Trivandrum and Waltair. Whereas at Delhi, a double peaking around 05:00 and 23:00 LT is observed during winter and equinoctial months. The neutral temperatures computed are compare well with those of the MSIS-90 model derived temperatures.

Wave ionosphere perturbations with small periods under magnetic storm conditions

The characteristics of total electron content (TEC) variations obtained using the data of middle-latitude stations for receiving GPS satellite signals were studied. An analysis showed that perturbations of diurnal TEC variations during a geomagnetic storm were determined by strengthening of harmonics with periods of 4–5 and 6–8 h. Perturbations in diurnal TEC variations were retained for several days after a geomagnetic storm. An analysis of the frequency and amplitude characteristics of perturbed variations showed that differences between variations at different latitudes could be explained by the propagation of Poincare planetary waves in an atmospheric wave channel. The atmospheric channel was formed because of increased heating of pole regions during geomagnetic storms. A comparative analysis of observations made at different stations was used to estimate the width of the atmospheric channel and Poincare wave frequencies and wave vector components.

Diurnal variations of the total electron content under quiet helio-geomagnetic conditions

The morphology of averaged diurnal variations of total electron content (TEC) under quiet helio-geomagnetic conditions for all latitudinal bands and various longitudes has been studied using Global Ionospheric Maps (GIMs) datasets. The diurnal TEC variation maximum is generally registered at 14–15 LT. The maximum is 38±5, 14±2, 10±2 TECU (TECU is generally accepted TEC unit) at the equatorial, middle and high latitudes. The nighttime TEC minimum is within 5–7 TECU regardless of a season, latitude and longitude. At the equatorial latitudes TEC exhibits the most significant daily/season variations and the asymmetry of its behavior in the hemispheres near the equinox. Abnormal diurnal TEC variations (evening maximum, near-noon minimum) are observed at middle and high latitudes in summer due to atmospheric wind effects. The comparison of the averaged diurnal TEC variations with the behavior of the ionospheric F2-layer critical frequency indicated that GIMs describe daily/annual TEC variations reasonably well.

Seasonal variation of total electron content at crest of equatorial anomaly station during low solar activity conditions

The variability of total electron content (TEC) over the crest of equatorial anomaly station Bhopal has been studied during the low solar activity period (2005–2006) using global positioning system (GPS) data. Diurnal variation of TEC is studied for different seasons. Interesting features like the winter anomaly, semiannual anomaly and noon bite out in TEC have been reported. GPS derived TEC is then compared with International Reference Ionosphere (IRI) 2001 model and the difference between predictions and observation is being studied. Using the variability index we have also studied the TEC variability for different seasons and also during quiet and disturbed conditions. A higher variability is observed on quiet days as compared to disturbed days during daytime and nighttime hours.

TEC variations during low solar activity period (2005–2007) near the Equatorial Ionospheric Anomaly Crest region in India

Abstract. The dual frequency signals from the GPS satellites recorded at Rajkot (22.29° N, 70.74° E, Geographic, 14.03° N Geomagnetic) near the Equatorial ionization anomaly crest in India have been analyzed to study the ionospheric variations in terms of Total Electron Content (TEC) for the low solar activity period from April 2005 to December 2007. In this study, we describe the diurnal and seasonal variations of TEC, solar activity dependence of TEC and effects of a space weather related event, a geomagnetic storm on TEC. The diurnal variation of TEC shows pre-dawn minimum for a short period of time, followed by a steep early morning increase and then reaches maximum value between 14:00 LT and 16:00 LT. The mean diurnal variations during different seasons are brought out. It is found that TEC at Rajkot is at its maximum during Equinoctial months (March, April, September, October), and minimum during the Winter months (November, December, January, February), with intermediate values during Summer months (May, June, July, August), showing a semi annual variation. TEC values have been decreasing since 2005, onwards showing positive correlation with solar activity. TEC variations during the geomagnetic storm commencing 24 August 2005 with Dst=−216 nT are analysed. TEC shows a positive ionospheric storm effect on the first day of the storm and negative ionospheric storm effect on the next day. The equatorial Electrojet control on the development of the equatorial anomaly is also demonstrated.