1 Introduction

It is well known that the variations of the Earth’s ionosphere are complicated and may behave quite differently from region to region. The application of trans-ionospheric communication systems such as satellites, aircraft and surface transportation systems have increased enormously in last few years. Therefore, the extensive knowledge of the morphology of the ionospheric TEC is of great importance for the scientific community. Total electron content is the total number of electrons in a vertical column with cross sectional area of one square meter centred on the signal path. TEC is measured in TEC Units (1 TECU=1016 electrons per m2). The group delay which the ionosphere contributes to the trans-ionospheric communication signals is directly related to the TEC of the ionosphere. The accurate values of TEC is required for making appropriate range corrections, as well as to account for errors introduced in the range delays owing to the effects of space weather related events, such as geomagnetic storms and scintillations due to ionospheric irregularities. The temporal and spatial characteristics of the TEC of the ionosphere in the global perspective are well known (Kane 1980). The ionization in the equatorial and Polar Region ionosphere is known to be high compared to relatively moderate levels of electron content in the mid-latitude ionospheric regions. Significant research has been done for the high and mid latitude regions of the ionosphere and there are several ionospheric models for these regions too. However for the equatorial and low latitude ionosphere, few corresponding research has been done. As the morphology of the equatorial ionosphere is quite different from that at other latitudes because the magnetic field lines are nearly parallel to the Earth’s surface. During daytime, the E-region dynamo electric field is eastward. An eastward electric field in the E region at off-equatorial latitudes maps along the Earth’s magnetic field to F-region heights above the magnetic equator. The resulting E×B drift transports F-region plasma upward at the magnetic equator. The uplifted plasma then moves along magnetic field lines in response to gravity and pressure-gradient forces. As a result, the equatorial (Appleton) anomaly is formed with minimum F-region ionization density at the magnetic equator and maxima at the two crests approximately 15° to 20° in magnetic latitude to the north and south. These physical processes contribute to make the equatorial and low latitude ionosphere highly dynamic and unpredictable. Several researchers have made significant contributions by making individual measurements of TEC from various locations in India (Rastogi and Sharma, 1971; Dasgupta and Basu, 1973; Rastogi et al., 1975; Rama Rao et al., 1977; Davies, 1980; Dabas et al., 1993). The results of their study have revealed the broad characteristic features in the morphological behaviour of the total electron content in the Indian sector. Rama Rao et al. (2006) reported the spatial and temporal variations of the equatorial ionosphere, using the simultaneous measurements of TEC, made with the GPS network of receivers from the equator to the northern Equatorial Ionization Anomaly (EIA) crest region and beyond in the Indian sector. They studied the diurnal, seasonal and latitudinal variations of TEC for a 16-month period during the low sunspot activity (LSSA) period from March 2004 to June 2005. Liu et al. (2009) analyzed the 11 years TEC data derived from GPS observations to investigate the overall climatological features of the ionosphere. They found that the mean TEC data show strong annual/semiannual, solar cycle, and 27-day variations and the mean TEC present stronger solar activity sensitivity at lower-latitude bands.

The application of GPS has increased enormously in navigation, surveillance, and recently, in the aircraft’s landing, by augmenting the GPS with a Space Based Augmentation System (SBAS) signal. Therefore, the TEC measurements have gained importance in estimating the range delays involved in the GPS-based navigation. As it is well known that the period since 2007 has been an ongoing solar minimum. Scientifically the debate has begun on whether the impact of this solar minimum is different from the recent 4 or 5 well documented solar minimum periods. The purpose of present study is to explore the climatological variations in ionospheric GPS-TEC recorded by dual frequency GPS receivers located at Trivandrum and Delhi during the extremely low solar activity periods of solar cycle 23 i.e. from January 2007 to June 2009. Also this study describes the solar cycle dependence of the TEC.

2 Data and method of analysis

Since October 2006, National Physical Laboratory, New Delhi is operating two dual frequency GPS receivers; one at NPL, New Delhi (28.6 °N, 77.2 °E, located beyond equatorial ionization anomaly crest region) and the second one at Space Physics Laboratory, Trivandrum (8.4 °N, 76.6 °E, an equatorial station) in order to study the equatorial and low latitude ionosphere in detail by using the continuous measurements of TEC. The GSV 4004B Ionospheric Scintillation and TEC Monitor receivers (GISTM) collect the TEC and scintillation data (Van Dierendonck et al. 1996) per minute from both the stations. Each GISTM can track up to 11 GPS C/A-code signals at the L1-frequency of 1.575 GHz and L2 frequency of 1.2 GHz. For accurate measurements of TEC associated with the GPS observations, It is required to correct the data for the inter frequency biases or satellite biases at the time of data logging or at the time of data analysis. The GPS-TEC data used in the present study was logged by updating the slog program with inter frequency biases on regular basis by using the university of Berne CODE website; http://www.aiub.unibe.ch/content/research/gnss/code_research/igs/differential_code_biases_dcb/index_eng.html. This procedure provides the proper values of slant TEC.

In the present study, the slant TEC data, recorded per minute from two Indian stations during December 2006 to June 2009 is used to describe the diurnal, seasonal and annual variations in equatorial and low latitude ionospheric TEC. As slant TEC is dependent upon the ray path geometry through the ionosphere, it is required to calculate an equivalent vertical value of TEC which is independent of the elevation of the ray path. The vertical TEC is obtained by applying a mapping function ‘m’ to slant TEC measurements. The mapping function is defined as (Mannucci et al. 1993; Langley et al. 2002);

where E is the elevation angle of the satellite in degrees; m is the mapping function with zenith angle α at the ionospheric pierce point; R EARTH is the mean radius of the earth in kms and h m is the ionosphere height above the earth’s surface.

The Vertical TEC data thus obtained is further processed for each of the satellite passes with an elevation mask angle greater than 40° in order to avoid the effects of low elevation angles, such as tropospheric, water vapour scattering and multi path effects. Figure 2(a) and 2(b) illustrates the sub-ionospheric coverage area of GPS satellites over Trivandrum and Delhi respectively with an elevation mask angle of 40° in fourth panel. Therefore the TEC observations are restricted to a latitude grid of ±2∘ and longitude grid of ±2∘ from the observing sites. The vertical TEC thus calculated is used in deriving the results presented in the following sections of this paper.

3 Results and discussion

3.1 Diurnal variations of TEC over equatorial and low latitude region

The slant TEC data recorded by GPS receivers at Trivandrum and Delhi are converted to vertical TEC as described in Sect. 2, in order to study the temporal, seasonal and annual variations of TEC in the equatorial and low latitude Indian region during the peculiar solar minimum of cycle 23. Figure 1 display variations of R12 since January 2001 to April 2012. Figures 2(a) and 2(b) displays the diurnal variation of vertical TEC for 7 January 2008 derived from all the visible satellites of two stations, Trivandrum and Delhi. The first panel of Fig. 2(a) shows the diurnal variation of raw VTEC data i.e. without applying any elevation mask angle. The second panel illustrates the VTEC variation after applying an elevation mask angle of 40°. These vertical TEC values are derived for a ±2∘ latitude and ±2∘ longitude bin from the Delhi and Trivandrum for all the visible satellites. It is observed from these figures the diurnal variation of TEC measured from different satellite passes spread significantly due to the spatial and temporal variations in TEC, and also the GPS satellites are spread in different parts of the sky at different local times. However, the spread in TEC at the equatorial station Trivandrum is small as compared to Delhi. At Delhi, the spread of VTEC values may be due to the ionization distribution around its latitudes. These figures clearly indicate the diurnal pattern of TEC exhibits a steady increase from about sunrise to an afternoon maximum and then falls again to attain a minimum just before sunrise.

Fig. 1
figure 1

Variations of R12 from January 2001 to April 2012

Full size image
Fig. 2
figure 2

(a) The first panel shows the diurnal variation of raw VTEC data at Trivandrum for 7 January 2008. The second panel shows VTEC variation after applying an elevation mask angle of 40°. The red line in third panel represents the average VTEC values with an elevation mask angle of 40°. The fourth panel shows the sub-ionospheric coverage of GPS over Trivandrum with an elevation mask angle of 40°. (b) Same as Fig. 1(a) but for Delhi

Full size image

In order to obtain a good average diurnal plot of TEC, hourly average of TEC of all satellites in view during that one hour period is taken and that particular value is putted in the middle of that hourly intervals. This procedure is repeated starting from 0 LT to 23 LT. In this way the average values of TEC are calculated for both the stations and the averaging is done both in space and time. The average values are plotted in third panel of Fig. 2(a) and 2(b) showing the diurnal variation of TEC over Trivandrum and Delhi respectively for 7 January 2008. Repeating this method, the average values of TEC are calculated for each of the day of every month from December 2006 to June 2009 for a detailed study of temporal, seasonal and annual variations of TEC over the equatorial and low latitude Indian region. Figures 3(a), 3(b), 4(a) and 4(b) show the mass plots of diurnal variations of VTEC for the months of year 2007, 2008 and 2009 for Trivandrum and Delhi respectively. These curves show appreciable day to day variability. On comparing the mass plots of years 2007, 2008 and 2009 for all the months, it is obvious from the figures that at both the locations, the magnitude of the daily maximum TEC decreases since 2007. The daytime peak value of TEC ranges from 5 to 40 TEC Units in 2007 and nearly equal daytime peak value of TEC (∼5 to 30 TEC Units) in 2008 and 2009 at Trivandrum. According to the study of Rama Rao et al. (2006), the daily peak value of TEC ranges from, 5 to 55 TEC Units at Trivandrum and 5 to 50 TEC Units at Delhi during the low solar activity period of year 2004–2005. Also, the diurnal spread in TEC values is larger at Trivandrum as compared to Delhi. In general, it may be seen from the figures that the diurnal variation shows a sharp and short-lived day minimum (∼5 TECU at Trivandrum and ∼10 TECU at Delhi) in TEC occurring around 0500 to 0600 LT. The variations can be grouped into three different sections, namely: the build up region, the day time plateau and the decay region. The diurnal variation in TEC at Trivandrum and Delhi exhibit many characteristics typical to equatorial low latitude ionosphere such as a TEC minimum at pre-dawn and gradual increase with the time of day attaining a maximum in the afternoon and a gradual decrease after sunset. The daily peak occurs around 1200–1400 LT at Delhi. A delayed (1600 to 1700 h LT) day maximum (40 TEC units) in TEC occurs at Trivandrum. From these figures, it is also observed that the TEC seasonal behavior depends on the solar cycle. The largest daily TEC values are observed during the equinoctial months at both the stations.

Fig. 3
figure 3

(a) and (b) Mass plot of diurnal variation of vertical TEC at Trivandrum from January 2007 to May 2009 with the exception of July 2007 to December 2007 due to instrument problem

Full size image
Fig. 4
figure 4

(a) and (b) Mass plot of diurnal variation of vertical TEC at Delhi from January 2007 to June 2009

Full size image

During the equinoctial months of March and April, it may also be noticed from Figs. 3(a), 3(b), 4(a) and 4(b) that the day minimum in TEC occur around 0500 to 0600 LT at equatorial region Trivandrum, whereas beyond the anomaly crest region, i.e. at Delhi, the day minimum is flat during most of the nighttime hours (2200 to 0600 LT). Secondly, the early morning increase in TEC is relatively fast at Trivandrum, compared to that at Delhi. During the winter months similar features are also seen but with reduced intensity, whereas during the summer months the nighttime values of TEC are almost constant at the minimum level during most of the nighttime hours. In all these figures, one noticeable feature is the day-to-day randomness in the variation of TEC, particularly at Delhi during the mid-day to pre-dawn hours, which is of serious concern in forecasting, as well as in navigation.

This randomness in the day-to-day variation in TEC over equatorial and low latitudes indicates that the different regions need different range corrections to be made in the navigational applications (Rama Rao et al. 2006). This randomness in day-to-day variation of TEC (Kane, 1980; Mendillo et al., 1980; Modi and Iyer, 1989) may be attributed to the changes in the activity of the Sun and to the associated changes in the intensity of the incoming radiations with the zenith angle at which they impinge on the Earth’s atmosphere. The day to day variability of TEC also depends upon the changes which take place in the Earth’s magnetic field and the strength of the equatorial electrojet (EEJ) strength and the local atmospheric conditions in the thermosphere (Dabas et al., 1984; Rama Rao et al., 1980).

3.2 Seasonal variations of TEC over equatorial and low latitude region

Adopting the procedure described in Sect. 3.1, the data for the diurnal variation of 30-month period, i.e. from January 2007 to June 2009, is used to calculate the monthly mean TEC values. These monthly mean TEC values are then used to study the seasonal variations of VTEC for each year by dividing a year into four seasons i.e. Spring equinox (February to April), summer solstice (May to July), autumn equinox (August to October) and winter solstice (November to January). The contour diagrams of monthly mean VTEC variations for Trivandrum and Delhi, representing two different latitude zones in the Indian sector are shown in Fig. 5. It may be seen from this figures that the diurnal maximum in TEC occurs approximately between 12:00 to 14:00 LT at Delhi, whereas at the equatorial station Trivandrum, the occurrence of the diurnal maximum is slightly delayed and occurs around 16:00 LT. It is evident from Fig. 5 that value of diurnal peak in VTEC is maximum during the spring equinox for both the stations. However, the minimum values of VTEC are found during summer solstice at Trivandrum and during winter solstice at Delhi. Thus spring equinoxes show larger value of VTEC than the autumn equinoxes for both the stations. As the solar flux decreases from 2007 onwards, the measured TEC also exhibits lower magnitude for the year 2008–2009 in comparison to 2007. Thus the daily maximum TEC values show a decreasing trend since year 2007 to 2009 both at Trivandrum and Delhi.

Fig. 5
figure 5

Seasonal variation of vertical TEC during 2007 to 2009 at Trivandrum and Delhi

Full size image

In addition to the contour plots, the diurnal variations of mean VTEC during different seasons i.e. winter, summer and equinox recorded at Trivandrum and Delhi are also shown separately in Fig. 6. Figure 6 clearly indicates higher TEC values in equinoctial months followed by more or less similar values in summer and winter at both the stations. It is well known that the thermospheic neutral composition has a direct control on the seasonal variation of TEC. During the daytime, the equator is hotter than the pole therefore meridional wind flows towards the pole from the equator. This flow of meridional wind changes the neutral composition and O/N2 decreases at equatorial and low latitude stations. In equinox, this decrease will be maximum. At 350 km altitude (F2 layer), N2 dissociation is the major process which removes ambient electrons. Hence, the decrease in O/N2 ratio will result in higher electron density and therefore in equinox TEC will be highest.

Fig. 6
figure 6

Contour plot of monthly mean VTEC variation over Trivandrum and Delhi during extreme low solar activity period of 2007–2009

Full size image

Thus the seasonal variation of VTEC over Trivandrum and Delhi shows a semiannual pattern, maximum in equinoxes and minimum in summer and winter. Ionospheric semi-annual anomaly has been known to exist for a long time. The presence of semi-annual anomaly indicates the role played by the solar zenith angle variation in the changes produced by the level of production of ionization, added to the role played by the equatorial ionization anomaly and the strength of the equatorial electrojet, which also maximizes during the equinoctial months. During the low solar activity period, the seasonal variability of daily peak TEC is comparatively low which agrees with the results shown by Modi and Iyer (1989) and Bagiya et al. (2009).

3.3 Solar activity dependence of TEC over equatorial and low latitude region

Solar activity has impacts on magnetospheric dynamics, which in turn influence the plasma density distribution with in the ionosphere. To quantify the variations in the TEC of the ionosphere with solar activity, along with the sunspot number, the flux of the Sun’s radio emission at a wavelength of 10.7 cm is a useful indicator of solar activity relevant for ionospheric effects. The 10 cm flux is the radio power of sun at a frequency of 2800 MHz. This flux is a good indicator of solar activity and is widely used in place of sunspot number. Rama Rao et al. (1985) reported the direct control of solar activity on the ionization level, with higher values during a high solar activity period and low value during a low solar activity period. However the range of solar flux variation during the period of observation used in the present study is very limited. Figure 7 shows the monthly mean variation of Trivandrum and Delhi TEC values during 2007 to 2009 with 12-month smoothed sunspot numbers R12. It is clear from Fig. 7 that the diurnal peak TEC shows semiannual variation with a peak during the spring equinox period and a trough during the summer solstice period. It is evident from the Fig. 7, as the R12 values show decreasing trend from 2007 onwards, in response, the measured TEC also exhibits lower magnitude. One more noticeable feature in Fig. 7 is that there is no variation in TEC day minimum values over Trivandrum with the seasons or the solar activity. The day minimum values of TEC are almost at constant level over Trivandrum. However Delhi TEC day minimum values show semi-annual variation likewise day maximum values. Moreover the day time peak value of TEC for all the seasons is correlated with solar F10.7 flux. Figure 8(a) illustrates the linear correlation analysis between the daytime peak values of TEC of Trivandrum and the solar F10.7 flux for the equinox, winter and summer respectively. Figure 8(b) displays the linear correlation analysis between the daytime peak values of TEC of Delhi and the solar F10.7 flux for all the three seasons. These figures clearly indicate good positive correlation (Correlation Coefficient R=0.99) between the two, for the equinoctial months as compared to summer and winter seasons of years 2007–2009 for Trivandrum and Delhi. During the period of a low sunspot number, the TEC builds up quite slowly, resulting in a low value of day maximum. Warnant et al. (2000) have reported higher values of TEC with increasing solar activity.

Fig. 7
figure 7

Month to month variation of daily mean peak of vertical TEC from 2007 to 2009 at Trivandrum and Delhi with R12

Full size image
Fig. 8
figure 8

(a) Solar cycle dependence of vertical TEC as observed by Trivandrum GPS receiver. (b) Same as Fig. 7(a) but for Delhi GPS data

Full size image

4 Summary

This paper describes the mean diurnal, seasonal and annual variations in TEC during the extended solar minimum period (2007–2009) of solar cycle 23 over the equatorial and low latitude stations namely Trivandrum and Delhi respectively. Since the beginning of 2007, the weak ionospheric activities were observed due to decreasing solar activity. The weak ionospheric activities are characterized by lower TEC values. From the observed results it may be concluded that the diurnal variation of the TEC at Delhi maximizes at around 1200 to 1400 LT, with a minimum in the pre-dawn period. However near the equator the day maximum is broad and its peak is delayed and occur around 1600 LT. Similarly, the day minimum in TEC occurs between 0500 and 0600 LT at both the stations. However, at Delhi an extended day minimum is found to occur which is flat during most of the nighttime hours, i.e. from 2200 to 0600 LT, a feature which is similar to that at mid latitudes. The diurnal variation in TEC shows a minimum to maximum variation of about 5 to 40 TEC units at Trivandrum and from 10 to 40 TEC units at Delhi. These TEC values correspond to range delay variations of about 1 to 8 m, at the GPS-L1 frequency of 1.575 GHz. These variations in the range delays will certainly increase in high sunspot activity periods. The day-to-day variability in TEC is also significant at both the stations, particularly during daytime hours, with maximum variation at Delhi during equinoctial months. The results discussed in the paper show a good positive correlation between solar activity and TEC values.