In this study, we analyse the climatology of ionosphere over Nepal based on
GPS-derived vertical total electron content (VTEC) observed from four stations as defined in Table 1: KKN4 (27.80
Total electron content (TEC) is a crucial parameter of ionosphere comprising
high concentrations of electrons and ions formed under the ionization of
extreme ultraviolet (EUV) radiation and solar X-rays. The lower atmospheric
disturbance also contributes to ionospheric variability (Anderson and
Fuller-Rowell, 1999; Prikryal et al., 2010). Numerous periodic and aperiodic variabilities identified in the ionosphere make the impact on the applications involving the radio link between satellites and the ground, which plays vital role in the communication, navigation and surveillance, with important consequences for the reliability and accuracy of the service (Guo et al., 2015). The global positioning system (GPS) is widely used in recent appliances which encounter the largest errors in the path due to disturbed ionospheric free electrons, emphasizing the need to study GPS–TEC variability. The application of GPS technology gives scientists insight into the shape and behaviour of the ionosphere. A list of factors affecting TEC includes ionospheric electron density, ion–electron temperature, composition, dynamic variations with altitude, latitude, longitude, local time, seasons, solar and magnetic activity. Because the equatorial
ionosphere is highly vulnerable, it poses major threats to communication signals. The ionosphere at the mid latitude is less variable; hence,
most of the observations and measurements are taken from this region, whereas the high latitude ionosphere is sensitive to outer space as it is connected by geomagnetic field lines (Akala et al., 2013; Parwani et al., 2019). The study of VTEC at the low–mid ionosphere showed solar activity dependence (Shimeis et al., 2014). TEC has been studied by a large number of researchers; Rama Rao et al. (1980) studied the diurnal variation in TEC at Waltair, India, and found a short-lived predawn minimum, a steep early morning rise followed by broad mid-afternoon maximum and a steep post-sunset fall. The relation between TEC and the sunspot number (SSN), F
TEC studied at the Jet Propulsion Laboratory for the years 1998–2008 found stronger annual TEC variation in the Southern Hemisphere, and the variation in phase and amplitude is more in the conjugate hemisphere (Liu et al., 2009). Galav et al. (2010) found semiannual periodicity in daytime TEC, the spring equinox shows the highest TEC, and winter solstices are the lowest in India. The winter anomaly, semiannual anomaly and annual anomaly are described by Liu and Chen (2009) and Rishbeth and Garriott (1998). Global-scale TEC research found that the effect on TEC was stronger during the day than at night and also at low latitudes than at high latitudes. The effect on TEC is seen more on the either side of the magnetic equator than at the magnetic equator (Liu et al., 2009). Dashora and Suresh (2015) analysed the characteristics of low latitude TEC data of solar cycles 23 and 24 over Indian sector using global ionospheric data. A double hump structure in the solar flux and in TEC was identified at the low latitude station of Varanasi, India, in the ionospheric response using the GPS TEC, IRI (International Reference Ionosphere) and TIE-GCM (Thermosphere–Ionosphere–Electrodynamics General Circulation Model) TEC of solar cycle 24 by Rao et al. (2019a). Parwani et al. (2019) studied the latitudinal variation in ionospheric TEC in the northern hemispheric region and found that the diurnal TEC has a higher value in low latitudes than in mid and high latitudes and in the seasonal variation maximum in spring and autumn than in summer and winter.
Many studies on TEC have been conducted in Asia; however, no result for the climatology of TEC over Nepal, for a long time series, about one solar cycle has been reported up to now. In this paper, we present, for the first time, characteristics of ionosphere in Nepal, such as the diurnal, annual, seasonal and solar cycle dependence of TEC on the local ionospheric conditions, using GPS TEC data obtained from the four GPS stations of KKN4, GRHI, JMSM and DLPA (see Table 1). Our study includes GPS TEC data from 2008 to 2018 of solar cycle 24, including all four phases of this sunspot cycle, the minimum phase of the years 2008–2009, the ascending phase of the years 2010–2011, the maximum phase from 2012 to 2014 and the descending phase of years 2015–2018. The second section of this paper includes the data set and methodology, and the third includes the results and discussion. The concluding remarks are discussed in the last section.
Total electron content (TEC) is the total number of electrons integrated along the path from the receiver to each GPS satellite which orbits the Earth at an altitude of 20 200
The TEC obtained by this method is called slanted TEC (STEC), which is a
measure of the total electron content of the ionosphere along the ray path from the satellite to receiver and has to be converted to vertical TEC (VTEC) using the equation (Titheridge, 1972).
For this study, data were carried out with GPS data taken from four GPS
stations (DLPA, JMSM, KKN4 and GRHI) from Nepal. The details of the stations,
including their geographical and geomagnetic coordinates, are shown in Table 1 and universal time is used for all time references. The GPS data of the four stations were downloaded from
The selected GPS stations and their coordinates, the data of which are used in the study.
The data for the solar indices sunspot number (SSN) and solar flux index (F10.7) to study long term solar activity are taken from Royal Observatory of Belgium, Brussels (
In this study, we use GPS-derived TEC from RINEX files, using this method to
obtain TEC calibrated at 15
Classification of selected years according to the solar cycle phases.
This study analyses variations in VTEC during different phases of solar cycle 24, along with the annual, seasonal and diurnal variations. For this, the local seasons are classified as winter (November–February), spring (March and April), summer (May–August) and autumn (September and October). The classifications of the selected years, as per solar cycle phases, are presented in Table 2.
In this section, we present the diurnal, monthly, seasonal, solar cycle and geomagnetic variation in GPS TEC over Nepal during the solar cycle-24. Figure 1 represents the position of chosen GPS stations in Nepal for this study and Fig. 2 represents the variation in the sunspot number and solar flux during the period 2008–2018.
A map of Nepal showing locations of GPS stations used in our study.
Display the variations in the sunspot numbers and solar flux for the year 2008 to 2018.
Figure 3a exemplifies the diurnal variation in VTEC in LT observed during
2 February 2009, 2012, 2014, 2016 and 2017 during the minimum, inclining,
maximum and declining phases of solar cycle 24 at the KKN4 station in Nepal. The plot shows that, before sunrise
The observed diurnal VTEC pattern reflects the signature of different solar events. The noon bite-out profile with asymmetric peaks, parabolic profile and wave profile with morning, evening and night peaks and a few complex structures are noted in the diurnal profile. The quiet day activity at the minimum phase, the fluctuating activity during the increasing phase, shock activity during the maximum phase and recurrent activity during the declining phase was noticed in the study of ionospheric parameters at the Ouagadougou ionosonde station data in West Africa by Ouattara et al. (2009).
The upward
Mountains generate relief waves which propagate to the stratosphere and lower thermosphere (Leutbecher and Volkert, 2000). Studies on these waves have been made in Nepal in the lower atmosphere (Regmi and Maharjan, 2015; Regmi et al., 2017). Other studies have shown the impact of relief waves on the ionosphere in the Andes (Torre et al., 2014) and Tibet (Khan and Jin, 2018). In Fig. 3a, we see oscillations which cannot be interpreted directly as the signature of the waves. In fact, for the processing of GPS data, we use pseudo-range signals which can be affected by reflections on surrounding reliefs and by waves.
Figure 4 shows the monthly variability in VTEC for the maximum phase of solar
cycle year 2014 at KKN4 station. The plot is obtained using the average of the daily data. The plot shows the maximum in equinoctial months (March and April) and the minimum in solstices (January and June). The rise or fall of TEC in each curve follows the diurnal pattern, which is the prominent peak in the midday with different peak amplitude. The lowest VTEC peak is observed during January and the highest in March. Late afternoon peak are seen in March, June and September, whereas the peak centred at
Monthly variation in vertical TEC in LT for each month of 2014 at KKN4 station.
Figure 5 shows a 2D diurnal plot of VTEC at JMSM station for all four phases (I minimum – 2009; II ascending – 2011; III maximum – 2014; IV
descending – 2015) of solar cycle 24, which explains how the diurnal VTEC varies hourly during the four phases. In the ionosphere over Nepal, the features of equinoctial asymmetry is distinctly noticed in 2D plots of years 2009, 2011, 2014 and 2015 in Fig. 5a–d, respectively. From Fig. 5a–d, it can be observed that equinoctial asymmetry is not noticed in 2009, in 2011 autumn is more intense than spring, and in 2014 and 2015 spring VTEC is greater than autumn. In the year 2009, equinoctial asymmetry is not noticed during low solar activities. But in the year 2011, the autumn is more intense than spring, which is a feature of the equatorial ionization anomaly (EIA) crest latitude, and in the year 2014, the difference between equinoctial asymmetry is less (spring
In Fig. 6a–e, each panel separately represents the VTEC variation during the
autumn, spring, summer and winter seasons for the years 2008, 2009, 2011, 2014 and 2015 at KKN4, GRHI, JMSM and DLPA, respectively. The plots show that the maximum value of VTEC is
The solar flux dependency of the winter anomaly in GPS TEC has been studied by Rao et al. (2019b). The result showed that, when the level of solar flux in winter month is greater than the corresponding summer month, the winter anomaly is observed irrespective of whether the phases of solar cycle are is high or low. Their study also pointed out that the winter anomaly in GPS-derived TEC may not be a feature of any geophysical significance. The winter or seasonal anomaly is introduced due to temperature changes (Appleton, 1935), interhemispheric transport of ionization (Rothwell, 1963), significant changes in the Sun–Earth distance (Yonezawa, 1959; Buonsanto, 1986), seasonal variation in
Seasonal variability in VTEC during years 2008
In Fig. 7, the top left panel represents the variation in VTEC during spring, bottom left during autumn, top right during summer and bottom right during winter from 2008 to 2017 at KKN4. In spring, the difference in VTEC between high and low solar activity is 65
Mean yearly seasonal variations in VTEC for 2008 to 2017 at KKN4.
VTEC variability in GRHI station during minimum, increasing, maximum and decreasing phases of solar cycle 24.
The important parameter for the semiannual variation in ionospheric ionization is
the variation in the atomic
Annual mean VTEC variability in the KKN4, GRHI, JMSM and DLPA stations for the SSN and solar flux during year 2008–2018.
In 2019, Ansari et al. (2019) found the minimum value of TEC in January, which becomes a maximum in April, decreases in June–July and is followed by an increase in magnitude of the second maximum in September–October and later a decrease until December at CHLM, JMSM and GRHI in year 2017. Referring to Fig. 8, our result for semiannual variation shows that the minimum value of VTEC is found in January, which becomes the maximum in March–April, decreases in June–July, is followed by an increase in magnitude of the second maximum in October–November and later decreases until December at GRHI in years 2009 to 2018.
The asymmetry between the two equinoxes is due to geophysical parameters as magnetic indices related to geomagnetic activity (Triskova, 1989) and the interplanetary magnetic field (IMF Bz) the interplanetary component of magnetic field (Russell and McPherron, 1973). The equinoctial asymmetry observed in VTEC is explained by (i) the axial hypothesis (ii) the Russell–McPherron (RM) effect and (iii) the equinoctial hypothesis (Lal, 1996; Shimeis et al., 2014).
Ouattara and Amory-Mazaudier (2012) made a statistical model of the F
Figure 9 shows the annual mean values of VTEC, solar flux index and sunspot
number during the solar cycle from years 2008 to 2018. The black, blue, green
and red lines represent the VTEC variation at stations KKN4, GRHI, JMSM and DLPA, whereas pink and light green lines represents variation in SSN and
solar flux index, respectively. The plot shows that VTEC gradually begins to
increase in 2009 and reaches a maximum in 2014. Then it begins to decrease
until 2018, which agrees with the sunspot number and solar flux variation in
the same plot. The figure shows that the maximum value of the peak of ionization in 2014 is about 37
Similarly, the solar flux increases are from 2011 onward; the measured VTEC also exhibits the highest magnitude for the year 2014. The maximum VTEC value shows a decreasing trend from years 2015 to 2018 at all the stations used for this study. It is observed from the graph that the average annual VTEC shows better synchronization with SSN and solar flux index.
The patterns of the solar cycles play a major role in the solar variability, i.e. solar radiation and sunspot number consequently influence the
ionosphere. Solar cycle 24 is the smallest solar cycle since the start of the spatial era (1957), in which a peak is noticed in 2014 and a few major solar flares erupted from the Sun in February and October 2014 (Kane, 2002), so the
maximum VTEC is noticed in February and October as shown in Fig. 8. Again, from Fig. 8, a higher value for sunspot and solar flux was reported in February 2011, corresponding to an X-class solar flare at which a higher value of VTEC was noted in station considered. Sharma et al. (2012) studied how the VTEC variation in Delhi lies near the equatorial crest region during low solar activities in years 2007 to 2009 and found that TEC has a short-lived day minimum between 05:00–06:00
In the African sector, Tariku (2015) observed, from 2008 to 2009 and 2012 to 2013, high values of VTEC during the low and high solar activity phases. According to their findings, the diurnal VTEC values attained a maximum in the time interval of 13:00 to 16:00
This paper investigates the diurnal, monthly, seasonal and solar cycle
variations in VTEC at four mid–low latitude stations, namely KKN4 (27.80
The following conclusions are found:
The shape of the mean diurnal variation in VTEC depends on the solar cycle phases, i.e. a flat diurnal peak is observed during minimum and descending phases of the solar cycle, whereas a Gaussian with different peak amplitude is noticed during the ascending and maximum phases of the solar cycle. The study may reveal that diurnal TEC maximizes at around 11:00 to 14:00 Day-to-day variation in VTEC is significant in all the station. The maximum is noticed at KKN4 and the minimum at DLPA. The mean diurnal profile in the years 2008, 2009 and 2010 exhibit a wave-like nature, whereas a parabolic nature is observed in the years 2011, 2012, 2013, 2014, 2015, 2016 and 2017. The week ionospheric activities are characterized by lower TEC values during the minimum phase, and strong activities are characterized by a higher value of VTEC during the maximum phase, i.e. VTEC has shown proper synchronization with SSN and solar flux. The monthly plot shows that, during the sunrise time in summer, the VTEC is linear, whereas it is steep during the winter. Equinoctial asymmetry is not noticed in 2009; in 2011, the autumn is more intense than the spring, and in 2014 and 2015, the spring VTEC is greater than the autumn. Equinoctial asymmetry peaks are noticed in spring (March and April) and autumn (September and October), with higher values being observed during spring. The equinoctial asymmetry is noticed in all the available stations due to difference in the F10.7 The spring maximum is smaller than autumn maximum, mainly during years 2011–2013 and also during year 2008 for one station; these years are years of the minimum or increasing phase of the sunspot cycle. The VTEC in winter is greater than the VTEC in summer and is observed in all the available stations at the maximum of the sunspot cycle in 2014 and in one other station during the year 2011. During the year 2009 of the sunspot minimum, the VTEC in winter is greater than the VTEC in summer and is not observed for all the stations. There is no equinoctial asymmetry, i.e. it is very weak (compare to the year of the maximum), except at JMSM. It seems that, in Nepal for some years, there is no semiannual variation, as we observe sometimes that the summer VTEC is larger than VTEC in the autumn.
The highest Himalayan mountains on Earth in Nepal are the source of landform waves that travel through the stratosphere and the lower thermosphere, where they deposit their energy and give birth to secondary gravity waves that can affect VTEC. In our climatology study, we analyse average behaviours that do not allow the study of these waves. Another study analysing each day individually and using the phase processing of GPS signals should be done in the future to analyse the impact of the Himalayas on VTEC and the impact of the low atmosphere on VTEC.
The data for this study are available at
DP developed the idea of this paper, prepared introduction, collected the data sets and contributed to the methodology and conclusions. BG contributed in writing the discussion of the results. CAM contributed to the data set and data analysis and conclusion, assisted with editing the paper and helped shape the paper. RF provided the software for this study and contributed to the methodology and editing of the text. NPC gave overall feedback on this paper by reviewing the paper thoroughly and giving the complete paper a shape. BA contributed to the results and discussion and reviewed the paper thoroughly.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We acknowledge
This paper was edited by Dalia Buresova and reviewed by S.S. Rao and two anonymous referees.