Diurnal and seasonal variation of GPS-TEC during a low solar activity period as observed at a low latitude station Agra

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1 Indian Journal of Radio & Space Physics Vol. 40, February 2011, pp Diurnal and seasonal variation of GPS-TEC during a low solar activity period as observed at a low latitude station Agra Vishal Chauhan 1,$, *, O P Singh 1 & Birbal Singh 2,# 1 Department of Physics, Faculty of Engineering & Technology, R B S College, Bichpuri, Agra , India 2 Department of Electronics & Communication Engineering, Faculty of Engineering & Technology, R B S College, Bichpuri, Agra , India $ vishalparam84@yahoo.co.in, vishalparam84@rediffmail.com; # bbsagra@gmail.com, bbsagra@yahoo.co.in Received 7 December 2009; revised received and accepted 20 January 2011 Employing a dual frequency GPS receiver at a low latitude station Agra (geographic lat N, long E, dip ), India (located just outside the equatorial anomaly crest), the measurement of ionospheric total electron content (TEC) has been carried out since 24 June Three years of data for a low solar activity period during 01 August July 2009 has been analysed. The temporal, seasonal and solar activity dependence of GPS-TEC data has been studied for the ionospheric region around Agra. The results show that the mean TEC varies from a minimum at 0500 hrs LT (LT=UT+5.5 hrs) to a peak value at about 1400 hrs LT and then decreases. The lowest values of TEC have been observed in winter whereas highest values have been observed in equinox and summer. The variation of the data has been compared with the variation of three solar indices, i.e. EUV flux, 10.7 cm solar radio flux (F 10.7 ), and sunspot numbers (SSN) during the period of analysis. It is found that all the solar indices show high correlation with day maximum TEC (TEC max ) in the summer than in equinox and low correlation is shown in winter season. It is also found that the data show high correlation with F 10.7 in the two seasons of summer and equinox while the same is found with EUV flux in winter. The correlation of TEC max and SSN is found to be low as compared to those of F 10.7 and EUV flux in all seasons. The GPS-TEC data obtained at Agra station for the year 2009 has also been compared with global ionospheric maps (GIMs) of TEC and a very good correlation is found between the two. Further, GIMs of TEC have been analysed for five different latitudes along the common meridian of 80 0 E longitude. The present study has been carried out for three months of the year 2009 in each season. The role of equatorial ionization anomaly (EIA) crest is clearly visible on TEC data. These results are significant for Agra station which is a new location in the low latitude sector of Indian region. Keywords: Ionospheric total electron content (TEC), GPS-TEC variation, Solar activity, Global ionospheric maps (GIMs) of TEC PACS Nos: dv; qd 1 Introduction The total electron content (TEC) is an imperative ionospheric parameter. The study of temporal, spatial and solar-induced variations in TEC is very useful for the users of satellite based radio systems. The changes in TEC are of serious concern at low and equatorial latitudes. In the last decade, global positioning system (GPS) based applications are being used widely 1,2 and they encounter largest errors due to the free electrons in the path of the satellite signal 3. Therefore, variations in GPS-TEC have been studied extensively 4-6. The GPS-TEC is the total number of electrons in a vertical column of 1 m 2 cross-section from the height of the GPS satellite (~ 20,000 km) to the receiver on the ground. It is measured in TEC Unit (TECU) and 1 TECU is equal to el m -2. Many researchers have studied the morphological features of TEC at low and equatorial latitudes 5,7-9. Solar activity dependence of TEC has also been studied by a large number of reseachers Rama Rao et al. 14 studied the diurnal variations in TEC at Waltair in India. They observed many characteristics typical to low latitude ionosphere such as short lived pre-dawn minimum, a steep early morning rise followed by broad mid afternoon maximum and a steep post sunset fall. Dabas et al. 11 studied the variations in TEC with different solar indices, i.e. EUV, F 10.7 solar flux and smoothed sunspot number (SSN) for summer, winter and equinoxes. They concluded that TEC exhibited nonlinear relationship with SSN in general and linear variations with EUV and F 10.7 solar flux. Rama Rao et al. 12 detected a good correlation of TEC with SSN (0.84) during the years Gupta & Singh 15 observed long term ionospheric TEC

2 CHAUHAN et al.: DIURNAL AND SEASONAL VARIATIONS OF GPS-TEC DURING LOW SOLAR ACTIVITY 27 variations over Delhi, India for the period and They concluded that winter anomaly in TEC appears only during high solar activity. They also found a positive correlation between TEC and F 10.7 solar flux. Rama Rao et al. 5 presented the temporal and spatial variations in TEC derived from the simultaneous and continuous measurements for the first time using the Indian GPS network of 18 receivers located from the equator to the northern crest of the equatorial ionization anomaly (EIA) region and beyond, covering a geomagnetic latitude range N. In the analysis, they used 16 month data for the low sunspot activity period March June In their findings, alongwith the diurnal and seasonal variations in TEC, the day-to-day variability was also significant at all the stations, particularly during the daytime, with maximum variations at the EIA crest regions. Chakraborty & Hajra 13 analyzed the TEC data at Calcutta, a station situated vertically below the northern crest of equatorial ionization anomaly. They studied the dependence of TEC on EUV radiation from the sun and found that besides day-to-day changes in TEC, monthly mean TEC was very well correlated to F 10.7 solar flux. By using a dual frequency GPS receiver, Bagiya et al. 6 investigated diurnal and seasonal variations of TEC during low solar activity period ( ) at Rajkot, a station near the equatorial ionization anomaly crest in India. It was found that TEC was maximum during equinoctial months (March, April, September and October) and minimum during winter months (November, December, January and February), with intermediate values during summer months (May, June, July and August). In this paper, the results of TEC measurement with a dual frequency GPS receiver for three years during 01 August July 2009 at Agra station (geographic lat N, long E, dip ) have been presented. The results show significant diurnal, seasonal and solar activity dependence of TEC. The GPS-TEC data of Agra station has been compared with GIM-TEC data for the year 2009 and a very good correlation, very close to 1, has been found. A latitudinal study of GIM-TEC data reveals the effect of EIA crest on TEC data. These results are very similar to those obtained by earlier researchers. 2 Experimental set up A dual frequency ( and MHz) GPS receiver system (GSV4004B) has been employed at Agra station for the measurements of TEC. The equipments for this purpose (imported from USA) include an L 1 /L 2 GPS antenna (Novatel s Model GPS 702), a GPS receiver (Novatel s Euro Pak 3-M) and relevant software. The TEC measurements have been carried out through combined frequencies of pseudorange and carrier phase measurements. The instrumental biases such as receiver and satellite biases are taken care of before final TEC calculations as seen in the following formula: TEC = [9.483*(PR L2 -PR L1 - C/A-P, PRN ) + TEC RX + TEC CAL ] TECU where, PR L2, is the L2 pseudo-range in meters; PR L1, the L1 pseudo-range in meters; and C/A-P, PRN, the input bias between SV C/A- and P-code chip transition in meters. There are 32 offset values (one for each satellite) which added to the C/A code pseudo-range measurements. TEC RX is the TEC result due to internal receiver L1/L2 delay and TEC CAL is the user defined TEC offset. The continuous monitoring of TEC data has been in progress since 24 June Retrieval of data and Method of analysis The above mentioned GPS equipments provide slant TEC data at a sampling rate of 60 s. From this data, vertical TEC (VTEC) values are obtained at different ionospheric pierce point (IPP) locations by using a mapping function 2, i.e. where, R E, is the mean radius of the earth in km; h S, the ionosphere (effective) height above the earth s surface; z, the zenith angle; and E, the elevation angle in degrees. Rama Rao et al. 16 have concluded in their study that sub-ionospheric height of 350 km is well suited for the latitudes of the Indian region. Therefore, sub-ionospheric height of 350 km is taken for the calculation of VTEC. Further, to avoid the abnormal values in TEC at low elevation angles due to the troposcatter, multipath, etc. 5, TEC values are taken at higher elevation angles > Since many TEC values are obtained at a time from different satellites, a running average is taken to get a single curve for a day. To study the solar activity dependence of TEC, the solar data have been obtained through internet from

3 28 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2011 the solar data services of National Geophysical Data Center (NDGC) and NOAA satellite information services (website: The details of quiet and disturb days are obtained from the website of World Data Center, Kyoto, Japan ( The global TEC maps in the IONEX format are accessible at the site ftp://cddis.gsfc.nasa.gov/gps/ products/ionex. The combined International GNSS Service (IGS) TEC maps denoted by the prefix igsg in the IONEX filename have been used in this study. It is to be noted that on the basis of observations from hundreds of ground-based GPS receivers distributed globally, IGS provides the global VTEC maps, namely the GIMs of TEC, to the public. These global TEC maps are generated routinely by the IGS community with the temporal resolution of 2 h and the spatial resolution of 5 0 and in longitude and latitude, respectively. The estimation accuracy for the TEC maps provided by IGS is about 10-20%. (ref. 21). The TEC data from GIMs of TEC has been denoted as GIM-TEC in this paper. 4 Results 4.1 Diurnal variation in TEC Diurnal variations in TEC have been studied by plotting mass plots of TEC for each month for a period of three years between 01 August 2006 and 31 July The data during magnetically quiet periods has been considered for this study. As an example, data for the month of March 2009 is shown in Fig. 1. The diurnal curves shown in this figure are obtained by the procedure mentioned above. It can be seen from the figure that day minimum in TEC is obtained around 0500 hrs LT while day maximum occurs around 1400 hrs LT. Almost similar diurnal pattern is observed for all the months of different seasons. In general, the diurnal variation of TEC shows a shortlived pre-dawn minimum, a steady early morning increase, followed by an afternoon maximum and gradual fall after sunset. The observations of diurnal variation in TEC show that the time at which TEC reaches the diurnal peak vary from day to day and month to month. In general, the diurnal peak in TEC is found during hrs LT. Large variations in TEC are observed in daytime, while nighttime variations are found to be almost constant. The variation of monthly diurnal mean of TEC for the above mentioned period of 36 months has been shown in the contour plot of Fig. 2. The diurnal features mentioned above are reflected very well in this figure. The equinoxial months of March, April, September and October show highest values of TEC. These are followed by the summer months of May, June, July and August. The lowest values of TEC are obtained in the winter months of November, December, January and February. It can also be seen that TEC values are in descending order from the year 2006 to Seasonal variation in TEC The seasonal variation in TEC for the year 2007 and 2008 has been shown in Figs 3(a and b), respectively. In this figure, local time variation of TEC during the period of equinox, summer and winter months has been shown. The two equinox months are combined together and summer and winter solstices are represented by groups of four months each. For example, summer solstice includes the months of Fig. 1 Monthly mass plots of the diurnal variation of TEC for the equinoxial month of March 2009

4 CHAUHAN et al.: DIURNAL AND SEASONAL VARIATIONS OF GPS-TEC DURING LOW SOLAR ACTIVITY 29 Fig. 2 Contour plot of the monthly average diurnal variation of TEC during 36 months (01 August 2006 to 31 July 2009) May, June, July and August and winter solstice is represented by November, December, January and February. As seen from the figure, the variation during equinox and summer are almost similar whereas the variation during winter differs markedly from the rest of the two periods. Fig. 3 Seasonal variations in TEC for the year: (a) 2007; (b) TEC variation during varying solar conditions The ionosphere is primarily affected due to solar radiations. A wide spectrum of radiation is emitted from the Sun. In general, these solar radiations are measured in terms of three solar indices, i.e. EUV flux, 10.7 cm solar radio flux (F 10.7 ), and sunspot numbers (SSN). Out of these indices, F 10.7 and SSN are used in the absence of EUV flux data. The reason to consider all three parameters of solar variability here is that a solar index can be found which correlates very well in all seasons at Agra so that it can be used for further studies. The correlation of these indices with GPS TEC has been studied for summer, winter and equinoxes. In the upper panel of Fig. 4, the day maximum value of TEC (TEC max ) has been shown for the above mentioned period of 36 months and the lower panel of this figure shows the variation of all three indices (EUV flux, F 10.7 and SSN) for the same period. In the upper panel, annual and semi-annual variations can be seen in TEC max data

5 30 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2011 Fig. 4 Variation during 36 months (01 August 2006 to 31 July 2009) of: (a) day maximum TEC (TEC max ); and (b) variation of three solar indices, i.e. SSN, F 10.7, EUV flux while these variations are not observed in the solar indices data. It can be seen from the figure that the values of all three indices are enhanced considerably in the years 2006 and 2007 and remain almost constant onwards. The TEC max is also found to be large in the years 2006 and 2007 and its values are lowered in the succeeding years. Figure 5 shows the correlation of TEC max with all three solar indices during the three seasons, i.e. summer, equinox and winter. For each season, the data of four months are grouped together. It can be seen from the figure that all three indices show highest correlation with TEC max in summer which is followed by equinox. The lowest correlation of both is obtained in winter. However, in summer and equinox, the TEC max show highest correlation with F 10.7 and in winter with EUV flux. The values of correlation coefficient (R) for EUV flux have been found to be 0.446, and in the summer, equinox and winter seasons, respectively. The values of R for F 10.7 have been observed as 0.509, 0.313, and in the above three seasons, respectively. The values of R for SSN are found to be 0.287, and for all three seasons, respectively. These results have been indicated in the corresponding figures also. Since this study is carried out for a low solar activity period, the range of solar indices is too small to obtain reliable correlation. However, it is proposed to compare these results with the data of high solar activity period in further study. The L-band scintillations studies are proposed to be carried out in the future work. 4.4 Comparison between GPS-TEC and GIM-TEC As mentioned earlier, global vertical TEC maps are provided by IGS. To compare the GPS-TEC data of Agra station, the GIM-TEC data for a location (very near to Agra) has been obtained. The geographic coordinates of this location are N latitude and 80 0 E longitude. The data for the year 2009 has been compared and a very good correlation throughout the year has been found. For brevity in the paper, only an example for the month of May 2009 has been presented in Fig. 6. In this figure, the GIM-TEC data has been shown by continuous solid lines while the GPS-TEC data has been represented by dashed lines. The GPS-TEC curves were not seen for few days at

6 CHAUHAN et al.: DIURNAL AND SEASONAL VARIATIONS OF GPS-TEC DURING LOW SOLAR ACTIVITY 31 Fig. 5 Plots of daily TEC vs EUV flux, F 10.7 and SSN for: (a-c) summer; (d-f) equinox; and (g-i) winter the end of the month due to non-availability of data because of power failure. However, a close variation can be seen on almost all the days of this month. In Fig. 7, correlation of GPS-TEC and GIM-TEC for the quietest day (Q1) of the month of May 2009 and most disturbed day (D1) of the same month have been shown. It can be seen from this figure that in both the conditions, the correlation coefficient is found about In a similar manner, correlation coefficients have been calculated between GPS and GIM TEC data for all the quiet and disturb days of the year 2009 and it is found above 0.9 throughout the year. This study shows that GIM-TEC data may be used for the locations where continuous GPS-TEC data are not available. 4.5 Latitudinal variation in TEC It has been shown above that GIM-TEC data may be used to study TEC for any location where observations are not available. Keeping in view the same, the GIM-TEC data has been analysed for five different locations in India along the same meridian of 80 0 E longitude. The latitudes of these locations are 10 0 N, 15 0 N, 20 0 N, 25 0 N and N. These latitudes start from very near the equator and reach the equatorial ionization anomaly (EIA) crest region at an interval of 5 0. The last location is outside the EIA crest region and lie near the Agra station. The TEC obtained at these locations have been represented by TEC 1, TEC 2, TEC 3, TEC 4 and TEC 5, respectively.

7 32 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2011 Fig. 6 Daily variations of GIM-TEC and GPS-TEC data for the month of May The solid continuous curves represent GIM-TEC and dashed lines show the GPS-TEC data Fig. 7 Plot of correlation between GPS-TEC and GIM-TEC for: (a) the quietest day (Q1) of May 2009, (b) most disturb day (D1) of May 2009 The data has been analysed for three typical months of year 2009, i.e. March, June and December representing three seasons of equinox, summer and winter, respectively. The mass plots of TEC variations at these locations for the months of March, June and December are shown in Figs 8, 9 and 10, respectively. Figure 11 shows the contour plots of monthly diurnal mean of the above mentioned three months. It can be seen from these figures that the highest values of TEC are obtained in the equinox month of March, particularly during daytime at all locations. However, out of the five locations considered here maximum values of TEC and maximum variations are obtained at the EIA crest, i.e N to 25 0 E in all the three months. The Agra station lies outside the EIA crest region and the values of TEC obtained here are found to be lower than other locations in all the three months. However, the diurnal peak is found to be relatively sharper than other locations at this station. It may be seen from Figs 9, 10 and 11 (b, c) that the locations near equator and inside EIA crest region show higher values of TEC in winter month of December in comparison to the summer month of June while the locations outside the EIA crest show lowest values in the winter month of December. This shows that there is an absence of winter anomaly at Agra station during the low solar activity period considered in the study. The other diurnal features are very similar to those obtained by earlier researchers 5.

8 CHAUHAN et al.: DIURNAL AND SEASONAL VARIATIONS OF GPS-TEC DURING LOW SOLAR ACTIVITY 33 Fig. 8 Mass diurnal curves of TEC at five different latitudes along the same meridian of 80 0 E longitude for the equinox month of March 2009 Fig. 9 Mass diurnal curves of TEC at five different latitudes along the same meridian of 80 0 E longitude for the summer month of June 2009

9 34 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2011 Fig. 10 Mass diurnal curves of TEC at five different latitudes along the same meridian of 80 0 E longitude for the winter month of December 2009 Fig. 11 Contour plots of monthly diurnal mean of TEC at five different latitudes along the same meridian of 80 0 E longitude for the three months of: (a) March, (b) June, and (c) December 5 Discussions The diurnal characteristics of TEC have seasonal, solar activity, geomagnetic, and latitudinal dependence. The large variability imposed on the low latitude ionosphere during sunrise and sunset transition period is well understood. As the total magnetic field tube is very small at equatorial and low latitudes, the electron contents in the field tubes collapse rapidly after sunset in response to the low temperature in the thermosphere in the nighttime. Following the sunrise, the magnetic field tubes again get filled up rapidly because of their low volume resulting steep increase in ionization 17. The day-today variability in TEC, which has also been observed by other researchers 5,18 may be due to the changes in the activity of the Sun itself and to the associated changes in the intensity of the incoming radiations and the zenith angle at which they are incident on the

10 CHAUHAN et al.: DIURNAL AND SEASONAL VARIATIONS OF GPS-TEC DURING LOW SOLAR ACTIVITY 35 Earth s atmosphere. Equatorial electrojet strength (EEJ), Earth s magnetic field, and the dynamics of the neutral wind are also some of the factors which may be responsible for this variability 5. Tyagi & Das Gupta 19 in their review paper, presented seasonal variation of electron content over Delhi for low, moderate, and high solar activity periods and concluded that there was absence of so called winter anomaly at low latitudes during solar minimum period. The same absence of winter anomaly in low solar activity period has been reported by Bagiya et al. 6 also. Rishbeth & Setty 20 suggested that the seasonal changes result from changes in the ratio of the concentration of atomic oxygen and molecular nitrogen in the F-region. Based on calculations of scale height of the absorbing gas, it is found that in equinoctical months, solar radiation is absorbed mainly by atomic oxygen. This is the reason for high values of TEC max in the equinoxes which are responsible for semi-annual variations. During low solar activity, lowest values of TEC max are observed in winters which give rise to annual variation in TEC max (ref. 15). Many earlier researchers have done work on latitudinal variations of electron density (references in Rama Rao et al. 5 ). Rama Rao et al. 5 have shown very good results related to the latitudinal variations of GPS-TEC. The equatorial ionization anomaly is a result of so-called fountain effect which gives rise to lifting of the equatorial plasma to higher altitudes, during most of the daytime hours. This plasma subsequently diffuses along the geomagnetic field lines to either side of the magnetic equator, owing to the effects of ambipolar diffusion, gravity and pressure gradients, giving rise to an accumulation of ionization at the F-region altitudes around ±15 0 geomagnetic latitudes, resulting in the formation of crests of ionization, while simultaneously depleting the ionization over the magnetic equator 5. 6 Conclusions The study of temporal, seasonal and solar activity dependence of TEC data has been carried out for a low solar activity period for three years (01 August July 2009). The results show that the mean TEC varies from a pre-dawn minimum to an afternoon maximum and then decreases. The low values of TEC are observed in winter whereas high values are observed in equinox and summer. By comparing the TEC data with the three solar indices, i.e. EUV flux, F 10.7, and SSN, it is found that day maximum TEC (TEC max ) show good correlation with all the solar indices in the summer season than in equinox. A low correlation is seen in winter season. Further, a better agreement is found between TEC max and F 10.7 than the other two indices. By comparing the GPS-TEC data with GIM-TEC for the year 2009, a good correlation is found between them. Latitudinal study of TEC shows the clear effect of EIA crest on the TEC variations in all the seasons. During this period of study, the so-called winter anomaly is seen near the equatorial and EIA crest region latitudes while it is found to be absent at low latitude outside the EIA crest. Acknowledgements The authors are thankful to the Ministry of Earth Sciences (MoES), Government of India, New Delhi for providing necessary funds for this research work in the form of a major research project. 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