Ionospheric TEC variations during the ascending solar activity phase at an equatorial station, Uganda

Transcription

1 Indian Journal of Radio & Space Physics Vol 42, February 2013, pp 7-17 Ionospheric TEC variations during the ascending solar activity phase at an equatorial station, Uganda S Oron $, F M D ujanga #,* & T J Ssenyonga + Department of Physics, Makerere University, Kampala, Uganda $ soron@physics.mak.ac.ug, # fdujanga@physics.mak.ac.ug, + tjssenyonga@physics.mak.ac.ug Received 17 September 2012; revised received and accepted 28 January 2013 The total electron content (TEC) is a vital and most dominant ionospheric parameter that can cause Global Positioning System (GPS) signal delays, signal degradation and in extreme cases loss of lock. This results into inefficient operations of ground and space based Global Navigation Satellite System (GNSS) applications. The study of TEC variability is, therefore, useful for GNSS users in order to minimize errors where high levels of accuracy in measurements are required. This paper presents the diurnal, seasonal and solar activity dependence of TEC at the GPS-SCINDA (SCIntillation Network Decision Aid) station in Kampala, Uganda (geographic coordinates: latitude 0.3 N, longitude 32.6 S; and geomagnetic coordinates: latitude -9.3, longitude ) for the years 2010 and The results obtained show that the diurnal variability in TEC at this station has a pre-dawn minimum followed by an early morning steady increase, an afternoon maximum and then a post sunset gradual reduction in TEC, with the equinoctial months depicting nighttime enhancements more prominently at around 2000 hrs UT (2300 hrs LT). Scintillation occurrence, a consequence of TEC fluctuations, was observed from about 1800 hrs UT (2100 hrs LT) to local midnight giving S 4 index values above 0.4, with the equinox months recording higher occurrences than other seasons. TEC variations were also seen to exhibit solar activity dependence. The sunspot numbers and the F10.7 solar flux exhibited a good correlation with TEC recorded over the two years. Keywords: Ionospheric scintillation, Total electron content (TEC) variation, Solar activity PACS Nos: dv; qd 1 Introduction The atmosphere, in which we live, is influenced by the dynamic and violent Sun that produces energy for life on earth but is also responsible for causing harmful effects to satellites and communication systems, a manifestation known as space weather. Space weather is defined as the conditions on the Sun and in the solar wind, magnetosphere, ionosphere and the thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life 1. The Sun, therefore, is at the core of space weather and continuously emits two main types of energy into space, namely, the electromagnetic and corpuscular radiations. The electromagnetic radiation emitted includes visible light, radio waves, microwaves, infrared, ultraviolet (UV), X-rays and gamma rays, while the corpuscular radiation is composed of charged atoms and sub-atomic particles (protons and electrons) in what is called solar wind. The velocity and density of the solar wind changes constantly and these changes affect the space environment causing the aurora, which manifest as large electric currents that disrupt satellite signals and thus, affect communication. The free electrons created in the upper region of the atmosphere, called the ionosphere, cause the inhomogeneity of the propagation medium 1, hence, affecting the propagation of the signals by changing their velocity and direction of travel. The electron density is determined by the total electron content (TEC), which is defined as the number of electrons in a column of cross-sectional area of 1 m 2 along a path of the signal through the ionosphere. TEC is expressed in TEC units (TECU), where 1 TECU = electrons m -2. Ionospheric refraction is a major error source in Global Positioning Systems (GPS) in real-time applications. The ionosphere causes GPS signal delays due to the TEC along the path from the GPS satellite, at a height of about 20,000 km, to a receiver on the ground. The variations in TEC cause fluctuations in the amplitude and phase of the GPS signal as they transit the ionosphere. This manifestation is the ionospheric scintillation, and

2 8 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2013 when intense, can cause signal degradation and in extreme cases, cause loss of lock which then severely affects the performance of global navigation satellite systems (GNSS). Earlier research has shown that the ionosphere is highly variable in space and time, according to geographical location (polar, auroral zones, mid latitudes and equatorial regions) and to certain solar related ionospheric disturbances. Licilla et al. 2 generalized ionospheric behaviour into auroral zones, mid latitudes and equatorial regions and reported that the two geographical regions, most affected by electron density fluctuations and scintillations, are the auroral zones and equatorial regions. The morphological features of TEC at low and equatorial latitudes have been widely studied, especially outside the African continent. Chauhan et al. 3 studied diurnal and seasonal variation of TEC at Agra in India and observed that the mean TEC varies from pre-dawn minimum to an afternoon maximum and then decreases. The low values were observed in winter, while high values were observed in equinox and summer months, and there was an absence of winter anomaly. Norsuzila et al. 1, from an equatorial station, used TEC maps and showed that the variations are such that the TEC values start increasing gradually from morning and reach its maximum at noon and decrease around 1800 hrs LT. The variability of TEC over the west African equatorial region have also been reported by Bolaji et al. 4 and results show that the value and diurnal range of variation is consistently lower in the June solstice than in other seasons during both high and low solar activities. The diurnal and seasonal behaviour of TEC depletions in the ionospheric electron content associated with amplitude scintillation was studied in the South American sector 5 during solar maximum and observations showed seasonal variations with asymmetric equinoctial maxima while the February - April period showed markedly higher occurrences. Scintillation studies in the Indian equatorial ionosphere have also been carried out by Manju et al. 6 during low solar activity period using ionosonde and GPS data. The findings revealed that the maximum scintillation index in the equatorial ionisation anomaly region was linearly dependent on the equatorial spread-f traces for both equinoxes. Bhattacharya et al. 7 also reported that in India, TEC is depleted at the equator and enhanced at regions located 15 o south and north of the magnetic equator and this raises the TEC gradient. A lot of attention has been directed towards understanding the dynamic behaviour of the equatorial ionosphere. This is aimed at enhancing the prediction capabilities of ionospheric irregularities within the equatorial and low latitude regions. TEC enhancements and depletions in the equatorial region have been studied in a number of regions, viz. the Indian sector 8 ; South American sector 9 ; and recently at a number of African stations 10,11. In all these findings, TEC depletions are well correlated with the scintillation index, S 4, with the TEC enhancement having no correlation with increased S 4 index. This study focuses on Kampala, an East African GPS station, which together with other African GPS stations, shall contribute to the understanding of the variability pattern for the equatorial African ionosphere in order to characterize the equatorial ionosphere. The GPS SCINDA receiver that provided data for this study is located at Makerere University, Department of Physics in Kampala, Uganda (geographic coordinates: latitude 0.3 N, longitude 32.6 S; and geomagnetic coordinates: latitude -9.3, longitude ). The TEC was determined from GPS measurements is the slant TEC (TEC along the line of sight) and then converted to vertical TEC (vtec) using a suitable mapping function. 2 Observations This paper presents analysis of GPS data for a period of two years, 2010 and The data used has been recorded by a dual frequency NovAtel GPS receiver model GSV 4004B installed in the late 2009 as one of the SCINDA projects in Africa at Makerere University, Kampala (geomagnetic coordinates: latitude -9.3, longitude ). The parameters TEC (carrier phase and pseudo-range) and the scintillation index, S 4, was recorded every second and averaged over sixty (60) seconds. GPS-SCINDA receivers are incorporated with software, WinTEC-P, which calibrates the GPS TEC. This software is integrated into the GPS data collection system and the technique used by the receiver to measure pseudo ranges dictates how the satellite and receiver biases should be downloaded and removed 12. The final TEC value is obtained after the removal of the satellite and receiver biases using the following equations: TEC = TEC ( b + b ) S R R TEC TEC ( TEC TEC ) R = ϕ + P ϕ ARC

3 ORON et al.: IONOSPHERIC TEC VARIATIONS DURING ASCENDING SOLAR ACTIVITY AT KAMPALA 9 where, b R, is the receiver station bias; b S, the satellite bias; TEC P, the differential pseudo range; TEC ϕ, the differential carrier phase; < > ARC, the average over phase connected arc; and TEC R, the relative TEC. The relative TEC R was then converted to vertical TEC (vtec) using the geomagnetic conversion: vtec = [ TEC ( b b )] S( E) R R + S where, S(E), is the single layer mapping function of the ionosphere defined as 12 : { 1 ( e ε ) e } S( E) = sec sin R cos / ( R + h) where, R e and h, are the Earth s radius and ionospheric height, respectively in km; and ε, the elevation angle in radians. For SCINDA, h is taken to be 350 km. In this work, vtec was used since it depends on the satellite position and the average vtec was recorded at a duration of 20 minutes each day. The scintillation index, S 4, was recorded at a rate of 60 s using the square root of the normalized variance of the signal intensity given by: S 4 = I 2 I 2 I 2 In order to eliminate errors in S 4 index due to multipath, only values of S 4 index with elevation angles > 30 were extracted and used for this study. A software in FORTRAN was developed to compute the daily averages of these parameters in order to give the annual (seasonal) variations, which were then averaged. The seasonal TEC was obtained by grouping the months for the seasons of equinox (March, April, September and October), summer solstice (May, June, July and August) and winter solstice (November, December, January and February). 3 Results 3.1 Day-to-day variability of TEC The overall day-to-day variation of vtec over Kampala with day-of-year (DoY) number for each year has been calculated. Figures 1(a) and 1(b) show variations of daily average vtec with DoY for the years 2010 and 2011, respectively. The values presented are daily averages (24 h running include both day and nigh time values) and therefore, are lower than the expected 60 s averages. A quick look at Figs 1(a) and 1(b) shows that the months of March-April and September-October recorded the highest daily average vtec values in both 2010 and This implies that the equinoctial months Fig. 1 vtec variation with DoY recorded at Kampala GPS station for the years: (a) 2010 and (b) 2011

4 10 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2013 registered higher TEC values than the solistice months. The year 2011 recorded vtec values as high as 35 TECU compared to the highest maximum values of about 23 TECU in This is because the year 2011 is in the ascending phase of solar activity, shown by high monthly averages of the F10.7 flux index that exceeded 150 in some months, whereas those in 2010 were all less than Diurnal variability of TEC Figures 2 and 3 show the diurnal variation of TEC on each day for the years 2010 and 2011, respectively. However, for the year 2011, the months of January and December are not presented due to lack of GPS data. It can be seen from Fig. 2 that the day time minimum TEC during 2010 is observed at around 0300 hrs UT (Local time, LT = UT +03 h). This gradually rises to a peak value from 1000 hrs UT to 1500 hrs UT and gradually falls after this time. The maximum TEC period lasts about 5 hours. The highest TEC values at this peak hours is about 45 TECU for the months of March, April, September and October while the rest of the months showed relatively lower peak values over the same period of Fig. 2 Diurnal variation of TEC during 2010 [highest values observed in March and October; lowest value observed in June and July during noontime]

5 ORON et al.: IONOSPHERIC TEC VARIATIONS DURING ASCENDING SOLAR ACTIVITY AT KAMPALA 11 time. These maximum diurnal values are evidently higher than the daily average vtec, shown in Fig. 1, that mainly showed general sesonal variations. This same trend is depicted in the year 2011 (Fig. 3) except that relatively higher values of TEC were recorded throughout the year with equinoctial months registering values as high as TECU, while the solistice months registered about 40 TECU maximum. There were TEC fluctuations depicted after sunset, with the equinoctial months showing nighttime enhancements most prominently at around 2000 hrs UT (2300 hrs LT). 3.3 Post-sunset variations of TEC Due to TEC fluctuations observed in Figs 2 and 3, from about 1700 to 2100 hrs UT (corresponding to 2000 hrs LT to midnight local time), it was necessary to investigate this post sunset variation of TEC. Since the months of April and October 2011 depicted very high S 4 indices (not shown), two days have been picked for analysis in this research. These were days for which the post sunset S 4 index values were distinctly higher than other times of the day. Data used was computed at an interval of 60 s for each satellite denoted by a pseudo random number (PRN) and at all possible elevation angles. Figures 4 and 5 show the analysis for the 8 April and 25 October 2011, respectively. The results presented show scintillation occurrence in relation to TEC fluctuation and satellite elevation angle. The satellite elevation angle is included in the study because electrons in a Fig. 3 Diurnal variation of TEC for the year 2011 [highest values observed in the months of March, April, September, October and November during noontime]

6 12 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2013 column of cross-sectional area of 1 m 2 depend on the slant length, and therefore, on the position of the satellite. Figures 4 and 5 show results for PRN 4 on 8 April 2011 and for PRN 1 on 25 October 2011, respectively. These particular days and PRNs were chosen since the onset of the TEC depletions could easily be established. Three panels are presented for each PRNs, where the panels show the change of the satellite elevation angle, the vtec and the S 4 index with time. On 8 April 2011, observations for PRN 4 (Fig. 4) depict a series of TEC reductions followed by some TEC enhancements, which lasted for 1 hour from 1730 to 1830 hrs UT. The sudden decreases in TEC Fig. 4 Variation of TEC and S 4 Index on 8 April 2011 for PRN 4 Fig. 5 Variation of TEC and S 4 Index on 25 October 2011 for PRN 1

7 ORON et al.: IONOSPHERIC TEC VARIATIONS DURING ASCENDING SOLAR ACTIVITY AT KAMPALA 13 have been identified as TEC depletions. Each of these depletions was seen to last for about 5 minutes, and the corresponding increase of the S 4 index was observed with no increase in the S 4 index when a TEC enhancement was observed. The TEC depletion at about 1730 hrs UT was about 10 TECU and resulted into the S 4 index of a value almost equal to 1, while the depletion of about 5 TECU observed at around 1818 hrs UT corresponded to S 4 index of about 0.6. Similarly, Fig. 5 shows TEC depletions that occurred on 25 October 2011 as seen by PRN 1. A significant TEC depletion was observed at about 1936 hrs UT with a TEC depletion of about 10 TECU which lasted for about 5 minutes. At this particular time, the S 4 index rises to values of about 0.6. The satellite was also at a high elevation angle of about 80 when observed at the Kampala station. There was a general broad reduction in TEC at this angle of elevation between 1930 and 2000 hrs UT. The observed TEC reduction at the highest elevation angle obtained is attributed to the reduction in the slant length of the receiver from the satellite 8. The observed TEC enhancement had no corresponding increase in S 4 index values for this PRN. TEC remained constant after 2100 hrs UT and the corresponding S 4 index value consistently remained lower than Seasonal variation A 20-day simple average of TEC was computed at an interval of 20 minutes for each month. The monthly values were then averaged and seasonal averages obtained. The TEC for the equinox, winter and summer seasons for the years 2010 and 2011 is presented in Fig. 6. It is evident that the highest values of TEC were observed in equinoctial months, moderate in the summer solstice and then least in the winter solstice. This trend is consistent for the two years in question but with relatively higher values recorded in the year 2011 than in the year 2010 for all the seasons. From Fig. 6, it is also observed that at Fig. 6 Seasonal variation of diurnal TEC for the years: (a) 2010; and (b) 2011

8 14 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2013 around 0300 hrs UT, the TEC in all the three seasons is almost the same in both years. The rate of increase of TEC is faster in the equinoctial months than in the other seasons, while the maximum of TEC in winter is observed earlier than in other seasons. The nighttime enhancement in TEC is observed during equinox and summer, with a higher rate of TEC increase in Variation of TEC with sunspot numbers and solar flux F10.7 The variation of TEC, sunspot numbers (SSN) and the 10.7 cm solar flux index (F10.7) for the years 2010 and 2011 have been presented and compared as shown in Fig. 7. Sunspot numbers and 10.7 cm solar flux have been obtained from the Solar Influences Data Analysis Centre (SIDC) ( sunspot-data/) and Earth Orientation Parameters (EOP) and Space Weather ( SpaceData/) websites, respectively. Monthly averages of vtec were computed and have been presented on the same plot with SSN and F10.7 solar flux indices in order to study their variation with TEC throughout the year. These indices have been considered since the ionosphere is primarily affected by solar radiation 3, which is measured in terms of F10.7 flux, SSN and the extreme ultraviolet (EUV) radiation. In the absence of EUV values, the SSN and F10.7 flux have been used. These indices were higher in the year 2011 as compared to 2010 and equally high TEC values as shown in Fig. 7. In particular, higher values of TEC recorded in 2011 [Fig. 7(b)] correlate well with these solar indices. This shows, therefore, that TEC is a consequence of the solar activity. Higher values of F10.7 flux and SSN were also observed in the equinoctial months than during the solstice months. 4 Discussions The day-to-day variability of TEC from Kampala station (0.3 N, 32,6 E) shown in Fig. 1 is consistent with other research findings in the equatorial region 3,13. This day-to-day behaviour of TEC is attributed to changes in the activity of the Sun, which therefore, is associated with the changes in the Fig. 7 Monthly noon-tec compared with solar indices: SSN and F10.7 flux for the years: (a) 2010; and (b) 2011

9 ORON et al.: IONOSPHERIC TEC VARIATIONS DURING ASCENDING SOLAR ACTIVITY AT KAMPALA 15 intensity of the incoming radiation and the zenith angle, at which the radiation is incident at the Earth s atmosphere 3. This day-to-day variability in TEC is further attributed to the equatorial electrojet (EEJ) strength, the Earth s magnetic field and the dynamics of the neutral winds 12. The EEJ is, as a result of the day side ionosphere neutral winds, setting up a polarization electric field pointing in the eastward direction and at the magnetic dip equator. For this Kampala station, where the magnetic field is almost horizontal, this effect is high, resulting into the upward E B drift, which is associated with the equatorial ionization anomaly (EIA) crest Generally, the diurnal variability in TEC at Kampala shows a pre-dawn minimum followed by an early morning steady increase, an afternoon maximum and then a post sunset gradual reduction in TEC, with the equinoctial months depicting nighttime enhancements more prominently at around 2000 hrs UT (2300 hrs LT). The observed diurnal variation of TEC trend is in conformity with the findings for other regions such as India, Malaysia and West Africa 1,3,4,5,17. The gradual increase in TEC to a maximum value at peak hours of the day at equatorial and low latitudes has been attributed to the solar extreme ultraviolet (EUV) ionization coupled with the upward vertical E B drift 4. The nighttime decrease is due to the size of the magnetic flux tubes which are so small that the electron content in these tubes collapses rapidly after sunset in response to the low temperatures in the thermosphere in the night, leading to low TEC values 3,13,17. During sunrise, the magnetic flux tubes again get filled up because of their small volume resulting into sudden increase in ionization due to increasing thermospheric temperatures during sunrise. From Figs 2 and 3, it is also evident that the year 2011 recorded relatively higher values of TEC than the year This difference is due to the difference in solar activity for two years which depends on the sunspot numbers and solar flux F10.7 as shown in Fig. 7. Higher values of sunspot numbers were recorded in the year 2011 than in This is because 2011 falls in the ascending phase of the solar cycle 24. This is further evidenced by the high levels of F10.7 solar flux of up to 150 sfu, the index which measures the noise level generated by the Sun at a wavelength of 10.7 cm at the Earth's orbit and is produced in response to sunspot numbers. The observed night time TEC enhancement (Figs 2 and 3) could be attributed to the tidal winds which blow the ionization across geomagnetic field. According to Hanson & Moffett 18, a large scale electrostatic field is produced at the low latitudes which is primarily eastward during the day and westward during the night, with the eastward fields being responsible for the upward plasma drift motion and the westward fields during the night, causing the downward drift motion. This plasma fountain reverses during the night hours and the northward motion of the crest of ionization during daytime reverses to southward motion during the night. The downward motion at the geomagnetic dip equator and the southward motion of ionization could be responsible for the nighttime enhancement of TEC observed at this station. Figures 4 and 5 show a series of TEC depletions followed by TEC enhancements for the selected days in the months of April and October These TEC depletions were of magnitudes 5 and 10 TECU, which resulted into high S 4 index values (> 0.2). The correspondence of TEC depletions with an increase in the S 4 index and elevation angle is in agreement with the findings of other researchers 7-9,11,19. These TEC depletions are associated with small scale plasma density irregularities, which manifest into an increase in the S 4 index. Such irregularities, which result into ionospheric scintillations, can cause trans-ionospheric signal fading, a potential threat to GNSS systems. According to Burke et al. 20, this behaviour could be attributed to plasma bubbles which are generated from sunset till sunrise. These plasma irregularities are due to the turbulent ionospheric conditions which give rise to the equatorial spread-f (Ref. 10), a phenomenon in which the F-layer trace becomes wide on ionograms. Factors responsible for this spread-f occurrence have been reported to be either due to the variations in the linear growth rate of the Rayleigh-Taylor instabilities as a result of the electrodynamics of the ionosphere, or to the presence of the seed perturbation mainly caused by the atmospheric gravity waves. It is, further, reported that for the African equatorial region, scintillation occurrence is most frequent when the solar terminator aligns with the geomagnetic field 10. The seasonal variation in TEC depicted in Fig. 6 could be attributed to changes in the ratio of concentrations of atomic oxygen and molecular nitrogen in the F2 layer since the ionosphere is composed of neutral oxygen and nitrogen gases. The equinoctial months have exhibited the high TEC values and high solar indices because solar radiation

10 16 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2013 is mainly absorbed by the atomic oxygen during the equinoctial months 21. The accumulation of the high electron concentration within this region of study (geomagnetic latitude, -9.3 ) could also be due to lifting of the equatorial plasma to high altitudes during day time, which subsequently diffuses along the geomagnetic field lines to either sides of the magnetic equator, leading to the accumulation of ionization at the F-region altitudes at around ±15 geomagnetic latitudes 3. Such phenomena could be responsible for the creation of crests of ionization at these latitudes and the depletion of ionization over the magnetic equator. 5 Conclusions This study has presented variations in TEC at Kampala, an African equatorial station, during 2010 and The daily average TEC values were seen to be highest during the months of March, April, September and October (equinoctial months), while lower TEC values occurred during the solstice months. It was observed that the mean TEC varies from a pre-dawn minimum to an afternoon maximum ( hrs UT), reducing after sunset, with a number of days exhibiting nighttime enhancements in TEC around 2000 hrs UT (2300 hrs LT). The observed nighttime TEC enhancements have been attributed to tidal winds which blow the ionization across geomagnetic fields. The gradual increase in TEC to a maximum value at peak hours of the day has been attributed to the presence of magnetic flux tubes which collapse rapidly after sunset in response to the low temperatures in the thermosphere, leading to low TEC values in the night. During sunrise, these magnetic flux tubes again get filled up resulting into sudden increase in ionization due to increasing thermospheric temperatures, leading to photo-ionization of particles. Further investigations into the nighttime variations of TEC revealed ionospheric scintillations that were observed between 1700 and 2200 hrs UT (2000 hrs LT to beyond midnight). This was depicted by TEC depletions which corresponded to increases in the S 4 index. Such ionospheric scintillations within the equatorial ionosphere can affect space-based navigation systems such as GNSS. By comparing TEC with solar activity indices, it was observed that TEC also depends on the activity of the sun (solar cycle). The sunspot numbers and the F10.7 solar flux exhibited a good correlation with TEC recorded over the two years. Also, seasonal variation was observed in TEC during this period. Highest values were recorded in daytime during the equinoctial months, viz. March, April, September and October; moderate values during the summer, i.e. May, June, July and August; and least values in the winter seasons, i.e. November, December, January and February. Acknowledgements One of the authors (SO) would like to thank the Belgian Technical Cooperation Agency (BTC), Royal Belgian Embassy, Kampala for the study grant. The authors would also like to extend their thanks to Boston College and the Air Force Research Laboratory (AFRL), who supplied the GPS receiver used in this research. References 1 Norsuzila Y, Mardina A & Mahamod I, Model validation for total electron content (TEC) at an equatorial region, Trends Telecom Tech (Malaysia), (2010) doi: / Lucilla A, Wernik A W, Materassi M, Spogli L, Bougard B & Monico J F G, Low latitude scintillation: a comparison of modeling and observations within the CIGALA project: IEEE Proc (IEEE, Italy), 2011, doi: /URSIGASS Chauhan V, Singh O P & Birbal S, Diurnal and seasonal variation of GPS-TEC during a low solar activity period as observed at a low latitude station Agra, Indian J Radio Space Phys, 40 (2011) Bolaji O S, Adeniyi J O, Radicella S M & Doherty P H, Variability of total electron content over an equatorial West African station during low solar activity, Radio Sci (USA), 47 (2012), doi: /2011RS Das Gupta A, Aarons J, Klobucher J A, Basu S & Bushby A, Ionospheric electron content depletions associated with amplitude scintillations in the equatorial region, Geophys Res Lett (USA), 9 (1982) Manju G, Sreeja V, Ravindran S & Thampi S V, Towards the prediction of L band scintillations in the equatorial ionization anomaly region, J. Geophys Sci (USA), 116 (2011) doi: /2010JA Bhattacharya S, Purohit P S & Gwal A K, Ionospheric time delay variations in the equatorial anomaly during low solar activity using GPS, Indian J Radio Space Phys, 38 (2009) Dashora N & Pandey R, Observations in the equatorial anomaly region of TEC enhancement and depletions, Ann Geophys (Germany), 23 (2005) Seemala G K & Valladares C E, Statistics of total electron content depletions observed over the South American continent for the year 2008, Radio Sci (USA), 46 (2011), doi: /2011RS Paznukhov V V, Carrano C S, Groves K M, Caton R G, Valladares C E, Semaala G K, Bridgwood C T, Adeniyi J,

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