Ionospheric variability of low and equatorial latitude regions over India A study using RaBIT on-board YOUTHSAT

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1 Indian Journal of Radio & Space Physics Vol 42, June 2013, pp Ionospheric variability of low and equatorial latitude regions over India A study using RaBIT on-board YOUTHSAT Perumalla Naveen Kumar 1,$,*, Nayab Rasool 2, K Madhu Krishna 2, A D Sarma 2, N Mridula 3, Tarun Kumar Pant 3, P Sreelatha 3, J Rosmi 3, Santosh Koli 4, Praveen Kumar 5 & R Sharma 6 1 Department of Electronics & Communication Engineering, Osmania University, Hyderabad , India 2 R &T Unit for Navigational Electronics, Osmania University, Hyderabad , India 3 Space Physics Laboratory, VSSC, Thiruvananthapuram , India 4 National Balloon Facility, TIFR, Hyderabad , India 5 Space Science Office, ISRO HQ, Bangalore , India 6 Master Control Facility, ISRO, Bhopal , India $ naveen.gps@gmail.com Received 24 November 2012; revised 19 March 2013; accepted 23 March 2013 The Indian small satellite, YOUTHSAT, was launched mainly for the study of the ionosphere-thermosphere region over the Indian longitudes. The ionospheric processes play an important role in influencing the position accuracies while using the GPS applications. Ionospheric tomography is an effective way of investigating the spatio-temporal variabilities of these ionospheric processes. The RaBIT (Radio Beacon for Ionospheric Tomography) is one of the three payloads on-board the YOUTHSAT. In this paper, first a case, viz. the analysis of the slant total electron content (STEC) variation measured at the three southern RaBIT stations, namely Trivandrum (8.5 N, 77.0 E), Bangalore (13 N, 77.6 E) and Hyderabad (17.8 N, 78.0 E) on 9 May, 7 and 10 June and 6 July 2011 is presented. It is found that the overall STEC variations measured at each station are different for different days. These variations are attributed to the day-to-day changes in the distribution of ionosphere owing to prevailing electrodynamic processes. To highlight this aspect, the RaBIT tomograms are presented, as case 2, for three different days for five stations, one each for the May, June and July months of 2011 to highlight the day and night differences in the overall distribution of the electron density. The tomograms, unambiguously, reveal the marked differences in the ionization distribution on these days not just during daytime but night time also. These differences are attributed to the overall evolution of the electrodynamics and the neutral dynamics over the equatorial region. Keywords: Electrodynamic process, Ionization distribution, Ionospheric tomography, Slant total electron content (STEC) PACS Nos: dv; Ua 1 Introduction The Low Earth Orbit (LEO) satellites such as the Indian small satellite (YOUTHSAT) are a boon to the young research community of India. This satellite is the outcome of the scientific collaboration between the Indian Space Research Organization (ISRO) and the Rokosmos Russian Federal Space Agency (RRFSA). Among the various scientific objectives of the YOUTHSAT, the prime objective is the study of the earth s upper atmosphere. The Radio Beacon for Ionospheric Tomography (RaBIT) is one of the payloads on-board the YOUTHSAT which aims at the investigation of the ionosphere through the tomography technique. The details of YOUTHSAT and RaBIT on-board have been already described 1. The basic data for ionospheric tomography is the line-of-sight total electron content (STEC) estimated along a number of ray paths from a chain of ground receivers aligned along the same longitude. The STEC, at the receiver, is obtained by employing the Differential Doppler technique. Here, the measured data is the relative phase between 150 and 400 MHz and is proportional to the relative slant TEC (STEC) along the propagation path of the signal. This can be given as: φ = CD STEC (1) where, φ, is measured in radians; STEC, in el m -2 ; and C D = for NNSS satellites 2. Since the phase measurements are accurate to < 3 when the ground receiver is in locked condition and the data

2 KUMAR et al.: IONOSPHERIC VARIABILITY OF LOW AND EQUATORIAL LATITUDE REGIONS OVER INDIA 137 sampling is at 100 Hz, these observations yield accurate estimates of the relative TEC with errors < 0.05%. As is known, the ionosphere, which is the part of the terrestrial upper atmosphere, delays the GPS signals through changes in the refractive index and degrades the positional accuracy of the Global Positioning System (GPS). The positional accuracy of GPS is limited by errors such as the satellite based errors, receiver based errors and atmospheric errors 3. Among the atmospheric errors, the time delay arising due to the variations in the ionospheric density is the most predominant one 4. In fact, this error depends on the TEC, which is also the parameter measured using RaBIT for the generation of ionospheric tomograms. The ionospheric time delay (τ), which depends on the TEC, is inversely proportional to the square of the frequency and is given as: 40.3 τ = TEC (sec) (2) 2 c f where, c, is the velocity of light in m s -1 ; f, the operating frequency in Hz; and TEC, in electrons m - 2. Since there is a lot of variation in the ionospheric TEC at high, mid and low latitude regions, the delay also differs characteristically for these regions 5. The ionospheric variability is a function of time, height, position and prevailing solar and geomagnetic activity levels 6. In this context, the observations on the variability of TEC as a function of the elevation angle of the satellite based beacon can highlight the characteristic latitudinal variability of ionosphere with respect to a given ground receiver location. These observations along a given ground based meridional chain of receivers enable one to generate the ionospheric tomograms. It has been realized that to understand the ionospheric variability especially in the low latitude regions in India where the unique geomagnetic configuration gives rise to significant spatio-temporal variations in the ionosphere on a dayto-day basis, ionospheric tomography is one efficient technique. In this paper, analysis of the ionospheric TEC for the Indian low latitude region, which has been measured using the YOUTHSAT s RaBIT payload, has been carried out and tomograms generated to understand the variability of the prevailing ionospheric processes. 2 YOUTHSAT The YOUTHSAT is a small satellite, an Indo- Russian endeavor, launched by ISRO and is 3-axis stabilized. The YOUTHSAT is the second satellite in the series of Indian Mini Satellite Series 7. The YOUTHSAT is launched using the PSLV-16 launch vehicle on 20 April 2011 (Ref. 8). The YOUTHSAT consists of three payloads on-board, namely, RaBIT, LiVHySI and SolRaD. The RaBIT (Radio Beacon for Ionospheric Tomography), LiVHySI (Limb Viewing Hyper Spectral Imager) are the Indian payloads and the SOLARD (Solar Radiation Detector) is the Russian payload. As already mentioned, the RaBIT payload is used for two-dimensional mapping of ionospheric structures. The RaBIT enables measurements of the TEC which are used to study the structure and dynamics of the equatorial ionosphere over the Indian region using tomographic techniques. The TEC is derived from the phase difference between two radio beacon signals at 150 and 400 MHz originating on-board the satellite and propagating through the same path to five ground receivers along the Indian longitude of 78 E. Table 1 shows the latitude and longitude of these ground stations where the RaBIT beacon signals are received. 3 Experimental data and Aanalysis Case 1: In Case 1, the RaBIT data used for the study of TEC variations are obtained at Trivandrum (8.5 N, 77.0 E), Bangalore (13 N, 78 E) and Hyderabad (17.3 N, 78.3 E) stations. The analysis is carried out for four days, i.e. 9 May, 7 and 10 June and 6 July of The variations of STEC and the elevation angle have been analysed with respect to the local time. The YOUTHSAT passages depending upon the location, on a typical day would be around 5 to 10 minutes. For example, the start and end times for RaBIT tracking on 10 June 2011 for Hyderabad, Bangalore and Trivandrum stations are: to 10.49; to and to hrs IST, respectively. Case 2: In Case 2, the tomographic reconstruction is carried out for three days, namely 9 May, 13 June and 15 July As the YOUTHSAT crosses the equator twice over Indian longitude every day, the local times of these crossings are ~ 10:30 and 22:30 hrs IST. As a Table 1 Ground receiving stations for RaBIT Station Northern latitude Eastern longitude Delhi 28.8º 77.2º Bhopal 23.2º 77.25º Hyderabad 17.3º 78.3º Bangalore 13.0º 78.0º Trivandrum 8.5º 77.0º

3 138 INDIAN J RADIO & SPACE PHYS, JUNE 2013 consequence, two tomograms are obtained every day, one corresponding to the former and the other latter. This enabled to investigate the day-night differences in the overall distribution of the ionization owing primarily to large scale processes like the equatorial ionization anomaly (EIA) over the Indian longitude. The RaBIT ground-track of the RaBIT passes considered in the present study have been shown in three panels in Fig. 1. The tomograms are presented in Fig. 2, panels (a), (c) and (e) representing the daytime at nearly same Indian Standard Time, i.e. between 10:50 and 11:20 hrs IST while panels (b), (d) and (f) represent the nighttime around ~20:46-22:28 hrs IST. Fig. 1 RaBIT passes used in the study [dots represent the ground receiving station; purple tracks represent daytime passes; the blue tracks indicate nighttime passes] Fig. 2 RaBIT tomograms: (left panels a,c,e) daytime tomograms; (right panels b,d,f) nighttime tomograms [white lines are magnetic field lines; TVM, BNG, HYD, BPL and DEL are the five ground stations]

4 KUMAR et al.: IONOSPHERIC VARIABILITY OF LOW AND EQUATORIAL LATITUDE REGIONS OVER INDIA 139 The tomograms are presented with the geocentric coordinates. The white lines overlaying the plot are the magnetic field lines. The location of each RaBIT ground station is marked on the plot as TVM, BNG, HYD, BPL, and DEL (Fig. 2). As mentioned, panels (a and b) represent ionosphere on 9 May during day and night, respectively. It is apparent from these panels that the overall ionization density is higher during daytime than that during the night. In addition, the crest of ionization appears to be around Hyderabad, i.e. 18 N latitude at ~ 11:20 hrs IST while the dip equatorial region around Trivandrum is marked by ionization trough. While during the nighttime, the location of the ionization trough remains fixed over Trivandrum latitude (8.5 N), the crest appears to have receded towards the equator. The nighttime crest resides around Bangalore. The daytime ionization density appears to be significant up to ~ 500 km of altitude while the same during night reduces by 100 km. Interestingly, panel (c), which represents 13 June daytime ionosphere, shows that the overall ionization density is reduced by 40%. The number density is found to be similar to what is experienced during nighttime. Nevertheless, the distribution is found to be markedly different during the day and the night. As on 9 May, the daytime ionization crest is found to be away from equator between Hyderabad and Bhopal, while the same during the night [panel (d)] clearly recedes to latitudes closer to equator. The trough ionization density is found to be very low during the night, with the overall distribution being similar to that on the night of 9 May. The ionosphere on 15 July is represented in panels (e and f) for day and nighttime, respectively. The daytime tomogram on this day reveals a completely different ionospheric distribution with a very broad ionization crest lying between Hyderabad and Delhi. In fact, the ionization appears to have much broader altitudinal extent than that on the other days. The trough region also seems to have extended much beyond the latitude of Bangalore. Interestingly, this aspect can still be seen in the nighttime tomogram, where the prominent ionization is beyond Bangalore. The nighttime ionization is seen to have a smaller altitudinal extent than that on other nights. The overall ionization density during the night is also found to be smaller that on other nights. On the whole, these six cases of day and nighttime tomograms reveal that not only the daytime tomograms are characteristically different on these days, the nighttime ionization also shows marked changes. The observed differences are attributed to the prevailing neutral and electrodynamics on these days. To comment on the prevailing electrodynamics on these three days, the ground-based surface magnetic field as measured at a station Alibag (18 N) away from the magnetic equator is also presented (Fig. 3). The rationale for presenting the off-equatorial magnetic field is that it is the electric field prevailing outside the equatorial electrojet region which maps to lower ionospheric altitudes over the dip equator giving rise to the daytime upward drift of plasma therein. 4 Results and Discussions Case 1 The STEC through RaBIT are acquired simultaneously at all the three stations on 10 June 2011 and are presented in Fig. 4. From Fig. 4, it is clear that minimum STEC for Hyderabad, Bangalore and Trivandrum station are observed at 10:54, 10:55 and 10:56 hrs IST. The STEC values for Bangalore and Hyderabad stations are very close during the period 10:55 10:56 hrs IST. More variations (kinks) in STEC are observed for Hyderabad station during 10:48 10:54 hrs IST, when compared to the other two stations. The maximum elevation angle over Hyderabad station is Fig. 3 Hourly variation of the surface measured magnetic field (provisional) at Alibag, an off-equatorial station in India [Source: and Indian Institute of Geomagnetism, Mumbai]

5 140 INDIAN J RADIO & SPACE PHYS, JUNE 2013 higher than other two stations. The elevation angle for the Hyderabad station during 10:48 10:55 hrs IST is found to increase from 0.6 to The maximum STEC value of 64.3 TECU at 11:03 hrs IST is observed for Hyderabad station. The elevation angle of the Hyderabad station during 10:48 10:55 hrs IST is higher than other stations. During 10:55 11:02 hrs IST, the elevation angles at Hyderabad station are less than that of other stations and the corresponding STEC values are more than other stations. The maximum elevation angle and corresponding STEC values for three stations are shown in Table 2. Among the stations, it is found that the YOUTHSAT is at the maximum elevation angle (71.2 ) for Hyderabad station having the lowest STEC value (0.623 TECU) as compared to the other stations. Figure 5(a to c) shows the plots of simultaneous STEC variations for three stations on 9 May, 7 June and 6 July It is observed that except on 6 July for Hyderabad station; and 9 May for Trivandrum station, when the YOUTHSAT is at the highest elevation angle, the corresponding STEC is not minimum for all other stations. Also, it is found that among the days considered here, only on 6 July 2011, the minimum STEC is observed for Hyderabad and Bangalore stations. These results may be considered as preliminary results obtained from YOUTHSAT. It can be clearly seen that the STEC variations are different for all considered days in the months of May, June and July Stations Table 2 Maximum elevation angle and corresponding STEC and local time of three stations Max elevation angle STEC value, TECU Time, hrs IST Hyderabad 71.2 o :54 Bangalore 69.9 o :56 Trivandrum 66.4 o :57 Fig. 4 Simultaneous STEC observations and elevation angle vs local time of three stations on 10 June 2011 Fig. 5 Variations of STEC and elevation angle for three stations on: (a) 9 May 2011; (b) 7 June 2011; and (c) 6 July 2011

6 KUMAR et al.: IONOSPHERIC VARIABILITY OF LOW AND EQUATORIAL LATITUDE REGIONS OVER INDIA 141 Case 2 It is a known fact that the global zonal electric field in the lower ionospheric region is generated through winds which are of tidal origin in the lower atmosphere. This primary zonal electric field is eastward during the day and westward in the night. It has been found that the wind in the lower ionospheric region is highly variable in both short (~h) and long (~days) time scales. Owing to these winds and the prevailing geomagnetic configuration, a global current system gets generated. Over the dip equator, this primary electric field leads to the generation of secondary field which is vertical and in turn leads to the enhanced current system known as the equatorial electrojet (EEJ). The ground based magnetic field measurements over the dip equator provide an indication on how strong the EEJ current system is on a given day. At the same time, the off-equatorial electric field, which gets mapped to ~ 150 km altitude over the equator leads to an upward drift of the overall plasma, only to diffuse downward along the magnetic field lines to latitudes away from equator. This process is known as the equatorial plasma fountain or equatorial ionization anomaly (EIA). In this context, a look at the temporal variability of the magnetic field on 9 May, 13 June, and 15 July clearly reveals that the ionospheric current system, in other words electric field, on the first and third day evolved almost the similar manner except that the magnetic field is stronger by ~20nT around noon on 15 July than that on 9 May (Fig. 3). The induced magnetic field on surface was least among the three on 13 June indicating that the current system was weak on this day. The daytime tomograms as presented here clearly corroborate with the observed variability of the magnetic field. The strongest magnetic field on 15 July indicates the strongest electric field, which in turn indicates strongest plasma fountain on this day. The stronger plasma fountain over equator leads to ionization crests forming at latitudes farther away from the dip equator. As mentioned earlier, on 15 July, the anomaly was strong and crest was farthest from equator. On the other hand, the anomaly as mentioned was weakest on 13 June, which also corroborates with the magnetic field variation on this day. However, the low levels of ionization on the whole cannot be ascribed to this. In the post-sunset hours over the equator, the ionosphere after a brief upward excursion, i.e. prereversal enhancement shows a downward trend. In this process, chemistry plays an important role in decreasing the electron/ion density through the recombinations, especially over altitudes below F- region peak of ionosphere. Owing to the presence of the ionization trough over the equatorial latitudes, the overall ionization in the nighttime is, in general, found to be less. This interplay of chemistry and diffusion process characterizes the dynamic nature of the F-region, which is coupled with the thermosphere and renders a typical distribution pattern for ionization in nighttime thermosphere. At latitudes away from the equator, where the magnetic field has significant dip, neutral wind, apart from chemistry, also becomes important. The nighttime tomograms in this context reveal a consistent behaviour which can be ascribed to the chemistry and the diffusion of plasma along the field lines (Fig. 2). However, the role of wind cannot be precluded. The present study is limited in discussing the role of wind for want of more data. On the whole, the overall distribution of the ionization, on a given day, is governed by the way in which the tidal wind varies, in turn varying the evolution of the overall electrodynamics. 5 Conclusions The data obtained from YOUTHSAT s RaBIT payload is very useful in the ionospheric variability study of low and equatorial latitude regions over India. The general observation is that the GPS satellite with highest elevation angle will have minimum STEC values. However, once in a while such as the case of the considered YOUTHSAT passes, when the YOUTHSAT is at the highest elevation angle (60.1 and 28.5 ), this trend is not observed (6 July 2011 for Hyderabad station). This could be due to ionospheric anomaly. The STEC variation trends due to YOUTHSAT for three stations are not same for the considered days of the months of May, June and July More specific conclusions may be made by extending this work for large amount of data and more number of stations. Also, the measurements on the distribution of the ionization in the equatorial and the low latitude ionosphere can facilitate understanding of the prevailing electrodynamics. Ionospheric tomography is the only experimental technique which can provide snapshots of the overall ionization distribution.

7 142 INDIAN J RADIO & SPACE PHYS, JUNE 2013 Acknowledgement The research work presented in this paper has been carried out under the project entitled, Coherent Radio Beacon Experiment (CRABEX), sponsored bythe Department of Space, Space Physics Laboratory, Vikram Sarabhai Space Center (VSSC), Thiruvanthapuram, India, vide sanction letter No: SPL/CRABEX/12/3, dated: 29 Oct References Pant T K, Sreelatha P, Mridula N, Trivedi S, Das R M, Koli S, Sharma R, Girija J, Alex Arun, Mukundan K K, Shukla S B, Purushottaman P, Santosh J N, Thomas Biju, Srikant M, Sridharan R, Krishnamoorthy K, Bisht Ratan, Raghavamurthy D V A, Chamy M P T & Rao J D, Radio Beacon for Ionospheric Tomography (RaBIT) onboard YOUTHSAT: Preliminary results, Indian J Radio Space Phys, 41 (2012) pp Kaplan Elliott D & Hegarty Christopher J, Understanding GPS: Principles and applications (Artech House, London), Hofmann-ellennof B, Lichtenegger H & Collins J, GPS theory and practice (Springer Wien, New York), Parkinson B W & Spilker J Jr, Global Positioning System Vol 1: Theory and Application (American Institute of Aeronautics and Applications, Washington, DC), Hall M P M, Barclay L W & Hewitt M T, Propagation of radio waves (The Institute of Electrical Engineers, Stevenage),