Spatial distribution of ionization in the equatorial and low-latitude ionosphere of the Indian sector and its effect on the pierce point altitude for GPS applications during low solar activity periods (original) (raw)
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Annales Geophysicae, 2006
The GPS data provides an effective way to estimate the total electron content (TEC) from the differential time delay of L1 and L2 transmissions from the GPS. The spacing of the constellation of GPS satellites in orbits are such that a minimum of four GPS satellites are observed at any given point in time from any location on the ground. Since these satellites are in different parts of the sky and the electron content in the ionosphere varies both spatially and temporally, the ionospheric pierce point (IPP) altitude or the assumed altitude of the centroid of mass of the ionosphere plays an important role in converting the vertical TEC from the measured slant TEC and vice versa. In this paper efforts are made to examine the validity of the IPP altitude of 350 km in the Indian zone comprising of the everchanging and dynamic ionosphere from the equator to the ionization anomaly crest region and beyond, using the simultaneous ionosonde data from four different locations in India. From this data it is found that the peak electron density height (h p F 2 ) varies from about 275 to 575 km at the equatorial region, and varies marginally from 300 to 350 km at and beyond the anomaly crest regions. Determination of the effective altitude of the IPP employing the inverse method suggested by did not yield any consistent altitude in particular for low elevation angles, but varied from a few hundred to one thousand kilometers and beyond in the Indian region. However, the vertical TEC computed from the measured GPS slant TEC for different IPP altitudes ranging from 250 to 750 km in the Indian region has revealed that the TEC does not change significantly with the IPP altitude, as long as the elevation angle of the satellite is greater than 50 degrees. However, in the case of satellites with lower elevation angles (<50 • ), there is a significant departure in the TEC computed using different IPP altitudes from both methods. Therefore, the IPP altitude of 350 km may be taken as valid even in the Indian sector but only in the cases of satellite passes with elevation angles greater than 50 • .
Journal of Geophysical Research: Space Physics, 2007
The paper presents the observations and modeling of an additional layer in the low‐latitude ionosphere in Indian longitudes. The signatures of the additional layer are observed as ledges or humps between the equatorial ionization anomaly trough and crest (EIA) in the latitudinal profiles of total electron content (TEC), measured using a single ground‐based beacon receiver located at Trivandrum (8.5°N, 77°E, dip 0.5°N) in India. The ground‐based ionograms also show the presence of the so‐called F3 layer for a short duration corresponding to these signatures, and the layer is found to drift upward to the topside ionosphere. The study provides first observational evidence that the so‐called “humps” in the latitudinal variation of TEC are nothing but the upward propagating F3 layer. This conclusion is supported by theoretical modeling using the Sheffield University Plasmasphere Ionosphere Model. It is shown that upward ExB drift and strong equatorward neutral wind (perturbed by atmosphe...
Effect of Ionospheric Perturbation on GPS Observation over Low Latitude Region, Bhopal
Journal of Scientific Research, 2012
Increased knowledge on the ionospheric structure is of important interest for precise positioning, since the ionosphere has an impact on global positioning system (GPS) L-band radio waves by its free electrons. Especially during perturbed geomagnetic conditions when the ionosphere differs from its undisturbed state, quasi real time data assimilation would be useful. On the other hand, these perturbations of the GPS signals are taken as scientific information to investigate ionospheric scenarios. In this paper we describe the occurrence of GPS phase fluctuations during January to December 2005 events on the basis of Bhopal GPS observations. This study concerns the analysis of strong phase fluctuations which were associated with geomagnetic storms. The intensive total electron content (TEC) fluctuations observed along GPS satellite passes, demonstrate a strong horizontal gradient of TEC and difficulties with the carrier phase ambiguity in relative GPS positioning. In turn, the phase f...
Journal of Earth System Science, 2011
In recent years, measurements of total electron content (TEC) have gained importance with increasing demand for the GPS-based navigation applications in trans-ionospheric communications. To study the variation in ionospheric TEC, we used the data obtained from GPS Ionospheric Scintillation and TEC monitoring (GISTM) system which is in operation at SVNIT, Surat, India (21.16 • N, 72.78 • E) located at the northern crest of equatorial anomaly region. The data collected (for the low sunspot activity period from August 2008-December 2009) were used to study the diurnal, monthly, seasonal semi-annual and annual variations of TEC at Surat. It was observed that the diurnal variation at the region reaches its maximum value between 13:00 and 16:00 IST. The monthly average diurnal variations showed that the TEC maximizes during the equinox months followed by the winter months, and are lowest during the summer months. The ionospheric range delay to TEC for the primary GPS signal is 0.162 m per TECU.
Variability of ionospheric TEC at low latitude station, Hyderabad during medium solar activity
Russian Journal of Earth Sciences, 2021
The successful operation of communicational and navigational applications requires knowledge of state and behaviour of ionosphere as well as spatial and temporal changes taking place in it. With this view, we have studied the variability and changes in the low latitude ionosphere by considering a low latitude Indian station Hyderabad (17.41 ∘ N, 78.55 ∘ E). The variability is studied during the year 2014, by using the space borne Global Positioning System (GPS) observations. From GPS observations an important parameter; Total Electron Content (TEC) is obtained, which represents the integral electron density of the ionosphere and is widely used in studies concerning ionospheric variability. We have studied how the ionospheric conditions at Hyderabad change from hour to hour, day to day, month to month and season to season. We found that ionosphere over Hyderabad exhibits significant, interesting and regular variability. Although, the trends followed by the ionospheric changes are similar, but the magnitude of peak values differ on different time scales. Finally we have compared the variability of observed or actual values of TEC with the corresponding values predicted by International Reference Ionosphere (IRI) model-2016 and found good agreement between hourly values, while IRI model overestimates or underestimates the daily values.
Annales Geophysicae, 2009
The dual frequency signals from the GPS satellites recorded at Rajkot (22.29 • N, 70.74 • E, Geographic, 14.03 • N Geomagnetic) near the Equatorial ionization anomaly crest in India have been analyzed to study the ionospheric variations in terms of Total Electron Content (TEC) for the low solar activity period from April 2005 to December 2007. In this study, we describe the diurnal and seasonal variations of TEC, solar activity dependence of TEC and effects of a space weather related event, a geomagnetic storm on TEC. The diurnal variation of TEC shows pre-dawn minimum for a short period of time, followed by a steep early morning increase and then reaches maximum value between 14:00 LT and 16:00 LT. The mean diurnal variations during different seasons are brought out. It is found that TEC at Rajkot is at its maximum during Equinoctial months (March, April, September, October), and minimum during the Winter months (November, December, January, February), with intermediate values during Summer months (May, June, July, August), showing a semi annual variation. TEC values have been decreasing since 2005, onwards showing positive correlation with solar activity. TEC variations during the geomagnetic storm commencing 24 August 2005 with D st =−216 nT are analysed. TEC shows a positive ionospheric storm effect on the first day of the storm and negative ionospheric storm effect on the next day. The equatorial Electrojet control on the development of the equatorial anomaly is also demonstrated.
Advances in Space Research, 2008
The electron density and temperature distribution of the equatorial and low latitude ionosphere in the Indian sector has been investigated by simultaneously solving the continuity, momentum and energy balance equations of ion and electron flux along geomagnetic field lines from the Northern to the Southern hemisphere. Model algorithm is presented and results are compared with the electron density and electron temperature measured in situ by Indian SROSS C2 satellite at an altitude of $500 km within 31°S-34°N and 75 ± 10°E that covers the Indian sector during a period of low solar activity. Equatorial Ionization Anomaly (EIA) observed in electron density, morning and afternoon enhancements, equatorial trough in electron temperature have been simulated by the model within reasonable limits of accuracy besides reproducing other normal diurnal features of density and temperature.
2009
The equatorial and low-latitude ionospheric response to three moderate geomagnetic storms (17, 18, and 22 January) during the period from 16 to 23 January 2005 is investigated in the context of development/inhibition of the Equatorial Ionization Anomaly (EIA) and the subsequent occurrence/nonoccurrence of Equatorial Spread F (ESF) irregularities on these days. The study is carried out using the Total Electron Content (TEC) measured with the GPS receivers along the 80°ElongitudesectorandtheF−layerbottomheightobtainedfromtheIonosondelocatedoverthedipequatoriallocationofTrivandrum(8.5°N,77°E,diplatitude80°E longitude sector and the F-layer bottom height obtained from the Ionosonde located over the dip equatorial location of Trivandrum (8.5°N, 77°E, dip latitude 80°ElongitudesectorandtheF−layerbottomheightobtainedfromtheIonosondelocatedoverthedipequatoriallocationofTrivandrum(8.5°N,77°E,diplatitude0.5°N) in India. It is observed that, for the storms on days 17 and 22, the development of the anomaly was inhibited, probably due to the westward disturbance dynamo electric fields. Subsequently, the post sunset enhancement in the vertical drift of the equatorial F region was also inhibited significantly compared to the quiet day pattern and, as anticipated, no ESF was observed on these days. A large vertical drift of the equatorial F region followed by nearly simultaneous onset of weak ESF was observed on day 18. The late development of the EIA on this day could be due to the eastward prompt penetration electric field associated with the southward turning of the interplanetary magnetic field. Also, strong and distinct F3 layer appeared for a short time in the morning, reappeared later in the noon time, and then quickly ascended to the topside ionosphere during the main phase of the storm on day 18.
2019
The relation between the occurrence of ionospheric irregularity and the spatial gradient of total electron content (TEC) during the post-sunset hours over the equatorial region is investigated. Different instruments and techniques have been applied to study the behavior of these ionospheric irregularities. In this study, the Global Positioning System (GPS) based derived TEC was employed to investigate the relation between the spatial gradient of TEC between two nearby located stations and the occurrence of ionospheric irregularity over the East Africa longitudinal sector. The gradient of TEC between the two 5 stations (ASAB: 4.34◦ N, 114.39◦ E and DEBK: 3.71◦ N, 109.34◦ E, geomagnetic) located within the equatorial region of Africa were considered in this study during the year 2014. The rate of change of TEC based derived index (ROTI , ROTIave) is also used to observe the relation between the spatial gradient of TEC and the occurrence of ionospheric irregularities over the stations....
Astrophysics and Space Science, 2011
The present paper analyzes the dual frequency signals from GPS satellites recorded at Varanasi (Geographic latitude 25°, 16 N, longitude 82°, 59 E) near the equatorial ionization anomaly (EIA) crest in India, to study the effect of geomagnetic storm on the variation of TEC, during the low solar active period of May 2007 to April 2008. Three most intense-but still moderate class-storms having a rapid decrease of Dst-index observed during the GPS recorded data have been analyzed, which occurred on