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Showing papers on "Total electron content published in 2006"


Journal ArticleDOI
TL;DR: In this paper, the ionosphere's total electron content (TEC) is a parameter widely used in studies of the near-Earth plasma environment, and a comprehensive summary of pre-GPS storm studies is needed to set the base for progress in the GPS era.
Abstract: [1] The ionosphere's total electron content (TEC) is a parameter widely used in studies of the near-Earth plasma environment. The scientific use of TEC appeared early in the artificial satellite era, and among its many contributions were fundamental insights into how the ionosphere responds to geomagnetic storms. While many excellent reviews of solar-terrestrial disturbances exist in the literature, none have concentrated on the TEC parameter per se. With new TEC data sets increasingly available from the Global Positioning System (GPS), a comprehensive summary of pre-GPS storm studies is needed to set the base for progress in the GPS era. This review summarizes past case studies, describes statistical occurrence pattern, and identifies responsible mechanisms validated via modeling. It presents a new set of results of TEC disturbance patterns during 180 geomagnetic storms to describe seasonal and solar cycle effects. It concludes with a set of open questions that require additional study.

442 citations


Journal ArticleDOI
TL;DR: The architecture of the MAPGPS software, which automates the processing of GPS data into global total electron density (TEC) maps, is described and three different methods for solving the receiver bias problem are described in detail.
Abstract: A software package known as MIT Automated Processing of GPS (MAPGPS) has been developed to automate the processing of GPS data into global total electron density (TEC) maps. The goal of the MAPGPS software is to produce reliable TEC data automatically, although not yet in real time. Observations are used from all available GPS receivers during all geomagnetic conditions where data has been successfully collected. In this paper, the architecture of the MAPGPS software is described. Particular attention is given to the algorithms used to estimate the individual receiver biases. One of the largest sources of error in estimating TEC from GPS data is the determination of these unknown receiver biases. The MAPGPS approach to solving the receiver bias problem uses three different methods: minimum scalloping, least squares, and zero-TEC. These methods are described in detail, along with their relative performance characteristics. A brief comparison of the JPL and MAPGPS receiver biases is presented, and a possible remaining error source in the receiver bias estimation is discussed. Finally, the Madrigal database, which allows Web access to the MAPGPS TEC data and maps, is described.

426 citations


Journal ArticleDOI
TL;DR: In this paper, the authors presented the temporal and spatial variations in TEC derived from the simultaneous and continuous measurements made, 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 of 1° S to 24° N, using a 16-month period of data for the low sunspot activity (LSSA) years of March 2004 to June 2005.
Abstract: . With the recent increase in the satellite-based navigation applications, the ionospheric total electron content (TEC) and the L-band scintillation measurements have gained significant importance. In this paper we present the temporal and spatial variations in TEC derived from the simultaneous and continuous measurements made, 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 of 1° S to 24° N, using a 16-month period of data for the low sunspot activity (LSSA) years of March 2004 to June 2005. The diurnal variation in TEC at the EIA region shows its steep increase and reaches its maximum value between 13:00 and 16:00 LT, while at the equator the peak is broad and occurs around 16:00 LT. A short-lived day minimum occurs between 05:00 to 06:00 LT at all the stations from the equator to the EIA crest region. Beyond the crest region the day maximum values decrease with the increase in latitude, while the day minimum in TEC is flat during most of the nighttime hours, i.e. from 22:00 to 06:00 LT, a feature similar to that observed in the mid-latitudes. Further, the diurnal variation in TEC show a minimum to maximum variation of about 5 to 50 TEC units, respectively, at the equator and about 5 to 90 TEC units at the EIA crest region, which correspond to range delay variations of about 1 to 8 m at the equator to about 1 to 15 m at the crest region, at the GPS L1 frequency of 1.575 GHz. The day-to-day variability is also significant at all the stations, particularly during the daytime hours, with maximum variations at the EIA crest regions. Further, similar variations are also noticed in the corresponding equatorial electrojet (EEJ) strength, which is known to be one of the major contributors for the observed day-to-day variability in TEC. The seasonal variation in TEC maximizes during the equinox months followed by winter and is minimum during the summer months, a feature similar to that observed in the integrated equatorial electrojet (IEEJ) strength for the corresponding seasons. In the Indian sector, the EIA crest is found to occur in the latitude zone of 15° to 25° N geographic latitudes (5° to 15° N geomagnetic latitudes). The EIA also maximizes during equinoxes followed by winter and is not significant in the summer months in the LSSA period, 2004–2005. These studies also reveal that both the location of the EIA crest and its peak value in TEC are linearly related to the IEEJ strength and increase with the increase in IEEJ.

249 citations


Journal ArticleDOI
TL;DR: The potential of broadband L-band SAR systems for ionospheric TEC mapping is studied and it is shown that phase advance and group delay can be measured by interferometric and correlation techniques, respectively.
Abstract: Ionospheric propagation effects have a significant impact on the signal properties of low-frequency synthetic aperture radar (SAR) systems. Range delay, interferometric phase bias, range defocusing, and Faraday rotation are the most prominent ones. All the effects are a function of the so-called total electron content (TEC). Methods based on two-frequency global positioning system observations allow measuring TEC in the ionosphere with coarse spatial resolution only. In this letter, the potential of broadband L-band SAR systems for ionospheric TEC mapping is studied. As a basis, the dispersive nature of the ionosphere and its effects on broadband microwave radiation are theoretically derived and analyzed. It is shown that phase advance and group delay can be measured by interferometric and correlation techniques, respectively. The achievable accuracy suffices in mapping small-scale ionospheric TEC disturbances. A differential TEC estimator that separates ionospheric from tropospheric contributions is proposed

197 citations


Journal ArticleDOI
TL;DR: In this article, the authors reproduce, with a 3D numerical modeling of the ocean-atmosphere-ionosphere coupling, the tsunami signature in the Total Electron Content (TEC) data measured by the Jason-1 and Topex/Poseidon satellite altimeters.
Abstract: [1] The Sumatra, December 26th, 2004, tsunami produced internal gravity waves in the neutral atmosphere and large disturbances in the overlying ionospheric plasma. To corroborate the tsunamigenic hypothesis of these perturbations, we reproduce, with a 3D numerical modeling of the ocean-atmosphere-ionosphere coupling, the tsunami signature in the Total Electron Content (TEC) data measured by the Jason-1 and Topex/Poseidon satellite altimeters. The agreement between the observed and synthetic TEC shows that ionospheric remote sensing can provide new tools for offshore tsunami detection and monitoring.

157 citations


Journal ArticleDOI
TL;DR: The Gauss-Markov Kalman filter (GMKF) as mentioned in this paper was developed as part of the Global Assimilation of Ionospheric Measurements (GAIM) program.
Abstract: [1] The Utah State University Gauss-Markov Kalman Filter (GMKF) was developed as part of the Global Assimilation of Ionospheric Measurements (GAIM) program. The GMKF uses a physics-based model of the ionosphere and a Gauss-Markov Kalman filter as a basis for assimilating a diverse set of real-time (or near real-time) observations. The physics-based model is the Ionospheric Forecast Model (IFM), which accounts for five ion species and covers the E region, F region, and the topside from 90 to 1400 km altitude. Within the GMKF, the IFM derived ionospheric densities constitute a background density field on which perturbations are superimposed based on the available data and their errors. In the current configuration, the GMKF assimilates slant total electron content (TEC) from a variable number of global positioning satellite (GPS) ground sites, bottomside electron density (Ne) profiles from a variable number of ionosondes, in situ Ne from four Defense Meteorological Satellite Program (DMSP) satellites, and nighttime line-of-sight ultraviolet (UV) radiances measured by satellites. To test the GMKF for real-time operations and to validate its ionospheric density specifications, we have tested the model performance for a variety of geophysical conditions. During these model runs various combination of data types and data quantities were assimilated. To simulate real-time operations, the model ran continuously and automatically and produced three-dimensional global electron density distributions in 15 min increments. In this paper we will describe the Gauss-Markov Kalman filter model and present results of our validation study, with an emphasis on comparisons with independent observations.

143 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated total electron content (TEC) perturbations associated with medium-scale traveling ionospheric disturbances (MSTIDs) and found that MSTIDs activity during daytime is different from that during nighttime with respect to seasonal, solar activity, longitudinal, and latitudinal dependences.
Abstract: [1] Using global positioning system (GPS) data taken from the International GNSS Service (IGS), we investigated total electron content (TEC) perturbations associated with medium-scale traveling ionospheric disturbances (MSTIDs). We analyzed TEC data taken from four or five GPS receivers in each of six regions (Japan, Europe, eastern United States, western United States, Australia, and South America) in 3 years (1998, 2000, and 2001). To derive perturbation components of TEC (I′), we subtracted the 1-hour running average form the time sequence of TEC for each satellite–receiver pair. Standard deviation of I′ within 1 hour, δI, was calculated every hour, and MSTIDs activity were defined as δI/, where is the 1-hour average of absolute vertical TEC. We found that MSTIDs activity during daytime is different from that during nighttime with respect to seasonal, solar activity, longitudinal, and latitudinal dependences. Daytime MSTIDs activity are high in winter in all six regions. On the other hand, seasonal variation of nighttime MSTIDs activity is coupled with its longitudinal variation. In the Japanese and Australian longitudinal sector, nighttime MSTIDs are most active near the June solstice, whereas it is most active near the December solstice in the European longitudinal sector. Nighttime MSTIDs activity at the Japanese and Australian longitudinal sector shows negative correlation with solar activity, whereas solar activity dependence is not seen in daytime MSTIDs activity. These results suggest that mechanisms causing MSTIDs could be different between daytime and nighttime.

136 citations


Journal ArticleDOI
TL;DR: In this paper, the authors examined 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.
Abstract: . 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 ever-changing 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 (hpF2) 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 Birch et al. (2002) 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 (

135 citations


Journal ArticleDOI
TL;DR: In this article, the total electron content (TEC) of ground-based receivers of the global positioning system (GPS) in the Indian Ocean area of the 26 December 2004 Mw 9.3 Sumatra earthquake was analyzed.
Abstract: [1] Tsunami ionospheric disturbances (TIDs) of the 26 December 2004 Mw 9.3 Sumatra earthquake are detected by the total electron content (TEC) of ground-based receivers of the global positioning system (GPS) in the Indian Ocean area. It is found that the tsunami waves triggered atmospheric disturbances near the sea surface, which then traveled upward with an average velocity of about 730 m/s (2700 km/hr) into the ionosphere and significantly disturbed the electron density within it. Results further show that the TIDs, which have maximum height of about 8.6–17.2 km, periods of 10–20 min, and horizontal wavelengths of 120–240 km, travel away from the epicenter with an average horizontal speed of about 700 km/hr (190 m/s) in the ionosphere.

129 citations


Journal ArticleDOI
TL;DR: In this paper, the authors explored the possibility to constrain the rupture process of the Sumatra-Andaman earthquake with the observed ionospheric disturbances by using the estimated TEC perturbations from individual point sources.
Abstract: [1] Near-field coseismic perturbations of ionospheric total electron content (TEC), caused by direct acoustic waves from focal regions, can be observed with Global Positioning System (GPS) They appear 10–15 min after the earthquake with typical periods of ∼4–5 min and propagate as fast as ∼1 km/s toward directions allowed by ambient geomagnetic fields Ionospheric disturbance, associated with the 2004 December 26 great Sumatra-Andaman earthquake, was recorded with nine continuous GPS receiving stations in Indonesia and Thailand Here we explore the possibility to constrain the rupture process of the earthquake with the observed ionospheric disturbances We assumed linearly distributed point sources along the zone of coseismic uplift extending ∼1300 km from Sumatra to the Andaman Islands that excited acoustic waves sequentially as the rupture propagate northward by 25 km/s TEC variations for several satellite-receiver pairs were synthesized by simulating the propagation of acoustic waves from the ground to the ionosphere and by integrating the TEC perturbations at intersections of line of sights and the ray paths The TEC perturbations from individual point sources were combined using realistic ratios, and the total disturbances were compared with the observed signals Prescribed ratios based on geodetically inferred coseismic uplifts reproduced the observed signals fairly well Similar calculation using a rupture propagation speed of 17 km/s degraded the fit Suppression of acoustic waves from the segments north of the Nicobar Islands also resulted in a poor fit, which suggests that ruptures in the northern half of the fault were slow enough to be overlooked in short-period seismograms but fast enough to excite atmospheric acoustic waves Coseismic ionospheric disturbance could serve as a new indicator of faulting sensitive to ruptures with timescale up to 4–5 min

124 citations


Journal ArticleDOI
TL;DR: In this article, the response of the ionosphere to the large earthquake that occurred in West Sumatra, Indonesia, at 0058 UT on December 26, 2004, is reported.
Abstract: We report the response of the ionosphere to the large earthquake that occurred in West Sumatra, Indonesia, at 0058 UT on December 26, 2004. We have analyzed Global Positioning System (GPS) data obtained at two sites in Sumatra and at three sites in Thailand to investigate total electron content (TEC) variations. Between 14 and 40 min after the earthquake, TEC enhancements of 1.6–6.9 TEC units (TECU) were observed at subionospheric points located 360–2000 km north of the epicenter. From the time delays of the observed TEC enhancements, we find that the TEC enhancements propagated northward from the epicenter. The time delays between the earthquake and rapid increases in TEC, which occurred near the epicenter, are consistent with the idea that acoustic waves generated by the earthquake propagated into the ionosphere at the speed of sound to cause the TEC variations. A small TEC enhancement of 0.6 TECU was observed south of the epicenter, while no TEC enhancements were seen east of the epicenter. From a model calculation, we find that this directivity of the TEC variations with respect to the azimuth from the epicenter could be caused partially by the directivity in the response of the electron density variation to the acoustic waves in the neutral atmosphere.

Journal ArticleDOI
TL;DR: In this article, the authors explore how the thermosphere-ionosphere system responded to the onset of the 20 November 2003 geomagnetic storm, using the NCAR TIMEGCM.
Abstract: [1] There is great interest in understanding how the thermosphere-ionosphere system responds to geomagnetic storms. New insights are possible using the new generation of fully coupled three-dimensional models, together with extensive ionospheric databases. The period of postsolar maximum geomagnetic storms in October and November 2003 were some of the largest storms ever recorded. In this paper, we explore how the thermosphere-ionosphere system responded to the onset of the 20 November 2003 geomagnetic storm, using the NCAR TIMEGCM. The model simulates dramatic changes in the thermospheric equatorward winds, O/N 2 , and corresponding ionospheric electron densities. The model is used as a framework to interpret an increase in the observed ionospheric total electron content, and F region electron density, in the European and North African sector, in terms of changes in the neutral gas. Corresponding compositional effects observed by the GUVI instrument on the TIMED satellite lend credence to the model results. We describe some of the important physical processes that will affect planning for the utilization of measurements from the Geospace investigations in NASA's Living With a Star Program. The study illustrates the value of measuring both the neutral and ionized gases, of obtaining quasi-global views from imaging instruments, and the synergy between satellite data, ground-based measurements, and models.

Journal ArticleDOI
TL;DR: In this paper, the occurrence times and locations of 11 solar flares were isolated from the 1-8 A X-ray radiations of the geosynchronous operational environmental satellite (GOES) and the SOHO Extreme Ultraviolet Imaging Telescope (EIT) images, respectively.
Abstract: [1] In this study, ionospheric solar flare effects on the total electron content (TEC) and associated time rate of change (rTEC) derived from ground-based global positioning system (GPS) receivers in the midday region are examined. The occurrence times and locations of 11 solar flares are isolated from the 1–8 A X-ray radiations of the geosynchronous operational environmental satellite (GOES) and the SOHO Extreme Ultraviolet Imaging Telescope (EIT) images, respectively, while the TEC and rTEC are obtained from the international GPS services (IGS). Results show that the maximum value of the TEC increase solely depends on the flare class, while the maximum value of the rTEC increase is related to not only the flare class but also the time rate of change in flare radiations. A statistical analysis further demonstrates that the two maximum values are inversely proportional to the cosine of the great circle angle between the center and flare locations on the solar disc.

Journal ArticleDOI
TL;DR: In this paper, the authors developed a model called the Middle East Technical University Neural Network (METU-NN) model, which is a data-driven neural network model of one hidden layer and several neurons.
Abstract: [1] Near-Earth space processes are highly nonlinear. Since the 1990s, a small group at the Middle East Technical University in Ankara has been working on a data-driven generic model of such processes, that is, forecasting and nowcasting of a near-Earth space parameter of interest. The model developed is called the Middle East Technical University Neural Network (METU-NN) model. The METU-NN is a data-driven neural network model of one hidden layer and several neurons. In order to understand more about the complex response of the magnetosphere and ionosphere to extreme solar events, we chose this time the series of space weather events in November 2003. Total electron content (TEC) values of the ionosphere are forecast during these space weather events. In order to facilitate an easier interpretation of the forecast TEC values, maps of TEC are produced by using the Bezier surface-fitting technique.

Journal ArticleDOI
TL;DR: In this article, the authors reviewed the different approaches, their implications for IRI, and their current status, and special emphasis is given to the coupling between topside and plasmaspheric models.
Abstract: [1] Shortcomings of the representation of the topside electron density profile in the International Reference Ionosphere (IRI) model have been noted in comparison with recently analyzed topside sounder data and also with total electron content (TEC) data. Various studies have proposed corrections of the IRI formulas or have introduced a new formalism. This paper reviews the different approaches, their implications for IRI, and their current status. An important challenge for topside modeling is the truthful representation of profiles in the equatorial anomaly (EA) region over the whole range of the EA fountain. This means that the latitudinal representation has to reproduce the merging of the double-peak signature at F region heights into a single peak at the top of the fountain. In this context, special emphasis is given to the coupling between topside and plasmaspheric models.

Journal ArticleDOI
TL;DR: In this article, a total electron content space weather study of the nighttime Weddell Sea Anomaly, overlooked by previously published TOPEX/Poseidon climate studies, and the nighttime ionosphere during the 1996/1997 southern summer was performed.
Abstract: This paper reports on a total electron content space weather study of the nighttime Weddell Sea Anomaly, overlooked by previously published TOPEX/Poseidon climate studies, and of the nighttime ionosphere during the 1996/1997 southern summer. To ascertain the morphology of spatial TEC distribution over the oceans in terms of hourly, geomagnetic, longitudinal and summer-winter variations, the TOPEX TEC, magnetic, and published neutral wind velocity data are utilized. To understand the underlying physical processes, the TEC results are combined with inclination and declination data plus global magnetic field-line maps. To investigate spatial and temporal TEC variations, geographic/magnetic latitudes and local times are computed. As results show, the nighttime Weddell Sea Anomaly is a large (∼1,600(°)2; ∼22 million km2 estimated for a steady ionosphere) space weather feature. Extending between 200°E and 300°E (geographic), it is an ionization enhancement peaking at 50°S–60°S/250°E–270°E and continuing beyond 66°S. It develops where the spacing between the magnetic field lines is wide/medium, easterly declination is large-medium (20°–50°), and inclination is optimum (∼55°S). Its development and hourly variations are closely correlated with wind speed variations. There is a noticeable (∼43%) reduction in its average area during the high magnetic activity period investigated. Southern summer nighttime TECs follow closely the variations of declination and field-line configuration and therefore introduce a longitudinal division of four (Indian, western/eastern Pacific, Atlantic). Northern winter nighttime TECs measured over a limited area are rather uniform longitudinally because of the small declination variation. TOPEX maps depict the expected strong asymmetry in TEC distribution about the magnetic dip equator.

Journal ArticleDOI
TL;DR: The Coherent Electromagnetic Radio Tomography (CERTO) constellation of radio beacons is available for measurements of ionospheric total electron content and radio scintillations.
Abstract: [1] The new constellation of radio beacons called Coherent Electromagnetic Radio Tomography (CERTO) will be available for measurements of ionospheric total electron content and radio scintillations. These beacons transmit unmodulated, phase-coherent waves, VHF, UHF, and L band frequencies. A fixed radio of 3/8 is used between successive frequencies. Total electron content (TEC) can be measured using the differential phase technique. The range between beacon and receiver is removed from the phase measurements, leaving a differential phase that is proportional to TEC. The three CERTO frequencies cover a wide range for determination of the radio scintillation effects caused by diffraction after propagation though ionospheric irregularities. All of the CERTO beacons are in low Earth orbit with inclinations ranging from equatorial to polar. Each satellite that carries CERTO has other plasma instruments that complement the beacon data. In addition, a Scintillation and Tomography Receiver in Space (CITRIS) instrument will be placed in orbit to detect signals from the CERTO beacons and from the array of 56 Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) VHF/S band radio beacons placed around the word by the French Centre National D'Etudes Spatiales. CITRIS will record ionospheric occultations and radio scintillations with a unique occultation and ground-to-space geometry. New algorithms have been developed for the multifrequency CERTO and CITRIS data to provide improved acquisition and analysis of TEC and scintillation data in ionospheric studies. The data from the CERTO constellation of beacons and receivers may be used to update space weather models.

Journal ArticleDOI
TL;DR: Validation of US-TEC indicates an accuracy of the line-of-sight electron content of between 2 and 3 TEC units, equivalent to less than 50 cm signal delay at L1 frequencies, which promises value for GPS users.
Abstract: [1] The potential of data assimilation for operational numerical weather forecasting has been appreciated for many years. For space weather it is a new path that we are just beginning to explore. With the emergence of satellite constellations and the networks of ground-based observations, sufficient data sources are now available to make the application of data assimilation techniques a viable option. The first space weather product at Space Environment Center (SEC) utilizing data assimilation techniques, US-TEC, was launched as a test operational product in November 2004. US-TEC characterizes the ionospheric total electron content (TEC) over the continental United States (CONUS) every 15 min with about a 15-min latency. US-TEC is based on a Kalman filter data assimilation scheme driven by a ground-based network of real-time GPS stations. The product includes a map of the vertical TEC, an estimate of the uncertainty in the map, and the departure of the TEC from a 10-day average at that particular universal time. In addition, data files are provided for vertical TEC and the line-of-sight electron content to all GPS satellites in view over the CONUS at that time. The information can be used to improve single-frequency GPS positioning by providing more accurate corrections for the ionospheric signal delay, or it can be used to initialize rapid integer ambiguity resolution schemes for dual-frequency GPS systems. Validation of US-TEC indicates an accuracy of the line-of-sight electron content of between 2 and 3 TEC units (1 TECU = 1016 el m−2), equivalent to less than 50 cm signal delay at L1 frequencies, which promises value for GPS users. This is the first step along a path that will likely lead to major improvement in space weather forecasting, paralleling the advances achieved in meteorological weather forecasting.

Journal ArticleDOI
TL;DR: In this article, the La Plata ionospheric model (LPIM) was updated to use modip latitude instead of the geomagnetic one, which significantly improved the agreement between GPS and Topex.

Journal ArticleDOI
TL;DR: In this paper, a near-real-time electron density retrieval technique based on the knowledge of the model driving parameter Az (ionization level) for the location considered is presented.
Abstract: [1] NeQuick is a three-dimensional and time-dependent quick run electron density model specifically designed for transionospheric propagation applications. It allows calculation of electron concentration values at any location in the ionosphere and the total electron content (TEC) along any ground station–to–satellite ray path. After specific adaptations, the model has been used to develop a near-real-time nontomographic electron density retrieval technique able to provide the electron density of the ionosphere above the geographic area of interest. The technique relies on the knowledge of the model driving parameter Az (ionization level) for the location considered. In the present study, the necessary Az values have been obtained through direct ingestion of Global Positioning System (GPS)–derived slant TEC data in two different ways: using data from a single GPS receiver and using data from multiple ground stations. Statistical comparisons between experimental and reconstructed slant TEC values and between experimental and retrieved maximum electron concentration values are shown.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the ionospheric electron content on the day of the great Sumatra-Andaman earthquake of December 26, 2004 and found that a significant perturbation of 1.5 to 2 TEC units over a smooth variation of TEC in the morning hours was observed within 45 minutes of the quake at stations situated near the east coast of the Indian subcontinent.
Abstract: Using the Total Electron Content (TEC) data recorded by the GPS receiver network, installed under the GPS and Geo Augmented Navigation (GAGAN) program, ionospheric electron content on the day of the great Sumatra-Andaman earthquake of December 26, 2004 was examined. A significant perturbation of 1.5 to 2 TEC units over a smooth variation of TEC in the morning hours was observed within 45 minutes of the quake at stations situated near the east coast of the Indian subcontinent. The disturbance was found to propagate northwestward with its origin situated about 2° northeast of the quake epicenter. Possible coupling mechanism of the crustal movement and the ionosphere are discussed.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the features of pre-earthquake ionospheric anomalies in the total electron content (TEC) data obtained on the basis of regular GPS observations from the IGS network.
Abstract: . This paper investigates the features of pre-earthquake ionospheric anomalies in the total electron content (TEC) data obtained on the basis of regular GPS observations from the IGS network. For the analysis of the ionospheric effects of the 26 December 2004 Indonesian earthquake, global TEC maps were used. The possible influence of the earthquake preparation processes on the main low-latitude ionosphere peculiarity – the equatorial anomaly – is discussed. Analysis of the TEC maps has shown that modification of the equatorial anomaly occurred a few days before the earthquake. For 2 days prior to the event, a positive effect was observed in the daytime amplification of the equatorial anomaly. Maximal enhancement in the crests reached 20 TECU (50–60%) relative to the non-disturbed state. In previous days, during the evening and night hours (local time), a specific transformation of the TEC distribution had taken place. This modification took the shape of a double-crest structure with a trough near the epicenter, though usually in this time the restored normal latitudinal distribution with a maximum near the magnetic equator is observed. It is assumed that anomalous electric field generated in the earthquake preparation zone could cause a near-natural "fountain-effect" phenomenon and might be a possible cause of the observed ionospheric anomaly.

Book ChapterDOI
01 Jan 2006
TL;DR: The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) is scheduled for launch in 2006 and each COSMIC satellite will carry three payloads: (1) a Global Positioning System (GPS) occultation receiver with two high-gain limb viewing antennas and two antennas for precision orbit determination, (2) a Tiny Ionospheric Photometer (TIP) for monitoring the electron density via nadir radiance measurements along the sub-satellite track, and (3) a Tri-Band Beacon (TBB
Abstract: The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) is scheduled for launch in 2006. COSMIC will consist of six low earth orbiting satellites in planes separated by 24° to provide global atmospheric and ionospheric observations. One of the goals is to demonstrate near real-time processing of data products for numerical weather prediction and space weather applications. Each COSMIC satellite will carry three payloads: (1) a Global Positioning System (GPS) occultation receiver with two high-gain limb viewing antennas and two antennas for precision orbit determination, (2) a Tiny Ionospheric Photometer (TIP) for monitoring the electron density via nadir radiance measurements along the sub-satellite track, and (3) a Tri-Band Beacon (TBB) transmitter for ionospheric tomography and scintillation studies. The data from all these payloads will be processed at the COSMIC Data Analysis and Archival Center (CDAAC). Here we give an overview of the ionospheric data products from COSMIC and focus on the plans and preliminary simulation studies for analyzing the ionospheric occultation data and combining them with ground-based GPS, TIP, and TBB observations.

Journal ArticleDOI
TL;DR: In this article, the authors observed exceptional intensive quasi-periodical perturbations of the total electron content (TEC) caused by the great Sumatra-Andaman earthquake on 26 December 2004.
Abstract: By using data from the GPS network, we observed exceptional intensive quasi-periodical perturbations of the total electron content (TEC) caused by the great Sumatra-Andaman earthquake on 26 December 2004. The time period of the variations was about 15 min, their duration was about 1 hour. The amplitude of the TEC oscillations exceeded the amplitude of “background” fluctuations in this range of periods by one order of magnitude, at a minimum. They were registered 2–7 hours after the main shock at a distance from 1000 to 5000 km, both on the northwest and northeast outward from the epicenter. The most probable source of the observed oscillations appeared to be a seismic airwave generated by the sudden vertical displacement of the Earth’s surface near the epicenter.

Journal ArticleDOI
TL;DR: In this article, the results from the USU Gauss-Markov Kalman filter (GMKF) for several cases with varying combinations of slant TEC and EDP data during a 30-day study period (days 080-110, 2004) are presented.

20 Jan 2006
TL;DR: In this paper, a methodology for real-time calibrated TEC estimation in the presence of scintillation and a highly structured ionosphere is described, where the inter-frequency biases of the GPS satellites are assumed known; they use estimates provided by the Center for Orbit Determination in Europe (CODE).
Abstract: The estimation of Total Electron Content (TEC) in the equatorial ionosphere using GPS presents a number of challenges due to the presence of strong spatio-temporal density gradients and scintillation of the satellite signals caused by F-region irregularities. In this paper we describe a methodology for real-time calibrated TEC estimation in the presence of scintillation and a highly structured ionosphere. The inter-frequency biases of the GPS satellites are assumed known; we use estimates provided by the Center for Orbit Determination in Europe (CODE). The inter-frequency bias associated with a particular receiver is estimated late at night when the ionosphere is minimally structured, using an iterative approach that minimizes the variance of verticalized TEC measured along the different satellite links. The nightly estimated receiver bias is shown to be insensitive to the assumed centroid height used in the single-layer approximation of the ionosphere. It is also relatively stable on a night to night basis, deviating from its running average most when nighttime gradients in density are largest (commonly associated with geomagnetic activity and/or equatorial spread F). A 14 day running average of the bias is used to minimize the effect of this variability on the calibrated TEC. The effectiveness of the technique is illustrated by comparing the calibrated TEC estimated using two GPS receivers connected to the same antenna. During quiescent ionospheric conditions the difference in TEC estimated with the two receivers is generally less than a couple of TECU, despite their substantially different internal biases. During scintillating conditions, the TEC from the two receivers exhibit substantial differences due to receiver errors in the measurement of pseudorange and phase, unless strict quality control techniques are applied to exclude this data from the analysis. Methods for the automated detection of receiver error due to scintillation are presented and are shown to yield reliable TEC estimates.


Journal ArticleDOI
TL;DR: In this paper, the authors present data on the occurrence of large-scale ionospheric irregularities observed at Antarctic stations McMurdo-MCM4 (−78N, 167E, Φ = 80°), Casey-CAS1 (−66N, 110E, ǫ = 80.5°), Mawson-MAW1 (−68N, 63E, æ = 71°), and Davis-DAV1 (−69N, 78E, ) in 2001.

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TL;DR: In this paper, the ionospheric total electron content (TEC) was observed to have anomalously increased following a severe daytime positive TEC storm at longitudes of Japan.
Abstract: [1] After sunset on 8 November 2004, the ionospheric total electron content (TEC) was observed to have anomalously increased following a severe daytime positive TEC storm at longitudes of Japan. The observation was made using a dense GPS receiver network, and covered a geographic latitudinal range of 27 to 45°N. There was a greater increase in TEC at higher latitudes in the evening, and the TEC reached 90 TEC units at 45°N (∼40°N magnetic latitude) at 1145 UT (2045 LT). The TEC enhancement exhibited features significantly different from those of positive TEC storms normally observed at Japan's longitudes. These features are interpreted as low-latitude signatures of a storm enhanced density (SED). Previously, SEDs were reported only at longitudes of America, and this led to the hypothesis that geomagnetic field configurations at these longitudes play a role in their formation. The present observations indicate that SEDs can be observed at other longitudes.

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TL;DR: Yizengaw et al. as mentioned in this paper used ground-based GPS total electron content (TEC), EISCAT incoherent scattering radar (ISR), and DMSP F15 ion drift meter observations to detect plasmaspheric plume signatures over Europe.
Abstract: [1] Previously, ionospheric signatures of plasmaspheric plumes were reported only at the North American longitude sector. This led to the hypothesis that the geomagnetic field configuration at those longitudes played a vital role in the observation of plasmaspheric plume signatures only over the American continent. Combining ground-based GPS total electron content (TEC), EISCAT incoherent scattering radar (ISR), and DMSP F15 ion drift meter observations we have observed greatly elevated density over the European continent during storm recovery phase on 12 September 2005. The TEC seen over Europe has a tongue of enhanced ionization extending to higher latitudes, which is identical to the plasmaspheric plume signatures that have been often observed over North America. Therefore, our observations clearly demonstrate that ionospheric signatures of plasmaspheric plumes are not limited to the North American sector and suggest that they may be observed at any longitude as long as a dense array of instruments are available to identify these signatures. Citation: Yizengaw, E., M. B. Moldwin, and D. A. Galvan (2006), Ionospheric signatures of a plasmaspheric plume over Europe, Geophys. Res. Lett., 33, L17103, doi:10.1029/2006GL026597.