Showing papers on "Total electron content published in 1998"

A global mapping technique for GPS‐derived ionospheric total electron content measurements

Abstract: A worldwide network of receivers tracking the transmissions of Global Positioning System (GPS) satellites represents a new source of ionospheric data that is globally distributed and continuously available. We describe a technique for retrieving the global distribution of vertical total electron content (TEC) from GPS-based measurements. The approach is based on interpolating TEC within triangular tiles that tessellate the ionosphere modeled as a thin spherical shell. The high spatial resolution of pixel-based methods, where widely separated regions can be retrieved independently of each other, is combined with the efficient retrieval of gradients characteristic of polynomial fitting. TEC predictions from climatological models are incorporated as simulated data to bridge significant gaps between measurements. Time sequences of global TEC maps are formed by incrementally updating the most recent retrieval with the newest data as it becomes available. This Kalman filtering approach smooths the maps in time, and provides time-resolved covariance information, useful for mapping the formal error of each global TEC retrieval. Preliminary comparisons with independent vertical TEC data, available from the TOPEX dual-frequency altimeter, suggest that the maps can accurately reproduce spatial and temporal ionospheric variations over latitudes ranging from equatorial to about ±65°.

High resolution mapping of TEC perturbations with the GSI GPS Network over Japan

Abstract: Two-dimensional total electron content (TEC) perturbations over Japan are mapped with the Geographical Survey Institute (GSI) GPS network, GEONET (GPS Earth Observation Network). Its spatial resolution is 0.15° latitude × 0.15° longitude and temporal resolution is 30 seconds. Two-dimensional TEC observations with these high resolutions revealed spatial structures and temporal evolutions of traveling ionospheric disturbances (TIDs) in the nighttime mid-latitude ionosphere on July 03, 1997. A preliminary result of the TEC perturbation mapping indicates that it would be a strong tool to investigate the ionospheric structures in detail. Coordinated observations with other ionospheric observation techniques, such as incoherent scatter (IS) radars, airglow imagers, and ionosondes, are needed to clarify the vertical structure of the ionosphere.

Tomography of the ionosphere: Four‐dimensional simulations

Abstract: Using a four-dimensional stochastic model of ionosphere perturbations, simulations are made of a tomography system based on data from the Global Positioning System and a low Earth-orbiting satellite. The perturbations are departures from a simple time-independent reference state. The spatial structure is parameterized in terms of empirical orthogonal functions (EOFs) in the vertical and spherical harmonics in the horizontal. The horizontal covariance structure is specified by variance and correlation length scales as functions of latitude and longitude. Time dependence is modeled as a first-order Markov process with a 6-hour timescale and white-noise forcing. A Sun-fixed coordinate system is used so that ionospheric features are more nearly steady in time. A Kaiman filter is used to objectively assimilate the simulated data into the simple time-dependent model. In addition to solving for the three-dimensional electron density field at each time step, the procedure solves for instrumental biases. The simulations show that the fractions of resolved variance for vertical EOF modes 1, 2, and 3 are 0.99, 0.93, and 0.73, respectively. The resolution of the vertically integrated total electron content is 0.99.

Ionospheric total electron content perturbations monitored by the GPS global network during two northern hemisphere winter storms

Abstract: The global evolution of two major ionospheric storms, occurring on November 4, 1993, and November 26, 1994, respectively, is studied using measurements of total electron content (TEC) obtained from a worldwide network of ground-based GPS receivers. The time-dependent features of ionospheric storms are identified using TEC difference maps based on the percent change of TEC during storm time relative to quiet time. The onset of each ionospheric storm is indicated by the appearance of auroral/subauroral TEC enhancements which occur within 1 hour of the beginning of the geomagnetic storm main phase. Significant TEC enhancements (> 100%) are observed in the winter northern hemisphere. The rate at which TEC enhancements appear is found to correlate with gradients in the Dst index. The large scale ionospheric structures identified during the storms are (1) nightside auroral/subauroral enhancements which surround the auroral oval, (2) dayside (around noon) high-latitude and middle-latitude enhancements associated with traveling ionospheric disturbances, and (3) conjugate latitudinal enhancements. For the November 1993 storm, a short positive phase (about 15 hours) is followed by a long negative phase (∼60 hours). In the November 1994 storm, we have identified the clear signature of a traveling ionospheric disturbance (TID) which propagated at a speed of ∼460 m/s from ∼60° N to ∼40° N. The motion of this disturbance appears to conserve angular momentum.

Ionospheric effects of the solar eclipse of March 9, 1997, as deduced from GPS data

Abstract: This paper presents data from first measurements of total electron content (TEC) and its gradient during the solar eclipse of March 9, 1997, obtained with the GPS-radio interferometer at Irkutsk. The interferometer consists of three receivers (one Turbo-Rogue SNR-8000, and two Ashtech Z-12) located at the vertices of a triangle and spaced by about 3–5 km. The measured TEC variations are indicative of profound changes in the ion production process in the ionosphere attendant on the solar eclipse, simultaneously in a large volume of space with a radius of at height 300 km at 300 km altitude. The delay of a minimum value of TEC with respect to the maximum phase of eclipse at 300 km altitude was about 10 min, and the depression depth of TEC growth varies from l÷3×1016m−2. By analyzing the data on TEC gradient variations, one is led to conclude that the depression of TEC growth during the eclipse is essentially independent of the longitude and of the latitude (within the observation ranges 52±6° N and 104±11° E).

Ionospheric forecasting technique by artificial neural network

Abstract: An artificial neural network method is applied to the development of an ionospheric forecasting technique for one hour ahead. Comparisons between the observed and predicted values of the critical frequency of the F2 layer, foF2, and the total electron content (TEC) are presented to show the appropriateness of the proposed technique.

Variations of total electron content during geomagnetic disturbances : A model/observation comparison

Abstract: This paper studies the ionospheric response to the major geomagnetic storm of October 18–19, 1995, using the NCAR TIE-GCM simulations and the global ionospheric maps (GIM) of total electron content (TEC) observations from the worldwide network of Global Positioning System (GPS) receivers. The TIE-GCM results show a good agreement with the GPS-GIM in terms of simulating storm-time TEC disturbances over the polar regions. The model indicates that the increase of electron density in the high-latitude E and lower F regions below 200 km is directly related to the magnetospheric energy input through auroral precipitation to the ionosphere, while the decrease of TEC is mainly due to the increase in O2 and N2 densities in the upper F region above 200 km. During the recovery phase, both the TIE-GCM and GPS-GIM reveal a distinct hemispheric asymmetry in the TEC integrated above |50°| magnetic latitude, with a 20% decrease in the southern (summer-like) hemisphere and a 30% increase in the northern (winter-like) hemisphere.

Global ionospheric TEC variations during January 10, 1997 storm

Abstract: The ionospheric storm evolution process was monitored during the January 10, 1997 magnetic cloud event, through measurements of the inonospheric total electron content (TEC) from 150 GPS stations. The first significant response of the inonospheric TEC to the geomagnetic storm was at 0300 UT as an auroral/subauroral enhancement around the Alaskan evening sector.

Measurements of transionospheric radio propagation parameters using the FORTE satellite

Abstract: We report initial measurements of ionospheric propagation parameters, particularly the total electron content (TEC), using the recently launched FORTE satellite. FORTE, which orbits the Earth at an altitude of 800 km and an inclination of 70{degree}, contains a set of wideband radio receivers whose output is digitally recorded. A specialized triggering circuit identifies transient, broadband radio events, which include radiation from lightning, transionospheric pulse pairs, and man-made sources. Event data are transmitted to the ground station for analysis. In this paper we examine signals transmitted from an electromagnetic pulse generator operated at Los Alamos. The transmitter produces nearly impulsive signals in the VHF range. The received signal is dispersed by the ionosphere, and the received signal can be analyzed to deduce the total electron content along the path. By comparing the slant TEC thus measured with results from a ray-tracing code, we can deduce the vertical TEC to 800 km. Data from eight passes are presented. These types of data (in larger quantities) are of interest to operators of radar altimeters, who need data to corroborate their corrections for the ionospheric TEC. The combination of FORTE TEC data to 800 km and TEC measurements to 20,000 km (the Global Positioningmore » System orbital altitude) can provide useful information for assessing the validity of models of plasmaspheric electron density. Initial estimates of the plasmaspheric density, on two daytime passes, are about 6 TECU. The signal received by FORTE, which is linearly polarized at the transmitter, is split into two magnetoionic modes by the ionosphere. The receiving antenna is also linearly polarized and therefore receives both modes. By measuring the beat frequency between the two modes, we can deduce the product of the geomagnetic field and the cosine of the angle between the field and the propagation vector. The possibility of using the measured slant TEC and the beat frequency to geolocate impulsive signals is discussed. {copyright} 1998 American Geophysical Union« less

Artificial neural network applications in ionospheric studies

Abstract: The ionosphere of Earth exhibits considerable spatial changes and has large temporal variability of various timescales related to the mechanisms of creation, decay and transport of space ionospheric plasma. Many techniques for modelling electron density profiles through entire ionosphere have been developed in order to solve the "age-old problem" of ionospheric physics which has not yet been fully solved. A new way to address this problem is by applying artificial intelligence methodologies to current large amounts of solar-terrestrial and ionospheric data. It is the aim of this paper to show by the most recent examples that modern development of numerical models for ionospheric monthly median long-term prediction and daily hourly short-term forecasting may proceed successfully applying the artificial neural networks. The performance of these techniques is illustrated with different artificial neural networks developed to model and predict the temporal and spatial variations of ionospheric critical frequency, f0F2 and Total Electron Content (TEC). Comparisons between results obtained by the proposed approaches and measured f0F2 and TEC data provide prospects for future applications of the artificial neural networks in ionospheric studies.

A Look Ahead: Expected Ionospheric Effects on GPS in 2000

Abstract: he next maximum in the approximate 11-year cycle of solar ultraviolet (UV) activity is expected to occur near 2000. Two major ionospheric effects on GPS signals are closely related to long-term solar UV activity, and also will maximize at that time. These are ionospheric range delays and amplitude fading and phase scintillation effects. Dual-frequency GPS receivers automatically correct for the ionospheric range delay by measuring the difference in this dispersive effect on both frequencies. Civilian users of singlefrequency LI GPS receivers must either rely on the ionospheric correction algorithm sent as part of the user message, designed to correct for only 50% rms of the range delay, or they must use a nearby, in time and space, actual measurement of the ionospheric range delay to provide a correction for the ionospheric range error. Ionospheric range delays are directly proportional to the total electron content (TEC), encountered along the path from each GPS satellite to the user. The TEC increases with increasing solar cycle activity. As the absolute values of range delay increase with the solar cycle, the need for improved corrections also will increase. Irregularities in the ionosphere that produce amplitude fading and phase scintillation effects on GPS frequencies can become significant as the solar cycle

Total electron content changes in the ionosphere during the January 10, 1997 disturbance

Abstract: We have used the CORS network of GPS receivers to record ionospheric TEC changes in the continental United States caused by the well observed geomagnetic disturbance of January 10, 1997. This event produced numerous worldwide consequences. Although the initial arrival of this disturbance was before sunrise in the U.S. the TEC levels were most significantly altered later during the daylight hours. We display temporal and spatial changes through time series at selected sites and maps of the entire network. The usual sequence of diurnal changes was not greatly altered, but most of the TEC values were increased by about a factor of one and a half to two. Stations in the northeast and upper north central were exceptions and showed a small decrease in TEC. We interpret the TEC changes as consequences of changes in neutral chemical composition resulting from storm heating.

Ionospheric calibration of radar altimeters using GPS tomography

Abstract: We compare TEC measurements from the NASA Radar Altimeter and DORIS instrument on board TOPEX/POSEIDON with GPS TEC estimates, and evaluate different GPS data analysis strategies. We verify that global tomographic GPS analysis using a voxel grid is well suited for ionospheric calibration of altimeters. We show that a 1-day fit of 20-second-averaged NRA ionospheric correction data versus GPS tomographic TEC data has a bias of 3.4 TECU and a root mean square deviation of 3.2 TECU. Tomographic inversion using simulated data from the Parametrized Ionospheric Model highlights the strong correlation between GPS bias constants, electronic densities at the highest layer, and unmodeled protonospheric TEC. This suggests that GPS TEC estimates at the TOPEX/POSEIDON altitude are more accurate if the bias constants are estimated and if a layer above TOPEX/POSEIDON is added to the grid.

Total electron content measurements in the southern hemisphere using GPS satellites, 1991 to 1995

Abstract: The seasonal, diurnal, and latitudinal variations of total electron content (TEC) were determined using Global Positioning System (GPS) satellite signals over approximately 5 days per month during almost half a sunspot cycle (July 1991 to June 1995) at Salisbury (latitude 34.77°S, longitude 138.63°E), South Australia. These are the only such extensive southern hemisphere data till 1995 that have been recorded and analyzed. A selection of the data is presented, discussed, and compared with other workers' observations. Examples of Australian Surveying and Land Information Group (AUSLlG) data in the Australian region are presented. Some model predictions for northern hemisphere ionospheric TEC are compared with the GPS southern hemisphere observations of the combined TEC of the ionosphere and protonosphere. (The two models employed are the international reference ionosphere (IRI-90) and the paramaterized ionospheric model (PIM)(version 1.4, February 1996)). They are considered to be global models, even though the IRJ model is based primarily, but not exclusively, on northern hemisphere TEC data, and PIM is based on a theoretical model and is thus not directly based on TEC data.

Regional ionospheric mapping and modelling over Antarctica

Abstract: The performance of regional and global models has been investigated when applied for long term mapping of ionospheric characteristics and modelling the Total Electron Content in the polar cap over an Antarctic region. Comparison results between modelled data and a short period of experimental values available for low solar activity in December 1993 and January 1994 are presented and discussed.

Polar plumes and fine-scale coronal structures: On the interpretation of coronal radio sounding data

Abstract: Variations in total electron content at high heliographic latitudes, observed during the 1995 solar conjunction of the Ulysses spacecraft when the radio ray path was embedded in the southern coronal hole, were interpreted by Woo and Habbal [1997b] as polar plume structures extending to at least 30 solar radii from the Sun. Somewhat surprisingly, we detected a 24-hour sinusoidal oscillation of the total electron content with a peak-to-valley amplitude of about 10%. Model calculations were performed under the assumption that these electron content variations are indeed the signature of plumes moving in and out of the radio ray path. If the density contrast between plume and interplume regions is low, then the plume structure must occupy a significant fraction of the radio ray path. If the plume extent along the line-of-sight is similar to their observed width in the plane of the sky, then the density contrast is very high (factor 8 or more). Neither scenario seems reasonable under the given geometry. A more likely explanation for this 24-hour variation would be an unaccounted bias at one of the ground stations which imposes an apparent diurnal periodicity onto the measurements. In short, there is no evidence for high latitude coronal plumes in radio sounding data near 30 solar radii.

Auroral zone ionospheric considerations for WADGPS

Abstract: Dual-frequency GPS receivers enable the estimation of absolute ionospheric delay and total electron content (TEC) along the signal path. By using a number of reference stations, each equipped with a dual-frequency receiver, it is possible to estimate values of the vertical ionospheric delay at a set of designated grid points. This type of ionosphere delay modeling is employed in Wide Area Differential GPS (WADGPS) networks, where grid accuracies generally depend on the temporal/spatial correlations of TEC. These models can suffer degraded performance in regions, such as the high-latitude auroral zone, where spatial gradients and temporal variations of electron density may differ significantly from assumptions. In this paper, TEC variations in the auroral zone are investigated during periods of enhanced ionospheric activity. Accuracies of corresponding wide area ionospheric grid corrections are also investigated for various grid spacings.

Imaging of structures in the high-latitude ionosphere: model comparisons

Abstract: The tomographic reconstruction technique generates a two-dimensional latitude versus height electron density distribution from sets of slant total electron content measurements (TEC) along ray paths between beacon satellites and ground-based radio receivers. In this note, the technique is applied to TEC values obtained from data simulated by the SheAeld/ UCL/SEL Coupled Thermosphere/Ionosphere/Model (CTIM). A comparison of the resulting reconstructed image with the 'input' modelled data allows for verifi- cation of the reconstruction technique. All the features of the high-latitude ionosphere in the model data are reproduced well in the tomographic image. Reconstruct- ed vertical TEC values follow closely the modelled values, with the F-layer maximum density (NmF2) agreeing generally within about 10%. The method has also been able successfully to reproduce underlying auroral-E ionisation over a restricted latitudinal range in part of the image. The height of the F2 peak is generally in agreement to within about the vertical image resolu- tion (25 km).

Three‐dimensional computerized ionospheric tomography using volumetric constraints

Abstract: A new algorithm for the reconstruction of three-dimensional images of ionospheric electron density using volumetric a priori information is presented in this paper. This method relies upon the total electron content (TEC) data measured by an ionospheric tomography system to derive necessary distribution-related information from a priori ionospheric images for projection domain and image domain volumetric correction processes. It involves extraction of shape information from a priori ionospheric images on an extremely localized basis to simultaneously satisfy measured TEC data and the need for coherent, convergent reconstruction guidance, necessary for the limited angle tomography system. The algorithm uses three-dimensional polar geometry for computerized ionospheric tomography (CIT) which permits direct use of slant TEC data without the interpolations that are necessary for conventional two-dimensional imaging of the ionosphere. Reconstructions based on data recorded in two recent CIT campaigns are also presented.

Detailed Analysis of Auroral Zone WADGPS Ionospheric Grid Accuracies during Magnetospheric Substorm Event

Abstract: By using a number of reference stations, each equipped with a dual frequency receiver, it is possible to estimate values of vertical ionospheric delay (or, equivalently, total electron content) at a set of designated grid points This type of ionosphere delay modelling is employed in Wide Area Differential GPS (WADGPS) Networks, where vertical TEC values at grid points are determined in an adjustment incorporating all receiver-satellite line-of- sight observations These models can suffer degraded performance in regions, such as the auroral zone, where large variations in total electron content (TEC) occur during geomagnetic disturbances The auroral zone is characterized by magnetospheric substorms and similar transient phenomena, in which localised enhancements and depletions of electron density occur Such events are often associated with small-scale variations in TEC, which are difficult to model The auroral oval is located at high latitudes, and can extend several degrees southward under significantly disturbed levels of activity TEC variations in the auroral zone are therefore significant concerns for WADGPS systems operating in Canada, Northern Europe, and Alaska In this paper, wide area ionospheric grid accuracies are investigated in the auroral zone during a well-defined magnetospheric substorm event This storm took place in April 1997, in the North American local time sector, and is representative of storm-time conditions anticipated during the next solar maximum (=year 2000) Detailed analysis of this substorm is conducted, in order to identifj both the magnitude and extent of geomagnetic disturbances, using a variety of space-borne instruments In this way, a clear picture of the substorm development will be presented, and grid accuracies assessed during different phases of substorm development GPS observations from the Natural Resources Canada (NRCan) wide area network are used in the grid adjustment

Observation of ionospheric disturbances with gps beacons

Abstract: The GPS method is rarely used in the investigation of ionospheric disturbances because of the movement of the observation point which leads to the mixture of both spatial and temporal changes of the observed total electron content(TEC). The present work proposes a new experiment method to modify the effects of the montion of observation points by the observation of a short baseline array of GPS receivers.The experimental results show that the wave parameters of the ionospheric disturbances derived from the observation of a short baseline GPS array are of high precision and reliable. The main advantage of the new experiment method is that the short baseline GPS array may used to observe ionospheric disturbances in a large range.

Investigations of the Nature and Behavior of Plasma Density Disturbances That May Impact GPS and Other Transionospheric Systems

Abstract: : As solar maximum approaches, the 'space-weather' vulnerability of systems that depend upon transionospheric radio propagation will increase. Likely effects include variable range errors in the Global Positioning System (GPS) due to variations in ionospheric 'total electron content' (TEC), spatial gradients in such errors, and the scintillation' of signals employed in a variety of navigation, communication, and other systems. This report summarizes research performed in the first year of a contract aimed at: (a) investigating the behavior of naturally occurring variations in TEC and the plasma-density irregularities that produce scintillation as solar activity increases, and (b) observing such phenomena and others that may be produced artificially by means of high-frequency (HF) heating of the ionosphere in the HF Active Auroral Research Program (HAARP). The first-year efforts included collection and processing of TEC data from USAF's Ionospheric Measuring System (IMS); campaign operation of a portable ionospheric monitor for measurement of TEC and scintillation at Ascension Island; preliminary assessment of plasmaspheric contribution to TEC; and coordinating development of a variety of diagnostic instruments for HAARP.