Showing papers on "TEC published in 1995"

Subdaily northern hemisphere ionospheric maps using an extensive network of GPS receivers

Abstract: Ionospheric total electron content (TEC) data derived from dual- frequency Global Positioning System (GPS) signals from 30 globally distributed network sites are fit to a simple ionospheric shell model, yielding a map of the ionosphere in the northern hemisphere every 12 hours during the January 1-15, 1993, period.

Observations of traveling ionospheric disturbances with a satellite-beacon radio interferometer : seasonal and local time behavior

Abstract: We have operated a very long baseline interferometer array at a northern midlatitude site, illuminated by VHF radio beacons from two geosynchronous satellites, quasi-continuously for over a year. The array can detect and measure the trace velocity of traveling ionospheric disturbances (TIDs) via their signatures in the line-of-sight total electron content (TEC). The system noise level is of the order of 1013 m−2 in the TEC, so that even very weak perturbations can be studied. We have used the year-long TID detection/velocimetry data set to describe local time and seasonal dependences of the wave parameters. The most striking finding is that the preferred azimuths of TIDs in the data set tend to belong to either of two modes: The first mode, strongest at midday and in the early afternoon, particularly around winter equinox, propagates southward. The second mode, strongest in the evening, especially during summer solstice through autumn equinox, propagates west-northwestward. The two modes are disposed in local time such as to suggest the agency of clockwise rotation of the TID preferred azimuths versus time, as expected by wind filtering in the thermospheric diurnal tide. However, there is a gap between the two modes' azimuth bands. Moreover, the two modes exist in all trace-speed quartiles of the data set TIDs, a finding which is at variance with the hypothesis of wind filtering being the primary explanation of these modes.

Interleukin 3 and erythropoietin induce association of Vav with Tec kinase through Tec homology domain.

Abstract: Although hematopoietic cytokine receptors lack tyrosine kinase domains, the binding of their ligands to the receptors induce rapid tyrosine phosphorylation of various cellular target proteins. The specific tyrosine kinases which phosphorylate these substrates, however, have not been identified, other than that JAK kinases which phosphorylate STAT proteins and the receptors. We found that the c-vav proto-oncogene product, Vav, is rapidly and transiently tyrosine-phosphorylated in response to erythropoietin and IL3 stimulations and that Tec kinase is also transiently activated by these cytokines. Immunoprecipitation experiments demonstrated that Tec kinase binds to Vav upon these cytokine stimulations and that Grb2 constitutively associates with Vav. In vitro binding assays showed that erythropoietin and IL3 stimulation induce the specific binding of Vav to Tec kinase through Tec homology domains. We therefore concluded that Tec kinase is one of the key enzymes in Epo and IL3 receptor-mediated signaling pathways and that Vav plays an important role in the cytokine receptor-mediated signal transduction.

Tomographic imaging of the ionosphere over Japan by the modified truncated SVD method

Abstract: A computerized tomography technique was applied to reconstruct two-dimensional electron density distributions from measurements of total electron content (TEC). We found that the modified truncated singular value decomposition (MTSVD) method, which is usually effective in ill-posed inversion problems, is effective in the ionospheric application. Furthermore, a main control parameter for a good reconstruction was examined by using several density distribution models, and the effects of noise in TEC and discretization error were also investigated. After the simulation study, the MTSVD method was applied to actual TEC data in a magnetically disturbed period. Total electron contents were observed, at four receiving stations covering a latitudinal range of 9.7 degrees with a longitudinal range of 2.2 degrees over Japan, by using signals transmitted from a Navy Navigational Satellite System (NNSS) satellite. The tomographic reconstruction revealed a large-scale structure of electron density in the disturbed ionosphere.

A common stem cell for murine cortical and medullary thymic epithelial cells

Abstract: We have addressed the question whether the epithelial stroma in the thymus is derived from a common stem cell or whether cortical and medullary epithelial cells are derived from different embryonic stem cells emerging, for example, from endoderm and ectoderm. By the use of rapidly expanding cultures of thymic epithelial cells (TEC) from 14 to 16 day-old murine fetuses and by specific antibodies against cortical and medullary epithelium, respectively, we were able to demonstrate a small subpopulation of double-labeled TEC in the cultures. These cells were not present in TEC cultures initiated from thymuses of neonatal mice. Double-labeled TEC were also found in tissue sections from fetal thymuses. These findings may indicate that TEC populations of the cortex and the medulla are derived from a common stem cell, with potential for differentiation toward both cortical and medullary TEC.

Detection of auroral activity using GPS satellites

Abstract: GPS (Global Positioning System) satellites and a receiver located at Fairbanks, Alaska are used to detect auroral activity. A technique for using GPS total electron content (TEC) data to detect auroral-E ionization (AEI) at all satellite line-of-sight elevations is presented. The location of AEI during auroral substorms is determined and is consistent with simultaneous magnetometer data. Maps of detected AEI events reveal the distribution of AEI in space and time. Additionally, a technique is presented for identifying the effects of the auroral oval E-layer on the TEC data. Particle precipitation measured by the TIROS satellite is closely related to variations in the TEC data. The effects of the oval are consistently seen in the TEC data for a variety of magnetic conditions. The location of the equatorward edge of the oval is determined during auroral substorms and compares well with a model of the oval and with individual TIROS passes.

Self-thyroid epithelial cell (TEC)-reactive CD8+ T cell lines/clones derived from autoimmune thyroiditis lesions. They recognize self-thyroid antigens directly on TEC to exhibit T helper cell 1-type lymphokine production and cytotoxicity against TEC.

Abstract: Self-thyroid epithelial cell (TEC)-reactive CD8+ and CD4+ T cell lines were established by culturing T cells that infiltrate in autoimmune thyroiditis lesions. We investigated the properties of CD8+ T cell lines and clones in comparison with previously characterized CD4+ T cell lines/clones. Although the recognition of self-Ag by anti-TEC CD4+ T cell lines/clones required the cooperation of syngeneic spleen cells as APC, a representative CD8+ line (N4C) was stimulated with syngeneic TEC in the absence of APC. Precise analysis of MHC restriction using N4C-derived clones revealed that CD8+ clones recognize self-Ag on TEC in the context of class I MHC molecules. Most CD8+ clones were also found to express TCR with V beta specificities that were different from those observed for anti-TEC CD4+ clones. N4C cells produced IL-2, IFN-gamma, and TNF-alpha beta, but not IL-4 and IL-5 after stimulation with TEC, thus exhibiting the profile of lymphokine production similar to that expressed by CD4+ Th1 on one hand, but on the other, they showed the functional property that has not been observed for anti-TEC CD4+ clones. Namely, they elicited appreciable levels of cytolytic effects on syngeneic TEC in a short-term (4-h) 51Cr release assay. Thus, these results indicate that self-TEC-reactive CD8+ T cell lines/clones recognize Ag directly on TEC in a class I MHC-restricted way so as to exhibit various functions including the Th1-like profile of lymphokine production and anti-TEC cytolysis. The results are also discussed in terms of the nature of self-Ag presented with class I MHC molecules on TEC, as well as the potential roles of anti-TEC CD8+ T cells in the pathogenesis of thyroiditis.

Human thymic epithelial cells produce TGF-β3 and express TGF-β receptors

Abstract: TGF-beta affects proliferation, differentiation and maturation of T cells; however, the effect of TGF-beta on thymic stromal cells has not been characterized. To better understand the role of TGF-beta in T cell development, we determined whether TGF-beta is present in the human thymus, and identified stromal cells that express TGF-beta receptors and respond to TGF-beta. We demonstrate that primary cultured human thymic epithelial cells (TEC) express TGF-beta 1, TGF-beta 2 and TGF-beta 3, as well as TGF-beta type I receptor (T beta RI) (ALK-5) and TGF-beta type II receptor (T beta RII) transcripts. In vitro, epidermal growth factor (EGF) increases transcript levels of TGF-beta 1, TGF-beta 3 and T beta RII, suggesting that EGF may modulate TGF-beta responses in TEC; however, TGF-beta 2 and T beta RI transcript levels were not affected. We also detect TGF-beta 3 and T beta RII protein in association with keratin-positive TEC in vitro and in vivo. TEC culture supernatants contain TGF-beta 3 as detected by Western blots and, upon heat and acid activation, display growth inhibitory activity on the CCL-64 cells that is neutralized by anti-TGF-beta mAb treatment. We further demonstrate that TGF-beta 1 increases leukemia inhibitory factor transcript levels in TEC, indicating that TEC express functional TGF-beta receptors. Thus, we have shown in the human thymus that TEC produce TGF-beta 3 and express T beta RI and T beta RII. The data suggest that TGF-beta is present in the human thymus and may indirectly affect T cell development by regulating TEC cytokine production.

Tomographic reconstruction of the ionosphere over North America with comparisons to ground‐based radar

Abstract: Data collection for the first ground-based ionospheric tomography campaign in North America was conducted over a 48-hour period in mid-November 1991. The data consist of records of ionospheric total electron content (TEC) from a number of passes of the U. S. Navy Navigation Satellite System spacecraft over a chain of ground-based receiving stations. Data collection and reduction techniques are discussed; these include the determination of absolute TEC from the different phase advances induced by the ionosphere in each component of the dual-frequency spacecraft signal. The use of tomographic methods to reconstruct ionospheric electron densities over a two-dimensional (2-D) region of the Earth's ionosphere at a number of different times is demonstrated. Specifically, two distinct tomographic methods, the algebraic reconstruction technique and a maximum entropy method, are used to mathematically invert the records of TEC. The resulting 2-D contour “maps” of ionospheric electron density are then compared to similar maps produced by the Millstone Hill incoherent backscatter radar facility located at Westford, Massachusetts. Both qualitative and quantitative measures of agreement among the different reconstructions and the radar maps are presented. The behavior of the ionosphere over the course of the experiment is discussed.

An investigation of TID evolution observed in MACE '93 data

Abstract: Analysis of the total electron content (TEC) data and reconstructed tomographic images from the Mid-America CIT (computerized ionospheric tomography) Experiment (MACE '93) is presented with the intent of examining and characterizing ionospheric irregularities. The TEC data and tomographic images are examined for structure and irregularities on top of the smooth background data. Structures seen in the TEC data by neighboring receivers over a single Transit pass are then correlated to obtain information about the approximate size and location of the structure. An attempt is made to analyze temporal and spatial evolution of structures from the TEC data using sequential satellite passes. The tomographic images generated from the TEC data are then examined for similar structures, with structure characteristics such as structure size and degree of electron density enhancement and depletion considered.

An Improved Ionospheric Correction Method for Wide-Area Augmentation Systems

Abstract: A powerful approach for generating ionospheric corrections in wide area differential GPS applications has been developed, that can be applied to the Federal Aviation Administration’s Wide Area Augmentation System (WAAS). This approach has been used to support ionospheric calibration for NASA’s Deep Space Network and will be supporting real-time operations for global ionospheric specification and, possibly, single-frequency satellite altimeter calibration. It is a real-time, grid-based technique relying on a computationally efficient Kalman- type filter to produce accurate, smoothly varying ionospheric correction maps over the coverage area. Formal error maps are also computed, providing vertical delay errors over the WAAS grid, which are useful in integrity monitoring. This solution to the ionospheric correction problem is self-calibrating, since GPS transmitter inter-frequency biases are obtained as a by- product of the mapping procedure. This contrasts with other techniques in which bias values must be provided from some additional source. Simulated data were generated for the proposed configuration of 24 WAAS reference GPS stations, using a well-tested climatological ionosphere model (Bent) to compute ionospheric total electron content (TEC) during conditions typical near the peak of the solar cycle. Slant TEC delays often exceed 30 meters over the continental US (CONUS) during solar maximum, but the simulations indicate that the corrections are accurate to 0.25-0.5 meter over the CONUS. Alaska and Hawaii (this includes any errors in estimating the transmitter biases). Our technique is therefore useful for en-route navigation and precision approach, the latter requiring 1.5 meter correction accuracy.

Passive detection of sporadic E using GPS phase measurements

Abstract: A passive sporadic E detection technique based on a Global Positioning System (GPS) receiving system has been developed and tested in a midlatitude environment. This system detects the small-scale total electron content (TEC) variations believed to be produced by electron density structures associated with sporadic E. The current GPS detection technique was able to detect ionosonde-detected sporadic E conditions for 73% of the cases at high-elevation look angles in a set of midlatitude summer observations. Several approaches have been identified that may significantly improve this detection ratio. These approaches include reducing GPS phase multipath, implementing time and space averaging, and investigating the use of high-speed GPS TEC measurements. This technique provides a basic sporadic E detection functionality for applications where an ionosonde is not available. It also provides complementary ionospheric information in regions outside the ionosonde viewing area for applications where an ionosonde is available.

Fiber optic assembly and method of making same

Abstract: There is provided an assembly having a light guiding medium sealed to a her. Preferably the holder is a metal shell and a light guiding medium is an optical fiber of glass or sapphire whisker. The assembly includes a sealing medium which sealingly engages the metal holder to the fiber. In the formation of the assembly, the seal is essentially hermetic having a capability of minimizing leakage having a helium leak rate of less than 1×10 -8 cubic centimeters per second and high strength having a capability of withstanding pressures of 100,000 psi or greater. The features of the assembly are obtained by a specific preparation method and by selection of specific starting materials. The fiber is selected to have a sufficiently high coefficient of thermal expansion which minimizes strains in the component during fabrication, as a result of fabrication, and during use. The other components are selected to be of a material having compatible coefficients of thermal expansion (TEC) where the TEC of the holder is greater than or equal to the TEC of the sealing material. The TEC of the sealing material is in turn greater than or equal to the TEC of the fiber. It is preferred that the materials be selected so that their respective coefficients of thermal expansion are as close as possible to one another and they may all be equal.

PRISM: A Parameterized Real-Time Ionospheric Specification Model, Version 1.5.

Abstract: : This report describes PRISM Version 1.5, a parameterized, real-time ionospheric specification model. PRISM was developed for use at the Air Force Space Forecast Center (50th Weather Squadron) at Falcon Air Force Base. PRISM consists of the theoretical climatology model, PIM (Parameterized Ionospheric Model), and an algorithm for updating PIM electron density profiles using ground based an spaced data. PIM is based on a set of diurnally reproducible runs of theoretical ionospheric models for broad range of geophysical conditions and for three seasons (June, December, and March/September). PRISM is capable of ingesting and utilizing data from digital or analog ionosondes, polarimeter or GPS based TEC measurements, in situ plasma measurements (electron density, electron temperature, ion composition, ion temperature, and plasma drift velocity), and auroral electron and ion precipitation measurements. PRISM 1.5 contains a number of improvements over PRISM 1.0, which was described in PL-TR-91-2299, including improved climatology, improved handling of in situ plasma data, and the ability to ingest and utilize TEC data.

Real-Time TEC Determination for Ionospheric Modeling in WADGPS

Abstract: In this paper, an algorithm for the real-time computati- on of Ionospheric Total Electron Content (TEC) will be presented. The computation is based on a sequential least-squares algorithm applied to dual frequency GPS pseudorange and carrier phase measurements. It will be shown that this combination enables a strong reduction of noise on the TEC determination as compared to TEC values computed from simple frequency differenced pseudoranges. Thanks to its real-time processing capability, this al- gorithm is mainly expected to be implemented in Wide Area Differential GPS Reference Stations or in Io- nosphere Monitoring Stations.

Real Time Estimation of Ionospheric Delays

Abstract: The estimation of the ionospheric delay in real time with GPS can be very useful, especially for high precision navigation applications. To estimate TEC in real time it is necessary to separate the ionospheric delay Tom the total error budget. Therefore, the reachable accuracy of the derived real time absolute values of slant TEC is determined by several parameters: number of epochs before the first ambiguity fixing, elevation cut-off, predicted or estimated satellite and receiver instrumental delays, and the multipath and its handling. Our method for real time TEC e&nation will be described and demonstrated with real observations, using two independently working GPS receivers. The influence of selected processing parameters on the reachable accuracy of the real time TEC will be shown. The evaluation of the quality will be obtained by comparison of the real time with the postprocessed TEC, using our reference receiver, and with the postprocessed TEC obtained using our station plus a subset of IGS stations. These comparisons will permit the determination of an optimized setup of real time estimation algorithms.

Longitudinal effects of ionospheric responses to substorms at middle and lower latitudes: a case study

Abstract: An ionospheric model is used to simulate total electron content (TEC) disturbance events observed at middle and lower latitude sites near 75○W and 7○E longitudes. Within this longitudinal range, daytime TEC disturbances show patterns that are correlated with substrom activity seen in both auroral electrojet and ring current behavior. In modeling studies of the observed ionospheric effects, both electric field and neutral wind perturbations are examined as possible mechanisms. The morphological features of the required electric field perturbations near drawn and dusk are compared with those at other times to examine the local time characteristics of magnetospheric influence. Large-scale traveling atmospheric disturbances (TADs), an alternative candidate for the disturbance source, are also characterized and compared with known thermospheric behavior.

An investigation of the feasibility of utilizing GPS/TEC "signatures" for near-real time forecasting of auroral-E propagation at high-HF and low-VHF frequencies

Abstract: VHF propagation on /spl les/5300 km polar paths has been documented during the maximum phase of sunspot cycle 19. Mode analysis on these polar paths has shown that auroral-E ionization (AEI) supported some modes. Electron densities and plasma frequencies which could support AEI modes at frequencies up to 46 MHz have also been measured. Long distance VHF propagation from AEI has also been reported by radio amateurs using frequencies in the 2 m band in a "sidescatter mode". An AEI experiment has been in operation between Wales (Alaska) and Fairbanks (Alaska) where a 75-watt CW transmitter located in Wales transmits the Morse letter "R" every 5 s, and a receiver in Fairbanks detects the 25.5 MHz signal whenever AEI is present near the midpoint of the 960 km path. Another experiment is underway using a GPS total electron content (TEC) receiving station at Fairbanks also using AEI data from the Wales-Fairbanks experiment. From this, the authors examine 58 passes of GPS satellites whose E-layer penetration points lie close to the midpoint of the Wales-Fairbanks path and find that there is a threshold value of TEC above which auroral-E (AE) propagation occurs. They also find that AEI propagation is strongly correlated with large- and medium-scale E-region structures in TEC determined by the GPS measurements. When TEC ionospheric structures are not present near the Wales-Fairbanks path midpoint, no AEI signal is received. The authors tentatively conclude that the occurrence of these specific TEC signatures may be utilized as predictors of AEI forward propagation on paths within and parallel to the auroral oval. >

Enhanced variability in the topside ionosphere

Abstract: Variability of total electron content (TEC) observed by the Faraday rotation method at Florence has been stud- ied with the same technique applied independently to the ionospheric parameters foF2 and M(3000)F2 of the ground-based vertical-incidence sounding database (VID). Results of daily and monthly TEC disturbance indices at sub-ionospheric point are compared with variability of the ionosphere at Rome and Gibilmanna (de- duced from VID) for a period of 1976 to 1991. During moderate and high solar activity the variability of TEC is greater than the variability of VID, whereas during solar minimum the situation is opposite. In this context joint TEC and VID observations distinguish either the F region peak or the topside ionosphere heights where the dynamic processes dominate at different times.

Auulication of SCORE Techniques to Improve Ionospheric Observations

Abstract: Self-Calibration Of pseudoRange Error (SCORE) techniques [l] can improve accuracy of ionospheric total electron content (TEC) covering 10 degrees of latitude from one station. SCORE uses self-consistency constraints on the TEC data to calibrate a two-frequency GPS receiver system for the sum of system contributions to pseudorange error. These errors come from components such as receiver, pre-amp, antenna and satellite transmitters. Frequent application of SCORE can thus reduce system component drift contributions to TEC measurement error and improve long-term measurement accuracy. SCORE was first developed out of data presentation techniques that permitted a single GPS station to simulate zenith TEC monitoring from stations spaced over 10 degrees of latitude, along the same longitude [2]. Applying these presentation techniques in conjunction with SCORE allows accurate parallel monitoring of the diurnal TEC behavior over this wide latitude region from a single station. Typically, in well-calibrated data, latitudinal TEC gradients of less than 1 TEC unit per degree may be identified. We will present results of SCORE application studies showing variation in TEC accuracy with the time interval and the number of satellites used in the process, and the effects of improved calibration on the observable TEC morphology in widely-spaced latitude regions. We will also examine how these results differ in high-latitude and equatorial regions, where the diurnal behavior of the ionosphere can be radically different from over CONUS. We will also present results from an initial application of the SCORE process to single-frequency measurements, compared with simultaneous two-frequency TEC data. These initial tests show potential for useful single- frequency ionospheric TEC observation.