Showing papers on "TEC published in 2007"

Medium‐scale traveling ionospheric disturbances detected with dense and wide TEC maps over North America

Abstract: [1] Nighttime and daytime medium-scale traveling ionospheric disturbances (MSTIDs) are detected with dense and wide detrended total electron content (TEC) maps over North America using multiple GPS receiver networks. The TEC maps cover a wide region of 60–130°W and 24–54°N (30–65°N in geomagnetic latitude), and have a spatial resolution of 1.05° × 1.05° in latitude and longitude (0.15° × 0.15° with 7 × 7 pixel smoothing) and a temporal resolution of 30 seconds. The TEC maps reveal, for the first time, that the nighttime MSTIDs propagate southwestward with 200–500 km wavelengths over North America and have wavefronts longer than ∼2,000 km. We also observe that daytime MSTIDs with 300–1,000 km wavelengths propagate southeastward until mid-afternoon and southwestward in the late afternoon. In the mid-to-late afternoon, these MSTIDs propagating in the different directions are superimposed. The TEC maps can be a new powerful tool to investigate the MSTIDs.

Longitudinal structure of the equatorial ionosphere: Time evolution of the four-peaked EIA structure

Abstract: [1] Longitudinal structure of the equatorial ionosphere during the 24 h local time period is observed by the FORMOSAT-3/COSMIC (F3/C) satellite constellation. By binning the F3/C radio occultation observations during September and October 2006, global ionospheric total electron content (TEC) maps at a constant local time map (local time TEC map, referred as LT map) can be obtained to monitor the development and subsidence of the four-peaked longitudinal structure of the equatorial ionosphere. From LT maps, the four-peaked structure starts to develop at 0800–1000 LT and becomes most prominent at 1200–1600 LT. The longitudinal structure starts to subside after 2200–2400 LT and becomes indiscernible after 0400–0600 LT. In addition to TEC, ionospheric peak altitude also shows a four-peaked longitudinal structure with variation very similar to TEC during daytime. The four-peaked structure of the ionospheric peak altitude is indiscernible at night. With global local time maps of ionospheric TEC and peak altitude, we compare temporal variations of the longitudinal structure with variations of E × B drift from the empirical model. Our results indicate that the observations are consistent with the hypothesis that the four-peaked longitudinal structure is caused by the equatorial plasma fountain modulated by the E3 nonmigrating tide. Additionally, the four maximum regions show a tendency of moving eastward with propagation velocity of several 10 s m/s.

Large-scale traveling ionospheric disturbances observed by GPS total electron content during the magnetic storm of 29-30 October 2003

Abstract: [1] We have investigated the propagation of large-scale traveling ionospheric disturbances (LSTIDs) during the super magnetic storm of 29 - 30 October 2003. Two-dimensional total electron content (TEC) perturbation maps over North America were built using TEC data provided by the American GPS network and the International GNSS Service. Three LSTID events were observed in the range of 30 degrees N - 50 degrees N, 60 degrees W - 110 degrees W during this period. The first two LSTIDs occurred consecutively during 0620 - 0800 UT on 29 October at the local time of midnight, right after the onset of the big substorm; the third one was found at noon during the expansion phase of another substorm on 30 October. The phase fronts of these LSTIDs passed over the United States and traveled southwestward to the distance of similar to 2000 km with the maximum front width of similar to 4000 km and the duration of less than 2 hours. The maximum amplitude of TEC perturbations attained 3 total electron content units (TECUs). The results differ from the former observation of Afraimovich and Voeykov ( 2004) and Afraimovich et al. ( 2006), who reported a solitary LSTID propagating southwestward over the United States with the amplitudes of up to 14 TECU on 30 October 2003. We have checked the magnetic H component observed at the geomagnetic observatories in North America and found it is most likely that the auroral westward electrojet was the cause of the LSTIDs on 29 October. The source region for these TIDs was likely to be located several hundred kilometers north of 50 degrees N. Cross-spectral analysis was conducted to obtain the global propagation characteristics of LSTIDs during this superstorm. Equatorward LSTIDs were found in all the three sectors of North America, Europe, and Asia, showing high correlation with the occurrence of auroral substorms.

Longitudinal Variability of Low-Latitude Total Electron Content: Tidal Influences

Abstract: [1] Recently, nighttime ultraviolet (UV) observations obtained by IMAGE FUV and TIMED GUVI instruments have revealed a longitudinal wave number four pattern in the nighttime airglow intensity and in the position of the equatorial anomalies during equinox and high solar flux conditions. In the present study, we have extended this work and determined the longitudinal variability of the low-latitude total electron content (TEC) climatology during different geophysical conditions with a special emphasis on the longitudinal wave number four structure in the low-latitude ionosphere. We have used more than 5 million low-latitude TOPEX TEC observations covering the entire 13 years of TOPEX TEC data from August 1992 until October 2005. This data set was used to determine the local time, seasonal, solar cycle, and geomagnetic activity dependence of the longitudinal variability of TEC at equatorial and low latitudes, and in particular, to address the existence and evolution of the wave number four longitudinal pattern under these conditions. Our study shows that the wave number four pattern is created during the daytime hours at equinox and June solstice but is absent, or washed out by other processes, during December solstice. During equinox the wave number four pattern is created around noon with well-defined longitudinal enhancements in the low-latitude TEC. These enhancements, which are symmetric about the geomagnetic equator during this season, last for many hours and can be clearly seen past midnight. The longitudinal patterns are found to be nearly identical between the vernal (March/April) and autumnal (September/October) equinoxes and largely independent of the solar cycle conditions. The wave number four pattern is also observed during geomagnetically active conditions, indicating that the processes that create this pattern are also present during active times. The variations between the well-defined longitudinal maxima and minima are of the order of 20%. During June solstice, the wave number four pattern is also observed in the afternoon hours but, in contrast to the equinox cases, it exhibits a strong hemispheric asymmetry and is not observed during the night. The low-latitude TEC exhibits clear longitudinal variations during December solstice, with large daytime enhancements over the east Asian and Pacific regions and a third enhancement emerging in the afternoon over the Atlantic Ocean, but a clear wave number four pattern is not observed during this season. Although the equatorial and low-latitude TEC values exhibit clear longitudinal patterns during all seasons, a significant amount of scatter remains in the TEC data that is not accounted for by changes in the solar cycle, the season, or the local time or by the longitudinal variability. This remaining scatter is largest near the poleward edges of the anomalies and is of the order of 40%.

Estimation of the Total Electron Content of the Martian Ionosphere using Radar Sounder Surface Echoes

Abstract: The Martian ionosphere's local total electron content (TEC) and the neutral atmosphere scale height can be derived from radar echoes reflected from the surface of the planet We report the global distribution of the TEC by analyzing more than 750,000 echoes of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) This is the first direct measurement of the TEC of the Martian ionosphere The technique used in this paper is a novel 'transmission-mode' sounding of the ionosphere of Mars in contrast to the Active Ionospheric Sounding experiment (AIS) on MARSIS, which generally operates in the reflection mode This technique yields a global map of the TEC for the Martian ionosphere The radar transmits a wideband chirp signal that travels through the ionosphere before and after being reflected from the surface The received waves are attenuated, delayed and dispersed, depending on the electron density in the column directly below the spacecraft In the process of correcting the radar signal, we are able to estimate the TEC and its global distribution with an unprecedented resolution of about 01 deg in latitude (5 km footprint) The mapping of the relative geographical variations in the estimated nightside TEC data reveals an intricate web of high electron density regions that correspond to regions where crustal magnetic field lines are connected to the solar wind Our data demonstrates that these regions are generally but not exclusively associated with areas that have magnetic field lines perpendicular to the surface of Mars As a result, the global TEC map provides a high-resolution view of where the Martian crustal magnetic field is connected to the solar wind We also provide an estimate of the neutral atmospheric scale height near the ionospheric peak and observe temporal fluctuations in peak electron density related to solar activity

Large variation in whole-plant water-use efficiency among tropical tree species

Abstract: It is well known that whole-plant water-use efficiency (transpiration efficiency of carbon gain, TEC) varies among plant species with different photosynthetic pathways. However, less is known of such variation among tree species within the C3 group. Here we measured the TEC of seven C3 tropical tree species. Isotopic analyses (δ13C, δ18O, and δ15N) and elemental analyses (carbon and nitrogen) were undertaken to provide insight into sources of variation in TEC. Plants were grown over several months in approx. 80% full sunlight in individual 38-l containers in the Republic of Panama. Soil moisture content was nonlimiting. Significant variation was observed in TEC among the C3 tree species. Values ranged from 1.6 mmol C mol−1 H2O for teak (Tectona grandis) to 4.0 mmol C mol−1 H2O for a legume, Platymiscium pinnatum. Variation in TEC was correlated with both leaf N concentration, a proxy for photosynthetic capacity, and oxygen-isotope enrichment, a proxy for stomatal conductance. The TEC varied with C-isotope discrimination within species, but the relationship broke down among species, reflecting the existence of species-specific offsets.

Total Electron Content Estimation with Reg‐Est

Abstract: [1] Total Electron Content (TEC) constitutes one of the key elements for observing the variable structure of the ionosphere. GPS provides a cost-effective alternative in TEC estimation through earth-based receivers. In this paper, one of the TEC estimation methods, namely Reg-Est, is investigated in detail in terms of its parameters and developed further to include improvements. Reg-Est estimates robust TEC using GPS measurements of 30 s time resolution. The method combines the vertical TEC computed from all the satellites in view over 10° horizon limit in the least squares sense through the minimization of a cost function which also includes a high pass penalty filter. Optional weighting functions and sliding window median filters are added to enrich the processing and smoothing of the data. In this study, the input to the Reg-Est is enlarged to include phase-corrected TEC. The best way of including the instrumental biases is investigated and the algorithm is updated to include the biases in the slant TEC computation. The effect of the thin shell height of the ionosphere in Reg-Est estimates is studied. It is concluded that the Reg-Est algorithm is very robust to the choice of thin shell height. The best weighting function to reduce the multipath effects and to minimize the non-ionospheric noise is selected. The improved Reg-Est algorithm can be used for all latitudes and for both quite and disturbed days of the ionosphere. The Reg-Est TEC are in excellent accordance with the estimates from IGS analysis centers.

Features of annual and semiannual variations derived from the global ionospheric maps of total electron content

Abstract: . In the present work we use the NASA-JPL global ionospheric maps of total electron content (TEC), firstly to construct TEC maps (TEC vs. magnetic local time MLT, and magnetic latitude MLAT) in the interval from 1999 to 2005. These TEC maps were, in turn, used to estimate the annual-to-mean amplitude ratio, A1, and the semiannual-to-mean amplitude ratio, A2, as well as the latitudinal symmetrical and asymmetrical parts, A' and A" of A1. Thus, we investigated in detail the TEC climatology from maps of these indices, with an emphasis on the quantitative presentation for local time and latitudinal changes in the seasonal, annual and semiannual anomalies of the ionospheric TEC. Then we took the TEC value at 14:00 LT to examine various anomalies at a global scale following the same procedure. Results reveal similar features appearing in NmF2, such as that the seasonal anomaly is more significant in the near-pole regions than in the far-pole regions and the reverse is true for the semiannual anomaly; the winter anomaly has least a chance to be observed at the South America and South Pacific areas. The most impressive feature is that the equinoctial asymmetry is most prominent at the East Asian and South Australian areas. Through the analysis of the TIMED GUVI columnar [O/N2] data, we have investigated to what extent the seasonal, annual and semiannual variations can be explained by their counterparts in [O/N2]. Results revealed that the [O/N2] variation is a major contributor to the daytime winter anomaly of TEC, and it also contributes to some of the semiannual and annual anomalies. The contribution to the anomalies unexplained by the [O/N2] data could possibly be due to the dynamics associated with thermospheric winds and electric fields.

Investigation of ionospheric electron content variations before earthquakes in southern California, 2003–2004

Abstract: [1] It has been proposed that earthquakes are preceded by electromagnetic signals detectable from ground- and space-based measurements. Ionospheric anomalies, such as variations in the electron density a few days before earthquakes, are one of the precursory signals proposed. Since Global Positioning System (GPS) data can be used to measure the ionospheric total electron content (TEC), the technique has received attention as a potential tool to detect ionospheric perturbations related to earthquakes. Here, we analyze 2 years (2003–2004) of data from the Southern California Integrated GPS Network (SCIGN), a dense network of 265 continuous GPS stations centered on the Los Angeles basin, for possible precursors. This time period encompasses the December 2003, M6.6, San Simeon and September 2004, M6.0, Parkfield earthquakes. We produce TEC time series at all SCIGN sites and apply three different statistical tests to detect anomalous TEC signals preceding earthquakes. We find anomalous TEC signals but no statistically significant correlation, in time or in space, between these TEC anomalies and the occurrence of earthquakes in southern California for the 2003–2004 period. This result does not disprove the possibility of precursory phenomena but show the signal-to-noise ratio of a hypothetical TEC precursor signature is too low to be detected by the analysis techniques employed here. Precursors may still be revealed for future large earthquakes in well instrumented areas such as California and Japan, if the tests can be developed into techniques that can better separate external influences from the actual TEC signal.

Medium-scale traveling ionospheric disturbances observed by GPS receiver network in Japan: a short review

Abstract: Medium-scale traveling ionospheric disturbances (MSTID) are wave-like perturbations of the ionospheric plasma with wavelengths of several hundred kilometres and velocities of several hundred metres per second. MSTID is one of the most common ionospheric phenomena that generally induce the perturbations of ionospheric total electron content (TEC) by ∼1016 electron/m2, which corresponds to ∼54 ns (16.2 cm) delay at GPS L1 signal. In the past decade, several new characteristics on MSTIDs have been revealed by the TEC observations using the dense GPS receiver network in Japan. In this paper, we provide a short review of these new observations and summarize the morphological characteristics of MSTIDs in Japan.

A numerical simulation of ionospheric and atmospheric variations associated with the Sumatra earthquake on December 26, 2004

Abstract: In the Sumatra earthquake that occurred on December 26, 2004, significant ionospheric variations were detected immediately after the earthquake in both the TEC (total electron content) data of GPS (Global Positioning System) and the ionosonde data. A magnetic pulsation with a period of about 4 min was also observed in Phimai, Thailand. Recent studies have suggested that these events are associated with acoustic waves excited by a sudden large-scale displacement of the sea surface around the epicenter. In order to study these phenomena quantitatively, a time-dependent two-dimensional nonhydrostatic compressible atmosphere-ionosphere model has been used and compared with relevant data. By modeling in sea surface perturbation, we were able to reproduce an atmospheric oscillation with a period of about 4 min in the upper atmosphere above the epicenter. The electron density variations observed by GPS/TEC and by ionosondes were also reproduced fairly well. We found that the observed TIDs (traveling ionospheric disturbances) with long periods are caused by the ducted thermospheric gravity waves produced in the thermosphere through acoustic pulse from the epicenter. The good overall agreement between the simulation results and observations indicates that numerical simulation with the nonhydrostatic compressible atmosphere-ionosphere model could be a useful tool to investigate the relationship between variations in the upper atmosphere and various sources of disturbances in the lower atmosphere.

A neural network approach for regional vertical total electron content modelling

Abstract: A Neural Network model has been developed for estim ating the total electron content (TEC) of the ionosphere. TEC is proportional to the delay suffered by electromagnetic signals crossing the ionosphere and is among the er rors that impact GNSS (Global Navigation Satellite Systems) observations. Ionosph eric delay is particularly a problem for single frequency receivers, which cannot elimin ate the (first-order) ionospheric delay by combining observations at two frequencies. Singl e frequency users rely on applying corrections based on prediction models or on region al models formed based on actual data collected by a network of receivers. A regiona l model based on a neural network has been designed and tested using data sets collected by the Brazilian GPS Network (RMBC) covering periods of low and high solar activity. An alysis of the results indicates that the model is capable of recovering, on average, 85% of TEC values. K e y w o r d s : total electron content, ionosphere, regi onal ionospheric model, neural network

Morphology in the total electron content under geomagnetic disturbed conditions: results from global ionosphere maps

Abstract: Using 8-year global ionosphere maps (GIMs) of TEC products from the Jet Propulsion Laboratory (JPL), we make a statistical study on the morphology of the global ionospheric behaviors with respect to the geomagnetic disturbances. Results show that the behaviors of TEC during geomagnetic storm present clear seasonal and local time variations under geomagnetic control in a similar way as those of NmF2 (Field and Rishbeth, 1997). A negative phase of TEC occurs with high probability in the summer hemisphere and most prominent near the geomagnetic poles, while a positive phase is obvious in the winter hemisphere and in the far pole region. A negative storm effect toward lower latitudes tends to occur from post-midnight to the morning sector and recedes to high latitude in the afternoon. A positive storm effect is separated by geomagnetic latitudes and magnetic local time. Furthermore, ionospheric responses at different local time sectors with respect to the storm commencement shows very different developing processes corresponding to the evolution of the geomagnetic storm. A daytime positive storm effect is shown to be more prominent in the American region than those in the Asian and European regions, which may suggest a longitudinal effect of the ionospheric storm.

Tec Kinases in T Cell and Mast Cell Signaling

Abstract: The Tec family of tyrosine kinases consists of five members (Itk, Rlk, Tec, Btk, and Bmx) that are expressed predominantly in hematopoietic cells The exceptions, Tec and Bmx, are also found in endothelial cells Tec kinases constitute the second largest family of cytoplasmic protein tyrosine kinases While B cells express Btk and Tec, and T cells express Itk, Rlk, and Tec, all four of these kinases (Btk, Itk, Rlk, and Tec) can be detected in mast cells This chapter will focus on the biochemical and cell biological data that have been accumulated regarding Itk, Rlk, Btk, and Tec In particular, distinctions between the different Tec kinase family members will be highlighted, with a goal of providing insight into the unique functions of each kinase The known functions of Tec kinases in T cell and mast cell signaling will then be described, with a particular focus on T cell receptor and mast cell Fc epsilon RI signaling pathways

Precursory phenomena observed in the total electron content measurements before great Hokkaido earthquake of September 25, 2003 (M = 8.3)

Abstract: In this paper the analysis of the ionospheric total electron content (TEC) variations obtained with using GPS measurements before the Hokkaido earthquake (M = 8.3) is presented. Anomalous behavior of TEC was detected within several days before the main event. Anomaly appeared as the local TEC enhancement situated in the vicinity of the forthcoming earthquake epicenter. These structures occurred during 5 days prior to the shock at the same interval of local time. At the process of the earthquake approach the amplitude of modification was increased, and it has reached the 85–90% level relative to the non-disturbed conditions 18 hours before the earthquake. The area of strong positive disturbance has extended over 1500 km in latitudes and 4000 km in longitudes. The analysis have shown that according to the series of characteristics (its locality, affinity with the epicenter, dome-shaped zone of manifestation, characteristic time of existence) the detected ionospheric anomaly may be associated to the precursors of seismic activity.

Proapoptotic activity of indoleamine 2,3-dioxygenase expressed in renal tubular epithelial cells.

Abstract: Exposure of renal tubular epithelial cells (TEC) to IFN-gamma/TNF-alpha leads to Fas/FasL-mediated self-injury, which contributes to allograft rejection. Indoleamine 2,3-dioxygenase (IDO) converts tryptophan to N-formyl-kynurenine and contributes to immune privilege in tissues by increasing Fas-mediated T cell apoptosis. However, renal expression of IDO and its role in promoting Fas-mediated TEC death have not been examined. IDO expression was analyzed by RT-PCR and Western blot. Apoptosis was measured by fluorescence-activated cell sorting analysis and terminal deoxytransferase-mediated dUTP nick end labeling. We demonstrated that functional IDO is expressed in TEC and is increased by IFN-gamma/TNF-alpha exposure. Increased IDO activity promoted TEC apoptosis, whereas inhibition of IDO by its specific inhibitor 1-methyl-d-tryptophan attenuated IFN-gamma/TNF-alpha-mediated TEC apoptosis and augmented TEC survival. Transgenic expression of IDO resulted in increased TEC apoptosis in the absence of proinflammatory cytokine exposure, supporting a central role for IDO in TEC injury. Inhibition of IDO-mediated TEC death by a caspase-8-specific inhibitor (Z-IETD-FMK), as well as the absence of an IDO effect in Fas-deficient and FasL-deficient TEC, supports a Fas/FasL-dependent, caspase-8-mediated mechanism for IDO-enhanced TEC death. These data suggest that renal IDO expression may be deleterious during renal inflammation, because it enhances TEC self-injury through Fas/FasL interactions. Thus attenuation of IDO may represent a novel strategy to promote kidney function following ischemia and renal allograft rejection.

GPS ionospheric tomography: A comparison with the IRI-2001 model over South Korea

Abstract: The International Reference Ionosphere model 2001 (IRI-2001) is one of the most comprehensive empirical models of the ionosphere and has been widely used to estimate the electron density profiles in the altitude ranging from about 60 to 2000 km and the total electron content (TEC) at any given location, time and date, which reflect smooth-average global ionospheric behaviors. However, whether it provides normal actual estimations in the ionosphere over some regions should be tested with real observation data. In this paper, the three-dimensional ionospheric electron density profiles over South Korea in 2003 are obtained using the ionospheric tomography reconstruction technique with the permanent Korean GPS Network (KGN) data, and its validity is further verified by another independent ionosonde data. The GPS ionospheric reconstruction results are used to compare then results obtained with the IRI-2001 model in South Korea in terms of NmF2 and TEC. The monthly averaged diurnal values of these key parameters in January, April, July and October 2003 are considered to represent the winter, spring, summer and autumn seasons, respectively. Compared with the GPS reconstruction results, averaged monthly NmF2 medians from the IRI-2001 are overestimated in daytime and underestimated in nighttime for all seasons, but the deviation magnitudes in autumn and winter are smaller than in spring and summer. In addition, averaged monthly TEC medians from the IRI-2001 are overestimated in daytime in winter, but almost always underestimated in other seasons.

Analysis of ionospheric scintillation spectra and TEC in the Chinese low latitude region

Abstract: GPS L-band scintillations and total electron content (TEC) were recorded at Sanya (18.33°N, 109.52°E) during the period July 2004–July 2005. Automatic recorded raw digital scintillation data are analyzed to obtain the spectral characteristics of irregularities producing ionospheric scintillations and to estimate the correlation between amplitude scintillation and power spectral density. Concurrent measurements of TEC are used to analyze ROTI, defined as the standard deviation of the rate of change of TEC. The statistical results of S4 indices and power spectral indices indicate that the power spectral indices increase with S4 indices for weak scintillation (0.1 ≤ S4 < 0.3), but for moderate and strong scintillation, spectral indices tend to be saturated. In the analyzed data set, the ratio of ROTI/S4 is found to vary between 0.3 and 6, and the variation in estimated zonal drift velocities during geomagnetic quiet days (K p < 3) shows that the motion of the irregularities is highly variable in the initial phase of irregularity development. After about 22:00 LT, the estimated drift velocities tend to follow the same pattern.

Time and event based one time password

Abstract: A system and method for generating a One Time Password (OTP) based upon a value TEC that can change based both upon the occurrence of an event and the passage of time. The OTP can be computed at a token and sent to a verifier. The verifier stores exact or estimated parameters necessary to compute one or more expected OTPs from the token, including TEC. The value TEC can be synchronized between the token and the verifier.

Software developed for obtaining GPS-derived total electron content values

Abstract: This paper describes a complex technique with its built-in cycle slip correction procedures that have been developed for ionospheric space research to obtain high-quality and high-precision GPS-derived total electron content (TEC) values. Thus, to correct GPS anomalies while the signatures of space weather features detected in the dual-frequency 30-s rate GPS data are preserved is the main aim of this technique. Its main requirement is to complete fully automatically all the tasks required to turn the observational data to the desired final product. Its major tasks include curve fitting, cycle slip detection and correction in the slant relative TEC data, residual error detection and correction in the vertical TEC data, and vertical TEC data filtering for quantifying data smoothness and GPS phase fluctuations. A detailed description of these two data correction methods is given. Validation tests showing weaknesses and strengths of the methods developed are also included and discussed. Versatility and accuracy of the methods are demonstrated with interesting and real-world examples obtained from smooth midlatitude and from dynamic low- and high-latitude data. Results indicate that errors can be detected and corrected more reliably in the vertical TEC data than in the slant TEC data because of the lower rate of change of vertical TEC over a 30-s sampling period. Future work includes the development of a complex software package wherein the individual FORTRAN algorithms, described in this paper, will be incorporated into one main (FORTRAN, Matlab, or C++) program to provide professional and customized GPS data processing for ionospheric space research.