TEC

About: TEC is a research topic. Over the lifetime, 5119 publications have been published within this topic receiving 84696 citations.

Variability of GPS/GLONASS differential code biases

Abstract: While estimating ionospheric total electron content (TEC) using both pseudorange and phase GPS/GLONASS data, there occurs a systematic error caused by the difference in processing times of L1 and L2 signals through radio frequency paths of satellites and receivers, known as differential code biases (DCBs). A 1-ns DCB causes an ∼2.9 TECU error in TEC estimation. Along with systematic DCB variations, seasonal variations, most likely related to variations in the receiver environment (temperature, humidity), also exist for some receivers and can reach in some cases up to ∼20 TECU.

EOF analysis and modeling of GPS TEC climatology over North America

Abstract: An empirical ionospheric model of the total electron content (TEC) over North America (20°–60°N, 40°–140°W) is constructed using the GPS TEC data collected by Massachusetts Institute of Technology (MIT) Haystack Observatory during the years 2001–2012. This model is based on an analysis of quiet time monthly averages using the empirical orthogonal function (EOF) decomposition technique, allowing for separation of spatial and temporal variations. The importance of different types of spatial-temporal variations to the overall TEC variability can be well represented by the characteristics of EOF basis functions and associated principal components coefficients, with various modes. The mode one EOF decomposition constitutes 97.5% of the total variance and therefore represents the essential feature of North America spatial and diurnal variation of the TEC. The mode two EOF, as reported in an earlier study, reveals a large and significant symmetric longitudinal variation of the ionosphere, organized with respect to magnetic declination. The mode three EOF decomposition shows midlatitude latitudinal structure that varies with season in a manner very similar to the so-called winter anomaly. Because of the quick convergence of EOF decomposition modes, the first four EOF modes are utilized for constructing a TEC empirical model. For each of the EOF modes, the temporal variations are expressed analytically in terms of local time, season, and solar activity, and the spatial variations by cubic-spline functions. An analysis of accuracy and quality indicates that this regional empirical TEC model can reflect the majority of the quiet time monthly means, and represent characteristic temporal-spatial variations in the North America.

Interleukin-6 overproduction by cultured thymic epithelial cells from patients with myasthenia gravis is potentially involved in thymic hyperplasia.

Abstract: Most patients with Myasthenia Gravis (MG) present a thymic hyperplasia characterized by the presence of lymphoid follicles. The acetylcholine receptor autoantigen, as well as autoantigen specific activated T and B cells found in the thymus, strongly suggest that auto-sensitization could take place in this organ. Since IL-6 is involved in T and B cell growth and differentiation, we thought that abnormal IL-6 expression by thymic epithelial cells (TEC) could be related to thymic hyperplasia in MG. In this paper, IL-6 protein and gene expression by cultured TEC from patients with MG were examined. TEC from patients presented a dramatic IL-6 hyperproduction phenotype as compared to controls when stimulated by exogenous signals such as LPS and cytokines (IL-1 beta, TNF-alpha) alone or in combination. Moreover, we observed a similar effect with a physiological signal such as the syngeneic lympho-epithelial cell contact. Autologous thymocytes stimulated normal and MG TEC IL-6 production in a time- and dose- dependent way, and with a higher magnitude in MG TEC compared to controls. In all stimulation conditions, induction of IL-6 production required protein synthesis and was associated with increased IL-6 mRNA level expression as assessed by computer-aided quantification after in situ mRNA hybridization. In addition, recombinant IL-6 induced in vitro growth of TEC, demonstrating that IL-6 is a possible autocrine growth factor for these cells. This deregulated IL-6 production as well as the ability of TEC to use it as a growth factor may be of pathophysiological relevance in MG. It provides an explanation for morphological changes of the thymus and may have a key role in initiation, exacerbation and ongoing of the autoimmune response in MG. Therefore this study extends our current understanding of the molecular pathophysiology of MG.

Seasonal and local time variation of ionospheric migrating tides in 2007-2011 FORMOSAT-3/COSMIC and TIE-GCM total electron content

Abstract: [1] This study examines the seasonal and interannual variation of the major migrating tidal components in midlatitude to low-latitude total electron content (TEC) observations from the FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellite constellation from 2007 to 2011. Although the absolute amplitudes of the TEC zonal mean and migrating tidal components show a strong positive relation to the increasing and decreasing phases of the solar cycle, the relative tidal amplitudes following normalization by maximum background values show a more varied response to solar activity levels. Features of ionospheric local time variation produced by individual migrating tidal components are consistent from year to year, with DW1 forming the equatorial daytime peak in TEC, SW2 corresponding to the generation of the equatorial ionization anomaly (EIA) crests, and TW3 contributing to the TEC trough between the EIA crests. Numerical experiments using Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) are also performed to determine the sensitivity of the ionospheric migrating tides to upward propagating migrating tidal components from the neutral mesosphere and lower thermosphere (MLT). Zonal mean TECs decrease when MLT tidal forcing is applied and are particularly sensitive to the MLT DW1. Most of the ionospheric SW2 response is attributable to MLT SW2 forcing, enhancing the EIA crests by amplifying the equatorial fountain. TW3 in the model is generated through both in situ photoionization and nonlinear interaction between DW1 and SW2.