TEC

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

Coseismic ionospheric disturbances triggered by the Chi-Chi earthquake

Abstract: [1] At 17:47 UT on 20 September 1999, a large earthquake of magnitude Mw 7.6 struck the central Taiwan near a small town of Chi-Chi. The ground-based receivers of the global positioning system (GPS) in the Taiwan area detected coseismic ionospheric disturbances (CIDs) in the total electron content (TEC) triggered by the Chi-Chi earthquake. When the CIDs travel away from the origin on the Earth surface and then propagate into the ionosphere, their amplitudes and periods generally become smaller and longer, respectively. Moreover, two global grid searches, adapting the ray-tracing and the beam-forming techniques, have been used to analyze the observed GPS TEC. We have not only estimated the average speed of the CIDs propagating in the atmosphere and ionosphere but also determined the location of CID origin on the Earth surface by using the two techniques. The results show that the observed CIDs result from shock-acoustic waves triggered by sudden and large vertical motions of the Chi-Chi earthquake.

Three-dimensional ionospheric electron density structure of the Weddell Sea Anomaly

Abstract: [1] This paper provides the first three-dimensional description of the ionospheric density structure of the Weddell Sea Anomaly (WSA). The WSA is characterized by a nighttime ionospheric density greater than that in daytime in the Weddell Sea region around the Antarctic Peninsula during the southern hemisphere summer. It was first observed by ground-based ionosondes located in the Antarctica back in the 1950s and was further investigated by two-dimensional maps over the oceans using TEC measurements collected by the TOPEX/Poseidon recently. Although these TEC maps have provided two-dimensional views for tracking the time-evolution and spatial coverage of the WSA, the vertical distribution of this peculiar feature is still unavailable. With the vertical ionospheric density profiles observed by the FORMOSAT-3/COSMIC, three-dimensional density structure of the WSA is presented here for the first time. Meanwhile, a similar WSA signature is observed in the northern and eastern hemisphere during June solstice by both the GPS-TEC and the FORMOSAT-3/COSMIC electron density observations. From the observed altitudinal structure of the WSA during 1800–2400 LT and the similar feature occurred in the opposite hemisphere suggest that the southward offset of the magnetic equator with respect to the geographic equator plays a major role for the WSA formation.

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.

Seasonal and solar activity variations of the Weddell Sea Anomaly observed in the TOPEX total electron content measurements

Abstract: [1] The Weddell Sea Anomaly (WSA) in the ionosphere is characterized by higher plasma densities at night than during the day in the region near the Weddell Sea. According to previous studies on the WSA, it is known to occur mostly in southern summer and has not been reported in other seasons. We have utilized more than 13 years of TOPEX total electron content (TEC) measurements in order to study how the WSA varies with seasons and how it changes with solar activity. The TOPEX TEC data have been extensively utilized for climatological studies of the ionosphere because of their excellent spatial and temporal coverage. We investigate the seasonal and solar activity variations of the WSA using four seasonal cases (March equinox, June solstice, September equinox, and December solstice) and two solar activity conditions (F10.7 120 for solar maximum conditions) for geomagnetically quiet periods. Our analysis shows that (1) the WSA occurs only in the southern summer hemisphere for low F10.7, as in previous studies, but (2) the WSA occurs in all seasons except for winter when F10.7 is high; it is most prominent during the December solstice (southern summer) and still strong during both equinoxes. The TOPEX TEC maps in the midlatitude and high-latitude ionosphere display significant global longitudinal variations for a given local time in the Southern Hemisphere, which varies with season and solar activity. The observed WSA appears to be an extreme manifestation of the longitudinal variations.