Showing papers on "TEC published in 2017"
TL;DR: In this paper, the geostationary Earth Orbit (GEO) observations of BeiDou Navigation Satellite System (BDS) are used to investigate the ionospheric responses to the geomagnetic storm.
Abstract: The most intense geomagnetic storm in solar cycle 24 occurred on March 17, 2015, and the detailed ionospheric storm morphologies are difficultly obtained from traditional observations. In this paper, the Geostationary Earth Orbit (GEO) observations of BeiDou Navigation Satellite System (BDS) are for the first time used to investigate the ionospheric responses to the geomagnetic storm. Using BDS GEO and GIMs TEC series, negative and positive responses to the March 2015 storm are found at local and global scales. During the main phase, positive ionospheric storm is the main response to the geomagnetic storm, while in the recovery phase, negative phases are pronounced at all latitudes. Maximum amplitudes of negative and positive phases appear in the afternoon and post-dusk sectors during both main and recovery phases. Furthermore, dual-peak positive phases in main phase and repeated negative phase during the recovery are found from BDS GEO observations. The geomagnetic latitudes corresponding to the maximum disturbances during the main and recovery phases show large differences, but they are quasi-symmetrical between southern and northern hemispheres. No clear zonal propagation of traveling ionospheric disturbances is detected in the GNSS TEC disturbances at high and low latitudes. The thermospheric composition variations could be the dominant source of the observed ionospheric storm effect from GUVI $$\hbox {[O]/[N}_{2}]$$
ratio data as well as storm-time electric fields. Our study demonstrates that the BDS (especially the GEO) observations are an important data source to observe ionospheric responses to the geomagnetic storm.
91 citations
TL;DR: A new approach is presented, named VARION (Variometric Approach for Real-Time Ionosphere Observation), and slant TEC (sTEC) variations in a real-time scenario are estimated and perturbation periods consistent with a tsunami typical deep ocean period are found.
Abstract: It is well known that tsunamis can produce gravity waves that propagate up to the ionosphere generating disturbed electron densities in the E and F regions. These ionospheric disturbances can be studied in detail using ionospheric total electron content (TEC) measurements collected by continuously operating ground-based receivers from the Global Navigation Satellite Systems (GNSS). Here, we present results using a new approach, named VARION (Variometric Approach for Real-Time Ionosphere Observation), and estimate slant TEC (sTEC) variations in a real-time scenario. Using the VARION algorithm we compute TEC variations at 56 GPS receivers in Hawaii as induced by the 2012 Haida Gwaii tsunami event. We observe TEC perturbations with amplitudes of up to 0.25 TEC units and traveling ionospheric perturbations (TIDs) moving away from the earthquake epicenter at an approximate speed of 316 m/s. We perform a wavelet analysis to analyze localized variations of power in the TEC time series and we find perturbation periods consistent with a tsunami typical deep ocean period. Finally, we present comparisons with the real-time tsunami MOST (Method of Splitting Tsunami) model produced by the NOAA Center for Tsunami Research and we observe variations in TEC that correlate in time and space with the tsunami waves.
86 citations
TL;DR: An algorithm for estimating a time-series of ionospheric phase delay that is useful for correcting InSAR time- series of ground surface displacement or for evaluating the spatial and temporal variations of the ionosphere’s total electron content (TEC).
Abstract: Repeat pass interferometric synthetic aperture radar (InSAR) observations may be significantly impacted by the propagation delay of the microwave signal through the ionosphere, which is commonly referred to as ionospheric delay. The dispersive character of the ionosphere at microwave frequencies allows one to estimate the ionospheric delay from InSAR data through a split range-spectrum technique. Here, we extend the existing split range-spectrum technique to InSAR time-series. We present an algorithm for estimating a time-series of ionospheric phase delay that is useful for correcting InSAR time-series of ground surface displacement or for evaluating the spatial and temporal variations of the ionosphere’s total electron content (TEC). Experimental results from stacks of L-band SAR data acquired by the ALOS-1 Japanese satellite show significant ionospheric phase delay equivalent to 2 m of the temporal variation of InSAR time-series along 445 km in Chile, a region at low latitudes where large TEC variations are common. The observed delay is significantly smaller, with a maximum of 10 cm over 160 km, in California. The estimation and correction of ionospheric delay reduces the temporal variation of the InSAR time-series to centimeter levels in Chile. The ionospheric delay correction of the InSAR time-series reveals earthquake-induced ground displacement, which otherwise could not be detected. A comparison with independent GPS time-series demonstrates an order of magnitude reduction in the root mean square difference between GPS and InSAR after correcting for ionospheric delay. The results show that the presented algorithm significantly improves the accuracy of InSAR time-series and should become a routine component of InSAR time-series analysis.
83 citations
TL;DR: In this article, the authors present quantitative predictions of the impact of the upcoming total solar eclipse on the ionosphere and plasmasphere using the Naval Research Laboratory (NRL) model Sami3 is Also a Model of the Ionosphere (SAMI3).
Abstract: We present quantitative predictions of the impact of the upcoming total solar eclipse on the ionosphere and plasmasphere using the Naval Research Laboratory (NRL) model Sami3 is Also a Model of the Ionosphere (SAMI3). The eclipse will occur over the continental United States on 21 August 2017. Our simulation results indicate that in the vicinity of the eclipse (1) the total electron content (TEC) decreases by up to ∼ 5 TEC units (TECU; 1 TECU = ×1016 m−2) which is a ∼ 35% decrease in TEC, (2) the electron density decreases by a factor of ∼ 50% in the F region, (3) the electron temperature decreases by up to ∼800 K in the plasmasphere, and (4) the O+ velocity changes from ∼40 m s−1 upward to ∼20 m s−1 downward in the F region. Interestingly, the continental size modification of the ionospheric conductance modifies the global electric field, which should lead to measurable changes in the TEC in the southern conjugate hemisphere ( ≲1 TECU).
72 citations
09 Jul 2017
TL;DR: A long short-term memory (LSTM) based model is proposed to predict ionospheric vertical TEC of Beijing and shows the root of mean square (RMS) error of test data can reach 3.50 and RMS error is less than this number during the period of low solar activity.
Abstract: Ionosphere is an important space environment near the earth. Its disturbance would result in severe propagation effects to radio information system, thus causing bad influences on communication, navigation, radar and so on. The total electron content (TEC) is an important parameter to present the disturbance of ionosphere, so TEC forecast is very meaningful in scientific research field. In this paper, we propose a long short-term memory (LSTM) based model to predict ionospheric vertical TEC of Beijing. The input of our model is a time sequence consisting of the vector of daily TECs and other closely related parameters. The output is TECs of future 24 hours. The result shows the root of mean square (RMS) error of test data can reach 3.50 and RMS error is less than this number during the period of low solar activity. Compared to multilayer perceptron network, LSTM is more promising and reliable to forecast ionospheric TEC.
60 citations
TL;DR: In this article, the authors compare changes in the total electron content (TEC) of the ionosphere with occurrences of M ≥ 6.0 earthquakes globally for 2000 - 2014.
Abstract: There are many reports on the occurrence of anomalous changes in the ionosphere prior to large earthquakes. However, whether or not these changes are reliable precursors that could be useful for earthquake prediction is controversial within the scientific community. To test a possible statistical relationship between ionospheric disturbances and earthquakes, we compare changes in the total electron content (TEC) of the ionosphere with occurrences of M ≥ 6.0 earthquakes globally for 2000 - 2014. We use TEC data from the global ionosphere map (GIM) and an earthquake list declustered for aftershocks. For each earthquake, we look for anomalous changes in GIM-TEC within 2.5° latitude and 5.0° longitude of the earthquake location (the spatial resolution of GIM-TEC). Our analysis has not found any statistically significant changes in GIM-TEC prior to earthquakes. Thus, we have found no evidence that would suggest that monitoring changes in GIM-TEC might be useful for predicting earthquakes.
51 citations
TL;DR: In this article, a two-stage TEC was proposed to achieve a larger temperature drop when TECs operate at steady state conditions, where pulse currents are respectively supplied to the hot and the cold stage of a cascaded TEC to seek further increase in temperature drop across the TEC.
50 citations
TL;DR: Thrombotic and embolic complication was not uncommon; risk factors for TEC were APC and atrial arrhythmias; most patients were treated successfully with warfarin alone.
50 citations
TL;DR: In this article, the influence of defects in the thermal expansion coefficient (TEC) of suspended graphene membranes was systematically investigated, and it was shown that the presence of low densities of mono-vacancies reduces the graphene TEC by up to one order of magnitude.
39 citations
TL;DR: This paper used dense GNSS networks in central Europe to detect and investigate a special type of plasma inhomogeneities, called travelling ionospheric disturbances (TID), and used GNSS sensors to reconstruct the main TID parameters, such as spatial dimensions, velocities, and directions of their movement.
Abstract: The technique of the orthogonal projection of ionosphere electronic content variations for mapping total electron content (TEC) allows us to visualize ionospheric irregularities. For the reconstruction of global ionospheric characteristics, numerous global navigation satellite system (GNSS) receivers located in different regions of the Earth are used as sensors. We used dense GNSS networks in central Europe to detect and investigate a special type of plasma inhomogeneities, called travelling ionospheric disturbances (TID). Such use of GNSS sensors allows us to reconstruct the main TID parameters, such as spatial dimensions, velocities, and directions of their movement. The paper gives examples of the restoration of dynamic characteristics of ionospheric irregularities for quiet and disturbed geophysical conditions. Special attention is paid to the dynamics of ionospheric disturbances stimulated by the magnetic storms of two St. Patrick’s Days (17 March 2013 and 2015). Additional opportunities for the remote sensing of the ionosphere with the use of dense regional networks of GNSS receiving sensors have been noted too.
38 citations
TL;DR: In this article, the performance of the Galileo broadcast NeQuick model was evaluated over both the continental and oceanic regions for the year 2013, and it was shown that it can correct 51.1, 61.2 and 68.6 % of the ionospheric delay.
Abstract: We evaluate the performance of Galileo broadcast NeQuick model by comparing it with GPS broadcast Klobuchar and the original NeQuick2 models. The broadcast coefficients of Galileo NeQuick model are computed from 23 globally distributed tracking stations of the International GNSS Service (IGS), by ingesting the Global Positioning System (GPS)-derived ionospheric total electron content (TEC) into the original NeQuick2 model. The accuracy of the three ionospheric models is evaluated over both the continental and oceanic regions for the year 2013. In continental regions, ionospheric TEC derived from 34 IGS stations is used as references for comparison. In oceanic regions, where the IGS stations are sparse, high-quality vertical TEC sources provided by JASON-1&2 altimeters are used as references. The evaluation results show that in continental regions, GPS broadcast Klobuchar and the original and broadcast NeQuick can mitigate the ionospheric delay by 56.8, 63.3 and 72.4 %, respectively. In oceanic regions, the three models can correct for 51.1, 61.2 and 68.6 % of the ionospheric delay. Galileo broadcast NeQuick model outperforms Klobuchar by 15.6 and 17.5 % over the continental and oceanic regions, respectively, for the test period. The broadcast NeQuick model can provide accurate ionospheric error corrections when Galileo begins full operational capability.
TL;DR: In this article, a self-consistent modeling study of the ionosphere-plasmasphere system response to the 17 March 2015 geomagnetic storm using the coupled SAMI3-RCM code is presented.
Abstract: We present a self-consistent modeling study of the ionosphere-plasmasphere system response to the 17 March 2015 geomagnetic storm using the coupled SAMI3-RCM code. The novel feature of this work is that we capture the important storm time dynamics of the ionosphere on a global scale and its manifestation in the plasmasphere. We find that the penetration electric fields associated with the magnetic storm lead to a storm time enhanced density in the low- to middle-latitude ionosphere. We compare the modeled total electron content (TEC) with GPS-measured TEC in the American sector. Additionally, we observe the development of polar cap “tongues of ionization” and the formation of subauroral plasma streams in the postsunset, premidnight sector, and its impact on the plasmasphere. However, we did not see the development of plasmaspheric plumes during this event which we attribute to the long main phase of the storm (∼18 h).
TL;DR: In this paper, the authors measured the total electron content (TEC) variation in the ionosphere from Global Navigation Satellite System (GNSS) data which might have been induced by earthquakes in the Himalayan region.
TL;DR: An approach for regional ionosphere modelling based on un-differenced multi-GNSS carrier phase data for total electron content (TEC) estimation, and thin plate splines for TEC interpolation is proposed and a methodology for ionospheric products self-consistency analysis based on calibrated slant TEC is proposed.
Abstract: The ionosphere is still considered one of the most significant error sources in precise Global Navigation Satellite Systems (GNSS) positioning. On the other hand, new satellite signals and data processing methods allow for a continuous increase in the accuracy of the available ionosphere models derived from GNSS observables. Therefore, many research groups around the world are conducting research on the development of precise ionosphere products. This is also reflected in the establishment of several ionosphere-related working groups by the International Association of Geodesy. Whilst a number of available global ionosphere maps exist today, dense regional GNSS networks often offer the possibility of higher accuracy regional solutions. In this contribution, we propose an approach for regional ionosphere modelling based on un-differenced multi-GNSS carrier phase data for total electron content (TEC) estimation, and thin plate splines for TEC interpolation. In addition, we propose a methodology for ionospheric products self-consistency analysis based on calibrated slant TEC. The results of the presented approach are compared to well-established global ionosphere maps during varied ionospheric conditions. The initial results show that the accuracy of our regional ionospheric vertical TEC maps is well below 1 TEC unit, and that it is at least a factor of 2 better than the global products.
TL;DR: The results suggested that tumor‐secreting factor increases ALDHhigh TEC populations that are resistant to 5‐FU in tumor blood vessels might be an important target to overcome or prevent drug resistance.
Abstract: Tumor blood vessels play an important role in tumor progression and metastasis. We previously reported that tumor endothelial cells (TEC) exhibit several altered phenotypes compared with normal endothelial cells (NEC). For example, TEC have chromosomal abnormalities and are resistant to several anticancer drugs. Furthermore, TEC contain stem cell-like populations with high aldehyde dehydrogenase (ALDH) activity (ALDHhigh TEC). ALDHhigh TEC have proangiogenic properties compared with ALDHlow TEC. However, the association between ALDHhigh TEC and drug resistance remains unclear. In the present study, we found that ALDH mRNA expression and activity were higher in both human and mouse TEC than in NEC. Human NEC:human microvascular endothelial cells (HMVEC) were treated with tumor-conditioned medium (tumor CM). The ALDHhigh population increased along with upregulation of stem-related genes such as multidrug resistance 1, CD90, ALP, and Oct-4. Tumor CM also induced sphere-forming ability in HMVEC. Platelet-derived growth factor (PDGF)-A in tumor CM was shown to induce ALDH expression in HMVEC. Finally, ALDHhigh TEC were resistant to fluorouracil (5-FU) in vitro and in vivo. ALDHhigh TEC showed a higher grade of aneuploidy compared with that in ALDHlow TEC. These results suggested that tumor-secreting factor increases ALDHhigh TEC populations that are resistant to 5-FU. Therefore, ALDHhigh TEC in tumor blood vessels might be an important target to overcome or prevent drug resistance.
TL;DR: In this article, the authors reported the first observation of concentric traveling ionospheric disturbances (CTIDs) triggered by the launch of a SpaceX Falcon 9 rocket on 17 January 2016, which showed a shock acoustic wave signature in the time rate change (time derivative) of total electron content (TEC), followed by CTIDs in the 8-15 minutes bandpass filtering of TEC.
Abstract: We report the first observation of concentric traveling ionospheric disturbances (CTIDs) triggered by the launch of a SpaceX Falcon 9 rocket on 17 January 2016. The rocket triggered ionospheric disturbances show shock acoustic wave signature in the time rate change (time derivative) of total electron content (TEC), followed by CTIDs in the 8–15 minutes bandpass filtering of TEC. The CTIDs propagated northward with phase velocity of 241–617 m/s and reached distances more than 1000 km away from the source on the rocket trajectory. The wave characteristics of CTIDs with periods of 10.5–12.7 minutes and wavelength ~200–400 km agree well with the gravity wave dispersion relation. The optimal wave source searching and gravity wave ray-tracing technique suggested that the CTIDs have multiple sources which are originated from ~38–120 km altitude before and after the ignition of the 2nd stage rocket, ~200 seconds after the rocket was launched.
TL;DR: The largest geomagnetic storm in solar cycle 24 occurred during March 17-18, 2015 where the main phase of the storm reached the negative minimum at 22:00 UT as discussed by the authors.
Abstract: The largest geomagnetic storm in solar cycle 24 occurred during March 17-18, 2015 where the main phase of the storm commenced from 07:00 UT of March 17, 2015 and reached the Dst negative minimum at 22:00 UT. The present paper reports observations of TEC, amplitude and phase scintillations from different GPS stations of India during the storm of March 17 and highlights its effects on GPS. It also presents the global ESF occurrence during the storm using total ion density drift measurements from C/NOFS satellite. TEC enhancements were noted from stations along 77oE meridian around 10:00 UT on March 17 compared to March 16 and 18 indicating positive storm effects arising out of equatorward neutral wind in the local morning-noon sector of the main phase. Intense scintillation observations from Calcutta were most extensive during 15:00-16:00 UT, March 17 and the receiver recorded a longitude deviation of 5.2 m during this time. Cycle slips of the order of 8 s could be observed during periods of intense phase scintillations on the same night. Intense scintillation observation from Palampur is an exceptional phenomenon attributed to the dramatic enhancement of the electric field due to PPEF leading to a very high upward ion velocity over the magnetic equator as recorded by C/NOFS. The total ion density measured globally by C/NOFS reveals two distinct longitude regions of ESF occurrence during the storm: i) East Pacific sector and ii) Indian longitude during the storm. The time and longitude of ESF occurrence could be predicted using the time of southward turning of IMF Bz.
TL;DR: In this article, the capability of the NeQuick 2 and IRI Plas models to predict vertical total electron content (VTEC) over the low latitude and South American sector is analyzed.
TL;DR: In this paper, the authors investigated the diurnal and monthly behavior of TEC and the relative TEC deviations along with the upper and lower quartiles to represent its spatio-temporal variability.
Abstract: The present study investigates the ionospheric Total Electron Content (TEC) variations in the lower mid-latitude Turkish region from the Turkish Permanent GNSS Network (TPGN) and International GNSS Services (IGS) observations during the period from January 2015 to December 2015. The corresponding TEC predicted by the International Reference Ionosphere (IRI 2012) and Standard Plasmasphere-Ionosphere Model (SPIM), and interpolated from Global Ionosphere Maps (GIMs) are evaluated to realize their reliability over the region. We studied the diurnal and monthly behavior of TEC and the relative TEC deviations along with the upper and lower quartiles to represent its spatio-temporal variability. The diurnal variation of GNSS-derived TEC indicates its maximum peak value around 10.00 UT which decreases gradually to attain minimum value after midnight. The monthly maximum value of TEC is observed in March followed by May and August, and the lowest value is seen during September. Studies show that the monthly relative deviation of TEC variability lies in the range of −1 to 4 units for all stations with the maximum difference between positive and negative variability remaining around 5. The studies also cover seasonal variation, grand-mean of ionospheric TEC and TEC intensity from the TPGN. The seasonal ionospheric VTEC pattern over all stations depicts slight increment in VTEC distribution during March equinox compared to September equinox. The December solstice perceived relatively higher VTEC than June solstice. The overall of VTEC values enhanced at all stations towards end of the year 2015 compare to mid of year due the high solar activity. The maximum grand-mean of VTEC is registered in March equinox while the lowest value is seen in September irrespective of all stations. The measured grand-mean intensity variations of VTEC values are in ascending phase during March, May, August and November months, but in descending phase during February, April, June and September months. The latitudinal study shows daytime TEC slowly decreasing with latitudes with a latitudinal gradient range of 0.1–0.2 TECU/degree. Additionally, the TEC analysis during the strong geomagnetic storm period (07–11 September 2015; SYM-H −120 nT) infers relatively better predictability of the SPIM model compared to the IRI 2012 model. The outputs of this study would complement towards a complete understanding of the lower mid-latitude ionospheric dynamics and its effects on radio propagations, particularly over the Turkish region.
TL;DR: In this article, a new model of the thermionic energy converter (TEC) configured with the graphene-based cathode is proposed, which includes the thermal radiation between the cathode and the anode electrodes and the heat losses from the anodes to the environment.
Abstract: A new model of the thermionic energy converter (TEC) configured with the graphene-based cathode is proposed, which includes the thermal radiation between the cathode and the anode electrodes and the heat losses from the anode to the environment. Analytic expressions for the power output density and efficiency of the TEC are derived. The performance characteristics of the TEC are analyzed by numerical calculations. It is found that the maximum efficiency and power density can, respectively, reach 30% and $\textsf {0.575}\,\,\textsf {Wcm}^{-\textsf {2}}$ when the TEC is operated between the two heat reservoirs at temperatures 1500 and 300 K. In addition, the optimum regions of the efficiency, power density, voltage output, electric current, and anode temperature are determined. The maximum efficiency and power density of the graphene-based TEC operated at different temperatures are calculated and compared with those of the metal-based TEC. It shows that the graphene-based TEC operated at 1200–1800 K displays the better performance than the metal-based TEC. The results obtained here may provide guidance for the appropriate selection of electrode materials and optimum design of practical TEC devices.
TL;DR: In this article, the authors used TEC data from a BeiDou geostationary orbit (GEO) receiver network to observe and investigate ionospheric longitudinal variations within the zonal scale of 1000 km over Central China (112°-122°E, 27°-31°N; 20°-24°N magnetic latitudes) during June 2015 - December 2016.
Abstract: Many studies have devoted to the longitudinal variations of the ionosphere globally. However, the ionospheric longitudinal variations in a small region are rarely reported. In this paper, we for the first time use TEC data from a BeiDou geostationary orbit (GEO) receiver network to observe and investigate ionospheric longitudinal variations within the zonal scale of 1000 km over Central China (112°-122°E, 27°-31°N; 20°-24°N magnetic latitudes) during June 2015 - December 2016. The BeiDou GEO TEC provides a good dataset to study longitudinal variations, compared with Non-GEO TEC, without contaminating the spatial variations and elevation changes due to satellite motion. Pronounced daytime longitudinal gradients within the distance of 1000 km are present in BeiDou GEO TEC. It was found that the TEC is generally larger in the west than in the east. In some cases, the TEC gradient magnitude is larger than 45 TECU. For most events, the obvious daytime longitudinal gradients are accompanied by the TEC enhancement. The occurrence rate of daytime longitudinal gradients under different geomagnetic activities is similar, whereas strong daytime longitudinal gradients mainly occur under the moderate and strong disturbance geomagnetic activities. These observations suggest that the electric field disturbances could have significant effects on producing the observed ionospheric longitudinal gradients.
TL;DR: In this article, the authors investigated the ionospheric total electron content (TEC) variations in the lower mid-latitude Turkish region from the Turkish Permanent GNSS Network (TPGN) and International GNSS Services (IGS) observations during the year 2016.
Abstract: The present study investigates the ionospheric Total Electron Content (TEC) variations in the lower mid-latitude Turkish region from the Turkish Permanent GNSS Network (TPGN) and International GNSS Services (IGS) observations during the year 2016. The corresponding vertical TEC (VTEC) predicted by Auto Regressive Moving Average (ARMA) and International Reference Ionosphere 2016 (IRI-2016) models are evaluated to realize their effectiveness over the region. The spatial, diurnal and seasonal behavior of VTEC and the relative VTEC variations are modeled with Ordinary Least Square Estimator (OLSE). The spatial behavior of modeled result during March equinox and June solstice indicates an inverse relationship of VTEC with the longitude across the region. On the other hand, the VTEC variation during September equinox and December solstice including March equinox and June solstice are decreasing with increase in latitude. The GNSS observed and modeled diurnal variation of the VTEC show that the VTEC slowly increases with dawn, attains a broader duration of peak around 09.00 to 12.00 UT, and thereafter decreases gradually reaching minimum around 21.00 UT. The seasonal variation of VTEC shows an annual mode, maxima in equinox and minima in solstice. The average value of VTEC during the June solstice is with slightly higher value than the March equinox though variations during the latter season is more. Moreover, the study shows minimum average value during December solstice compared to June solstice at all stations. The comparative analysis demonstrates the prediction errors by OLSE, ARMA and IRI remaining between 0.23 to 1.17%, 2.40 to 4.03% and 24.82 to 25.79% respectively. Also, the observed VTEC seasonal variation has good agreement with OLSE and ARMA models whereas IRI-VTEC often underestimated the observed value at each location. Hence, the deviations of IRI estimated VTEC compared to ARMA and OLSE models claim further improvements in IRI model over the Turkish region. Although IRI estimations are well accepted over the mid-latitudes but the performance over the lower mid-latitudes is not satisfactory and needs further improvement. The long-term TEC data from the TPGN network can be incorporated in the IRI under laying database with appropriate calibration for further improvement of estimation accuracy over the region.
TL;DR: In this article, the atmospheric ionospheric anomalies triggered by the Calbuco volcano on 22th and 23th of April 2015 are investigated from 50 continuous GPS stations observations. And the results of SO 2 and temperature anomalies obtained from Aura/Ozone Monitoring Instrument (OMI) and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) vertical profiles are consistent with results from GPS TEC.
TL;DR: In this paper, a statistical analysis on time error of synchronous electric clocks around the world is presented using the power grid frequency measurements recorded by the wide-area frequency monitoring network FNET/GridEye.
Abstract: Synchronous electric clocks utilize power grid frequency as their timing reference. Power grid frequency deviation away from its nominal value results in synchronous electric clocks running fast or running slow (also known as the time error). In this article, statistical analysis on time error of synchronous electric clocks around the world is firstly presented using the power grid frequency measurements recorded by the wide-area frequency monitoring network FNET/GridEye. Then, the time error correction (TEC) process provided by electric utilities is analyzed and the worldwide TEC practice is investigated. Eventually, regions of the world where electric utilities provide TEC service are differentiated from those without TEC services. Analytical results demonstrate that the average time error of synchronous electric clocks in North America seems to be less than five seconds, and it has not changed very much over the past few years. On the other hand, the identification results present that up to the end of 2016, many electric utilities around the world, especially in North America and Europe, provided the TEC service to periodically remove the accumulative time error of synchronous electric clocks.
TL;DR: In this article, a simplified source model for the 2011 Tohoku earthquake is constructed from estimates of initial ocean surface responses to approximate the vertical motions over realistic spatial and temporal scales.
Abstract: Numerical models of ionospheric coupling with the neutral atmosphere are used to investigate perturbations of plasma density, vertically integrated total electron content (TEC), neutral velocity, and neutral temperature associated with large-amplitude acoustic waves generated by the initial ocean surface displacements from strong undersea earthquakes. A simplified source model for the 2011 Tohoku earthquake is constructed from estimates of initial ocean surface responses to approximate the vertical motions over realistic spatial and temporal scales. Resulting TEC perturbations from modeling case studies appear consistent with observational data, reproducing pronounced TEC depletions which are shown to be a consequence of the impacts of nonlinear, dissipating acoustic waves. Thermospheric acoustic compressional velocities are ∼±250–300 m/s, superposed with downward flows of similar amplitudes, and temperature perturbations are ∼300 K, while the dominant wave periodicity in the thermosphere is ∼3–4 min. Results capture acoustic wave processes including reflection, onset of resonance, and nonlinear steepening and dissipation—ultimately leading to the formation of ionospheric TEC depletions “holes”—that are consistent with reported observations. Three additional simulations illustrate the dependence of atmospheric acoustic wave and subsequent ionospheric responses on the surface displacement amplitude, which is varied from the Tohoku case study by factors of 1/100, 1/10, and 2. Collectively, results suggest that TEC depletions may only accompany very-large amplitude thermospheric acoustic waves necessary to induce a nonlinear response, here with saturated compressional velocities ∼200–250 m/s generated by sea surface displacements exceeding ∼1 m occurring over a 3 min time period.
TL;DR: The upward-looking ionospheric total electron content (TEC) from the MetOp-A and TSX satellites during 2008-2015 has been used to systematically study the longitudinal variations of the topside ionosphere and plasmasphere as mentioned in this paper.
Abstract: The upward-looking ionospheric total electron content (TEC) from the MetOp-A and TSX satellites during 2008-2015 has been used to systematically study the longitudinal variations of the topside ionosphere and plasmasphere The results of this study are summarized as follows: (1) There are significant longitudinal variations in the topside ionosphere and plasmasphere at low latitudes The TEC maximum during the June solstice over the Western and Central Pacific Ocean corresponds to a TEC minimum at the same location during the December solstice; but the opposite behavior occurs over South America and the Atlantic Ocean (2) During the solstices, the relative longitudinal variations in the geomagnetic equatorial region do not have a strong dependence on local time and solar activity (3) The TEC in the winter hemisphere decreases with increasing solar activity, especially at higher altitudes and at night The topside TEC depletion with solar activity depends on longitude (4) The solstice-like longitudinal pattern lasts much longer than the equinox-like patterns, with the June solstice-pattern lasting the longest Furthermore, the equinox-like longitudinal patterns occur in March when expected, whereas they extend from the autumnal equinox until the end of October (5) The longitudinal variations of upward-looking TEC are different from the corresponding longitudinal variations of electron densities around the F2 peak and orbital altitudes This indicates that the topside ionosphere structure is strongly influenced by the physical processes in the topside region, rather than being a pure reflection of the ionospheric F2 peak structure
TL;DR: In this article, the authors report on the adaptation and modification of a climatological model, the International Reference Ionosphere (IRI-2012 model) with the use of total electron content (TEC) data derived from the Global Navigation Satellite System (GNSS), and most importantly its subsequent validation with both radio occultation from Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) and ionosonde data.
Abstract: This paper reports on the adaptation and modification of a climatological model, the International Reference Ionosphere (IRI-2012 model) with the use of total electron content (TEC) data derived from the Global Navigation Satellite System (GNSS), and most importantly its subsequent validation with both radio occultation from Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) and ionosonde data. By adjusting the solar activity indices used within the standard IRI 2012 model with the aim of minimising error differences between IRI TEC and GNSS TEC, the adjusted indices are used as drivers of the IRI 2012 model on a regional scale and results for electron density (Ne) profiles, maximum height of the F2 layer (hmF2), TEC and critical frequency of the F2 layer (foF2) generated. Validation was done by direct comparison with ionosonde and COSMIC derived data parameters. By averaging results over low, equatorial and mid-latitude regions, the modified IRI 2012 gave an improvement of about 18% in estimating TEC during the storm period of 09 March 2012. An important result observed during our candidate period of study is that COSMIC data provides maximum electron density of the F2 layer (NmF2) and hmF2 closer to ionosonde data even during the disturbed period, and is hence a suitable dataset that can be incorporated into the climatological IRI model to improve its performance. The Ne profiles from the modified IRI 2012 model accurately approximates ionosonde Ne profiles especially below 300 km altitude, but underestimates the ionosonde NmF2 and hence foF2 in mid-latitude regions. In most cases both standard and modified IRI 2012 models match COSMIC data for topside electron density representation, making the COSMIC dataset a valuable resource for the improvement of ionospheric climatological models.
TL;DR: In this article, an analysis of the TEC derived from the GPS Ionospheric Scintillation and TEC Monitor (GISTM) receiver at the Langkawi station, Malaysia, located at the geographic coordinates of 6.19°N, 99.51°E and the geomagnetic coordinates of 3.39°S, 172.42°E.
TL;DR: In this article, the authors used the global navigation satellite system (GNSS) total electron content (TEC) sequences to capture the arrival time and location of the ionosphere disturbances in response to the 2015 Typhoon Dujuan.
Abstract: The global navigation satellite system (GNSS) total electron content (TEC) sequences were used to capture the arrival time and location of the ionosphere disturbances in response to the 2015 Typhoon Dujuan. After removing the de-trended TEC variation, the clear ionosphere disturbances on the typhoon landing day could be distinguished, and these disturbances disappeared from the TEC sequences before and after the typhoon landing day. The foF2 data observed by Xiamen ionosonde station also show ionosphere disturbances. Based on the advantages of GNSS multi-point observations, the disturbances horizontal velocity in the ionosphere were estimated according to the linear theory for a dispersion relation of acoustic gravity waves (AGWs) in an isothermal atmosphere. The average horizontal velocity (
$$\sim $$
240 m/s) and the radial velocity (
$$\sim $$
287 m/s) were used in the two-dimensional grid search for the origin point on the Earth’s surface. The origin area was determined to be on the eastern side of Taiwan. Lastly, a possible physical mechanism is discussed in this study. When typhoons land on Taiwan, the severe convective storms and the drag effect from the Central Mountains create an ideal location for development of AGWs. Topographic conditions, like the high lapse rate, contribute to the formation of AGWs, which then propagates into the ionosphere altitude.