Showing papers on "TEC published in 2010"

A three-dimensional time-dependent algorithm for ionospheric imaging using GPS

Abstract: Global Positioning System (GPS) satellite receivers provide a world-wide network of phase and group delay measurements. The combination of two-frequency measurements can be used to derive the integral of the electron concentration along each satellite-to-receiver path, a parameter known as the Total Electron Content (TEC). At this stage these slant TEC data are diffi cult to interpret as they originate from a combination of a temporally changing ionosphere and spatially changing observation geometry. In this paper TEC data are inverted to evaluate the underlying distribution and time evolution of electron concentration. Accordingly, a new three-dimensional, time-dependent algorithm is presented here for imaging ionospheric electron concentration using GPS signals. The inversion results in a three-dimensional movie rather than a static image of the electron-concentration distribution. The technique is demonstrated using simulated ground-based GPS data from actual measurement geometry over Europe.

GPS TEC, scintillation and cycle slips observed at high latitudes during solar minimum

Abstract: . High-latitude irregularities can impair the operation of GPS-based devices by causing fluctuations of GPS signal amplitude and phase, also known as scintillation. Severe scintillation events lead to losses of phase lock, which result in cycle slips. We have used data from the Canadian High Arctic Ionospheric Network (CHAIN) to measure amplitude and phase scintillation from L1 GPS signals and total electron content (TEC) from L1 and L2 GPS signals to study the relative role that various high-latitude irregularity generation mechanisms have in producing scintillation. In the first year of operation during the current solar minimum the amplitude scintillation has remained very low but events of strong phase scintillation have been observed. We have found, as expected, that auroral arc and substorm intensifications as well as cusp region dynamics are strong sources of phase scintillation and potential cycle slips. In addition, we have found clear seasonal and universal time dependencies of TEC and phase scintillation over the polar cap region. A comparison with radio instruments from the Canadian GeoSpace Monitoring (CGSM) network strongly suggests that the polar cap scintillation and TEC variations are associated with polar cap patches which we therefore infer to be main contributors to scintillation-causing irregularities in the polar cap.

Quiet time ionospheric variability over Arecibo during sudden stratospheric warming events

Abstract: [1] We present observations of the F-region ionosphere over Arecibo, Puerto Rico (18.34°N, 66.75°W), during the January–February 2008 and January–February 2009 sudden stratospheric warming (SSW) events. For the first period (2008), we have used incoherent scatter radar (ISR) electron density and temperature measurements from the Arecibo Observatory (AO), as well as relative total electron content (TEC) derived from a dual-frequency GPS receiver. For the second event (2009), during which we observed the largest recorded stratospheric warming, we have used the relative GPS TEC. Our analysis indicates that the ionosphere over Arecibo exhibits perturbations after the SSW, the effects are most visible during the daytime. The strongest signatures are observed in the TEC measurements, represented by large enhancements (with respect to non SSW days), particularly during daytime hours. However, the local time dependence of these enhancements is not the same in the two events. In addition, the data show that our results are consistent with the larger than normal daytime vertical drift differences observed at the magnetic equator over Jicamarca. The electron temperature is also affected during the daytime due to changes in electron density, indicating that the electron temperatures is influenced, indirectly, by changes in planetary wave activity in the lower altitudes.

Assessment of GPS global ionosphere maps (GIM) by comparison between CODE GIM and TOPEX/Jason TEC data: Ionospheric perspective

Abstract: [1] We performed a comprehensive comparison between GPS global ionosphere map (GIM) and TOPEX/Jason (T-J) total electron content (TEC) data for the periods of 1998–2009 in order to assess the performance of GIM over the global ocean where the GPS ground stations are very sparse. Using the GIM model constructed by the Center for Orbit Determination in Europe at the University of Bern, the GIM TEC values were obtained along the T-J satellite orbit at specific locations and times of measurements and then binned into various geophysical conditions for direct comparison with the T-J TEC. On the whole, the GIM model was able to reproduce the spatial and temporal variations of the global ionosphere as well as the seasonal variations. However, the GIM model was not accurate enough to represent the well-known ionospheric structures such as the equatorial anomaly, the Weddell Sea Anomaly, and the longitudinal wave structure. Furthermore, a fundamental limitation of the model seems to be evident in the unexpected negative differences (i.e., GPS T-J) at night represent the plasmaspheric contribution to GPS TEC, which is maximized, reaching up to 100% of the corresponding T-J TEC values in the early morning sector. In particular, the relative differences decreased with increasing solar activity, and this may indicate that the plasmaspheric contribution to the maintenance of the nighttime ionosphere does not increase with solar activity, which is different from what we normally anticipate.

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.

A global positioning system–based climatology for the total electron content in the ionosphere

Abstract: [1] The total electron content (TEC) in the ionosphere is an important factor in the propagation of radio waves. Since 1998 the coverage global positioning system (GPS) observations has been sufficient to monitor the TEC globally. We have used the global ionosphere maps provided by the Jet Propulsion Laboratory to devise a new ionosphere climatology (NIC09). The climatology fits the GPS maps to 4.5 TECU, more than twice as well as the IRI2007 climatology. The use of the global mean TEC as the input parameter reduces scaling errors and long-term mean errors in the model. For climatic studies of sea level measured by satellite radar altimeters, it is necessary to go back before 1998. During the earlier years of radar altimetry, we use TOPEX dual-frequency altimeter data to reconstruct the global mean TEC or use the solar radio flux (F10.7) as a proxy. The comparison of dual-frequency altimeter data with the GPS maps demonstrated that about 8% of the TEC extends above 1350 km and about 14% above 800 km. The root mean square error of the NIC09 climatology was found to be approximately 18% of the TEC, compared to 14% for the GPS TEC maps, and 35% for IRI2007.

A general approach in evaluating and optimizing thermoelectric coolers

Abstract: In this paper, a general approach in evaluating and optimizing thermoelectric cooler (TEC) performance is presented. Analytical solutions for performance parameters such as cooling capacity and device temperature at both the TEC pellet level and module level are derived in simplified formulation based on the TEC thermal balance equations. The major advantage of the present solutions lies in the fact that the optimal TEC performances can be determined in a straightforward manner, as against the iterative procedure mostly reported in literature. In addition, the module level analysis facilitates the TEC optimization without prior knowledge of pellet details and dimensions. The validity of the present analysis approach is demonstrated by examining TECs of different cooling capacities. Furthermore, the optimization of device temperature for microprocessors under TEC enhanced air cooling and liquid cooling conditions is examined. The effect of hot side to cold side temperature difference is discussed and the thermal resistances at both hot and cold sides are also analyzed within given performance parameters.

Ionospheric response to the initial phase of geomagnetic storms: Common features

Abstract: [1] Ionospheric responses to the initial phases of three geomagnetic storms: 2–5 April 2004, 7–9 November 2004, and 13–16 December 2006, were compared using both ground-based GPS total electron content (TEC) data and coupled magnetosphere ionosphere thermosphere (CMIT) model simulations. The onset times for these storms all occurred at local daytime in the North American sector. This similarity of onset times and other factors resulted in some common features in their ionospheric response. These common features include (1) enhanced TEC (positive response) at low and middle latitudes in the daytime, (2) depleted TEC (negative response) around the geomagnetic equator in the daytime, (3) a north-south asymmetry in the positive response as the northern hemispheric response appeared to be more pronounced, and (4) negative response at high latitudes as the storms progressed. The CMIT model captured most of these features. Analysis of model results showed that storm-time enhancements in the daytime eastward electric field were the primary cause of the observed positive storm effects at low and middle latitudes as well as the negative response around the geomagnetic equator in the daytime. These eastward electric field enhancements were caused by the penetration of high latitude electric fields to low latitudes during southward interplanetary magnetic field (IMF) periods, when IMF Bz oscillated between southward and northward direction in the initial, shock phase of the storms. Consequently, the ionosphere was lifted up at low and middle latitudes to heights where recombination was weak allowing the plasma to exist for a long period resulting in higher densities. In addition, the CMIT model showed that high-latitude negative storm responses were related to the enhancements of molecular nitrogen seen in TIMED/Global Ultraviolet Imager observations, whereas the negative storm effects around the geomagnetic equator were not associated with thermospheric composition changes; they were the result of plasma transport processes.

Temporal and spatial precursors in the ionospheric global positioning system (GPS) total electron content observed before the 26 December 2004 M9.3 Sumatra–Andaman Earthquake

Abstract: [1] We report seismo-ionospheric precursors of anomalous decreases in the total electron content (TEC) appearing day 5 prior to an M9.3 earthquake, the largest one in the last five decades, which occurred in Sumatra-Andaman, Indonesia on 26 December 2004. Sequences of global ionosphere maps of the TEC derived from worldwide ground-based receivers of the global positioning system (GPS) are used to statistically study the temporal and spatial precursors of the earthquake. It was found that the temporal precursor of the GPS TEC around the epicenter was significantly reduced during the afternoon period on d 5 before the earthquake. The spatial precursors prominently, persistently, and simultaneously appear around the epicenter and its conjugate areas of the Sumatra-Andaman earthquake.

Tec‐Kinase‐Mediated Phosphorylation of Fibroblast Growth Factor 2 is Essential for Unconventional Secretion

Abstract: Fibroblast growth factor 2 (FGF2) is a potent mitogen that is exported from cells by an endoplasmic reticulum (ER)/Golgi-independent mechanism. Unconventional secretion of FGF2 occurs by direct translocation across plasma membranes, a process that depends on the phosphoinositide phosphatidylinositol 4,5-biphosphate (PI(4,5)P2) at the inner leaflet as well as heparan sulfate proteoglycans at the outer leaflet of plasma membranes; however, additional core and regulatory components of the FGF2 export machinery have remained elusive. Here, using a highly effective RNAi screening approach, we discovered Tec kinase as a novel factor involved in unconventional secretion of FGF2. Tec kinase does not affect FGF2 secretion by an indirect mechanism, but rather forms a heterodimeric complex with FGF2 resulting in phosphorylation of FGF2 at tyrosine 82, a post-translational modification shown to be essential for FGF2 membrane translocation to cell surfaces. Our findings suggest a crucial role for Tec kinase in regulating FGF2 secretion under various physiological conditions and, therefore, provide a new perspective for the development of a novel class of antiangiogenic drugs targeting the formation of the FGF2/Tec complex.

TEC response to the 2008 Wenchuan Earthquake in comparison with other strong earthquakes

Abstract: We registered near-field global positioning system (GPS) total electron content (TEC) response to the Wenchuan Earthquake on 12 May 2008. The Wenchuan Earthquake (magnitude 8.0) occurred at 06:28 UT as the result of motion on a northeast striking reverse fault (thrust fault) on the northwestern margin of the Sichuan Basin. The earthquake reflects tectonic stresses resulting from the convergence of crustal material slowly moving from the high Tibetan Plateau, to the west, against a strong crust underlying the Sichuan Basin and southeastern China. We found that intensive N-shaped shock-acoustic waves with a plane waveform and with a half-period of about 200 s propagated south-eastwards with a velocity 580 m s-1 for a distance of about 1000 km from the epicentre. The wavefront of N-shaped disturbance was parallel with the earthquake rupture direction (from southwest to northeast). The main directional lobe of shock-acoustic wave emitter is directed southeastwards, i.e. transversely to the rupture. We speculate that the above properties of TEC response are determined by the geodynamics of the Wenchuan Earthquake. No noticeable TEC response to that earthquake was found in far-field regions in South Korea and Japan. We compared TEC response to the 2008 Wenchuan earthquake with other strong earthquakes.

Ionosphere around equinoxes during low solar activity

Abstract: [1] The seasonal behaviors of the ionosphere have been investigated for several decades, but the differences of the ionosphere between the March and September equinoxes are still an open question. In this analysis we utilize the data of ionospheric electron density (Ne) profiles from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission radio occultation measurements, total electron density (TEC) from TOPEX and Jason-1, and TEC from Global Positioning System (GPS) receivers as well as global ionosonde measurements of the F2 layer peak electron density (NmF2) to investigate the behaviors of the daytime ionosphere around equinoxes during low solar activity (LSA). The analysis reveals that during LSA the equinoctial asymmetry in ionospheric plasma density is mainly a low-latitude phenomenon. The differences of equinoctial TEC and NmF2 have considerable amplitudes at low latitudes in both hemispheres and less significant at higher latitudes. With increasing altitude, the asymmetry in COSMIC Ne becomes weaker in the Southern Hemisphere, and the northern pronounced asymmetry regions move toward the magnetic equator. The ionospheric equinoctial asymmetry may be considered as a manifestation of the annual variation, whose annual phase significantly shifts away from the solstices. The F layer peak height (hmF2) extracted from COSMIC Ne profiles also shows an equinoctial asymmetry at low latitudes, indicating the existence of equinoctial differences in low-latitude neutral winds, specifically in the Northern Hemisphere. It reveals that, besides the important effect of the neutral wind, other processes should play roles in the forming of the observed equinoctial asymmetry in the ionosphere.

JPL/USC GAIM: On the impact of using COSMIC and ground‐based GPS measurements to estimate ionospheric parameters

Abstract: [1] The University of Southern California (USC) and the Jet Propulsion Laboratory (JPL) have jointly developed the Global Assimilative Ionospheric Model (GAIM) to monitor space weather, study storm effects, and provide ionospheric calibration for space weather applications. JPL/USC GAIM is a physics-based 3-D data assimilation model that uses both four-dimensional variational analysis and Kalman-filter techniques to solve for the ion and electron density state and key drivers such as equatorial electrodynamics, neutral winds, and production terms. Here we report on GAIM Kalman filter-based assimilation results using ground-based GPS and COSMIC-derived total electron count (TEC) measurements. We find that assimilating COSMIC measurements into GAIM improves critical ionospheric parameters such as NmF2 and HmF2. Assimilating COSMIC data produces higher-accuracy vertical electron density profile "shapes," as verified by comparisons to independent electron density profiles measured at Arecibo, Jicamarca, and Millstone Hill incoherent scatter radar (ISR). We also find significant improvement in global vertical TEC (VTEC) maps when assimilating COSMIC measurements, verified by comparing GAIM output with VTEC measurements from the Jason ocean altimeter. For 3 days in June 2006, improvement in accuracy compared to ground-data-only assimilation is found to be 30%, 28%, and 44%, respectively.

GPS TEC response to the 22 July 2009 total solar eclipse in East Asia

Abstract: [1] The longest total solar eclipse of this century occurred in East and South Asia on 22 July 2009. The eclipse was accompanied with a medium magnetic storm, whose main phase onset occurred ∼27 min after the passage of the Moon's umbral shadow. Using TEC data from 60 GPS stations, we construct differential TEC maps to investigate the ionosphere response to the solar eclipse in central China in the range of 26°N–36°N, 108°E–118°E (i.e., the magnetic latitude 15°N–25°N). During the eclipse's totality, a “shadow” in the ionosphere shown as TEC depletion area was formed ∼100 km south of the Moon's umbral path with a maximum decrease of 5 TECU. The TEC depletion area moved eastward, following the movement of the totality area with a time lag of ∼10 min. Enhancements of TEC due to the storm are observed after the main phase onset. The relative drop of TEC due to the solar eclipse is evidently larger at lower latitudes than that at higher ones and around noontime than that in the morning. By modeling work, we find that the latitudinal dependence of the TEC response may result from latitudinal variation of magnetic inclination, which influences the diffusion of ionization among different layers. Besides, the local time dependence of TEC response is closely related to the local time variation of background atmosphere density, which affects the electron loss efficiency in the ionosphere.

Latitudinal variations in Saturn's ionosphere: Cassini measurements and model comparisons

Abstract: [1] We present a study of latitudinal variations in Saturn’s ionosphere using Cassini Radio Science Subsystem (RSS) measurements and Saturn‐Thermosphere‐Ionosphere‐Model (STIM) simulations. On the basis of Cassini RSS observations, the peak electron density (NMAX) and the total electron content (TEC) both exhibit a clear increase with latitude, with a minimum at Saturn’s equator. When compared with these RSS trends, current model simulations overestimate NMAX and TEC at low latitudes and underestimate those parameters at middle and high latitudes. STIM is able to reproduce the RSS values for NMAX and TEC at low latitude when an additional low‐latitude loss process, such as a water influx, is introduced near Saturn’s equator. The lack of auroral precipitation processes in the model likely explains some model/data discrepancies at high latitude; however, most of the high‐latitude RSS data are from latitudes outside of Saturn’s typical main auroral oval. Using Cassini RSS electron density altitude profiles combined with ion density fractions and neutral background parameters calculated in STIM, we also present estimates of the latitudinal variations of Saturn’s Pedersen conductance, SP. We find SP to be driven by ion densities in Saturn’s lower ionosphere and to exhibit a latitudinal trend with a peak at mid‐latitude. Model calculations are able to reproduce low‐latitude conductances when an additional loss process is introduced, as before, but consistently underestimate most of the mid‐ and high‐latitude conductances derived from Cassini observations, perhaps indicating a missing ionization source within the model.

Verification of the concept of seismoionospheric coupling under quiet heliogeomagnetic conditions, using the Wenchuan (China) earthquake of May 12, 2008, as an example

Abstract: The variability of the ionosphere during April–May, 2008, has been analyzed in detail in order to reveal anomalous variations related to seismic activity, initiated by the strongest Wenchuan earthquake (M = 7.9) in the Sichuan province on May 12, 2008. Information about the total electron content (TEC) from the network of GPS receivers in the earthquake region, the global IONEX TEC maps, and the reconstructed vertical profiles of electron density according to the data of GPS receivers were used as a data source. The spatial and time localization of the observed anomalies, their morphological features, and the absence of geomagnetic disturbances during the observation period undoubtedly demonstrate that the observed variations were caused by seismic activity.

Time delay and duration of ionospheric total electron content responses to geomagnetic disturbances

Abstract: . Although positive and negative signatures of ionospheric storms have been reported many times, global characteristics such as the time of occurrence, time delay and duration as well as their relations to the intensity of the ionospheric storms have not received enough attention. The 10 years of global ionosphere maps (GIMs) of total electron content (TEC) retrieved at Jet Propulsion Laboratory (JPL) were used to conduct a statistical study of the time delay of the ionospheric responses to geomagnetic disturbances. Our results show that the time delays between geomagnetic disturbances and TEC responses depend on season, magnetic local time and magnetic latitude. In the summer hemisphere at mid- and high latitudes, the negative storm effects can propagate to the low latitudes at post-midnight to the morning sector with a time delay of 4–7 h. As the earth rotates to the sunlight, negative phase retreats to higher latitudes and starts to extend to the lower latitude toward midnight sector. In the winter hemisphere during the daytime and after sunset at mid- and low latitudes, the negative phase appearance time is delayed from 1–10 h depending on the local time, latitude and storm intensity compared to the same area in the summer hemisphere. The quick response of positive phase can be observed at the auroral area in the night-side of the winter hemisphere. At the low latitudes during the dawn-noon sector, the ionospheric negative phase responses quickly with time delays of 5–7 h in both equinoctial and solsticial months. Our results also manifest that there is a positive correlation between the intensity of geomagnetic disturbances and the time duration of both the positive phase and negative phase. The durations of both negative phase and positive phase have clear latitudinal, seasonal and magnetic local time (MLT) dependence. In the winter hemisphere, long durations for the positive phase are 8–11 h and 12–14 h during the daytime at middle and high latitudes for 20≤Ap

Response of the thermosphere and ionosphere to an ultra fast Kelvin wave

Abstract: [1] Ultra Fast Kelvin (UFK) waves are eastward propagating planetary waves with periods between 3 and 5 days, which are capable of penetrating into the thermosphere and ionosphere where they may modulate phenomena occurring in this region. A sensitivity study has been conducted to examine the effect of an Ultra Fast Kelvin wave on the thermosphere and ionosphere using the NCAR Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) under June solstice solar minimum conditions. It is found that realistic ultra fast Kelvin waves with amplitudes in the MLT region of approximately 20–40 m s−1 in zonal wind fields and 10–20 K in temperature fields, can result in approximately 8–12% perturbations in hourly neutral density at 350 km, as well as hourly total electron content (TEC) perturbations of 25–50% in regions corresponding to the equatorial ionization anomalies (EIAs), with the largest relative changes resolved during the nighttime due to the lower electron densities. The electrodynamical calculations in the model were then disabled to identify the relative importance of ionospheric electrodynamics and direct wave propagation in generating the aforementioned changes. The subsequent results show that changes in thermospheric neutral density are relatively insensitive to the presence of the dynamo electric field, while UFK wave modulation of the dynamo accounts for most of the TEC perturbations due to changes of ionospheric vertical plasma drift.

Effects of typhoon Matsa on ionospheric TEC

Abstract: The effects of typhoon Matsa on the ionosphere are studied by using GPS-TEC data observed at about 50 GPS stations It is shown that the ionosphere has already influenced and TEC tends to increase before the landing of Matsa, and the difference of TEC from its monthly median over the typhoon area is about 5 TECU With the landing of Matsa, both the magnitude and the area of increased TEC decrease One day after the landing of Matsa, TEC reaches its minimum and is lower than the monthly median In comparison of TEC along the typhoon’s path with that along three reference paths far from the typhoon, it is found that typhoon’s impact on TEC can be fully distinguished The evolution of TEC variation has the same tendency as reported typhoon-induced foF2

Solar proxies pertaining to empirical ionospheric total electron content models

Abstract: [1] Solar proxies and indices exhibiting extreme ultraviolet (EUV) irradiance that affects the ionospheric total electron content (TEC) were examined through training an artificial neural network (ANN). A TEC database was constructed from a dense GPS receiver network over Japan from April 1997 to March 2008, covering an entire 11 year solar activity period. In empirical models of upper atmospheric parameters, such as the International Reference Ionosphere model and the Mass Spectrometer and Incoherent Scatter thermospheric model, the 10.7 cm solar radio flux (F10.7) or the sunspot number (R) is used as a proxy for determining the solar activity. In the present study, ANN training for predicting TEC as a target parameter was done by including new solar proxies/indices in the input space that were based on direct measurements of solar EUV/UV flux, SOHO_SEM26–34 (the integrated 26–34 nm EUV emission), and Mg II cwr (the core-to-wing ratio of Mg II 280 nm line), as well as the traditional indices F10.7 and R. Root mean square errors (RMSEs) of TEC were compared after the training was completed using a variety of combinations of solar proxies. When a single proxy was used, SOHO_SEM26–34 yielded the smallest RMSE, or it was the best proxy for modeling ionospheric TEC. Further, general improvements were obtained by combining different types of proxies and short- and long-term means of them. The best combination was the 3 day smoothed daily, 7 day and 27 day backward mean values of Mg II cwr, SOHO_SEM26–34, and the 10.7 cm radio flux.

Scintillation‐producing Fresnel‐scale irregularities associated with the regions of steepest TEC gradients adjacent to the equatorial ionization anomaly

Abstract: [1] Using ground-based GPS and digital ionosonde instruments, we have built up at latitudes of the equatorial ionization anomaly (EIA), in the Brazilian sector, a time-evolving picture of total electron content (TEC), L-band amplitude scintillations, and F region heights, and we have investigated likely reasons for the occurrence or suppression of equatorial scintillations during the disturbed period of 18–23 November 2003. During the prestorm quiet nights, scintillations are occurring postsunset, as expected; however, during the storm time period, their spatial-temporal characteristics and intensity modify significantly owing to the dramatic changes in the ionospheric plasma density distribution and in the temporal evolution of TEC. The two-dimensional maps showing both TEC and amplitude scintillations revealed strong evidence of turbulences at the Fresnel length (causing scintillations) concurrent with those regions of steepest TEC gradients adjacent to the crests of the EIA. The largest density gradients have been found to occur in an environment of increased background electron density, and their spatial distribution and location during the disturbed period may differ significantly from the magnetic quiet night pattern. However, in terms of magnitude the gradients at equatorial and low latitudes appear to not change during both magnetic quiet and disturbed conditions. The scenarios for the formation or suppression of scintillation-producing Fresnel-scale irregularities during the prestorm quiet nights and disturbed nights are discussed in view of different competing effects computed from numerical simulation techniques.

Accuracy analysis of the GPS instrumental bias estimated from observations in middle and low latitudes

Abstract: With one bias estimation method, the latitude-related error distribution of instrumental biases estimated from the GPS observations in Chinese middle and low latitude region in 2004 is analyzed statistically It is found that the error of GPS instrumental biases estimated under the assumption of a quiet ionosphere has an increasing tendency with the latitude decreasing Besides the asymmetrical distribution of the plasmaspheric electron content, the obvious spatial gradient of the ionospheric total electron content (TEC) along the meridional line that related to the Equatorial Ionospheric Anomaly (EIA) is also considered to be responsible for this error increasing The RMS of satellite instrumental biases estimated from mid-latitude GPS observations in 2004 is around 1 TECU (1 TECU = 1016/m2), and the RMS of the receiver's is around 2 TECU Nevertheless, the RMS of satellite instrumental biases estimated from GPS observations near the EIA region is around 2 TECU, and the RMS of the receiver's is around 3–4 TECU The results demonstrate that the accuracy of the instrumental bias estimated using ionospheric condition is related to the receiver's latitude with which ionosphere behaves a little differently For the study of ionospheric morphology using the TEC derived from GPS data, in particular for the study of the weak ionospheric disturbance during some special geo-related natural hazards, such as the earthquake and severe meteorological disasters, the difference in the TEC accuracy over different latitude regions should be paid much attention

Total electron content in the Mars ionosphere: Temporal studies and dependence on solar EUV flux

Abstract: [1] Total electron content (TEC) derived from radar signal distortions is a useful tool in probing the ionosphere of Mars. We consider 26 months of data from the subsurface mode of the Mars Express MARSIS instrument and confirm that the TEC dependence on solar zenith angle (SZA) approximately matches Chapman theory. After detrending this dependence, we find no clear trend with Martian season or dust activity but find that disturbed solar and space weather conditions can produce prolonged higher TEC values and that isolated solar energetic particle events are coincident with short-lived increases in TEC of ∼1015 m−2 at all SZAs. We present the first comparison between TEC and directly measured solar EUV flux in the 30.4 nm He-II line. We find that the relationship between TEC and both He-II line irradiance and F10.7 solar radio flux (a long-used EUV proxy) can be expressed as power laws with exponents of 0.54 and 0.44, respectively, in approximate agreement with Chapman theory.

Ionosphere response to recurrent geomagnetic activity: Local time dependency

Abstract: [1] Observations of global positioning system total electron content (TEC) and in situ electron densities at altitudes of ~350-370 km from the CHAMP satellite are used to illustrate the local time and latitude dependence of 9 day periodicities in the ionosphere due to recurring high-speed solar wind streams and geomagnetic activity during 2005. A local time dependence is found, with nighttime TEC oscillations concentrated at high latitudes and close to ±40% of background levels. The largest oscillations in daytime TEC occur at midlatitudes and are ±25% of background levels. Furthermore, the daytime response is generally symmetric about the geomagnetic equator with anticorrelation between high and low latitudes, whereas at night the high-latitude Northern Hemisphere is generally in-phase with low latitudes and anticorrelated with the high-latitude Southern Hemisphere. A combination of enhanced equatorward neutral winds and changes in neutral composition are thought to be the primary mechanisms responsible for the observed ionospheric response. Although similar mechanisms are driving the response, the local time dependency arises because of the presence (lack) of photoionization during the daytime (nighttime). Similar trends are observed in CHAMP in situ electron densities; however, the oscillations at a near-constant altitude are ~10-15% larger than the TEC oscillations. Additionally, the CHAMP observations reveal possible variations in the strength of the equatorial ionization anomaly, indicating that disturbance dynamo electric fields may also contribute to the ionospheric response to recurrent geomagnetic activity. The results presented are the first to reveal the significant differences between the daytime and nighttime response of the ionosphere to periodic forcing from solar wind high-speed streams.

Seismo-ionospheric anomalies of the GPS-TEC appear before the 12 May 2008 magnitude 8.0 Wenchuan Earthquake

Abstract: In this paper, the total electron content (TEC) of Global Ionosphere Maps (GIMs) is normalized and employed to study the seismo-ionospheric anomalies at the time of the 12 May 2008 M 8.0 Wenchuan Earthquake. The space weather conditions are taken into account. It is found that remarkable reductions appear locally around the epicentre and their conjugate points during the daytime of 29 April and 6-10 May 2008. A global study and a strict criterion are applied to detect anomalies. Results show that the anomalies on 29 April and 6 and 7 May 2008, which are respectively days 13, 6 and 5 before, are possibly related to the earthquake. The conjugate signature implies the seismo-generated electric field is essential.