Showing papers on "Total electron content published in 2008"

Estimation of single station interfrequency receiver bias using GPS-TEC

Abstract: [1] Dual-frequency Global Positioning System (GPS) receivers present a plausible and cost-effective way of computing Total Electron Content (TEC) For accurate estimates of TEC, frequency-dependent satellite and receiver instrumental biases should be removed from GPS measurements properly Although instrumental satellite bias values are widely available through the internet from various International GPS Service (IGS) analysis centers, receiver biases (also known as differential code biases or interfrequency biases) are provided only for a very few GPS stations and a select number of days This makes it very difficult to compute TEC for a single station In this study, an online, single station receiver bias estimation algorithm, IONOLAB-BIAS, is developed and implemented to obtain daily and monthly averages of receiver bias The algorithm is successfully applied to both quiet and disturbed days of the ionosphere for stations positioned in high-latitude, midlatitude, and equatorial regions The receiver bias estimates are compared with two of the basic methods in the literature that can be applied off-line, and also with the receiver bias values provided from the IGS centers for a select number of stations It is observed that IONOLAB-BIAS is in excellent accordance with the sparse estimates from the IGS centers for all ionospheric states and regions IONOLAB-BIAS has a high potential to be an alternative receiver bias computation algorithm with its ease of implementation and accurate estimates for any single station GPS-TEC

Wavenumber-4 patterns of the total electron content over the low latitude ionosphere

Abstract: [1] The global ionospheric maps (GIMs) produced by JPL are used to investigate the longitudinal structure of the low latitude ionosphere. As a proxy of the ionization parameter at low latitudes, the latitudinally integrated total electron content (ITEC) is first extracted from low latitude GIMs and then Fourier filtered to obtain the wavenumber-4 components. We then study in detail the diurnal, seasonal and solar cycle variations of the wave patterns. It is found that the wavenumber-4 patterns are intense and well developed in boreal summer and early boreal autumn, but quite weak in boreal winter. This seasonal variation is consistent with that of the zonal wind of the non-migrating tide mode DE3. We also found that the wavenumber-4 patterns shift eastward with a shifting speed that is smaller in daytime than at night. This is attributed to the contribution of both the eastward propagation of DE3 in E-region and the zonal E ×B ion drifts in F-region. Our results support the suggestion that the longitudinal wavenumber-4 structure of the low latitude ionosphere should be originated from the non-migrating tide mode DE3.

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%.

Observations and simulations of the ionospheric and thermospheric response to the December 2006 geomagnetic storm : Initial phase

Abstract: [1] We have investigated the thermospheric and ionospheric response to the 14–15 December 2006 geomagnetic storm using a Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 20 model simulation In this paper we focus on observations and simulations during the initial phase of the storm (about 8 h), when the shock was driving changes in geospace The global ionospheric maps of total electron content (TEC), ionosonde data at four stations and Millstone Hill incoherent scatter radar (ISR) observations are compared with the corresponding simulation results from the CMIT model The observations showed significant positive storm effects occurred in the Atlantic sector after the onset of this storm The CMIT model is able to capture the temporal and spatial variations of the ionospheric storm effects seen in the GPS TEC observations, although the model slightly underestimates the daytime positive ionospheric storm in the South American sector The simulations are also in agreement with the ionosonde and ISR ionospheric measurements Term analysis of the ion continuity equation demonstrates that changes in the electric fields play a dominant role in generating the observed ionospheric positive storm effect in the American sector during the initial phase, although neutral winds and composition changes also contribute The difference in the strength of the enhancements over North and South America can be explained by the slope of the topside electron density profiles in the two hemispheres In the southern hemisphere electron densities decrease slowly with altitude, whereas the decrease is much more rapid in the northern (winter) hemisphere The electric fields, therefore, cannot cause large increases in electron density by uplifting the plasma, so positive storm effects are small in the southern hemisphere compared with the northern hemisphere, even though the increase in hmF2 is greater in the southern hemisphere Nighttime changes in electron density in other longitude sectors are small, because the topside electron densities also decrease slowly with altitude at night

Ionosphere response to solar wind high-speed streams

Abstract: [1] We present 9- and 7-day periodic oscillations in the global mean Total Electron Content (TEC) from 1 January 2005 to 31 December 2006. Spectral analysis indicates that the pronounced periodicities of 9 and 7 days observed in TEC are associated with variations in solar wind high-speed streams and geomagnetic activity. Neutral temperature and winds near 250 km, measured by a Fabry-Perot Interferometer at Resolute Bay, also exhibit 9- and 7-day periodicities. These pronounced periodicities support simultaneous observations of 9- and 7-day periodicities in thermosphere neutral density (Lei et al., 2008a; Thayer et al., 2008). It is anticipated that the ionospheric response at 9 and 7 days represents some combination of effects due to chemical loss, neutral winds, and disturbance dynamo-driven electric fields.

Geomagnetic dependence of ionospheric disturbances induced by tsunamigenic internal gravity waves

Abstract: SUMMARY A series of ionospheric anomalies following the Sumatra tsunami has been recently reported in the literature. These anomalies show the signature in the ionosphere of tsunami-generated internal gravity waves (IGW) propagating in the neutral atmosphere over the ocean. All these anomalies, observed in the total electron content (TEC) measured by GPS or altimeters, show geographical heterogeneity in the perturbed TEC amplitude and suggest a dependence on geomagnetic latitude. This latitudinal dependence has been taken into account in the previous 3-D modelling used for the interpretation of the TEC Topex and Jason data. Here we present an accurate description of the ocean‐atmosphere‐ionosphere coupling method, and focus on the properties of the propagation of tsunamigenic IGW in the neutral atmosphere and their interaction with the ionospheric plasma. The analytical dependence on the geomagnetic field in the neutral‐plasma coupling discussed in detail and quantitative modelling is used to describe the propagation of a simple tsunami wave at the global scale. What emphasize the role of geomagnetic field within the neutral‐plasma coupling at the equatorial and mid-latitude regions. The results, presented here in terms of electron density and TEC variations, show a strong geometric dependance involving the magnetic field inclination and the propagative direction of the tsunami. If the strongest electron density and TEC perturbations are located around −15 ◦ , 0 ◦ and 15 ◦ North, the structure and amplitude of the modelled perturbation changes in the two

Estimate of higher order ionospheric errors in GNSS positioning

Abstract: [1] Precise navigation and positioning using GPS/GLONASS/Galileo require the ionospheric propagation errors to be accurately determined and corrected for. Current dual-frequency method of ionospheric correction ignores higher order ionospheric errors such as the second and third order ionospheric terms in the refractive index formula and errors due to bending of the signal. The total electron content (TEC) is assumed to be same at two GPS frequencies. All these assumptions lead to erroneous estimations and corrections of the ionospheric errors. In this paper a rigorous treatment of these problems is presented. Different approximation formulas have been proposed to correct errors due to excess path length in addition to the free space path length, TEC difference at two GNSS frequencies, and third-order ionospheric term. The GPS dual-frequency residual range errors can be corrected within millimeter level accuracy using the proposed correction formulas.

Superposed epoch analysis of the dayside ionospheric response to four intense geomagnetic storms

Abstract: [1] Prompt daytime ionospheric responses are presented for the following four intense geomagnetic storms: 29 October 2003, 30 October 2003, 20 November 2003, and 7 November 2004. We perform a superposed epoch analysis of the storms by defining the start time of the epoch when the Kan-Lee interplanetary electric field (proportional to the reconnection electric field) first reaches 10 mV/m during a period of continuously southward Bz. Measurements from the GPS receiver onboard the CHAMP satellite at 400 km altitude indicate significant low- to middle-latitude daytime total electron content (TEC) increases above the satellite within 1–2 h of the defined start time for three of the storms (∼1400 local solar time). The 20 November 2003 data follow a different pattern: the largest TEC increases appear several hours (∼5–7) following the interplanetary magnetic field Bz event onset. TEC data obtained from ground-based GPS receivers for the November 2003 storm tend to confirm a “late” TEC increase for this storm at ∼1400 LT. Estimates of vertical plasma uplift near the equator at Jicamarca longitudes (∼281 E) using the dual-magnetometer technique suggest that variability of the timing of the TEC response is associated with variability in the prompt penetration of electric fields to low latitudes. It is also found that for the November 2003 magnetic storm the cross-correlation function between the SYM-H index and the interplanetary electric field reached maximum correlation with a lag time of 4 h. Such a large lag time has never been noted before. The long delays of both the ionosphere and magnetosphere responses need to be better understood.

A statistical study of large-scale traveling ionospheric disturbances observed by GPS TEC during major magnetic storms over the years 2003–2005

Abstract: [1] In this paper, we plot two-dimensional total electron content (TEC) perturbation maps and investigate the statistical characteristics of large-scale traveling ionospheric disturbances (LSTIDs) during major magnetic storms from 2003 to 2005. The TEC data were obtained from more than 600 GPS receivers in North America within the geographical latitudes of 25°N–55°N. We found a total of 135 cases of LSTIDs, with amplitudes of up to 3.5 TECU and a maximum front width of ∼4000 km. The mean value of periods, horizontal velocities, and azimuths are 1.8 h, 300 m/s, and 187° (7° west of south), respectively. The mean velocity is obviously slower than that observed at lower latitudes such as Japan. Of all the 135 LSTID events, 35 cases (26%) occurred in the nighttime with their possible source within the region of North America, according to the variation of magnetic H component observed in this region. In addition, the occurrence of LSTIDs peaks at 1200 LT and at 1900 LT. It is also pointed out that the UT dependence of the occurrence of auroral geomagnetic disturbances plays a major role in the forming of UT and LT dependence of the occurrence of LSTIDs observed at midlatitudes.

TEC anomalies—Local TEC changes prior to earthquakes or TEC response to solar and geomagnetic activity changes?

Abstract: A number of papers have reported on deviations of daily values of the maximum electron concentration of the ionospheric F2 layer and/or total electron content (TEC) in the vicinity of an earthquake’s epicenter some time prior to the quake. Owing to the importance of this problem, a question of a “locality” of those effects is emerging. To study this issue we have developed a method based on the calculation of global electron content and of local electron content in “check-region” with low seismic activity. The effect of TEC day-to-day changes before strong earthquakes is analyzed in this work. It is shown that in some cases this effect might be a reflection of global changes of the ionization caused by the 27-day variations as well as other fast alterations due to solar and geomagnetic activity changes. We discuss the problem of certain data corrections that permit local changes to be distinguished from global ones.

An empirical orthogonal function model of total electron content over China

Abstract: [1] In this paper a climatology model of total electron content (TEC) over China has been developed on the basis of the empirical orthogonal function (EOF) analysis using Global Positioning System (GPS) data from the International Global Navigation Satellite System Service (IGS) and Crust Movement Observation Network of China (CMONOC) covering almost the whole Chinese sector during 1996–2004. The model well represents observational data with mean bias of −0.00994 TECU (1 TECU = 1.0 × 1016 el· m−2) and standard deviation of 5.42 TECU. Then the EOF model and IRI have been used in three-dimensional variational (3DVAR) data assimilation experiments separately, and results reveal that the ability of assimilation nowcasting for the EOF model is better as it provides a more authentic background.

Oscillations of global mean TEC

Abstract: [1] The global mean of TEC (total electron content) is retrievable from signals of the global navigation satellite system (GNSS) received by the world-wide network of GNSS ground stations. The time series of global mean TEC show dominant fluctuations with periods of 11-year, 1-year, 0.5-year, and 27-day. The annual variation of global mean TEC confirms the unsolved puzzle that "there is more ionosphere in January than in July". Global mean TEC varies from +12.5% in January to -12.5% in July while the annual variation of solar irradiance due to the orbital eccentricity of the Earth is only ±3.3%. The oscillations of global mean TEC can be related to oscillations of the ionizing, solar EUV (extreme ultraviolet) irradiance as represented by the Mg II index. Comparing the dominant spectral components of EUV and global mean TEC, we find different amplification factors ATEC/AMg II for each spectral component. Depending on periodicity, a 1% -variation of solar EUV irradiance can induce variations of global mean TEC from 3.8 to 22%. While the 27-day oscillation of EUV perfectly correlates with the 27-day oscillation of global mean TEC (r = 0.90), we find a weak correlation (r = 0.31) between the 13.5-day oscillations of global mean TEC and EUV. The amplitude modulations of the spectral components of global mean TEC during solar cycle-23 are derived, and influences of atmospheric dynamics are discussed. The typical response of global mean TEC to solar flares is derived by a superposed epoch analysis of 240 major flares of solar cycle-23 and gives new insights into the Sun-Earth connection chain.

Prestorm enhancements in NmF2 and total electron content at low latitudes

Abstract: [1] The enhancement of electron concentrations in the ionosphere before geomagnetic storms is one of the open questions. Using ionosonde observations and total electron content (TEC) from Global Positioning System (GPS) measurements along longitude 120°E, we analyzed three low latitude pre-storm enhancement events that occurred on 21 April (day 111) 2001, 29 May (day 149) 2003, and 22 September (day 265) 2001, respectively, in the Asia/Australia sector. All three events (and other two cases on 9 August 2000 and 10 May 2002) show quite similar features. The strong prestorm enhancements during these events are simultaneously presented in foF2 and TEC and enhancements have latitudinal dependence, tending to occur at low latitudes with maxima near the northern and southern equatorial ionization anomaly (EIA) crests and depletions in the equatorial region. This is quite different from what reported by Buresova and Lastovicka (2007) for middle latitudes. They found no systemic latitudinal dependence in prestorm enhancements over Europe. It is argued that solar flares are not the main drivers for the enhancements, at least for low-latitude events. Main features of low-latitude prestorm enhancements do not coincide with the solar flare effects. We postulate that the vertical plasma drift or zonal electric field is a likely cause for the low-latitude prestorm enhancements. Its existence is supported by the facts of stronger EIA, the latitudinal coverage of the enhancements as well as the lift of the F layer peak height at an equatorward station during the prestorm enhancements. Moreover, the behaviors of hmF2 at low latitudes during the prestorm enhancements may possibly be explained in terms of the coupling nature of parallel and perpendicular dynamics at low latitudes (see, e.g., Behnke and Harper, 1973; Rishbeth et al., 1978).

Theoretical limits on SAR imposed by the ionosphere

Abstract: The ultimate theoretical limitations on space-based synthetic aperture radar (SAR) image formation that are imposed by the ionosphere are examined. The effects on the SAR image are derived from first principles, and it is shown that the ionosphere will cause defocusing in both the range and along track directions. The performance of an autofocus procedure is then examined, and it is shown that the range defocusing can always be removed, but the range time delay can only be determined for high percentage bandwidths and high signal-to-noise plus clutter ratios. It is also shown that the performance limits of autofocus are not determined by the absolute total electron content, but are given by the amount of ionospheric turbulence, which limits the along track resolution. The relationship between the requirement for a focussed SAR image and the S4 index and the integrated strength of turbulence CkL is derived.

Derivation of a planetary ionospheric storm index

Abstract: The planetary ionospheric storm index, Wp, is de- duced from the numerical global ionospheric GPS-IONEX maps of the vertical total electron content, TEC, for more than half a solar cycle, 1999-2008. The TEC values are ex- tracted from the 600 grid points of the map at latitudes 60 N to 60 S with a step of 5 and longitudes 0 to 345 E with a step of 15 providing the data for 00:00 to 23:00 h of lo- cal time. The local effects of the solar radiant energy are filtered out by normalizing of the TEC in terms of the solar zenith angle at a particular time and the local noon value 0. The degree of perturbation, DTEC, is computed as log of TEC relative to quiet reference median for 27 days prior to the day of observation. The W-index map is generated by segmentation of DTEC with the relevant thresholds specified earlier for foF2 so that 1 or 1 stands for the quiet state, 2 or 2 for the moderate disturbance, 3 or 3 for the moder- ate ionospheric storm, and 4 or 4 for intense ionospheric storm at each grid point of the map. The planetary iono- spheric storm Wp index is obtained from the W-index map as a latitudinal average of the distance between maximum positive and minimum negative W-index weighted by the lat- itude/longitude extent of the extreme values on the map. The threshold Wp exceeding 4.0 index units and the peak value Wpmax 6.0 specify the duration and the power of the plan- etary ionosphere-plasmasphere storm. It is shown that the occurrence of the Wp storms is growing with the phase of the solar cycle being twice as much as the number of the magnetospheric storms with Dst 100 nT andAp 100 nT.

Large magnetic storm-induced nighttime ionospheric flows at midlatitudes and their impacts on GPS-based navigation systems

Abstract: [1] Analysis of GPS phase fluctuations in conjunction with regional total electron content (TEC) maps, in situ measurements of subauroral polarization streams (SAPS) and auroral convection from several Defense Meteorological Satellite Program spacecraft, and dynasonde measurements at the Bear Lake Observatory obtained during the intense magnetic storm of 7–8 November 2004 have indicated the serious impact of large ionospheric velocities on GPS-based navigation systems within the midlatitude region in the North American sector. The major difference between this superstorm and the others observed during the earlier October-November 2003 events is the absence of appreciable storm-enhanced density gradients, with the midlatitude region being enveloped by either the auroral oval or the ionospheric trough within which the SAPS were confined during the local dusk to nighttime hours. This shows that it is possible to disable GPS-based navigation systems for many hours even in the absence of appreciable TEC gradients, provided an intense flow channel is present in the ionosphere during nighttime hours. The competing effects of irregularity amplitude ΔN/N, the background F region density, and the magnitude of SAPS or auroral convection are discussed in establishing the extent of the region of impact on such systems.

Observations and simulations of quasiperiodic ionospheric oscillations and large‐scale traveling ionospheric disturbances during the December 2006 geomagnetic storm

Abstract: [1] A numerical simulation was performed to investigate quasiperiodic ionospheric oscillations that were observed with periods of 4–5 h by the ionosonde network (Okinawa, Yamagawa, Kokubunji, and Wakkanai) in Japan during the 15 December 2006 magnetic storm. This simulation used the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 2.0 model. The CMIT model reproduced the main characteristics of the observed ionospheric oscillations, although it remains a challenging task to simulate the observations in a quantitative sense. Term analysis of the ion continuity equation demonstrated that the ionospheric oscillations in this event were mainly induced by the disturbed neutral winds, which were associated with the large scale thermospheric circulation and traveling atmospheric disturbances (TADs) during the storm. The TADs simulated from the model were then compared with those observed by the GPS Earth Observation Network (GEONET) in Japan to validate the simulation results. A prominent northward propagating large-scale traveling ionospheric disturbance (LSTID) during daytime, seen by the GEONET total electron content (TEC) data, was captured by the CMIT model. Two southward LSTIDs observed by GEONET GPS network were also reproduced by the CMIT model. However, the model gave faster phase speeds for the southward propagating LSTID occurred during 0620–0800 UT and the northward propagating LSTID; furthermore, the model missed the LSTID seen in the TEC perturbation data during 0140–0220 UT. Finally, both observations and simulations showed a strong hemispheric asymmetry for the TAD propagation that occurred during 0000–0400 UT, which may be associated with the hemispheric asymmetry of the change of Joule heating at high latitude.

An Evaluation of Interpolation Techniques for Reconstructing Ionospheric TEC Maps

Abstract: Maps of the total electron content (TEC) of the ionosphere can be reconstructed using data extracted from global positioning system (GPS) signals. For historic and other sparse data sets, the reconstruction of TEC images is often performed using a multivariate interpolation technique. Although there are many interpolation methods available, only a limited number, for example kriging, have been applied to TEC data. This paper presents a quantitative comparison of various commonly used algorithms for scattered-data interpolation over a range of sparsi- ties. Techniques evaluated include a relatively new approach called Adaptive Normalized Convolution (ANC) that has not previously been applied to ionospheric reconstruction. The proposed evaluation scheme employs a quantitative methodology applied to both simulated and real TEC data. Results show that, although the performance of kriging is good in many cases, it is several times worse than the best performing techniques at some sparsities. Natural-neighbor interpolation has a better overall performance than kriging for both simulated and TEC data. Although its performance is a few percent worse than other methods for the simulated data, ANC produces the best performance for the TEC reconstructions.

Ionosphere disturbances observed throughout Southeast Asia of the superstorm of 20–22 November 2003

Abstract: [1] Ionospheric disturbances in the Southeast Asian region during the super magnetic storm of 20–22 November 2003 were investigated through an ionosonde chain and a GPS network assisted by the space-borne instruments. At early stage of the storm in the postsunset sector, large enhancements in the critical frequency of F2 layer and total electron content were observed at northern crest region of the equatorial ionization anomaly (EIA), which might be produced by both the storm meridional wind superimposed with traveling atmospheric disturbances and prompt penetration electric field (PPE). During the main phase of the storm when interplanetary magnetic field started a 12-h southward turning, equatorial ionosphere was elevated to a very high level which should be most probably caused by a long-duration PPE event. Meanwhile, at mid-low latitudes, ionosphere witnessed an initial simultaneous decrease then followed by drastic increases, which is very different from the past observations in this region (Reddy and Nishida, 1992). Combined analysis of the data from the ionosonde and other space-based measurement shows that for the present case the penetration efficiency of the interplanetary electric field (IEF) to the equatorial ionosphere was larger at night than in the daytime, which agrees with the results of Fejer et al. (2007) showing the ratios of PPE and IEF changes were highly variable with the local time. During the recovery phase, EIA was severely inhibited owing to a wind convergence and possibly because of the westward disturbance dynamo electric field.