Relative Total Electron Content Research Articles

Verification and Accuracy Analysis of Single-Frequency Occultation Processing Based on the BeiDou Navigation System

GNSS single-frequency occultation processing technology has the advantage of simple instrumentation, but it is not clear about the accuracy of the Beidou-based single-frequency occultation processing. This paper verifies the single-frequency occultation processing algorithm of the BeiDou navigation system (BDS) and analyzes its accuracy based on occultation observation data from the FY3E satellite. The research aimed to verify the single-frequency ionospheric relative total electron content (relTEC), analyze the accuracy of the reconstructed second frequency B3∗’s excess phase Doppler, and analyze the accuracy of the refractive index products. Results: (1) As for relTEC and excess phase Doppler, the correlation coefficient between single-frequency occultation processing and dual-frequency occultation processing is greater than 0.95. (2) The relative average deviations of the excess phase Doppler of B3∗ are mostly less than 0.2%, and the relative standard deviations are mostly around 0.5%. (3) The bias index and root mean square index of single/dual-frequency inversion have good consistency compared with ERA5 data. All the results show that the single- and dual-frequency inversion refractive index products have comparable accuracies, and the accuracy of the standard deviation of single-frequency inversion refractive index products over 25 km being slightly lower than that of dual-frequency inversion refractive index products.

Driver of the Positive Ionospheric Storm over the South American Sector during 4 November 2021 Geomagnetic Storm

During geomagnetic storms, ionospheric storms can be driven by several mechanisms. Observations performed using ground- and space-based instruments were used to reveal the driver of the positive ionospheric storm over the South American sector during the 4 November 2021 geomagnetic storm. The positive storm appeared from 10:30 UT to 18:00 UT and covered the region from 40°S to 20°N. The maximum magnitudes of TEC (Total Electron Content) enhancement and relative TEC enhancement were about 20 TECU and 100%, respectively. Defense Meteorological Satellite Program (DMSP) also observed a significant electron density increase over South America and the eastern Pacific Ocean. In the meantime, about 50% ∑O/N2 enhancement was observed by the Global-scale Observations of the Limb and Disk (GOLD) satellite at low latitudes. Ionosonde observations (AS00Q and CAJ2M) registered an ~80 km uplift in F2 peak height (HmF2) and a prominent F2 peak electron density (NmF2) increase ~3 h after the uplift. A prominent enhancement in the cross-polar cap potential (CPCP) in the southern hemisphere was also observed by Super Dual Auroral Radar Network (SuperDARN) one hour earlier than the HmF2 uplift. Measurements of the Ionospheric Connection Explorer satellite (ICON) showed that the outward E×B drift was enhanced significantly and that the horizontal ion drift was poleward. According to the ICON ion drift observations, the HmF2 uplift was caused by an electric field rather than equatorward neutral wind. We propose that the enhanced eastward electric field dominated the positive ionospheric storm and that the thermospheric composition variation may have also contributed.

Storm-Time Relative Total Electron Content Modelling Using Machine Learning Techniques

Accurately predicting total electron content (TEC) during geomagnetic storms is still a challenging task for ionospheric models. In this work, a neural-network (NN)-based model is proposed which predicts relative TEC with respect to the preceding 27-day median TEC, during storm time for the European region (with longitudes 30°W–50°E and latitudes 32.5°N–70°N). The 27-day median TEC (referred to as median TEC), latitude, longitude, universal time, storm time, solar radio flux index F10.7, global storm index SYM-H and geomagnetic activity index Hp30 are used as inputs and the output of the network is the relative TEC. The relative TEC can be converted to the actual TEC knowing the median TEC. The median TEC is calculated at each grid point over the European region considering data from the last 27 days before the storm using global ionosphere maps (GIMs) from international GNSS service (IGS) sources. A storm event is defined when the storm time disturbance index Dst drops below 50 nanotesla. The model was trained with storm-time relative TEC data from the time period of 1998 until 2019 (2015 is excluded) and contains 365 storms. Unseen storm data from 33 storm events during 2015 and 2020 were used to test the model. The UQRG GIMs were used because of their high temporal resolution (15 min) compared to other products from different analysis centers. The NN-based model predictions show the seasonal behavior of the storms including positive and negative storm phases during winter and summer, respectively, and show a mixture of both phases during equinoxes. The model’s performance was also compared with the Neustrelitz TEC model (NTCM) and the NN-based quiet-time TEC model, both developed at the German Aerospace Agency (DLR). The storm model has a root mean squared error (RMSE) of 3.38 TEC units (TECU), which is an improvement by 1.87 TECU compared to the NTCM, where an RMSE of 5.25 TECU was found. This improvement corresponds to a performance increase by 35.6%. The storm-time model outperforms the quiet-time model by 1.34 TECU, which corresponds to a performance increase by 28.4% from 4.72 to 3.38 TECU. The quiet-time model was trained with Carrington averaged TEC and, therefore, is ideal to be used as an input instead of the GIM derived 27-day median. We found an improvement by 0.8 TECU which corresponds to a performance increase by 17% from 4.72 to 3.92 TECU for the storm-time model using the quiet-time-model predicted TEC as an input compared to solely using the quiet-time model.

Introduction of TROPS ionospheric TEC products for FORMOSAT-7/COSMIC-2 mission

Six low Earth orbit (LEO) satellites were launched on June 25th, 2019 for a radio occultation (RO) mission for the FORMOSAT-7/COSMIC-2 (F7/C2) program. The GPS and GLONASS RO signals received by these F7/C2 satellites can be used to retrieve atmospheric and ionospheric parameter profiles for atmospheric and ionospheric research. In order to process the received RO signal, the processing system named Taiwan Radio Occultation Processing System (TROPS) is built. TROPS is developed by National Space Organization, Taiwan Analysis Center for COSMIC, and GPS Science and Application Research Center in Taiwan. The ionospheric products of TROPS are electron density profile, ionospheric scintillation index (S4 index), and absolute total electron content (TEC). S4 index has been calculated on board the satellites and other two products are retrieved by TROPS after the observation data downlink to ground. TEC is the linear integration of electron density along the signal propagation path. The electron density profile is retrieved from the relative TEC when the elevation angle of GNSS satellite is negative from F7/C2 satellite. The absolute TEC is the TEC from GNSS satellite to F7/C2 satellite. The difference between absolute and relative TEC is the TEC with/without differential code bias (DCB) correction. Currently, the data for the electron density profile and absolute TEC are provided by TROPS. Users can obtain the products freely from the internet. In this study, the retrieval method and the preliminary F7/C2 ionospheric TEC products retrieved by TROPS are presented in detail.

Africa mid and low latitude ionosphere response observed during the geomagnetic storms of July 15 and 9 March 2012 using GPS

In this paper, the initial and recovery phases of July 15 and March 9, 2012 geomagnetic storms in African mid and low latitudes ionosphere has been studied using GPS. We employ relative total electron content (rTEC) variations using 2 stations from the Africa Geodetic Reference Frame (AFREF) to characterize African sector ionosphere responses during both storms. To characterize rTEC, we employ 15-day median-average sliding-window during the storm. Both storms lasted 18 h with Dst minima -139 nT for July 15 and -145 nT for March 9, when solar plasma wind speed recorded 545 km/s and 712 km/s respectively. The recovery phase lasted 48 h for -139 nT storm and 46 h for -145 nT when solar plasma wind speed recorded 485 km/s and 428 km/s respectively. It may be attributed that storm recovery phases do not depend on storm severity but the response of ionosphere during storms. Results show Positive storm dominates during the recovery phase and interplanetary electric field and solar plasma wind speed contribute to storm enhanced density. Ionospheric disturbances observed due to prompt penetration electric field shaped the magnetic field and prompted pre-storm rTEC enhancement. Plasma convection at mid-latitudes of African sector observed rTEC enhancements which did not appear in other studied sector results. Further observations should be carried out using other storms.

Capacities of TEC measurements by the low-cost GNSS receiver based on the u-blox ZED-F9P for ionospheric research

A prototype of a low-cost GNSS receiver assembled on base of the u-blox ZED-F9P module is described in the paper. The original low-cost GNSS receiver (OGNSS-R) allows made the parallel registration of 184 channels in the entire frequency range (1176 ÷ 1610 MHz) of the main GNSS (GPS, GLONASS, Galileo, Beidou) and recording raw data of carrier-phase, pseudorange (code) and signal strength measurements. The obtained data processing allows to calculate the slant total electron content (TEC), TEC variations and other most popular TEC-based indices of ionospheric activity. The comparative studies of these parameters and similar parameters obtained by simultaneous measurements on the professional geodetic GNSS receiver Trimble Alloy (KZN2 station of the IGS network) were preformed. The analysis showed the median value of the signal strength at the L1 frequency for the KZN2 station are 8% higher than that at the low-cost GNSS receiver based on the u-blox ZED-F9P module (KZN2–48.75 dBHz; OGNSS-R – 45dBHz). For the L2 frequency that difference is less than 2% (KZN2–49.4 dBHz; OGNSS-R – 48.5 dBHz). The median levels of relative slant TEC and TEC-based indexes of the ionospheric activity obtained by the Trimble Alloy receiver at KZN2 station are 25÷50% lower than for the u-blox ZED-F9P receiver. Thus, the using of starter kits such as the simpleRTK2b debug board based on the u-blox ZED-F9P module described in the paper, is really actual for creating a permanent network of GNSS-stations and/or for carrying out field measurements in experimental campaigns.

A method for automatic detection and characterization of plasma bubbles using GPS and BDS data

Detecting and characterizing Total Electron Content (TEC) depletion is important for studying the ionospheric threat due to the Equatorial Plasma Bubble (EPB) when applying the Ground-Based Augmentation System (GBAS) at low latitudes. This paper develops a robust method to automatically identify TEC depletion and derive its parameters. The rolling barrel algorithm is used to automatically identify the TEC depletion candidate and its parameters. Then, the depletion candidates are screened by several improved techniques to distinguish actual depletions from other phenomena such as Traveling Ionospheric Disturbance (TID) or abnormal data. Next, based on the depletion signals from three triangular receivers, the method derives EPB parameters such as velocity, width and gradient. The time lag and front velocity are calculated based on cross-correlation using TEC depletions and the geometrical distribution of three triangular receivers. The width and gradient of slope are then determined by using TEC depletion from a single receiver. By comparison, both the station-pair method and proposed method depend on the assumption that the EPB morphology is frozen during the short time when the plasma bubble moves between the receivers. However, our method relaxes the restriction that the baseline length should be shorter than the width of slope required by the station-pair. This relaxation is favorable for studying small-scale slope of depletions using stations of a longer baseline. In addition, the accuracy of the width and gradient is free of impact from hardware biases and small-scale disturbance, as it is based only on the relative TEC variation. The method is demonstrated by processing Global Positioning System (GPS) and BeiDou Navigation Satellite System (BDS) data on 15 August, 2018, in a solar minimum cycle.

Quality analysis of dual-frequency smartphone-based ionospheric TEC measurements: what can be achieved?

The growing quality of smartphone-based Global Navigation Satellite Systems (GNSS) chipsets opens a new frontier for scientific research in positioning, navigation and timing ap- plications. The portability and affordability of these instruments could enhance the current GNSS receiver global network for atmospheric monitoring purposes. However, the quality of the measurements gathered from smartphones have not yet been fully assessed. In this paper, an analysis of the quality of smartphone-based Total Electron Content (TEC) mea- surements is performed. The primary focus of this work is to provide a general analysis on the potential of using smartphone observations for ionospheric sciences. Dual-frequency phase observations are used to measure the relative TEC. For this experiment, GPS L1/L5 and Galileo E1/E5a observations acquired with the Xiaomi Mi8 and Huawei Mate20 X smartphones were considered. Both devices are equipped with the Broadcom BCM47755 chipset, which enables GNSS dual-frequency measurements. More than 100 hours of phase observations at mid-latitude during a low solar activity period were gathered. Three differ- ent setup configurations were defined to assess the effects multipath or signal strength may have in the quality of the phase observations. In addition, to detect and discard unrealistic fluctuating phase observations, a quality-check was performed. In the results, good agree- ment between the slant TEC (sTEC) measurements from the smartphone and the sTEC obtained from a co-located geodetic receiver is presented. Furthermore, the amount and quality of observations discarded by the quality-check are reported, which emphasises the use of the signal strength to indicate the quality of phase observations. The results indicate that the C/N0 and multipath are important —when gathering the data from a geodetic antenna, around 80% of the collected data passed a quality threshold. However, collecting data with the addition of an attenuator, or directly from the smartphone antenna, reduced the valid data to below 50%. However, given the ease of use of a smartphone for data collection, even at 50% of data being usable, this shows potential as a useful course of TEC for ionospheric observations.

Seismogenic Disturbances of the Ionosphere During High Geomagnetic Activity

Herein, we analyze the variations in the ionosphere for the period of two weeks before the M6.7 earthquake in India on January 3, 2016. The earthquake occurred after a series of magnetic substorms on December 31, 2015 and January 1, 2016. The relative total electron content (TEC) disturbances have been estimated using global TEC maps and calculated numerically using the 3D global first-principle Upper Atmosphere Model (UAM) for the whole period including the days before, during, and after the substorms. Numerical simulations were repeated with the seismogenic vertical electric currents switched on at the earthquake epicenter. The UAM calculations have reproduced the general behavior of the ionosphere after the main phase of the geomagnetic storm on January 1, 2016 in the form of negative TEC disturbances propagating from high latitudes, being especially strong in the Southern (summer condition) Hemisphere. It was shown that the local ionospheric effects of seismic origin can be identified in the background of the global geomagnetic disturbances. The seismo-ionospheric effects are visible in the nighttime regions with the additional negative TEC disturbances extending from the eastern side of the epicenter meridian to the western side, both in the observations and in the UAM simulations. It was found that the vertical electric field and corresponding westward component of the electromagnetic [E × B] drift played a decisive role in the formation of the ionospheric precursors of this earthquake.

Comparison of TEC Estimation Techniques using S1 and L5 Signals of IRNSS

Indian Regional Navigation Satellite System (IRNSS) is a satellite based navigation system developed by Indian Space Research Organization (ISRO). The positional accuracy of IRNSS is limited by several errors including ionospheric delay. Ionospheric delay can be estimated with precise calculation of total electron content (TEC). However, TEC is biased by multipath and thermal noise. Therefore, multipath and thermal noise free relative TEC measurements are made and compared with two other methods, namely, code TEC and IRNSS Receiver Software (IRS) techniques. In this investigation L5 (1176.45 MHz) and S1 (2492.028 MHz) signals from IRNSS 1A to 1G satellites are considered. The results indicate that relative TEC estimation technique removes the multipath and thermal noise from the TEC measurements. Further, the TEC for signals coming from various IRNSS satellites are evaluated by calculating the standard deviation (SD). In the analysis the data is divided into segments with duration of 1 h each. The results indicate that SDs for the relative TEC estimation (2.5 TECU in case of IRNSS 1B) are lesser than the other two techniques indicating the relative TEC can be used in the receiver for the estimation of ionospheric delay. Further, the high ratio of the operating frequencies indicates a better TEC estimation.

Lunar tides in total electron content over Brazil

AbstractUsing data from Global Navigation Satellite Systems dual frequency receivers, the lunar tides were studied in the ionospheric total electron content (TEC) over Brazil from 2011 to 2014. In order to calculate the amplitudes and phases of the lunar tides, quiet magnetic days within a 27 day (window) have been used to remove the solar rotation effects. Relative residuals TEC were calculated by performing a spectral analysis by removing the solar tide contributions. Lastly, a lunar month size window was used to calculate the amplitude and phase of the lunar components. Lunar diurnal tide amplitudes showed a semiannual variation with maximum in the equinox months for almost all latitudes, while lunar semidiurnal amplitudes were larger in December and January and may be related to sudden stratospheric warming events. Both components did not display significant longitudinal variation what could be due to the short range of longitude (30°). Lunar diurnal amplitudes were basically uniform over Brazil, while lunar semidiurnal amplitudes showed some peaks over the equatorial ionization anomaly crest and minimum values near the magnetic equator.

Analysis of ionospheric TEC from GNSS observables over the Turkish region and predictability of IRI and SPIM models

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.

Analysis of ionosphere variability over low-latitude GNSS stations during 24th solar maximum period

Global Positioning System (GPS) is a remote sensing tool of space weather and ionospheric variations. However, the interplanetary space-dependent drifts in the ionospheric irregularities cause predominant ranging errors in the GPS signals. The dynamic variability of the low-latitude ionosphere is an imperative threat to the satellite-based radio communication and navigation ranging systems. The study of temporal and spatial variations in the ionosphere has triggered new investigations in modelling, nowcasting and forecasting the ionospheric variations. Hence, in this paper, the dynamism in the day-to-day, month-to-month and seasonal variability of the ionospheric Total Electron Content (TEC) has been explored during the solar maximum period, January-December 2013, of the 24th solar cycle. The spatial and temporal variations of the ionosphere are analysed using the TEC values derived from three Indian low-latitude GPS stations, namely, Bengaluru, Guntur and Hyderabad, separated by 13–18° in latitude and 77–81° in longitude. The observed regional GPS-TEC variations are compared with the predicted TEC values of the International Reference Ionosphere (IRI-2012 and 2007) models. Ionospheric parameters such as Vertical TEC (VTEC), relative TEC deviation index and monthly variations in the grand-mean of ionosphere TEC and TEC intensity, along with the upper and lower quartiles, are adopted to investigate the ionosphere TEC variability during quiet and disturbed days. The maximum ionospheric TEC variability is found during March and September equinoxes, followed by December solstice while the minimum variability is observed during June solstice. IRI models are in reasonable agreement with GPS TEC but are overestimating during dawn hours (01:00–06:00 LT) as compared to the dusk hours. Higher percentage deviations are observed during equinoctial months than summer over EIA stations, Guntur and Hyderabad. GPS TEC variations are overestimated during dawn hours for all the seasons over Bengaluru. It has also been observed that positive storm effect (enhancement of TEC) is observed during the main phase of the March storm, 2013 (March 16–18, 2013) while both positive and negative storm effects (depletion of TEC) are registered during the main phase of the June storm, 2013 (June 28–30, 2013) at Bengaluru and Guntur, respectively. IRI-2012 model has slightly large discrepancies with the GPS-VTEC compared with the IRI-2007 model during the June storm, 2013 over Guntur station. This analysis highlights the importance of upgrading the IRI models due to their discrepancies during quiet and disturbed states of the ionosphere and developing an early warning forecast system to alert about ionosphere variability.

Determination of Differential Code Bias of GNSS Receiver Onboard Low Earth Orbit Satellite

The uncertainty of differential code bias (DCB) is one of the main error sources in the low Earth orbit (LEO) based total electron content (TEC) retrieval, whereas the derivation of the LEO DCB is not systematically studied. In this paper, we propose an improved DCB estimation method (ZERO method) based on the assumption that the LEO-based TEC can reach zero and also optimize the parameter configuration in the commonly used least square method (LSQ method). In the improved ZERO method, the combination of the lower quartile minimum relative TEC during each orbital revolution with the daily minimum relative TEC gives a stable and reliable DCB estimation. For the LSQ method, the 3-TECU cutoff vertical TEC with 10° cutoff elevation is considered to offer a reasonable DCB estimation. Subsequently, Global Positioning System (GPS) observations from multiple LEO satellites at different altitudes are used to study the variability of the LEO DCBs. Our results revealed that the LEO DCBs underwent obvious long-term variation and periodic oscillations of months. Moreover, the CHAMP data illustrated that the long-term variation of LEO DCBs is partly associated with the GPS satellite replacement, and the periodic variation can be attributed to the variation of the hardware thermal status, represented by the receiver CPU temperature in this study.

Relative importance of horizontal and vertical transports to the formation of ionospheric storm‐enhanced density and polar tongue of ionization

AbstractThere are still uncertainties regarding the formation mechanisms for storm‐enhanced density (SED) in the high and subauroral latitude ionosphere. In this work, we deploy the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) and GPS total electron content (TEC) observations to identify the principle mechanisms for SED and the tongue of ionization (TOI) through term‐by‐term analysis of the ion continuity equation and also identify the advantages and deficiencies of the TIEGCM in capturing high‐latitude and subauroral latitude ionospheric fine structures for the two geomagnetic storm events occurring on 17 March 2013 and 2015. Our results show that in the topside ionosphere, upward E × B ion drifts are most important in SED formation and are offset by antisunward neutral winds and downward ambipolar diffusion effects. In the bottomside F region ionosphere, neutral winds play a major role in generating SEDs. SED signature in TEC is mainly caused by upward E × B ion drifts that lift the ionosphere to higher altitudes where chemical recombination is slower. Horizontal E × B ion drifts play an essential role in transporting plasma from the dayside convection throat region to the polar cap to form TOIs. Inconsistencies between model results and GPS TEC data were found: (1) GPS relative TEC difference between storm time and quiet time has “holes” in the dayside ion convection entrance region, which do not appear in the model results. (2) The model tends to overestimate electron density enhancements in the polar region. Possible causes for these inconsistencies are discussed in this article.

Observation of the neutral‐ion coupling through 6 day planetary wave

AbstractA westward 6 day oscillation with zonal wave number 1 is identified in the global maps of the ionospheric total electron content (TEC) during May 2003. This signature coincides with the wave activities in the mesosphere and lower thermosphere (MLT) region with a similar zonal structure and period, deduced from the temperature and wind observations from the Thermosphere Ionosphere and Mesosphere Electric Dynamics (TIMED) satellite. The vertical wavelength of this 6 day wave is estimated to be ~65 km, which enables it to propagate up into the lower thermosphere and therefore modulate the ionosphere through E region wind dynamo. The zonal wind perturbations of the 6 day wave maximize in the equatorial region with an amplitude of ~30 m/s and the meridional wind perturbations peak at middle latitudes with an amplitude of 15–20 m/s. The 6 day oscillation in TEC peaks at the geomagnetic equatorial ionization anomaly (EIA) crests in both hemispheres with a deep minimum at the equator. The absolute TEC perturbations maximize at ~1400–1800 LT with an amplitude of ~9 (~7) total electron content unit (TECU; 1 TECU = 1016 m−2) in the southern (northern) hemisphere, which account for ~16% (~10%) of the background TEC. Larger relative TEC perturbations of ~20% are found at 0200–0400 LT. The similar wave number‐period spectra and the consistent temporal variations of the 6 day periodical signatures in both neutral atmosphere and ionosphere suggest a strong neutral‐ion coupling through planetary wave.

An ensemble average method to estimate absolute TEC using radio beacon-based differential phase measurements: Applicability to regions of large latitudinal gradients in plasma density

A GNU Radio Beacon Receiver (GRBR) system for total electron content (TEC) measurements using 150 and 400 MHz transmissions from Low-Earth Orbiting Satellites (LEOS) is fabricated in house and made operational at Ahmedabad (23.04°N, 72.54°E geographic, dip latitude 17°N) since May 2013. This system receives the 150 and 400 MHz transmissions from high-inclination LEOS. The first few days of observations are presented in this work to bring out the efficacy of an ensemble average method to convert the relative TECs to absolute TECs. This method is a modified version of the differential Doppler-based method proposed by de Mendonca (1962) and suitable even for ionospheric regions with large spatial gradients. Comparison of TECs derived from a collocated GPS receiver shows that the absolute TECs estimated by this method are reliable estimates over regions with large spatial gradient. This method is useful even when only one receiving station is available. The differences between these observations are discussed to bring out the importance of the spatial differences between the ionospheric pierce points of these satellites. A few examples of the latitudinal variation of TEC during different local times using GRBR measurements are also presented, which demonstrates the potential of radio beacon measurements in capturing the large-scale plasma transport processes in the low-latitude ionosphere.

Earthquakes and global electrical circuit

Based on recent experimental and theoretical model results, the role of earthquakes and processes of their preparation as electricity sources in the global electric circuit (GEC) is discussed. In addition to the traditional elements of the GEC, such as thunderstorm currents, ionosphere currents, fair weather currents, and telluric currents, hypothetical seismogenic currents flowing between the faults and the ionosphere are considered. The ionization sources for these currents are presumably the radiation of radioactive gases and the ionization by the electric field of so-called “positive holes” created by the compression of tectonic plates, whereas transportation of electric charges between the Earth and the ionosphere occurs under the action of electric fields and turbulent diffusion (for heavy charged species). Seismogenic currents deliver electric charges into the ionosphere, which give rise to electric fields in it and in the magnetically conjugated region. The drift of magnetized plasma in the ionosphere F2-region and plasmasphere plasma under the action of these fields causes disturbances in the electron density and total electron content (TEC) of the ionosphere, which are observed by GPS satellites before strong earthquakes. The typical features of these disturbances (magnitudes, dimensions, stability, nighttime predominance of the relative TEC disturbances, geomagnetic conjugacy) are well reproduced in theoretical model calculations based on the solution of the equation for the electric ionosphere potential with specified seismogenic electric current at the lower boundary of the ionosphere if this current is strong enough (comparable with thunderstorm currents). The feasibility of such seismogenic currents is discussed. It is argued that the TEC disturbances observed before strong earthquakes cannot be explained by neutral atmosphere disturbances. These TEC disturbances can be treated as ionospheric earthquake precursors created by seismogenic GEC disturbances.

Coordinated observations of a high‐altitude auroral electrojet by UHF/VHF receivers, magnetometers and meridian scanning photometer

UHF/VHF beacon receivers deployed in Alaska measure the latitude profile of relative total electron content over Alaska and frequently across the auroral oval. On March 9, 2008, these receivers observed a plasma density enhancement at 147 km. These observations are supported by collocated scanning photometer measurements. A simple model current consistent with magnetic field perturbations observed by a chain of magnetometers across Alaska corresponds spatially to the enhanced density observed by the receivers. Hence, these measurements suggest that the auroral electrojet is flowing at 147 km, which is unusually high. Indeed, the ratio of the 630.0 nm and 427.8 nm emissions and the strength of the magnetic perturbation suggest the electrojet is flowing at a lower altitude. We attribute this difference to thermospheric compositional changes associated with particle and Joule heating that could result in a lower ratio of the 630.0 nm and 427.8 nm emissions and higher conductivity corresponding to observed density enhancement.

Inversion of backscatter ionograms and TEC data for over‐the‐horizon radar

Over‐the‐horizon radar (OTHR) uses ionospheric reflection to propagate HF transmissions to long range (∼500–5000 km). The ionosphere acts as a dynamic “mirror” that varies diurnally, seasonally, and with the solar cycle. Geolocation of targets observed by OTHR (Coordinate Registration (CR)) requires accurate real‐time ionospheric modeling and HF propagation calculations to convert radar‐measured target signal delays and beam steers to geographical position. We merged our backscatter ionogram (BI) leading edge inversion algorithm CREDO with our more advanced ionospheric data assimilation capability, GPS Ionospheric Inversion (GPSII). The combined algorithm produces a dynamic model of electron density for a fixed geographical region. The model is consistent with BI leading edge data, vertical sounding data, as well as with absolute and relative total electron content (TEC) data from a number of GPS/LEO receivers. Incorporation of additional ionospheric data beyond conventional OTHR vertical and oblique backscatter soundings is expected to enhance the fidelity of real‐time ionosphere models, resulting in improved OTHR Coordinate Registration metric accuracy. Initial tests of the OTHR CR supported by the new ionospheric inversion algorithm indicate noticeable improvement of CR accuracy in comparison with legacy techniques.