Reconstructing Regional Ionospheric Electron Density: A Combined Spherical Slepian Function and Empirical Orthogonal Function Approach

Global ionospheric mapping (GIM) is a new and emerging technique for determining global ionospheric TEC (total electron content) based on measurements from a worldwide network of Global Positioning System (GPS) receivers. In this study, GIM accuracy in specifying TEC is investigated by comparison with direct ionospheric measurements from the TOPEX altimeter. A climatological model (Bent model) is also used to compare with the TOPEX altimeter data. We find that the GIM technique has much better agreement with TOPEX in TEC measurements, compared with the predictions of the climatological model. The difference between GIM and TOPEX in TEC measurements is very small (less than 1.5 TEC units (TECU)) within a 1500‐km range from a reference GPS station. The RMS gradually increases with increasing distance from the station, while the Bent model shows a constant large RMS, unrelated to any station location. Within a 1000‐km distance of a GPS site (elevation angle > 25°), GIM has a good correlation (R > 0.93) to TOPEX with respect to TEC measurements. The slope of the linear fitting line to the data set from two TOPEX cycles is 44.5° (near the ideal 45°). In the northern hemispheric regions, ionospheric specification by GIM appears to be accurate to within 3‐10 TECU up to 2000+ km away from nearest GPS station (corresponding to ∼1° elevation angle cutoff). Beyond 2000 km, GIM accuracy, on average, is reduced to the Bent model levels. In the equatorial region, the Bent model predictions are systematically lower (∼5.0 TECU) than TOPEX values and often show a saturation at large TEC values. During ionospheric disturbed periods, GIM sometimes shows differences from TOPEX values due to transient variations of the ionosphere. Such problems may be improved by the continuous addition of new GPS stations in data‐sparse regions. Thus, over a GPS station's measurement realm (up to 2000 km in radius), GIM can produce generally accurate TEC values. Through a spatial and temporal extrapolation of GPS‐derived TEC measurements, the GIM technique provides a powerful tool for monitoring global ionospheric features in near real time.

Inter-hourly variability of Total Electron Content during the quiet condition over Nigeria, within the Equatorial Ionization Anomaly region

On November 22–23, 1997, a geomagnetic storm occurred during a period of excellent viewing conditions over the Arecibo Observatory in Puerto Rico. Here we explore the total electron content (TEC) registered by Global Positioning System (GPS) receivers located close to the Cornell All‐Sky Imager (CASI) at the Arecibo Observatory. The storm began with the equatorward surge of a very high (100% increase) TEC enhancement stretching for many hours of local time on the dayside. At dusk the TEC over the Caribbean remained elevated with levels equal to the noontime monthly averages. During the event the TEC was highly structured and clearly correlated with high and low airglow emission levels. In one fortuitous instance a common ionospheric penetration point (15 km apart), shared by two GPS satellites viewed from two receiving stations, registered an 8 TEC unit difference during the active period. We show that a GPS station can be calibrated using the pseudorange method and a reliable data‐driven technique during quiet conditions and still have absolute TEC capability within 2 TEC units (RMS) 5 days later. We compare the observations to a climatological model which, although reasonable for quiet times, is very poor during the storm period. We also present an independent evaluation of the GPS TEC. This study is an initial step toward quality control of this database, needed before it is used in an assimilation model.

Abstract. In this paper the Total Electron Content (TEC) data of eight Global Positioning System (GPS) stations of the EUREF network (AUT1, Thessaloniki, TUC2, Crete in Greece, MATE, Matera, LAMP, Lampedusa in Italy, GAIA, in Portugal, RABT, Rabat, EVPA, Evpatoria in Ukrain and TRAB, Trabson in Turkey) were analysed using wavelet analysis in order to detect any frequency dependence of the correlation between TEC over different stations. In the same time frequency dependence of Dst (Global geomagnetic field disturbances) and TEC variations over each GPS station are searched in order to detect any correlation between them. The main conclusion of this analysis is that the components of TEC variation with periods <3 h are more suitable in searching for earthquake precursors. On the base of this conclusion the analyzed TEC series are searched for possible precursory phenomena on the occasion of the seismic activity of the last quarter of 2005 in the area of Greece. An exalting (i.e. an increase in the amplitude) of variations with periods up to the tidal ones (period of 6 h,8 h,12 h) may be observed a month before and during the seismic activity over the stations TUC2 and AUT1 and may be attributed to this tectonic activity. Statistical properties of the 1.5 h component of the Total Vertical Electron Content (TVEC) over the nearest GPS stations (TUC2 and AUT1) of the areas of the seismic activity indicate that this component present characteristic exalting in the time period of 15 days before the shock.

We report an example of total electron content (TEC) variations, using Global Positioning System (GPS) measurements, associated with a polar cap arc detected by an all‐sky imager and ionosonde at a polar cap station. During the transit of the arc, GPS signals along the arc length showed an increase of ∼2 TEC units whereas GPS signals perpendicular to the arc showed only an increase of ∼0.3 TEC units. This indicates that the GPS TEC variations associated with a polar cap arc are dependent on the geometry of the GPS raypaths with respect to the arc. Ionosonde measurements confirm that this arc was an E region feature at ∼110 km with peak electron density of 5.4 × 1011 el/m3. Estimated speed of the arc using a GPS triangulation method was about 300 m/s.

A fast algorithm for automatic detection of ionospheric disturbances: DROT

The long‐duration positive ionospheric storm effect that occurred on 15 December 2006 is investigated using a combination of ground‐based Global Positioning System (GPS) total electron content (TEC), TOPEX and Jason‐1 TEC, and topside ionosphere/plasmasphere TEC, GPS radio occultation, and tiny ionospheric photometer (TIP) observations from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites. This multi‐instrument approach provides a unique view of the ionospheric positive storm effect by revealing the storm time response in different altitude regions. The ground‐based GPS TEC, TOPEX/Jason‐1 TEC, and topside ionosphere/plasmasphere TEC all reveal significant enhancements at low latitudes to midlatitudes over the Pacific Ocean region during the initial portions of the storm main phase from 0000–0400 universal time (UT) on 15 December. At low latitudes, the topside ionosphere/plasmasphere TEC increase represents greater than 50% of the TEC enhancement that is observed by ground‐based GPS receivers. Moreover, electron density profiles obtained using the technique of GPS radio occultation demonstrate that the F layer peak height increased by greater than 100 km during this time period. The effects of soft particle precipitation are also apparent in the COSMIC observations of topside ionosphere/plasmasphere TEC. The positive storm effects over the Pacific Ocean region remain present in the equatorial ionization anomaly crest regions beyond 1200 UT on 15 December. This long‐lasting positive storm effect is most apparent in ground‐based GPS TEC and COSMIC TIP observations, while only a small increase in the topside ionosphere/plasmasphere TEC after 0400 UT is observed. This indicates that the long‐lasting positive storm effect occurs predominantly at F region altitudes and, furthermore, that refilling of the topside ionosphere and plasmasphere is not the primary mechanism for producing the long‐lasting positive storm phase during this event. The observations suggest that the enhanced eastward electric field and equatorward neutral wind are likely to play a significant role in the generation of long‐lasting positive storm effects.

Ionospheric imaging at mid-latitudes using both GPS and ionosondes

Global ionosphere models may not be accurate enough in regions where there are insufficient observations. Therefore, there is strong interest in regional ionosphere modeling using observations from dual-frequency Global Navigation Satellite System (GNSS) receivers in such regions. This paper presents a data assimilation method for hourly total electron content (TEC) modeling along with estimation of the receivers’ differential code biases (DCBs). For this purpose, first, initial values for slant TEC (STEC) and the receivers’ DCBs are estimated utilizing global ionosphere maps (GIMs). Second, STEC is expanded to a linear combination of spherical radial basis functions with time-dependent coefficients to reduce the number of unknowns, and a constrained least-squares adjustment is applied to resolve the corrected STEC and DCB values in the study area using observations from dual-frequency GNSS receivers. Third, the resolved STEC at ionospheric pierce points (IPPs) are converted to vertical TEC (VTEC) using a mapping function and assimilated into the background provided by spatial and temporal interpolation of GIM. Retrieved VTECs are weighted according to the satellites’ elevation angles, and an objective function is defined to determine the background’s covariance matrix. In this study, the Iranian permanent Global Positioning System (GPS) network is used to construct the regional hourly VTEC model and calculate hourly DCB variations. The proposed method is validated in two ways. First, some of the VTEC values are excluded from the dataset for use as test data and compared with modeling results. The root mean square (RMS) of the constructed regional assimilative model on test data is significantly better than GIM. Second, the RINEX file of Tehran station, whose observations did not play a role in modeling, is corrected using the regional model. The single-frequency positioning results with corrected observations show much better accuracy than GIMs.

[1] We observed two-dimensional structure and time evolution of ionospheric irregularities caused by midlatitude sporadic E (Es) over Japan as positive anomalies of total electron content (TEC) by analyzing the data from the nationwide Global Positioning System (GPS) array. In this paper we report a case study of strong Es observed in the local evening of 21 May 2010, over Tokyo, Japan. In the slant TEC time series, Es showed a characteristic pulse-like enhancement of ~1.5 TEC units lasting for ~10 min. We plotted these positive TEC anomalies on the subionospheric points of station-satellite pairs to study the horizontal structure of the Es irregularity. We confirmed that the irregularity existed at the height of ~106 km by comparing the data of multiple GPS satellites, which is consistent with the local ionosonde observations. The horizontal shapes of the Es irregularity showed frontal structures elongated in E-W, spanning ~150 km in length and ~30 km in width, composed of small patches. The frontal structure appears to consist of at least two parts propagating in different directions: one moved eastward by ~60 m s−1, and the other moved southwestward by ~80 m s−1. Similar TEC signatures of Es were detected by other GPS satellites, except one satellite that had line of sight in the N-S direction which dips by 40–50° toward north, which indicates the direction of plasma transportation responsible for the Es formation. We also present a few additional observation results of strong Es irregularities.

Seismo‐ionospheric anomalies have been reported, while its disturbance modes, patterns, and mechanism are very complex for different kinds of earthquakes. The earthquake with Mw = 7.2 occurred off the coast of Northern California with strike‐slip faulting on 15 June 2005, which may provide a new insight on ionospheric disturbance waves and patterns from dense Global Positioning System (GPS) observations. In this paper, the detailed seismic ionospheric disturbance modes and characteristics are investigated from dense GPS observations following the 2005 Northern California offshore earthquake. Around 10 min after the earthquake, significant ionospheric disturbances are observed by GPS total electron content (TEC). Furthermore, two‐mode ionospheric disturbances are clearly found with two speeds, 1.51 and 2.31 km/s. The slow‐propagating mode has an amplitude of 0.02–0.04 total electron content unit (TECU), while the fast‐propagating mode has an amplitude of 0.1–0.2 TECU. The frequency spectrogram of the slow mode is around 3.7 mHz, while typical frequency of the fast mode is about 5.3 mHz. The fast‐propagating mode is close to the speed of the seismic Rayleigh wave, which is the up propagating secondary acoustic wave triggered by the Rayleigh wave. The slow mode ionospheric disturbance is propagating horizontally as the acoustic wave at the ionospheric level, which is caused by the fault rupture in focal regions.

Global Positioning System (GPS) total electron content (TEC) measurements, although highly precise, are often rendered inaccurate due to satellite and receiver differential code biases (DCBs). Calculated satellite DCB values are now available from a variety of sources, but receiver DCBs generally remain an undertaking of receiver operators and processing centers. A procedure for removing these receiver DCBs from GPS-derived ionospheric TEC at high latitudes, using Canadian Advanced Digital Ionosonde (CADI) measurements, is presented. Here we will demonstrate the inadequacy of common numerical methods for estimating receiver DCBs in high-latitude regions and compare our CADI-calibrated vertical TEC (vTEC) measurements to corresponding IGS IONEX interpolated vTEC map data. We demonstrate that the bias values determined using the CADI method are largely independent of the topside model (exponential, Epstein, and α-Chapman) used. We further confirm our results via comparing CADI-estimated biases with those derived from incoherent scatter radar (ISR) measurements. These CADI-method results are found to be within 1.0 TEC units (TECU) of ISR measurements, where the numerical methods used are found to be in excess of 5.0 TECU in error when compared to ISR measurements.

The joint Taiwan-United States FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) mission, hereafter called COSMIC, is the first satellite constellation dedicated to remotely sense Earth's atmosphere and ionosphere using a technique called Global Positioning System (GPS) radio occultation (RO). The occultations yield abundant information about neutral atmospheric temperature and moisture as well as space weather estimates of slant total electron content, electron density profiles, and an amplitude scintillation index, S4. With the success of COSMIC, the United States and Taiwan are moving forward with a follow-on RO mission named FORMOSAT-7/COSMIC-2 (COSMIC-2), which will ultimately place 12 satellites in orbit with two launches in 2016 and 2019. COSMIC-2 satellites will carry an advanced Global Navigation Satellite System (GNSS) RO receiver that will track both GPS and Russian Global Navigation Satellite System signals, with capability for eventually tracking other GNSS signals from the Chinese BeiDou and European Galileo system, as well as secondary space weather payloads to measure low-latitude plasma drifts and scintillation at multiple frequencies. COSMIC-2 will provide 4–6 times (10–15X in the low latitudes) the number of atmospheric and ionospheric observations that were tracked with COSMIC and will also improve the quality of the observations. In this article we focus on COSMIC/COSMIC-2 measurements of key ionospheric parameters.

Ionosphere is an atmospheric layer that has a high electron concentration and anisotropic, dispersive, inhomogeneous nature. Total Electron Content (TEC) is one of the most important parameters that provides necessary information to observe the ionospheric variability. In this study, spatial-temporal variability of the ionosphere is investigated using Symmetric Kullback Leibler Distance (SKLD), Metric Distance (L2N) and Cross Correlation Coefficient (CC) on Slant Total Electron Content (STEC) data provided by a midlatitude Global Positioning System (GPS) network. STEC data that are used are obtained by using IONOLAB-STEC method on Turkish National Permanent GPS Network Active (TNPGN-Active) stations during 2011 for quiet and disturbed days of ionosphere.

Wavelet modelling in support of IRI