Global Positioning System Total Electron Research Articles

Multi-instrument observations of plasma features in the Arctic ionosphere during the main phase of a geomagnetic storm in December 2006

Arctic ionospheric variations during the main phase of a magnetic storm on 14–15 December, 2006 were investigated to characterize the high energy particle precipitation caused effects, based on multi-instrument observations. These include electron density observations provided by the Global Positioning System (GPS) total electron content (TEC) measurements, European Incoherent Scatter (EISCAT) radar, the radio occultation (RO) from both the CHAMP satellite and the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellite, as well as the ionospheric absorption of cosmic radio noise measured by the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi in the northern Finland (69.05°N, 20.79°E). Significant increases in the electron density for these different observations were found in the Arctic ionosphere during the main phase of the magnetic storm. These increase occurred in Scandinavian, Northwest part of Russia and Svalbard (SNRS) region, primarily at an altitude of about 110km. These results are first reported for the SNRS region, and our study contributes to a more complete description of this space weather event during 14–15 December, 2006. Our observations also provide direct evidence that the stormtime E-layer electron density enhancement (e.g., the sporadic E) can form a nearly dominant portion in the observed TEC increase. These increases were accompanied by the ionospheric absorption enhancement at the altitude of about 90km. The Y-component of magnetic field to the south of SNRS decreased, indicating strong upward field aligned electric current in the Arctic ionosphere. These features are interpreted as the effect of the high energy electron precipitation during the magnetic storm, which is caused by the sub-storm reflected on AL index and the measurements of IMAGE (International Monitor for Auroral Geomagnetic Effects) chain. The average energy of the precipitation electrons reached to about 10keV and the boundary of the high energy electron precipitation was also found to move poleward with a speed of about 800m/s.

Response of the equatorial and low‐latitude ionosphere to an intense X‐class solar flare (X7/2B) as observed on 09 August 2011

AbstractIn this paper, we present response of equatorial and low‐latitude ionosphere to an intense solar flare of class X7/2B that peaked at 08:05 UT on 09 August 2011 in the solar cycle 24. Global positioning system total electron content (TEC) observations in the sunlit hemisphere show enhancement of ~3 TEC units, while geomagnetic H component observations indicate sudden decrease and increase in their strength at equatorial and low‐latitude stations, respectively, at several stations in the sunlit hemisphere. In addition, equatorial electrojet strength over Indian region reveals commencement of counter electrojet. Simultaneous Canadian Advanced Digital Ionosonde observations at Tirunelveli, an equatorial station in India, show the disappearance of ionogram echoes during the flare event indicating absorption of radio signals in the D region. Strong equatorial blanketing type Es layer was observed in the ionogram records at Tirunelveli prior to the occurrence of the solar flare that continued for several hours though it became weak/absent during the flare event. Ionogram records on the control day show regular F layer movement without any blanketing type Es layer. Very low frequency (VLF) observations at Allahabad, an Indian low‐latitude station, show enhanced VLF amplitude signal during the same time revealing the sudden enhancement of D region ionization. Using the observations presented here, an attempt has been made to study the impact of the solar flares on the electrodynamics of the equatorial and low‐latitude ionosphere.

Temporal and spatial distribution of GPS-TEC anomalies prior to the strong earthquakes

Earthquakes are one of the most destructive and harmful natural disasters, especially in recent years, the 2008/5/12 Wenchuan M7.9 earthquake, the 2011/3/11 Tohoku M9.0 earthquake and the 2012/4/11 Sumatra M8.6 earthquake have caused a significant impact to the human life. In this paper, we make a study of the temporal and spatial distribution of the Global Positioning System Total Electron Content (GPS TEC) anomalies prior to the three strong earthquakes by the method of statistical analysis. Our results show that the pre-earthquake ionospheric anomalies are mainly positive anomalies and take the shape of a double-crest structure with a trough near the epicenter. The ionospheric anomalies do not coincide with the vertical projection of the epicenter of the subsequent earthquake, but mainly localize in the near-epicenter region and corresponding ionospheric anomalies are also simultaneously observed in the magnetic conjugate region prior to the three earthquakes. In addition, the amplitude and scale-size of the ionospheric ΔTEC are different with the magnitude of the earthquake, and the horizontal scale-size of the greatest anomalies before the Tohoku M9.0 earthquake is ∼30∘ in longitude and ∼10∘ in latitude, with the maximum amplitude of TEC disturbances reaching ∼20 TECu relative to the background. The peak of anomaly enhancement usually occurs in the afternoon to sunset (i.e. between 14:00 and 18:00 local time) which lasts for approximate 2 hours. Possible causes of these anomalies are discussed, and after eliminating the effect of solar activities and magnetic storms it can be concluded that the detected obvious and regular anomalous behavior in TEC within just a few days before the earthquakes is related with the forthcoming earthquakes with high probability.

Anomalous variation in GPS TEC prior to the 11 April 2012 Sumatra earthquake

On 11 April 2012, a strong earthquake of magnitude Ms8.6 occurred near the west coast of Northern Sumatra, Indonesia. In this paper, we investigated the morphological characteristics of anomalous variations in Global Positioning System Total Electron Content (GPS TEC) prior to the earthquake by the method of the statistical analysis. It was found the TEC anomaly was firstly decreased, then, it became more enhanced, finally, it decreased, the peak of anomaly enhancement arose from 13:00–17:00 LT on April 5 lasted for ∼4 hours and the anomalous ionospheric regions extended to ∼40∘ in longitude and ∼20∘ in latitude, its location did not coincide with the vertical projection of the epicenter, but lies at the north and south of the geomagnetic equator, meanwhile, corresponding ionospheric anomalies are also observed in the magneto conjugate region. Potential causes of these results are discussed,eliminating the ionospheric anomalies that may be caused by solar activities and magnetic storms, it can be concluded that the observed obvious anomalous variation in GPS TEC on April 5 were possibly related to the earthquake.

Determining receiver biases in GPS-derived total electron content in the auroral oval and polar cap region using ionosonde measurements

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 test the applicability of common numerical methods for estimating receiver DCBs in high-latitude regions and compare our CADI-calibrated GPS vertical TEC (vTEC) measurements to corresponding International GNSS Service 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 ?-Chapman) used. We further confirm our results via comparing bias-calibrated GPS vTEC 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. The numerical methods tested demonstrate agreement varying from within 1.6 TECU to in excess of 6.0 TECU when compared to ISR measurements.

Longitudinal differences of ionospheric vertical density distribution and equatorial electrodynamics

Accurate estimation of global vertical distribution of ionospheric and plasmaspheric density as a function of local time, season, and magnetic activity is required to improve the operation of space‐based navigation and communication systems. The vertical density distribution, especially at low and equatorial latitudes, is governed by the equatorial electrodynamics that produces a vertical driving force. The vertical structure of the equatorial density distribution can be observed by using tomographic reconstruction techniques on ground‐based global positioning system (GPS) total electron content (TEC). Similarly, the vertical drift, which is one of the driving mechanisms that govern equatorial electrodynamics and strongly affect the structure and dynamics of the ionosphere in the low/midlatitude region, can be estimated using ground magnetometer observations. We present tomographically reconstructed density distribution and the corresponding vertical drifts at two different longitudes: the East African and west South American sectors. Chains of GPS stations in the east African and west South American longitudinal sectors, covering the equatorial anomaly region of meridian ∼37°E and 290°E, respectively, are used to reconstruct the vertical density distribution. Similarly, magnetometer sites of African Meridian B‐field Education and Research (AMBER) and INTERMAGNET for the east African sector and South American Meridional B‐field Array (SAMBA) and Low Latitude Ionospheric Sensor Network (LISN) are used to estimate the vertical drift velocity at two distinct longitudes. The comparison between the reconstructed and Jicamarca Incoherent Scatter Radar (ISR) measured density profiles shows excellent agreement, demonstrating the usefulness of tomographic reconstruction technique in providing the vertical density distribution at different longitudes. Similarly, the comparison between magnetometer estimated vertical drift and other independent drift observation, such as from VEFI onboard Communication/Navigation Outage Forecasting System (C/NOFS) satellite and JULIA radar, is equally promising. The observations at different longitudes suggest that the vertical drift velocities and the vertical density distribution have significant longitudinal differences; especially the equatorial anomaly peaks expand to higher latitudes more in American sector than the African sector, indicating that the vertical drift in the American sector is stronger than the African sector.

Study of total electron content variations over equatorial and low latitude ionosphere during extreme solar minimum

In recent years with the advancement in satellite based navigational applications, study of Total Electron Content (TEC) has gained significant importance. It is well known that due to dynamical behaviour of equatorial and low latitude ionosphere, the levels of ionization is relatively high herein. The sustained decrease in solar extreme ultraviolet radiations during the current minimum is greater than any in recent history. This gives us the opportunity to study the observations of global positioning system total electron content (GPS-TEC) dual frequency signals from the GPS satellites continuously recorded at Trivandrum (an equatorial station) and Delhi (a low latitude station) during the extremely low solar activity period from January 2007 to June 2009. This study illustrates the diurnal, seasonal and annual variations of TEC during the extended solar minimum period. This study also investigates the behaviour of daytime ionosphere around spring and autumn equinoxes at low solar activity period. The results clearly reveal the presence of equinoctial asymmetry which is more pronounced at equatorial station Trivandrum. The diurnal variation of TEC shows a short-lived day minimum which occurs between 0500 to 0600 LT at both the stations. Delhi TEC values show its steep increase and reach at its peak value between 1200 and 1400 LT, while at the equator the peak is broad and occurs around 1600 LT. Further, the daily maximum TEC ranges from about 5 to 40 TEC units at Trivandrum and about 10 to 40 TEC units at Delhi, which correspond to range delay variations of about 1 to 8 m at the GPS L1 frequency of 1.575 GHz. The Maximum values of TEC were observed during spring equinox rather than autumn equinox, showing presence of semi annual variation at both the locations. The minimum values of TEC were observed during the summer solstice at Trivandrum indicating the presence of winter anomaly at equatorial region while Delhi TEC values were minimum during winter solstice showing absence of winter anomaly. Also the TEC values at both the locations have been decreasing since 2007 onwards exhibit good positive correlation with solar activity.

Positive ionospheric storm effects at Latin America longitude during the superstorm of 20–22 November 2003: revisit

Abstract. Positive ionospheric storm effects that occurred during the superstorm on 20 November 2003 are investigated using a combination of ground-based Global Positioning System (GPS) total electron content (TEC), and the meridian chain of ionosondes distributed along the Latin America longitude of ~280° E. Both the ground-based GPS TEC and ionosonde electron density profile data reveal significant enhancements at mid-low latitudes over the 280° E region during the main phase of the November 2003 superstorm. The maximum enhancement of the topside ionospheric electron content is 3.2–7.7 times of the bottomside ionosphere at the locations of the ionosondes distributed around the mid- and low latitudes. Moreover, the height of maximum electron density exceeds 400 km and increases by 100 km compared with the quiet day over the South American area from middle to low latitudes, which might have resulted from a continuous eastward penetration electric field and storm-generated equatorward winds. Our results do not support the conclusions of Yizengaw et al. (2006), who suggested that the observed positive storm over the South American sector was mainly the consequence of the changes of the bottomside ionosphere. The so-called "unusual" responses of the topside ionosphere for the November 2003 storm in Yizengaw et al. (2006) are likely associated with the erroneous usage of magnetometer and incomplete data.

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

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

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.

Evidence for stratosphere sudden warming‐ionosphere coupling due to vertically propagating tides

Observations of the Global Positioning System (GPS) total electron content (TEC) are used to study the coupling between the 2009 sudden stratospheric warming (SSW) and ionospheric perturbations. The observations reveal both migrating and nonmigrating perturbations to the semidiurnal tide in the equatorial ionization anomaly crest region that are associated with changes in electric fields induced by the tidal dynamo. In particular, a significant enhancement is observed in the nonmigrating semidiurnal westward propagating tide with zonal wavenumber 1 (SW1) in GPS TEC during the SSW. The SW1 perturbations in the low‐latitude ionosphere are found to oscillate with a similar period as planetary wave‐1 activity in the Northern Hemisphere stratosphere. This connection is attributed to the nonlinear interaction between tides and planetary waves and strongly supports the theory that planetary wave‐tide interaction is the primary mechanism coupling SSWs to ionospheric variability. Enhancements are also observed in the nonmigrating semidiurnal tide with zonal wavenumber 0 (S0) during this time period and may be related to the nonlinear interaction between the migrating semidiurnal tide and planetary wave‐2. The connection between planetary wave‐2 and S0 is, however, less clear which may be attributed to differences in the zonal mean zonal winds in the mesosphere and lower thermosphere during the times of peak planetary wave‐1 and planetary wave‐2 activity. We conclude that the changing zonal winds during SSWs play an important role in the coupling between ionospheric variability and the forcing from planetary waves of lower atmospheric origin during SSWs.

Global structure of the lunar tide in ionospheric total electron content

The global structure of the lunar tide in the ionosphere is studied based on observations of the global positioning system (GPS) total electron content (TEC). The unprecedented spatial and temporal resolution afforded by the GPS TEC observations enables illustration of the latitude, longitude, solar local time, and seasonal variability of the lunar tide in the ionosphere. Based on analysis of the multi‐year mean from 1999–2008, the dominant component is generally the semidiurnal lunar tide. However, a significant diurnal variation is also observed in some instances. The semidiurnal component achieves a maximum amplitude of ∼6% in the equatorial ionization anomaly crest regions and a slight hemispheric asymmetry is present with larger amplitudes in the Northern Hemisphere. Furthermore, the maximum amplitudes in the semidiurnal lunar tide are found to occur during Northern Hemisphere winter and during 9–15 solar local time. During certain years, a secondary maxima in solar local time is also observed demonstrating the influence of the lunar tide on the prereversal enhancement. Lastly, the observations reveal significant longitudinal variability in the semidiurnal lunar tide that is most prominent between November and February. The presence of a longitudinal variation reveals the existence of nonmigrating components in addition to the dominant migrating semidiurnal lunar tide.

Ionosphere response to recurrent geomagnetic activity: Local time dependency

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.

A physical mechanism of positive ionospheric storms at low latitudes and midlatitudes

A physical mechanism of the positive ionospheric storms at low latitudes and midlatitudes is presented through multi‐instrument observations, theoretical modeling, and basic principles. According to the mechanism, an equatorward neutral wind is required to produce positive ionospheric storms. The mechanical effects of the wind (1) reduce (or stop) the downward diffusion of plasma along the geomagnetic field lines, (2) raise the ionosphere to high altitudes of reduced chemical loss, and hence (3) accumulate the plasma at altitudes near and above the ionospheric peak centered at around ±30° magnetic latitudes. Daytime eastward prompt penetration electric field (PPEF), if it occurs, also shifts the equatorial ionization anomaly crests to higher than normal latitudes, up to approximately ±30° latitudes. The positive ionospheric storms are most likely in the longitudes where the onset of the geomagnetic storms falls in the ionization production dominated morning‐noon local time sector when the plasma accumulation due to the mechanical effects of the wind largely exceeds the plasma loss due to the chemical effect of the wind. The mechanism agrees with the multi‐instrument observations made during the supergeomagnetic storm of 7–8 November 2004, with 18 h long initial phase (IP) and 10 h long main phase (MP). The observations, which are mainly in the Japanese‐Australian longitudes where the MP onset was in the morning (0600 LT, 2100 UT), show (1) strong positive ionospheric storms (in Ne, Nmax, hmax, Global Positioning System–total electron content (GPS‐TEC), and 630 nm airglow intensity) in both Northern and Southern hemispheres started at the morning (0600 LT) MP onset and lasted for a day, (2) repeated occurrence of strong eastward PPEF events penetrated after the MP onset and superposed with westward electric field started before the MP onset, and (3) storm time equatorward neutral winds (inferred from 1 and 2). Repeated occurrence of an unusually strong F3 layer with large density depletions around the equator was also observed during the morning‐noon MP.

Interannual variability in the longitudinal structure of the low-latitude ionosphere due to the El Niño-Southern Oscillation

[1] The ratio of monthly median foF2 values between the ionosondes at Maui (20.8°N, −156°E geographic; 21.2°N, −90.4°E geomagnetic) and Yamagawa (31.2°N, 130.6°E geographic; 20.6°N, −160.9°E geomagnetic) from January 1960 to June 1993 are used to investigate interannual variability in the wave-4 longitudinal structure of the low-latitude F region ionosphere. Analysis of Global Positioning System total electron content between January 1998 and December 2008 reveals that the ratio between these two locations is a suitable proxy for the amplitude of the wave-4 longitudinal structure in the Northern Hemisphere. Significant interannual variability is present in the foF2 ratio after removing the solar cycle and intra-annual variability. The remaining variability is thought to be due, in part, to changes in atmospheric and oceanic circulations arising from the El Nino–Southern Oscillation (ENSO). Wavelet analysis reveals that similar periodicities and occurrence times exist for the foF2 ratio monthly anomalies and sea surface temperature anomalies, represented by the Oceanic Nino Index (ONI). Furthermore, the yearly ONI extreme value is well correlated with the extreme value of the foF2 ratio monthly anomalies in the subsequent 5 months. This surprising connection is attributed to changes in tropospheric convection, and hence latent heating, associated with changing sea surface temperatures due to the ENSO. Changing distributions of latent heat tidal forcing are thought to induce changes in the strength of the upward and eastward propagating diurnal tide with zonal wave number s = −3 (DE3) which subsequently produces the observed changes in the amplitude of the wave-4 longitudinal structure through modulation of the electric fields generated by the dynamo mechanism in the ionospheric E region. Our results demonstrate that there is significant interannual variability in the ionospheric wave-4 longitudinal structure and further indicate that the ENSO phenomenon represents a source of ionospheric variability which has previously not been considered.

Observations of the ionospheric response to the 15 December 2006 geomagnetic storm: Long‐duration positive storm effect

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.

Characterizing the 10 November 2004 storm‐time middle‐latitude plasma bubble event in Southeast Asia using multi‐instrument observations

The development and dynamics of ionospheric plasma bubble (PB) irregularity during the super storm of 7–11 November 2004 are investigated using the data from a multi‐instrument network operated in Southeast Asia. Analysis of fluctuations in Global Positioning System total electron content (GPS TEC), ionosonde, GPS scintillation, and in situ satellite density data indicates a series of intense PB‐associated irregularities at equatorial, low, and middle latitudes in the Japanese longitude on 10 November. However, in the Chinese sector, the scintillations and PB irregularities are confined within the range of 20–50°N in geographic latitude and 110–125°E in geographic longitude. The absence of equatorial PB irregularities in this sector shows a major difference from that in the close‐by longitude Japanese sector. In the Southern Hemisphere Australian sector, the irregularities occurrence is present as a symmetrical distribution at conjugate latitudes. Combined analysis of the data from Osan and Wuhan ionosondes illustrates that the middle‐latitude spread F irregularities initially develop at the lower part of the F region and then distribute in the whole F region. This initiation of spread F at lower altitudes indicates that the middle‐latitude PB‐associated irregularities are locally generated. These results together with the irregularities occurrence sequence from higher to lower latitudes, and the onset time delay of several hours implies that the presence of PB‐associated irregularities within a latitude range of 20–50°N in the Chinese sector cannot be attributed to the effects of prompt penetration electric fields (PPEFs), although the equatorial PBs in the close‐by longitude are seen to be associated with PPEFs. The possible mechanism is the F region plasma instabilities triggered by wave structures, which act as an external driving force and seed active plasma dynamics and instability growth at middle latitude.

A recurrent neural network approach to quantitatively studying solar wind effects on TEC derived from GPS; preliminary results

Abstract. This paper attempts to describe the search for the parameter(s) to represent solar wind effects in Global Positioning System total electron content (GPS TEC) modelling using the technique of neural networks (NNs). A study is carried out by including solar wind velocity (Vsw), proton number density (Np) and the Bz component of the interplanetary magnetic field (IMF Bz) obtained from the Advanced Composition Explorer (ACE) satellite as separate inputs to the NN each along with day number of the year (DN), hour (HR), a 4-month running mean of the daily sunspot number (R4) and the running mean of the previous eight 3-hourly magnetic A index values (A8). Hourly GPS TEC values derived from a dual frequency receiver located at Sutherland (32.38° S, 20.81° E), South Africa for 8 years (2000–2007) have been used to train the Elman neural network (ENN) and the result has been used to predict TEC variations for a GPS station located at Cape Town (33.95° S, 18.47° E). Quantitative results indicate that each of the parameters considered may have some degree of influence on GPS TEC at certain periods although a decrease in prediction accuracy is also observed for some parameters for different days and seasons. It is also evident that there is still a difficulty in predicting TEC values during disturbed conditions. The improvements and degradation in prediction accuracies are both close to the benchmark values which lends weight to the belief that diurnal, seasonal, solar and magnetic variabilities may be the major determinants of TEC variability.

Modulation of the equatorial F‐region by the quasi‐16‐day planetary wave

In situ electron density measurements from the CHAMP satellite and global positioning system total electron content (TEC) observations are used to illustrate how quasi‐16‐day planetary wave activity influences the structure of the low‐latitude ionosphere. SABER temperature measurements reveal that quasi‐16‐day oscillations in the dynamo region (110 km) during the time interval of 1 December 2005 to 1 March 2006 are most dominant in the zonal mean. Enhancements in equatorial zonal mean temperatures in the E‐region are connected with enhancements at ±10–20° magnetic latitude in zonal mean CHAMP electron densities and TEC. At a near‐constant altitude of 350 km, the oscillations in daytime electron densities are greater than 40 percent of background levels. A local time effect is also observed in the planetary wave influence on the F‐region electron densities. These results demonstrate that vertically propagating planetary waves induce significant variability in the low‐latitude F‐region ionosphere and that these effects are seen at all longitudes.

Canadian High Arctic Ionospheric Network (CHAIN)

Polar cap ionospheric measurements are important for the complete understanding of the various processes in the solar wind‐magnetosphere‐ionosphere system as well as for space weather applications. Currently, the polar cap region is lacking high temporal and spatial resolution ionospheric measurements because of the orbit limitations of space‐based measurements and the sparse network providing ground‐based measurements. Canada has a unique advantage in remedying this shortcoming because it has the most accessible landmass in the high Arctic regions, and the Canadian High Arctic Ionospheric Network (CHAIN) is designed to take advantage of Canadian geographic vantage points for a better understanding of the Sun‐Earth system. CHAIN is a distributed array of ground‐based radio instruments in the Canadian high Arctic. The instrument components of CHAIN are 10 high data rate Global Positioning System ionospheric scintillation and total electron content monitors and six Canadian Advanced Digital Ionosondes. Most of these instruments have been sited within the polar cap region except for two GPS reference stations at lower latitudes. This paper briefly overviews the scientific capabilities, instrument components, and deployment status of CHAIN. This paper also reports a GPS signal scintillation episode associated with a magnetospheric impulse event. More details of the CHAIN project and data can be found at http://chain.physics.unb.ca/chain.