Showing papers on "TEC published in 2009"

Seismoionospheric GPS total electron content anomalies observed before the 12 May 2008 Mw7.9 Wenchuan earthquake

Abstract: [1] The global ionospheric map (GIM) is used to observe variations in the total electron content (TEC) of the global positioning system (GPS) associated with 35 M ≥ 6.0 earthquakes that occurred in China during the 10-year period of 1 May 1998 to 30 April 2008. The statistical result indicates that the GPS TEC above the epicenter often pronouncedly decreases on day 3–5 before 17 M ≥ 6.3 earthquakes. The GPS TEC of the GIM and electron density profiles probed by six microsatellites of FORMOSAT3/COSMIC (F3/C) are further employed to simultaneously observe seismoionospheric anomalies during an Mw7.9 earthquake near Wenchuan, China, on 12 May 2008. It is found that GPS TEC above the forthcoming epicenter anomalously decreases in the afternoon period of day 6–4 and in the late evening period of day 3 before the earthquake, but enhances in the afternoon of day 3 before the earthquake. The spatial distributions of the anomalous and extreme reductions and enhancements indicate that the earthquake preparation area is about 1650 km and 2850 km from the epicenter in the latitudinal and longitudinal directions, respectively. The F3/C results further show that the ionospheric F2 peak electron density, NmF2, and height, hmF2, significantly decreases approximately 40% and descends about 50–80 km, respectively, when the GPS TEC anomalously reduces.

TEC variations during low solar activity period (2005–2007) near the Equatorial Ionospheric Anomaly Crest region in India

Abstract: . The dual frequency signals from the GPS satellites recorded at Rajkot (22.29° N, 70.74° E, Geographic, 14.03° N Geomagnetic) near the Equatorial ionization anomaly crest in India have been analyzed to study the ionospheric variations in terms of Total Electron Content (TEC) for the low solar activity period from April 2005 to December 2007. In this study, we describe the diurnal and seasonal variations of TEC, solar activity dependence of TEC and effects of a space weather related event, a geomagnetic storm on TEC. The diurnal variation of TEC shows pre-dawn minimum for a short period of time, followed by a steep early morning increase and then reaches maximum value between 14:00 LT and 16:00 LT. The mean diurnal variations during different seasons are brought out. It is found that TEC at Rajkot is at its maximum during Equinoctial months (March, April, September, October), and minimum during the Winter months (November, December, January, February), with intermediate values during Summer months (May, June, July, August), showing a semi annual variation. TEC values have been decreasing since 2005, onwards showing positive correlation with solar activity. TEC variations during the geomagnetic storm commencing 24 August 2005 with Dst=−216 nT are analysed. TEC shows a positive ionospheric storm effect on the first day of the storm and negative ionospheric storm effect on the next day. The equatorial Electrojet control on the development of the equatorial anomaly is also demonstrated.

Propagation of tsunami‐driven gravity waves into the thermosphere and ionosphere

Abstract: [1] Recent observations have revealed large F-region electron density perturbations (∼100%) and total electron content (TEC) perturbations (∼30%) that appear to be correlated with tsunamis. The characteristic speed and horizontal wavelength of the disturbances are ∼200 m/s and ∼400 km. We describe numerical simulations using our spectral full-wave model (SFWM) of the upward propagation of a spectrum of gravity waves forced by a tsunami, and the interaction of these waves with the F-region ionosphere. The SFWM describes the propagation of linear, steady-state acoustic-gravity waves in a nonisothermal atmosphere with the inclusion of eddy and molecular diffusion of heat and momentum, ion drag, Coriolis force, and height-dependent mean winds. The tsunami is modeled as a deformation of our model lower boundary traveling at the shallow water wave speed of 200 m/s with a maximum vertical displacement of 50 cm and described by a modified Airy function in the horizontal direction. The derived vertical velocity spectrum at the surface describes the forcing at the lower boundary of the SFWM. A steady-state 1-D ionospheric perturbation model is used to calculate the electron density and TEC perturbations. The molecular diffusion strongly damps the waves in the topside (>300-km altitude) ionosphere. In spite of this, the F-region response is large, with vertical displacements of ∼2 to 5 km and electron density perturbations of ∼100%. Mean winds have a profound effect on the ability of the waves to propagate into the F-region ionosphere.

Two-mode long-distance propagation of coseismic ionosphere disturbances

Abstract: [1] Using GPS total electron content (TEC) measurements, we analyzed ionosphere response to the great Kurile earthquake of 4 October 1994. High spatial resolution of the Japanese dense array of GPS receivers (GEONET) provided us the unique opportunity to observe the evolution of coseismic ionospheric disturbances (CID), which propagated for more than 1800 km away from the epicenter. Plotting a traveltime diagram for the CID and using an “array processing” technique within the approximation of a spherical CID wavefront, we observed a phenomenon of CID separation into two modes and we found that characteristics of the CID depend on the distance from the epicenter. The maximum of the CID amplitude was observed at ∼500 km from the epicenter. Within the first 600–700 km, the CID propagation velocity was about 1 km/s, which is equal to the sound speed at the height of the ionospheric F-layer. Starting from ∼600 to 700 km out from the epicenter, the disturbance seems to divide into two separate perturbations, with each propagating at a different velocity, about 3 km/s for the one and about 600 m/s for the other. Apparently, the TEC response in the far-field of the CID source is a mixture of signals that further “splits” into two modes because of the difference in their velocities. Our observations are in good agreement with the results of space-time data processing in the approximation of a spherical wavefront of CID propagation.

Climatology of the mean total electron content derived from GPS global ionospheric maps

Abstract: [1] We analyzed 11 years' (1998–2008) worth of the total electron content (TEC) data derived at the Jet Propulsion Laboratory (JPL) from Global Positioning System (GPS) observations to investigate the overall climatological features of the ionosphere in a new way. The global ionospheric maps (GIM) of JPL TEC are averaged globally and over low-, middle-, and high-latitude ranges in the southern (northern) hemisphere and both hemispheres to identify their capability of capturing the overall features of the ionosphere. These mean TEC data show strong annual/semiannual, solar cycle, and 27-day variations. The mean TEC presents stronger solar activity sensitivity at lower-latitude bands. Moreover, the saturation effect exists in these mean TEC versus solar index F10.7, more pronounced at low latitudes, while the mean TEC increases faster with higher solar EUV fluxes, being evident at high latitudes. The annual asymmetry (differences in June and December solstices) can be detected in the mean TEC averaged globally and at low latitudes under all solar epochs as well as at middle and high latitudes under most solar activities. The hemispheric asymmetry of the TEC in conjugate hemispheres follows the control of solar declination. Both the hemispheric differences and annual asymmetry are more marked with increasing solar activity. The annual components of the mean TEC are stronger in the southern hemisphere, and the semiannual components are of similar phases and comparable amplitudes in conjugate hemispheres, which suggest close couplings of the ionosphere in both hemispheres. Further, the mean TEC averaged in one hemisphere can reliably be used as ionospheric indices to monitor the solar activity variabilities and to capture the overall climatological features of the ionosphere over specified regions, while it should be cautioned that the mitigation of the dominant annual components with opposite phases in conjugate hemispheres leaves a significant semiannual component in the mean TEC averaged in both hemispheres, especially under low solar activity.

Storm time density enhancements in the middle-latitude dayside ionosphere

Abstract: [1] Enhancements of the total electron content (TEC) in the middle-latitude dayside ionosphere have often been observed during geomagnetic storms. The enhancements can be as large as a factor of 2 or more, and many sightings of such structures have occurred over the United States. Here we investigate the effectiveness of an expanded convection electric field as a mechanism for producing such ionospheric enhancements. As a test case, we examine the storm period of 5-7 November 2001, for which observations from the DMSP F 13 are used to drive the Time Dependent Ionospheric Model (TDIM). Our findings indicate that at favorable universal times, the presence of the expanded electric field is sufficient to create dayside TEC enhancements of a factor of 2 or more. The modeled enhancements consist of locally produced plasma; we do not find it necessary to transport high-density plasma northward from low latitudes.

Statistical analysis of solar activity variations of total electron content derived at Jet Propulsion Laboratory from GPS observations

Abstract: [1] We analyzed the data series of the total electron content (TEC) derived at Jet Propulsion Laboratory from Global Positioning System (GPS) observations to investigate the solar activity effects of TEC on a global scale. The daily values of the solar extreme ultraviolet (EUV) fluxes in 0.1–50 nm wavelengths, 10.7 cm radio flux F10.7, and F10.7P (the average of daily F10.7 and its 81-day mean F10.7A) are adopted to represent the solar EUV variability, respectively. The EUV fluxes are measured by the Solar EUV Monitor (SEM) spectrometer aboard Solar Heliospheric Observatory (SOHO). Three kinds of patterns (linearity, saturation, and amplification) can be detected in TEC versus F10.7P and EUV. A saturation feature exists in TEC versus F10.7 in the daytime, more pronounced at low latitudes than at middle and high latitudes. The saturation in the equatorial anomaly regions is strongest in equinoxes and weakest in the June solstice. In contrast, the amplification in TEC, as a novel feature, is mainly distributed in the northern middle, and high latitudes in the December solstice and in the Southern Hemisphere in the June solstice and the March equinox. It is the first time to determine where and when the linear, saturation, and amplification patterns are distributed in TEC. Further, the solar activity sensitivity of TEC is stronger at day than at night and more evident at lower latitudes than at higher latitudes. The solar activity dependent rates of TEC in the equatorial and low-latitude regions have a minimum around the dip equator and maxima on both sides of the dip equator (near the crest of the equatorial anomaly). This structure is roughly aligned along the dip equator, being strongest in equinoxes and weakest in the June solstice, which highlights the importance of ionospheric dynamics related with E × B drift. In addition, this analysis confirms that in a statistical sense, a quadrate polynomial can well capture the long-term solar activity dependency of TEC at specified local time.

Three-dimensional ionospheric electron density structure of the Weddell Sea Anomaly

Abstract: [1] This paper provides the first three-dimensional description of the ionospheric density structure of the Weddell Sea Anomaly (WSA). The WSA is characterized by a nighttime ionospheric density greater than that in daytime in the Weddell Sea region around the Antarctic Peninsula during the southern hemisphere summer. It was first observed by ground-based ionosondes located in the Antarctica back in the 1950s and was further investigated by two-dimensional maps over the oceans using TEC measurements collected by the TOPEX/Poseidon recently. Although these TEC maps have provided two-dimensional views for tracking the time-evolution and spatial coverage of the WSA, the vertical distribution of this peculiar feature is still unavailable. With the vertical ionospheric density profiles observed by the FORMOSAT-3/COSMIC, three-dimensional density structure of the WSA is presented here for the first time. Meanwhile, a similar WSA signature is observed in the northern and eastern hemisphere during June solstice by both the GPS-TEC and the FORMOSAT-3/COSMIC electron density observations. From the observed altitudinal structure of the WSA during 1800–2400 LT and the similar feature occurred in the opposite hemisphere suggest that the southward offset of the magnetic equator with respect to the geographic equator plays a major role for the WSA formation.

Seasonal and solar activity variations of the Weddell Sea Anomaly observed in the TOPEX total electron content measurements

Abstract: [1] The Weddell Sea Anomaly (WSA) in the ionosphere is characterized by higher plasma densities at night than during the day in the region near the Weddell Sea. According to previous studies on the WSA, it is known to occur mostly in southern summer and has not been reported in other seasons. We have utilized more than 13 years of TOPEX total electron content (TEC) measurements in order to study how the WSA varies with seasons and how it changes with solar activity. The TOPEX TEC data have been extensively utilized for climatological studies of the ionosphere because of their excellent spatial and temporal coverage. We investigate the seasonal and solar activity variations of the WSA using four seasonal cases (March equinox, June solstice, September equinox, and December solstice) and two solar activity conditions (F10.7 120 for solar maximum conditions) for geomagnetically quiet periods. Our analysis shows that (1) the WSA occurs only in the southern summer hemisphere for low F10.7, as in previous studies, but (2) the WSA occurs in all seasons except for winter when F10.7 is high; it is most prominent during the December solstice (southern summer) and still strong during both equinoxes. The TOPEX TEC maps in the midlatitude and high-latitude ionosphere display significant global longitudinal variations for a given local time in the Southern Hemisphere, which varies with season and solar activity. The observed WSA appears to be an extreme manifestation of the longitudinal variations.

Geomagnetic storm effects on GPS based navigation

Abstract: . The energetic events on the sun, solar wind and subsequent effects on the Earth's geomagnetic field and upper atmosphere (ionosphere) comprise space weather. Modern navigation systems that use radio-wave signals, reflecting from or propagating through the ionosphere as a means of determining range or distance, are vulnerable to a variety of effects that can degrade the performance of the navigational systems. In particular, the Global Positioning System (GPS) that uses a constellation of earth orbiting satellites are affected due to the space weather phenomena. Studies made during two successive geomagnetic storms that occurred during the period from 8 to 12 November 2004, have clearly revealed the adverse affects on the GPS range delay as inferred from the Total Electron Content (TEC) measurements made from a chain of seven dual frequency GPS receivers installed in the Indian sector. Significant increases in TEC at the Equatorial Ionization anomaly crest region are observed, resulting in increased range delay during the periods of the storm activity. Further, the storm time rapid changes occurring in TEC resulted in a number of phase slips in the GPS signal compared to those on quiet days. These phase slips often result in the loss of lock of the GPS receivers, similar to those that occur during strong(>10 dB) L-band scintillation events, adversely affecting the GPS based navigation.

Characteristics of the ionospheric total electron content of the equatorial ionization anomaly in the Asian-Australian region during 1996-2004

Abstract: . Ionospheric total electron content (TEC) of the equatorial ionization anomaly (EIA) is studied by analyzing dual-frequency signals of the Global Position System (GPS) acquired from a network of receivers around the Asian-Australian region during 1996–2004. The latitude, occurrence time, strength of the most developed EIA crest, and crest-to-trough ratio (CTR) for both the noon and post-sunset sector obtained from a daily TEC contour map have been used to study the solar cycle variations of EIA in the Asian-Australian region. The results reveal that semiannual and seasonal variations were the dominant factor that controls the morphology of the EIA structure which can be identified in the past studies (e.g. Wu et al., 2008). It is also found that the latitude and local time position of the anomaly crest show a hemispheric asymmetry because (a) The northern crest of EIA is expanded during the equinox indicating a weak semiannual variation while the southern crest is inhibited during June–August presenting a strong seasonal variation, and (b) The local time of the northern crest appears ~1.3 h earlier than that of the southern crest in June while showing no difference at December. Solar activity dependence is more evident in the EIA crest region than in the EIA trough region and least in the post-sunset sector at equinox. A seasonal linear relationship is derived between the post-sunset CTR and solar flux, which should be caused by the solar-dependant equatorial E×B vertical drift.

Medium-scale traveling ionospheric disturbances observed with the SuperDARN Hokkaido radar, all-sky imager, and GPS network and their relation to concurrent sporadic E irregularities

Abstract: [1] We present midlatitude medium-scale traveling ionospheric disturbances (MSTIDs) observed with a Super Dual Auroral Radar Network (SuperDARN) HF radar at around 10 MHz in Hokkaido, Japan, in combination with a 630-nm all-sky imager and a GPS network (GEONET) that provides total electron content (TEC) data. MSTIDs propagating southward from high latitudes are detected at first with the HF radar and then with the imager and GEONET. We analyze two MSTID events, one in winter (event 1) and the other in summer (event 2), to find that MSTIDs appear simultaneously, at least, at 55°–25°N. It is shown that nighttime MSTIDs propagate toward the southwest over a horizontal distance of about 4000 km, and daytime MSTIDs do so toward the southeast. Daytime radar echoes are due to ground/sea surface (GS) scatter, while nighttime echoes in event 1 return from 15-m-scale F region field-aligned irregularities (FAIs) and those in event 2 are due to GS scatter. Doppler velocities of the nighttime F region FAI echoes in event 1 are negative (motion away from the radar) within strong echo regions and are positive (motion toward the radar) within weak echo regions. This fact suggests that the strong (weak) echoes return from suppressed (enhanced) airglow/TEC areas, in line with previous observations over central Japan. The nighttime MSTIDs in events 1 and 2 are often accompanied by concurrent coherent echoes from FAIs in sporadic E (Es) layers. The Es echo areas in event 2 rather coincide with suppressed airglow/TEC areas in the F region that are connected with the echo areas along the geomagnetic field, indicating the existence of E and F region coupling at night.

Dependence of waveform of near-field coseismic ionospheric disturbances on focal mechanisms

Abstract: Using Total Electron Content (TEC) measurements with Global Positioning System we studied ionospheric responses to three large earthquakes that occurred in the Kuril Arc on 04 October 1994, 15 November 2006, and 13 January 2007. These earthquakes have different focal mechanisms, i.e. high-angle reverse, low-angle reverse, and normal faulting, respectively. TEC responses to the 2006 and 2007 events initiated with positive and negative changes, respectively. On the other hand, the initial TEC changes in the 1994 earthquake showed both positive and negative polarities depending on the azimuth around the focal area. Such a variety may reflect differences in coseismic vertical crustal displacements, which are dominated by uplift and subsidence in the 2006 and 2007 events, respectively, but included both in the 1994 event.

Comparison of a CFD Analysis and a Thermal Equivalent Circuit Model of a TEFC Induction Machine With Measurements

Abstract: For a totally enclosed fan-cooled induction machine, two methods of numerical analysis are compared with measurements. The first numerical method is based on computational fluid dynamics (CFDs) and the second one uses a thermal equivalent circuit (TEC). For the analysis based on CFD, a 3-D induction machine including housing is modeled. The numeric solution of the flow equations is determined for stationary temperature distributions. For the TEC, a discretized one-and-a-half-dimensional model of the induction machine is considered. With the TEC model, stationary and transient operating conditions can be simulated. Measurement results are determined by iron-copper-nickel sensors embedded in the stator winding and the housing, as well as by an IR sensor for measuring the rotor temperature. With these measurement signals, stationary and transient operating conditions can be analyzed. For stationary operating conditions, additionally, the housing temperatures are determined by an IR camera. The investigated simulation and measurement methods reveal different local and global temperatures, and thus, only certain aspects and characteristics of the obtained temperatures can be compared. Nevertheless, certain conclusions can be drawn from comparing these aspects considering the actual restrictions of each of the applied methods.

Storm induced large scale TIDs observed in GPS derived TEC

Abstract: . This work is a first statistical analysis of large scale traveling ionospheric disturbances (LSTID) in Europe using total electron content (TEC) data derived from GNSS measurements. The GNSS receiver network in Europe is dense enough to map the ionospheric perturbation TEC with high horizontal resolution. The derived perturbation TEC maps are analysed studying the effect of space weather events on the ionosphere over Europe. Equatorward propagating storm induced wave packets have been identified during several geomagnetic storms. Characteristic parameters such as velocity, wavelength and direction were estimated from the perturbation TEC maps. Showing a mean wavelength of 2000 km, a mean period of 59 min and a phase speed of 684 ms−1 in average, the perturbations are allocated to LSTID. The comparison to LSTID observed over Japan shows an equal wavelength but a considerably faster phase speed. This might be attributed to the differences in the distance to the auroral region or inclination/declination of the geomagnetic field lines. The observed correlation between the LSTID amplitudes and the Auroral Electrojet (AE) indicates that most of the wave like perturbations are exited by Joule heating. Particle precipitation effects could not be separated.

Latitudinal dependence of the ionospheric response to solar eclipses

Abstract: [1] In this study, we statistically analyze the latitudinal dependence of F2-layer peak electron densities (NmF2) and total electron content (TEC) responses to solar eclipses by using the ionosonde observations during 15 eclipse events from 1973 to 2006 and the GPS TEC observations during six solar eclipse events from 1999 to 2006. We carried out a model study on the latitudinal dependence of eclipse effects on the ionosphere by running a theoretical ionospheric model with the total eclipse occurring at 13 latitudes from 0 Nt o 60N at intervals of 5. Both the observations and simulations show that the NmF2 and TEC responses have the same latitudinal dependence: the eclipse effects on NmF2 and TEC are smaller at low latitudes than at middle latitudes; at the middle latitudes (>40), the eclipse effect decreases with increasing latitude. The simulations show that the smaller NmF2 responses at low latitudes are mainly because of much higher heights of hmF2 at low latitudes and electron density response decreasing rapidly with increasing height. For the eclipse effects at the middle latitudes (>40), the simulations show that the smaller NmF2 or TEC response at higher latitude is mainly ascribed to the larger downward diffusion flux induced by the larger dip angle at this region, which can partly make up for the plasma loss and alleviate the depression of electron density in the F region. The simulated results show that there is an overall decrease in electron temperature throughout the entire height range at the middle latitude, but for the low latitudes the eclipse effect on electron temperature is much smaller at high heights, which is mainly because of the much smaller reduction of photoelectron production rate at its conjugate low heights where only a partial eclipse with small eclipse magnitude occurs.

An investigation of thermoelectric cooling devices for small-scale space conditioning applications in buildings

Abstract: This paper presents the study of a thermoelectric cooler (TEC) designed for small-scale space conditioning applications in buildings. A theoretical study was undertaken to find the optimum operating conditions, which were then applied in the laboratory testing work. A TEC unit was assembled and tested under laboratory conditions. Eight pieces of UltraTEC were shown to generate up to 220 W of cooling with a COP of 0.46 under the input current of 4.8 A for each module. Thermo-economical analysis was carried out and results showed that a system with PV panel can compete with an equivalent system without a PV panel when PV costs fall down to or lower than £1.25 per Watt. For the cases without a PV panel, the system with a high level of TEC power input delivered a better performance in terms of the average cooling energy price than that system with a low level of TEC power input after critical interest rate (currently 4%).

First observations of large‐scale wave structure and equatorial spread F using CERTO radio beacon on the C/NOFS satellite

Abstract: [1] First observations of large-scale wave structure (LSWS) and the subsequent development of equatorial spread F (ESF), using total electron content (TEC) derived from the ground based reception of beacon signals from the CERTO (Coherent Electromagnetic Radio Tomography) radio beacon on board C/NOFS (Communications/Navigation Outage Forecasting System) satellite, are presented. Selected examples of TEC variations, using measurements made during January 2009 from Bac Lieu, Vietnam (9.2°N, 105.6°E geographic, 1.7°N magnetic dip latitude) are presented to illustrate two key findings: (1) LSWS appears to play a more important role in the development of ESF than the post-sunset rise (PSSR) of the F-layer, and (2) LSWS can appear well before E region sunset. Other findings, that LSWS does not have significant zonal drift in the initial stages of growth, and can have zonal wavelengths of several hundred kilometers, corroborate earlier reports.

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

Abstract: [1] 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.

First tomographic observations of the Midlatitude Summer Nighttime Anomaly over Japan

Abstract: [1] Recently, a chain of digital beacon receivers has been established over Japan, mainly for the tomographic imaging of the ionosphere. These receivers are installed at Shionomisaki (33.45°N, 135.8°E), Shigaraki (34.85°N, 136.1°E), and Fukui (36.06°N,136°E), which continuously track the Low Earth Orbiting Satellites (LEOS), and the simultaneous line-of-sight Total Electron Content (TEC) data are used for tomographic reconstruction. In the images obtained during July 2008, it is seen that the nighttime electron densities exceed the daytime values on almost all days over latitudes >33–34°N. On several days, these northern latitudes show enhanced electron densities compared to the low-latitude region during nighttime. These are the prominent features of the “Midlatitude Summer Nighttime Anomaly (MSNA)” that is recently observed in the northern hemisphere and is considered similar to the nighttime Weddell Sea Anomaly (WSA). This is the first study of the MSNA using tomographic technique and found its significant day-to-day variability. The ionosonde data from Wakkanai (45.4°N, 141.7° E), ground-based GPS TEC observations using the GEONET, CHAMP in situ electron density measurements, and Formosat3/COSMIC (F3/C) occultation measurements are also used to confirm the presence of MSNA over this region and to examine its variability. It is seen that, in general, during the local summer period, electron density over the northern latitudes is highest at ∼2000–2100 LT and the latitudinal enhancement in electron density also begins to appear around the same time, which continues to exist even at later hours.

Simultaneous observation of traveling ionospheric disturbances in the Northern and Southern Hemispheres

Abstract: . Measurements of total electron content (TEC) using 263 GPS receivers located in the North and South America continents are presented to demonstrate the simultaneous existence of traveling ionospheric disturbances (TID) at high, mid, and low latitudes, and in both Northern and Southern Hemispheres. The TID observations pertain to the magnetically disturbed period of 29–30 October 2003 also known as the Halloween storm. The excellent quality of the TEC measurements makes it possible to calculate and remove the diurnal variability of TEC and then estimate the amplitude, wavelength, spectral characteristics of the perturbations, and the approximate velocity of the AGW. On 29 October 2003 between 17:00 and 19:00 UT, there existed a sequence of TEC perturbations (TECP), which were associated with the transit of atmospheric gravity waves (AGW) propagating from both auroral regions toward the geographic equator. A marked difference was found between the northern and southern perturbations. In the Northern Hemisphere, the preferred horizontal wavelength varies between 1500 and 1800 km; the propagation velocity is near 700 m/s and the perturbation amplitude about 1 TEC unit (TECu). South of the geographic equator the wavelength of the TECP is as large as 2700 km, the velocity is about 550 m/s, and the TECP amplitude is 3 TECu. Concurrently with our observations, the Jicamarca digisonde observed virtual height traces that exhibited typical features that are associated with TIDs. Here, it is suggested that differences in the local conductivity between northern and southern auroral ovals create a different Joule heating energy term. The quality of these observations illustrates the merits of GPS receivers to probe the ionosphere and thermosphere.

First observations of large-scale wave structure and equatorial spread F using CERTO radio beacon on the C/NOFS satellite

Abstract: [1] First observations of large-scale wave structure (LSWS) and the subsequent development of equatorial spread F (ESF), using total electron content (TEC) derived from the ground based reception of beacon signals from the CERTO (Coherent Electromagnetic Radio Tomography) radio beacon on board C/NOFS (Communications/Navigation Outage Forecasting System) satellite, are presented. Selected examples of TEC variations, using measurements made during January 2009 from Bac Lieu, Vietnam (9.2°N, 105.6°E geographic, 1.7°N magnetic dip latitude) are presented to illustrate two key findings: (1) LSWS appears to play a more important role in the development of ESF than the post-sunset rise (PSSR) of the F-layer, and (2) LSWS can appear well before E region sunset. Other findings, that LSWS does not have significant zonal drift in the initial stages of growth, and can have zonal wavelengths of several hundred kilometers, corroborate earlier reports.

Large enhancements in low latitude total electron content during 15 May 2005 geomagnetic storm in Indian zone

Abstract: . Results pertaining to the response of the equatorial and low latitude ionosphere to a major geomagnetic storm that occurred on 15 May 2005 are presented. These results are also the first from the Indian zone in terms of (i) GPS derived total electron content (TEC) variations following the storm (ii) Local low latitude electrodynamics response to penetration of high latitude convection electric field (iii) effect of storm induced traveling atmospheric disturbances (TAD's) on GPS-TEC in equatorial ionization anomaly (EIA) zone. Data set comprising of ionospheric TEC obtained from GPS measurements, ionograms from an EIA zone station, New Delhi (Geog. Lat. 28.42° N, Geog. Long. 77.21° E), ground based magnetometers in equatorial and low latitude stations and solar wind data obtained from Advanced Composition Explorer (ACE) has been used in the present study. GPS receivers located at Udaipur (Geog. Lat. 24.73° N, Geog. Long. 73.73° E) and Hyderabad (Geog. Lat. 17.33° N, Geog. Long. 78.47° E) have been used for wider spatial coverage in the Indian zone. Storm induced features in vertical TEC (VTEC) have been obtained comparing them with the mean VTEC of quiet days. Variations in solar wind parameters, as obtained from ACE and in the SYM-H index, indicate that the storm commenced on 15 May 2005 at 02:39 UT. The main phase of the storm commenced at 06:00 UT on 15 May with a sudden southward turning of the Z-component of interplanetary magnetic field (IMF-Bz) and subsequent decrease in SYM-H index. The dawn-to-dusk convection electric field of high latitude origin penetrated to low and equatorial latitudes simultaneously as corroborated by the magnetometer data from the Indian zone. Subsequent northward turning of the IMF-Bz, and the penetration of the dusk-to-dawn electric field over the dip equator is also discernible. Response of the low latitude ionosphere to this storm may be characterized in terms of (i) enhanced background level of VTEC as compared to the mean VTEC, (ii) peaks in VTEC and foF2 within two hours of prompt penetration of electric field and (iii) wave-like modulations in VTEC and sudden enhancement in hmF2 within 4–5 h in to the storm. These features have been explained in terms of the modified fountain effect, local low latitude electrodynamic response to penetration electric field and the TIDs, respectively. The study reveals a strong positive ionospheric storm in the Indian zone on 15 May 2005. Consequences of such major ionospheric storms on the systems that use satellite based navigation solutions in low latitude, are also discussed.

Excitation of 4-min periodic ionospheric variations following the great Sumatra-Andaman earthquake in 2004

Abstract: [1] Periodic variations of the ionosphere were detected by ground-based GPS total electron content (TEC) measurements after the Sumatra-Andaman earthquake in 2004. The observational data showed that the 4-min periodic TEC variations occurred 1 h after the earthquake and continued for longer than 4 h. At the PHKT station, about 30 cycles of the 4-min periodic TEC variations were observed from 0230 to 0430 UT. The maximum peak-to-peak amplitude of the variations was about 0.6 total electron content unit (TECU; 1 TECU = 1016 el m−2) around 0320 UT. The frequency of these periodic variations was 3.9 mHz. They were detected by the SAMP station in the northern Sumatra and the PHKT and BNKK stations in Thailand with the signal from seven GPS satellites: PRN 8, 11, 13, 19, 23, 27, and 31. They were observed in a limited area from 4°N to 15°N in latitude and from 96°E to 101°E in longitude, although the western boundary was not certain because of the limit of the observational field of view. The amplitude of these TEC variations showed the dependence on the zenith angle of the path between the GPS receiver and satellite. The amplitude had a maximum when the zenith angle was the smallest. This could be caused by the vertical structure of the electron density variations. This also suggested that the electrons were oscillating along the geomagnetic field. The 4-min periodic TEC variations were interpreted to be induced by the long-lasting free oscillation of the atmosphere set up by the earthquake. Their long duration also indicates that they were generated by a nontransient process like resonance.

Regional TEC mapping using neural networks

Abstract: [1] Characterization and modeling of ionospheric variability in space and time is very important for communications and navigation. To characterize the variations, the ionosphere should be monitored, and the sparsity of the measurements has to be compensated by interpolation algorithms. The total electron content (TEC) is a major parameter that can be used to obtain regional ionospheric maps. In this study, neural networks (NNs), specifically multilayer perceptrons (MLPs) and radial basis function networks (RBFN), are investigated for the merits of their nonlinear modeling capability. In order to assess the performance of MLP and RBFN structures with respect to mapping and ionospheric parameters, these algorithms are applied to synthetically generated TEC surfaces representing various ionospheric states. Synthetic TEC data are sampled homogenously and randomly for a varying number of data points. The reconstruction errors show that the performance improves significantly when homogenous sampling is preferred to random station distribution. The best MLP and RBFN structures for any possible realistic scenario are determined by examining the performance parameters for all possible cases. It is also observed that RBFN with local receptive fields relies more on the number of training data points. In contrast to RBFN, MLP as a global approximator depends strongly on ionospheric trends. Finally, chosen MLP and RBFN models are applied to a set of real GPS-TEC values obtained from central Europe, and their performances are compared with well known Global Ionospheric Maps produced by the International GNSS Service. The resolution and interpolation quality of the generated maps indicate that NNs offer a powerful and reliable alternative to the conventional TEC mapping algorithms.