Showing papers on "Total electron content published in 2001"

Variations of ionospheric total electron content during the Chi-Chi earthquake

Abstract: On 20 September1999 UT (21 Septemberin local time, LT) a large earthquake Mw=7.7 struck central Taiwan nearthe small town of Chi-Chi. The greatest plasma frequency in the ionosphere, foF2, observed by the Chung- Li ionosonde (25.0 ◦ N, 121.2 ◦ E) reveals three clear precur- sors at 1, 3, and 4 days prior to the earthquake. This paper examines the ionospheric total electron content (TEC) ob- served by a network of the global positioning system (GPS) receivers in Taiwan area. It is found that variations in foF2 and overhead TEC recorded at Chung-Li have a similar ten- dency. Combining the data of the network of 13 GPS re- ceivers, time, and spatial variations of TEC prior to the Chi- Chi earthquake are examined. Results show that the equa- torial anomaly crest moves equatorward and its TEC value significantly decreases 1, 3, and 4 days before the earth- quake. A comparison between the disturbed and reference (previous 15-day median) days confirms that TEC decreases significantly around the epicenter in the afternoons of these days. Finally, possible mechanisms are proposed and dis- cussed.

Ionospheric effects of major magnetic storms during the International Space Weather Period of September and October 1999: GPS observations, VHF/UHF scintillations, and in situ density structures at middle and equatorial latitudes

Abstract: In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22–23, 1999, with the hourly Dst index being −167 nT at ∼2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of −231 nT at ∼0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS)-based navigation system called Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration for use in the continental United States and their impact on global VHF/UHF communication systems. It is shown that the penetration of transient magnetospheric electric fields equatorward of the shielding region at midlatitudes, which have been well-correlated in the past with rapid changes in the well-known Dst index (or through its recently available high resolution 1-min counterpart the SYM-H index), can cause large increases of total electron content (TEC), TEC fluctuations, and saturated 250-MHz scintillation, and these, in turn, may have significant impacts on WAAS. The local time of Dst changes (and not just Dst magnitude) was found to be very important for WAAS, since the largest effects on TEC are seen near dusk. The prompt penetration of these magnetospheric electric fields all the way to the magnetic equator causes augmentation or inhibition of equatorial spread F. The global ionospheric response to these storms has been obtained from ground-based TEC observations with a GPS network and space-based in situ density and electric field measurements using the Republic of China Satellite-1 (ROCSAT-I) and several Defense Meteorological Satellite Program satellites. These prompt penetration electric fields cause VHF/UHF scintillations and GPS TEC variations at low latitudes in the specific longitude sector for which the early evening period corresponds to the time of rapid Dst variations and maximum Dst phase. The effects of the delayed ionospheric disturbance dynamo and those of decreased magnetospheric convection on postmidnight irregularity generation are shown to be confined to a part of the same longitude range that actively responded to the prompt penetration of electric fields in the early evening sector.

Response of the equatorial ionosphere in the South Atlantic Region to the Great Magnetic Storm of July 15, 2000

Abstract: The effects of the great magnetic storm of July 15, 2000 on the equatorial ionosphere have been studied by ground-based and satellite in-situ measurements. A large westward plasma drift in the evening equatorial ionosphere was observed as a result of the ionospheric disturbance dynamo. In that environment, the IMF Bz turned southward and presumably caused penetration of E-fields to low latitudes. This E-field initiated the onset of 250 MHz and L-band scintillations at Ascension Island (15°W) and precipitous TEC decrease at Fortaleza, Brazil (38°W), bounding the narrow longitude region in the South Atlantic. These impulsive ionospheric effects were extremely well correlated with abrupt decreases of SYM-H (1-min resolution Dst). The DMSP in-situ measurements showed the presence of severe ion density bite-outs extending over 30° latitude in the South Atlantic Magnetic Anomaly region. The ROCSAT-1 satellite measured upward and large southward ion drifts in the same sector.

Vertical electron content from ionograms in real time

Abstract: A new technique for calculating the vertical total electron content (TEC) from ground-based ionosonde measurements is introduced. The ionogram provides the information to directly calculate the vertical electron density profile up to the peak of the F2 layer. The profile above the peak is approximated by an α-Chapman function with a scale height that is derived from the profile shape around the F2 peak. The ionosonde TEC, or ITEC, is then calculated as the integral from 0 to ∞ over the entire profile. ITEC values from Digisonde observations at Millstone Hill, Wallops Island, and Jicamarca are compared with incoherent scatter radar and with Faraday and TOPEX satellite TEC measurements, showing very good agreement at middle latitudes and the magnetic equator.

The shock-acoustic waves generated by earthquakes

Abstract: . We investigate the form and dynamics of shock-acoustic waves generated by earthquakes. We use the method for detecting and locating the sources of ionospheric impulsive disturbances, based on using data from a global network of receivers of the GPS navigation system, and require no a priori information about the place and time of the associated effects. The practical implementation of the method is illustrated by a case study of earthquake effects in Turkey (17 August and 12 November 1999), in Southern Sumatra (4 June 2000), and off the coast of Central America (13 January 2001). It was found that in all instances the time period of the ionospheric response is 180–390 s, and the amplitude exceeds, by a factor of two as a minimum, the standard deviation of background fluctuations in total electron content in this range of periods under quiet and moderate geomagnetic conditions. The elevation of the wave vector varies through a range of 20–44°, and the phase velocity (1100–1300 m/s) approaches the sound velocity at the heights of the ionospheric F-region maximum. The calculated (by neglecting refraction corrections) location of the source roughly corresponds to the earthquake epicenter. Our data are consistent with the present views that shock-acoustic waves are caused by a piston-like movement of the Earth’s surface in the zone of an earthquake epicenter. Key words. Ionosphere (ionospheric disturbances; wave propagation) – Radio science (ionospheric propagation)

Traveling ionospheric disturbances detected in the FRONT Campaign

Abstract: The F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign was conducted to clarify the non-classical features of traveling ionospheric disturbances (TIDs) at mid-latitudes in May, 1998 and August, 1999 A cluster of all-sky CCD cameras and a GPS receiver network observed a wide area of the ionosphere over Japan to detect the spatial structure and temporal evolution of TIDs The propagation direction of the nighttime TID detected during the FRONT campaign periods is restricted to the southwest The time evolution of their amplitude indicates that the TID structure is intensified as it travels from high-latitudes to low-latitudes The significant coincidence between the structures of 630 nm band airglow and total electron content indicates that the perturbations take place in the bottomside of the ionospheric F region Coherent echoes from the field-aligned irregularities were observed by the MU radar in the nights when the TID activity was high

Measurement of the latitudinal distributions of total electron content during equatorial spread F events

Abstract: We have constructed latitudinal profiles of the total electron content (TEC) using measurements from six GPS receivers conducted during 1998. The TEC profiles have been divided into two groups: One corresponds to days when plumes or equatorial spread F (ESF) develops, and the second group portrays days of no-ESF condition. The presence/absence of ESF is based on the signature of the coherent echoes measured by the Jicamarca Unattended Long-Term Investigation (JULIA) radar and records of scintillations from two sites spaced in latitude. One scintillation station is located near the magnetic equator (Ancon) and the other 12° southward (Antofagasta). The TEC profiles display the typical day-to-day and seasonal variability seen at low latitudes. During the equinoxes, we observed quite often the crests of the anomaly located between 12° and 20° away from the magnetic equator and a trough in-between. The monthly distribution of the appearance of the anomaly and the local time of their appearance are in very good agreement with the reported variability of the upward vertical drifts and the current theory of the equatorial fountain effect. During the equinoxes and the December solstice, the TEC anomaly is observed almost every day, sometimes when there is no ESF activity. Nevertheless, fine inspection of the TEC latitudinal profiles suggests the existence of a close relationship between the temporal evolution of the TEC profiles near sunset and the onset of ESF. We have examined the TEC latitudinal distributions in two different ways. First, we calculated time difference profiles using the distributions corresponding to 1800 and 2000 LT. Second, we used a parameterization of the TEC distributions obtained at 2000 LT. The first method indicates quite drastic increases of the crest values and sharp decreases near the trough during ESF days. In contrast, during days of no ESF there exist almost uniform TEC decreases at all latitudes. The second method displays a preferred high crest/trough ratio (>2), small TEC values at the trough, and large latitudinal integrated values during ESF events.

Geomagnetic control of the spectrum of traveling ionospheric disturbances based on data from a global gps network

Abstract: . In this paper an attempt is made to verify the hypothesis of the role of geomagnetic disturbances as a factor in determining the intensity of traveling ionospheric disturbances (TIDs). To improve the statistical validity of the data, we have used the method involving a global spatial averaging of disturbance spectra of the total electron content (TEC). To characterize the TID intensity quantitatively, we suggest that a new global index of the degree of disturbance should be used, which is equal to the mean value of the rms variations in TEC within the selected range of spectral periods (of 20– 60 min, in the present case). The analysis has been made for a set of 100 to 300 GPS stations for 10 days with a different level of geomagnetic activity (Dst from 0 to –350 nT; the Kp index from 3 to 9). It was found that power spectra of daytime TEC variations in the range of 20–60 min periods under quiet conditions have a power-law form with the slope index k = –2.5. With an increase in the level of magnetic disturbance, there is an increase in the total intensity of TIDs, with a concurrent kink of the spectrum caused by an increase in oscillation intensity in the range of 20–60 min. The TEC variation amplitude is found to be smaller at night than during the daytime, and the spectrum decreases in slope, which is indicative of a disproportionate increase in the amplitude of the small-scale part of the spectrum. It was found that an increase in the level of geomagnetic activity is accompanied by an increase in the total intensity of TEC; however, it does not correlate with the absolute level of Dst, but rather with the value of the time derivative of Dst (a maximum correlation coefficient reaches –0.94). The delay of the TID response of the order of 2 hours is consistent with the view that TIDs are generated in auroral regions, and propagate equatorward with the velocity of about 300–400 m/s. Key words. Ionosphere (ionospheric disturbances; auroral ionosphere; equatorial ionopshere)

A quantitative study of ionospheric density gradients at midlatitudes

Abstract: The subauroral ionosphere, at the magnetic latitudes which characterize the northeastern United States, is subject to severe F region ionospheric density structuring due to the space weather effects of magnetospheric disturbance electric fields. Communications and navigation systems relying on transionospheric propagation must be able to compensate for the effects of the sharp changes (>10X) in total electron content (TEC) associated with the ionospheric trough and storm time disturbance effects at midlatitudes. The Millstone Hill incoherent scatter radar database has been used to investigate the spatial extent and temporal evolution of TEC and density altitude/latitude structure at middle and subauroral latitudes as a function of solar cycle, local time, and level of geomagnetic activity. More than 11,000 radar elevation scans covering >20° of latitude and altitudes between 150 and 750 km have been used to identify the characteristics of the density gradient near the equatorward edge of the ionospheric trough in a variety of circumstances spanning 20 years and two solar cycles. Pronounced density gradients can be identified in ∼35% of the Millstone Hill scans, and we present a statistical characterization of average magnitude and location for these steepest TEC gradients. In some cases (especially near noon) the equatorward edge of the trough lies poleward of our observational field of view, and gradients associated with phenomena other than the trough contribute to our statistics. On most days the trough appears in the radar scans between 1600 and 2000 magnetic local time (MLT). Larger TEC gradients occur at solar maximum and when the background TEC is higher. The steepest gradients occur in an environment of high TEC (at solar maximum and adjacent to regions of storm-enhanced density (SED)), when the processes which generate the trough are strongest (high Kp). High gradient values occur in the sunlit sector, with maximum values of TEC gradient (∼10 TEC/deg latitude, with 1 TEC unit = 10 16 el m -2 ) found in the postnoon ionosphere. Mean solar maximum TEC gradient at 1600 MLT is 3-4 TEC/deg for Kp 100 over New England and TEC gradients of ∼50 TEC/deg.

Comparisons of GPS/MET retrieved ionospheric electron density and ground based ionosonde data

Abstract: The Global Positioning System/Meteorology (GPS/MET) mission has been the first experiment to use a low Earth orbiting (LEO) satellite (the MicroLab-1) to receive multi-channel Global Positioning System (GPS) carrier phase signals and demonstrate active limb sounding of the Earth’s atmosphere and ionosphere by radio occultation technique. Under the assumption of spherical symmetry at the locality of the occultation, the dual-band phase data have been processed to yield ray-path bending angle profiles, which have then been used to yield profiles of refractive index via the Abel integral transform. The refractivity profiles can then, in turn, yield profiles of ionospheric electron density and other atmospheric variables such as neutral atmospheric density, pressure, and temperature in the stratosphere and upper troposphere, and water vapor in the lower troposphere with the aid of independent temperature data. To approach a near real-time process, electron density profiles can also be derived by the Abel transform through the computation of total electron content (TEC) assuming straight-line propagation (neglecting bending). In order to assess the accuracy of the GPS/MET ionospheric electron density retrievals, coincidences of ionosonde data with GPS/MET occultations have been examined. The retrieved electron density profiles from GPS/MET TEC observations have been compared with ionogram inversion results derived from digital ionospheric sounders operated by the National Central University (the Chung-Li digisonde; 24.6°N, 121.0°E) and by Utah State University (the Bear-Lake dynasonde; 41.9°N, 111.4°W). A fuzzy classification method for the automatic identification and scaling of ionogram traces has been applied to recorded ionograms, and then bottomside ionospheric electron density profiles are determined from true-height analysis. The comparison results show better agreement for both of the derived electron density profiles and the F2-layer critical frequency ( foF2) at mid-latitude observations than at low-latitude observations. The rms foF2 differences from the GPS/MET retrievals are 0.61 MHz to the Bear-Lake dynasonde measurements and 1.62 MHz to the Chung-Li digisonde measurements.

More total electron content climatology from TOPEX/Poseidon measurements

Abstract: We have used the TOPEX satellite data set to compare climatologies of total electron content (TEC) measurements from the dual-frequency altimeter with the Doppler orbitography and radiopositioning integrated by satellite (DORIS) TEC results and with the international reference ionosphere (IRI) and Bent model predictions corresponding to the satellite measurements. We have used the TOPEX measurements from launch in 1992 through 1997 to build a database that includes time, geographic and geomagnetic coordinates of the measurement, geomagnetic indices (Kp, previous Kp, hemispheric power, and integral of hemispheric power over the previous 36 hours), solar index (F 10.7), TOPEX and DORIS TEC measurements, and empirical model (IRI and Bent) results corresponding to the TOPEX measurements. We have binned the measurements in a magnetic local time, magnetic latitude coordinate system to produce global maps of TEC. We present climatological differences between TOPEX TEC and DORIS measurements as well as between TOPEX and the two empirical models. The maps were constructed using all appropriate TOPEX TEC data, DORIS TEC measurements, and the corresponding Bent and IRI model results for solar minimum conditions.

Polarization observations of broadband VHF signals by the FORTE satellite

Abstract: Coherent very high frequency (VHF) radio observations with the pair of orthogonal log-periodic array antennas of the FORTE satellite allow us to study thoroughly the polarization properties for a received signal. Eighty-one broadband VHF pulses that were generated by the Los Alamos Portable Pulser (LAPP) have been analyzed. The data are analyzed by computing the Stokes parameters in the time-frequency domain. We first examine the LAPP pulses at high time resolution so as to separate the ordinary and extraordinary ionospheric modes. The two modes have been found to be mirror images of each other in terms of polarization, as would be expected. For each mode the polarization degrades from circular toward elliptical as the nadir angle increases. Antenna pattern effects on this observation are discussed. The tilt of the detected polarization ellipse is found to be tightly associated with the azimuthal direction of the pulse source. The same set of data are then examined with much lower time resolution to intentionally mix together the two split modes, so that the ionospheric Faraday rotation can be detected. With the known geomagnetic field the total electron content (TEC) is computed, which shows good agreement with the TEC computed by dechirping the signal. A case study of an impulsive lightning emission shows that it is highly polarized, indicating that the associated breakdown processes are highly coherent and organized. Finally, we discuss the potential use of the polarization observations for locating terrestrial radio signals.

GPS normalization and preliminary modeling results of total electron content during a midlatitude space weather event

Abstract: 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.

Generalization of ionospheric tomography on diverse data sources: Reconstruction of the three-dimensional ionosphere from simultaneous vertical ionograms, backscatter ionograms, and total electron content data

Abstract: An ionospheric reconstruction method is developed that is capable of simultaneously utilizing information from all possible sources of ionospheric measurement. We describe the method and demonstrate its performance on ionospheric data collected by an over-the-horizon radar (vertical and backscatter ionograms) and total electron content data collected by receivers operated in the Caribbean by the Applied Research Laboratories of the University of Texas at Austin. For each data set the method produces a smooth three-dimensional ionospheric model that is consistent with all the measurements. The method is based on the Newton-Kontorovich method for nonlinear operator equations and Tikhonov's regularization technique for ill-posed problems. We demonstrate here the ability of our technique to reconstruct ionospheric spatial variations from global-scale to medium-scale irregularities or traveling ionospheric disturbances. Overall results of this study demonstrate that the technique suggested makes it possible to unite data from various ionospheric instruments to provide monitoring of the ionosphere over a large geographical area in a wide range of spatial scales.

Analysis of FORTE data to extract ionospheric parameters

Abstract: The ionospheric transfer function is derived for a spherically symmetric ionosphere with an arbitrary radial electron density profile in the limit where the radio frequencies of interest are much larger than the plasma frequency pe .A n expansion of the transfer function to second order in the parameter X ( pe 2 / 2 )i s carried out. In this limit the dispersive properties of the ionosphere are manifested as a frequency-dependent time of arrival that includes quadratic, cubic, and quartic terms in 1/. The coefficients of these terms are related to the total electron content (TEC) along the slant path from transmitter to receiver, the product of TEC and the longitudinal magnetic field strength along the slant path, and refractive bending and higher-order electron density profile effects, respectively. By fitting the time of arrival versus frequency of a transionospheric signal to a polynomial in 1/ it is possible to extract the TEC, the longitudinal magnetic field strength, the peak electron density, and an effective thickness for the ionosphere. This exercise was carried out for a number of transionospheric pulses measured in the VHF by the FORTE satellite receiver and generated by the Los Alamos Portable Pulser. The results are compared with predictions derived from the International Reference Ionosphere and the United States Geological Survey geomagnetic field model.

Ionospheric Corrections for VLA Observations Using Local GPS Data

Abstract: : We have conducted an experiment to evaluate the usefulness of ionospheric data produced using the Global Positioning System (GPS) for making Faraday rotation and interferometer phase corrections at the NRAO Very Large Array (VLA). Four GPS receivers were installed at the VLA site -- one at the array center and one at the end of each arm. A simple ionospheric model consisting of a vertical TEC, a horizontal gradient, and the azimuth of that gradient was developed and fitted to the GPS Total Electron Content (TEC) data from each receiver. The model was then used to predict the TEC in the observing direction. Ionospheric Faraday rotation and phase gradients were then estimated and compared with VLA measurements taken at frequencies of 322 and 333 MHz. We find that we can normally make Faraday rotation corrections that are accurate to less than or approximately 2 deg, although one unexplained discrepancy remains. The interferometer phase shifts caused by large-scale (greater than or approx. 1000 km) ionospheric structures can be predicted by our model. However the phase shifts caused by smaller (less than or approx. 100 km) structures can be estimated only when the direction of observation lies within a few degrees of one of the GPS satellites.

Assimilation Ionosphere Model: Development and testing with Combined Ionospheric Campaign Caribbean measurements

Abstract: Assimilation Ionosphere Model (AIM) is a physics-based, global, ionospheric specification model that is currently under development. It assimilates a diverse set of real-time (or near-real-time) measurements, such as ionograms, GPS slant total electron content (TEC), and in situ plasma measurements. This study focuses on a middle latitude ionosonde assimilation capability in both local and regional forms. The models described are capable of using theƒ0F2 and hmF2 from ionograms to generate either a local or a regional distribution of the induced plasma drift. This induced drift is usually caused by the meridional neutral wind. Results from a local model (AIM1.03L) and a regional model (AIM1.03R) are presented and compared with the international reference ionosphere (IRI) climatological predictions as well as GPS slant TEC measurements. Results from year-long studies during solar maximum show that the accuracy of the AIM1.03L model is about a factor of 2 better than that of IRI. An initial month-long regional study is also presented, and the results are almost as good. A study is also carried out using observations taken during the Combined Ionospheric Campaign (CIC) held in November, 1997, in the Caribbean. The digisonde located at Ramey Solar Observatory is used to drive the AIM1.03L model, and the predicted GPS slant TECs are compared to those observed by a GPS receiver located at St. Croix. This study confirms that this first step in preparing a weather-sensitive ionospheric representation is superior to a climatological representation. This sets the stage for the development of full assimilation of GPS TEC, in situ density measurements, etc., and it is anticipated that the AIM1.03L-R ionospheric representation will provide an accurate ionospheric specification.

TEC monitoring by GPS - a possible contribution to space weather monitoring

Abstract: The world-wide use of GPS satellites for navigation and positioning offers a unique chance for permanent monitoring the total electron content (TEC) of the ionosphere on regional/global scale. Using permanent operating GPS ground station networks (e.g. that from IGS) the derived TEC maps may contribute to monitor the actual development of large scale structures in electron content and their dynamics during ionospheric perturbations (e.g.www.kn.nz.dlr.de/ → Ionosphere Impact → Storm). Based on GPS measurements at European IGS ground stations and subsequent TEC estimations and mapping, both individual as well as common features of ionospheric storms shall be discussed for the European ionosphere. The paper demonstrates the power of this new GPS tool to detect and to study the dynamics of large-scale spatial structures (e.g. ionospheric response of solar eclipse on 11 August 1999). Detected TEC-perturbations are closely related to space weather characterizing quantities such as solar radio emission, solar wind and geomagnetic activity indices. Due to close relationships between TEC and space weather parameters a permanent space weather monitoring can help to control the ionospheric impact on operational navigation satellite systems.

Regional GPS Mapping of Storm Enhanced Density During the 15-16 July 2000 Geomagnetic Storm

Abstract: During geomagnetic disturbances, intense storm-time electric fields of magnetospheric origin extend across mid-latitudes and redistribute the ionospheric plasma through advection across both latitude and local time. Strong increases and sharp spatial gradients in total electron content (TEC) characterize the ionospheric storm response. In this paper, TEC measurements from GPS receivers scattered throughout the eastern and southeastern parts of the U.S., including a few located in the Caribbean, are used to monitor the ionospheric response to both the 15-16 July 2000 geomagnetic storm, one of the largest geomagnetic storms in recent history, and the more recent 31 March 2001 storm. The GPS data have been used to construct a time history of TEC perturbations in two dimensions. The data clearly show plasma advection from the lower latitudes bringing in storm-enhanced density (SED) [1] with total electron content (TEC) ~ 100 TEC units into the region immediately equatorward of the trough. During these geomagnetic storms, plasma is transported to higher latitudes and to earlier local times – approaching the noon meridian. Additional maps of storm-induced ionospheric perturbations, constructed with Millstone Hill incoherent scatter radar data spanning the mid-latitude ionosphere between 35° and 65° invariant latitude, support the GPS observations. In-situ DMSP satellite observations of electric fields and particle precipitation are used to further study the structure and dynamics of the magnetospheric and ionospheric response to these storms. During both storms, very large enhancements in the TEC are seen over the eastern part of the US. This is due to bulk transport of the F region ionosphere to mid latitudes from the vicinity of the equatorial anomaly. Large TEC gradients are observed at several individual GPS sites, impacting standard GPS data processing techniques at some sites. The fusion of these different data sources, GPS, Incoherent Scatter, and DMSP satellite observations, allows comprehensive monitoring of the magnetospheric and ionospheric interaction and of the development in time and space of the ionospheric SED phenomenon.

Computerized ionospheric tomography analysis of the Combined Ionospheric Campaign

Abstract: Results from the June 1998 Combined Ionospheric Campaign (CIC) will be presented. The CIC represents an attempt to focus a large number of different instruments on one interesting geophysical region. One of the objectives of these campaigns is to develop suitable data sets for ingestion into data assimilative models and also to serve as validation sets for these models. The Center for Ionospheric Research (CIR) at Applied Research Laboratories, University of Texas at Austin, has had several computerized ionospheric tomography (CIT) receivers deployed in the Caribbean region since July 1997. Analysis of the CIT data from the first CIC will be presented, and comparison with other data sets will be made. Analysis will initially focus on examining the total electron content (TEC) data from the CIT receivers and ground-based GPS TEC data and correlating it with other data sets. Subsequently, the analysis will shift to performing four-dimensional electron density estimations using the Ionospheric Data Assimilation 3D (IDA3D) algorithm developed at CIR. The resulting electron density estimates will be compared with other data sources both for accuracy of the technique and scientific investigations.

Ionospheric effects of the solar flares as deduced from global GPS network data

Abstract: Results derived from analysing the ionosphere response to faint and bright solar flares are presented. The analysis used technology of a global detection of ionospheric effects from solar flares as developed by the authors, on the basis of phase measurements of the total electron content (TEC) in the ionosphere using an international GPS network. The essence of the method is that use is made of appropriate filtering and a coherent processing of variations in the TEC which is determined from GPS data, simultaneously for the entire set of visible GPS satellites at all stations used in the analysis. This technique is useful for identifying the ionospheric response to faint solar flares (of X-ray class C) when the variation amplitude of the TEC response to separate line-on-sight to GPS satellite is comparable to the level of background fluctuations. The dependence of the TEC variation response amplitude on the flares location on the Sun is investigated.

A regional GPS receiver network for monitoring equatorial scintillation and total electron content

Abstract: A network of Global Positioning System (GPS) single-frequency (L1) receivers has been installed in Australia and Southeast Asia for the purpose of monitoring equatorial ionospheric scintillation during the current peak in solar cycle activity. Dual-frequency receivers to measure total electron content are also operating at some stations and will be described in a later paper. With respect to equatorial scintillation our long-term intention is to characterize its statistical properties and its effect on GPS link performance within the Asian-Australian longitude zone (approximately from 100° to 150° E) as a function of time of day, season, sunspot number, and magnetic latitude, for comparison with available models such as WBMOD and in order to quantify the potential for navigational degradation, especially during times of elevated solar activity. In this paper, the scintillation receiver network is described, and preliminary results are presented from the first two years of operation (1998 and 1999). Our initial results include measurements of diurnal and seasonal variations in S4 and an analysis of the performance of multiple satellite links which indicates that just prior to solar maximum, GPS can approach marginal performance because of scintillation.

Variations of midlatitude ionospheric plasma density in response to an interplanetary shock

Abstract: We report on a direct, prompt effect of the solar wind on the midlatitude ionosphere On November 7, 1997, an interplanetary shock impinged on the magnetopause around 1614 UT The solar wind dynamic pressure dropped by 70% across the shock, and the interplanetary magnetic field (IMF) changed from southward to northward The Millstone Hill incoherent scatter radar measured a significant decrease of 25-30% in the dayside ionospheric F region plasma density over 37° and 44° geographic latitudes within 15 hours and a drop of 23% in the total electron content (TEC) We propose that the decrease of the ionospheric density is associated with the decompression of the magnetosphere and/or the direct penetration of interplanetary electric field into the inner magnetosphere