X. Pi - Academia.edu (original) (raw)
Papers by X. Pi
We are in the midst of a revolution in ionospheric remote sensing driven by the illuminating powe... more We are in the midst of a revolution in ionospheric remote sensing driven by the illuminating powers of ground and space-based GPS receivers, new UV remote sensing satellites, and the advent of data assimilation techniques for space weather. In particular, the COSMIC 6-satellite constellation launched in April 2006. COSMIC will provide unprecedented global coverage of GPS occultations (~5000 per day),
The Federal Aviation Administration (FAA) is implementing a satellite-based navigation system for... more The Federal Aviation Administration (FAA) is implementing a satellite-based navigation system for aircraft using the Global Positioning System (GPS). Positioning accuracy of a few meters will be achieved by broadcasting corrections to the direct GPS signal. These corrections are derived using the wide-area augmentation system (WAAS), which includes a ground network of at least 24 GPS receivers across the Continental US (CONUS). WAAS will provide real-time total electron content (TEC) measurements that can be mapped to fixed grid points using a real-time mapping algorithm. These TECs will be converted into vertical delay corrections for the GPS L1 frequency and broadcast to users every five minutes via geosynchronous satellite. Users will convert these delays to slant calibrations along their own lines-of-sight (LOS) to GPS satellites. Uncertainties in the delay calibrations will also be broadcast, allowing users to estimate the uncertainty of their position. To maintain user safety without reverting to excessive safety margins an empirical model of user calibration errors has been developed. WAAS performance depends on factors that include geographic location (errors increase near WAAS borders), and ionospheric conditions, such as the enhanced spatial electron density gradients found during ionospheric storms.
2011 IEEE International Geoscience and Remote Sensing Symposium, 2011
The InSAR Scientific Computing Environment (ISCE) is a flexible, extensible software tool designe... more The InSAR Scientific Computing Environment (ISCE) is a flexible, extensible software tool designed for the end-to-end processing and analysis of synthetic aperture radar data. ISCE inherits the core of the ROI_PAC interferometric tool, but contains improvements at all levels of the radar processing chain, including a modular and extensible architecture, new focusing approach, better geocoding of the data, handling of multi-polarization data, radiometric calibration, and estimation and correction of ionospheric effects. In this paper we describe the characteristics of ISCE with emphasis on the ionospheric modules. To detect ionospheric anomalies, ISCE implements the Faraday rotation method using quadpolarimetric images, and the split-spectrum technique using interferometric single-, dual-and quad-polarimetric images. The ability to generate co-registered time series of quad-polarimetric images makes ISCE also an ideal tool to be used for polarimetricinterferometric radar applications.
36th COSPAR Scientific …, 2006
ABSTRACT
The first of NASA's Earth Science Decadal Survey missions, the Soil Moisture Active-Passive ... more The first of NASA's Earth Science Decadal Survey missions, the Soil Moisture Active-Passive (SMAP) mission, is planned to launch in 2014, Its primary objective is to measure global surface soil moisture and freeze/thaw conditions using a scanning L-Band radar/radiometer system. In this paper, we show how a technique derived to correct SMAP soil moisture measurements for Faraday rotation effects caused by propagation through the ionosphere, can be used to map Total Electron Content (TEC) variations at unprecedented spatial scales of 3-10 km. The technique relies on robust characteristics of polarimetric scattering at the surface, the signatures of which can be used to estimate distortions due to 2-way Faraday rotation. The Faraday rotation effect depends on the angle the radar look direction makes with the Earth's magnetic field and the TEC along the path of propagation. It has been validated using much higher resolution L-band radar data from JAXA's PalSAR system. App...
Journal of Geophysical Research, 2005
Model (JPL/USC GAIM) uses two data assimilation techniques to optimally combine ionospheric measu... more Model (JPL/USC GAIM) uses two data assimilation techniques to optimally combine ionospheric measurements with the physics model: a sparse, traditional Kalman filter to estimate the three-dimensional density state, and a four-dimensional variational approach (4DVAR) to estimate ionospheric drivers such as the equatorial E Â B drift or neutral winds. In this paper we study a specific implementation of the JPL/USC GAIM Kalman filter (single ion, low-resolution, and input data from 200 ground GPS sites) and validate its global accuracy over 137 days by comparisons to independent GPS slant total electron content (TEC) observations (''missing site'' tests) and independent JASON vertical TEC observations. The assimilation accuracy is robust with a slant TEC spatial prediction RMS error of 4 TECU (Total Electron Content Unit, 1 Â 10 16 e-/m 2) on average and a vertical TEC JASON RMS error of 7 TECU. Removing what appears to be a positive %4.4 TECU bias from the JASON observations, we obtain an improved performance of 5.3 TECU over the oceans. Comparisons with a single, thin shell global ionospheric map model and the International Reference Ionosphere and Bent ionospheric models are also provided.
Journal of Geophysical Research, 2006
There is great interest in understanding how the thermosphere-ionosphere system responds to geoma... more There is great interest in understanding how the thermosphere-ionosphere system responds to geomagnetic storms. New insights are possible using the new generation of fully coupled three-dimensional models, together with extensive ionospheric databases. The period of postsolar maximum geomagnetic storms in October and November 2003 were some of the largest storms ever recorded. In this paper, we explore how the thermosphere-ionosphere system responded to the onset of the 20 November 2003 geomagnetic storm, using the NCAR TIMEGCM. The model simulates dramatic changes in the thermospheric equatorward winds, O/N 2 , and corresponding ionospheric electron densities. The model is used as a framework to interpret an increase in the observed ionospheric total electron content, and F region electron density, in the European and North African sector, in terms of changes in the neutral gas. Corresponding compositional effects observed by the GUVI instrument on the TIMED satellite lend credence to the model results. We describe some of the important physical processes that will affect planning for the utilization of measurements from the Geospace investigations in NASA's Living With a Star Program. The study illustrates the value of measuring both the neutral and ionized gases, of obtaining quasi-global views from imaging instruments, and the synergy between satellite data, ground-based measurements, and models.
Journal of Geophysical Research, 2011
Journal of Atmospheric and Solar-Terrestrial Physics, 1999
The accuracy of single-frequency ocean altimeters benefits from calibration of the total electron... more The accuracy of single-frequency ocean altimeters benefits from calibration of the total electron content (TEC) of the ionosphere below the satellite. Data from a global network of GPS receivers provides timely, continuous, and globally well-distributed measurements of ionospheric electron content. For several months we have been running a daily automatic Global Ionospheric Map process which inputs global GPS data and climatological ionosphere data into a Kalman filter, and produces global ionospheric TEC maps and ocean altimeter calibration data within 24 hours of the end-of-day. Other groups have successfully applied this output to altimeter data from the GFO satellite and in orbit determination for the TOPEXPoseidon satellite. Daily comparison of the global TEC maps with independent TEC data from the TOPEX altimeter is performed as a check on the calibration whenever the TOPEX data are available. Comparisons of the global TEC maps against TOPEX data will be discussed. Accuracy is best at mid-to-high absolute latitudes (Ilatitudel > 30 degrees) due to the better geographic distribution of GPS receivers and the relative simplicity of the ionosphere. Our highly data-driven technique is relatively less accurate at low latitudes and especially during ionospheric storm periods, due to the relative scarcity of GPS receivers and the structure and volatility of the ionosphere. However, it is still significantly more accurate than climatological models.
Journal of Atmospheric and Solar-Terrestrial Physics, 1999
The Global Positioning System GPS satellites and global as well as regional networks of hundreds ... more The Global Positioning System GPS satellites and global as well as regional networks of hundreds to thousands of ground-based permanent GPS receiver stations have become an unprecedented space weather observation system since the beginning of the new millennium The GPS observables namely carrier phase and pseudorange as well as signal power can be processed to retrieve ionospheric measurements along the receiver-to-satellite line-of-sight such as total electron content TEC and scintillation indices Acquired simultaneously in various directions towards multiple satellites from a single site such data provides ionospheric weather information in an area extending up to 2000 km distance from the station Combining these global simultaneous measurements to produce time sequences of images such as Global Ionospheric Maps GIM makes it possible to observe and understand space weather effects in the ionosphere on global-scales This space weather observation system has also been augmented with...
Geophysical Research Letters, 1996
For the first time, measurements from the Global Positioning System (GPS) worldwide network are e... more For the first time, measurements from the Global Positioning System (GPS) worldwide network are employed to study the global ionospheric total electron content (TEC) changes during a magnetic storm (November 26, 1994). These measurements are obtained from more than 60 worldwide GPS stations which continuously reccivc dualfrequcncy signals. Based on the delays of these signals, we have generated high resolution global ionospheric TEC maps at 15 minute intervals. Using a differential method comparing storm time maps with quiet time maps, we find that the ionosphere during this storm has increased significantly (the percent change relative to quiet times is greater than 150%). IIuring this particular storm, there is almost no negative phase. A possible traveling ionospheric disturbance (TID) event is identified that propagates from high latitudes (subauroral region) to low latitudes covering a large latitude range (-300). This "1'111 is coincident with increases in the THC and the, peak density of the F region (NnlF). Two regions of strong "J%C enhancement occurred in the prc-dawn and noon sectors in the northern hemispheric subauroral latitudes, immediately after the storm main phase onset. This prc-dawn peak then migrated into a subauroral (-60"N) enhancement region during nighttime, while the daysidc peak. which is caused by the "J']]> shifted equatorward down to 30°N with a speed of-460 rids. "l'he TEC increase in the subauroral region is also noted in the southern hemisphere, but the positive effect at other latitudes is weak. l'he global TEC increases lasted for about 2 days, and then a weak decrease followed, These preliminary results indicate that the differential mapping method, which is based on GPS network measurements, appears to be a useful tool for studying the global pattern and evolution process of the entire ionospheric perturbation.
Geophysical Research Letters, 2008
The global ionospheric maps (GIMs) produced by JPL are used to investigate the longitudinal struc... more The global ionospheric maps (GIMs) produced by JPL are used to investigate the longitudinal structure of the low latitude ionosphere. As a proxy of the ionization parameter at low latitudes, the latitudinally integrated total electron content (ITEC) is first extracted from low latitude GIMs and then Fourier filtered to obtain the wavenumber-4 components. We then study in detail the diurnal, seasonal and solar cycle variations of the wave patterns. It is found that the wavenumber-4 patterns are intense and well developed in boreal summer and early boreal autumn, but quite weak in boreal winter. This seasonal variation is consistent with that of the zonal wind of the non-migrating tide mode DE3. We also found that the wavenumber-4 patterns shift eastward with a shifting speed that is smaller in daytime than at night. This is attributed to the contribution of both the eastward propagation of DE3 in E-region and the zonal E Â B ion drifts in F-region. Our results support the suggestion that the longitudinal wavenumber-4 structure of the low latitude ionosphere should be originated from the non-migrating tide mode DE3.
Three magnetic storms occurred during April 17-23, 2002 as an important SEC effect driven by seve... more Three magnetic storms occurred during April 17-23, 2002 as an important SEC effect driven by several solar flares and CMEs. FUV auroral data from the IMAGE spacecraft have-85% coverage for the double main phases of the second magnetic storm that occurred during April 19-20, and have 57% coverage for the main phase of the third magnetic storm that occurred on April 23. During the second storm, when the IMF Bz was strongly southward, northsouth auroras, auroral torches and double auroral ovals occurred within substorm expansion phases. Simultaneously, observations of energetic particles at the geosynchronous altitude showed the feature of "sawtooth injections" at every substorm expansion onset. During the third storm, when
Annales Geophysicae, 2004
A new ionospheric sounding station using a Canadian Advanced Digital Ionosonde (CADI) was establi... more A new ionospheric sounding station using a Canadian Advanced Digital Ionosonde (CADI) was established for routine measurements by the "Universidade do Vale do Paraiba (UNIVAP)" at São José dos Campos (23.2 • S, 45.9 • W), Brazil, in August 2000. A major geomagnetic storm with gradual commencement at about 01:00 UT was observed on 31 March 2001. In this paper, we present and discuss salient features from the ionospheric sounding measurements carried out at S. J. Campos on the three consecutive UT days 30 March (quiet), 31 March (disturbed) and 1 April (recovery) 2001. During most of the storm period, the foF2 values showed negative phase, whereas during the two storm-time peaks, large F-region height variations were observed. In order to study the longitudinal differences observed in the F-region during the storm, the simultaneous ionospheric sounding measurements carried out at S.
Advances in Space Research, 2014
We present a method to estimate the total neutral atmospheric density from precise orbit determin... more We present a method to estimate the total neutral atmospheric density from precise orbit determination of Low Earth Orbit (LEO) satellites. We derive the total atmospheric density by determining the drag force acting on the LEOs through centimeter-level reduceddynamic precise orbit determination (POD) using onboard Global Positioning System (GPS) tracking data. The precision of the estimated drag accelerations is assessed using various metrics, including differences between estimated along-track accelerations from consecutive 30-h POD solutions which overlap by 6 h, comparison of the resulting accelerations with accelerometer measurements, and comparison against an existing atmospheric density model, DTM-2000. We apply the method to GPS tracking data from CHAMP, GRACE, SAC-C, Jason-2, TerraSAR-X and COSMIC satellites, spanning 12 years (2001-2012) and covering orbital heights from 400 km to 1300 km. Errors in the estimates, including those introduced by deficiencies in other modeled forces (such as solar radiation pressure and Earth radiation pressure), are evaluated and the signal and noise levels for each satellite are analyzed. The estimated density data from CHAMP, GRACE, SAC-C and TerraSAR-X are identified as having high signal and low noise levels. These data all have high correlations with anominal atmospheric density model and show common features in relative residuals with respect to the nominal model in related parameter space. On the contrary, the estimated density data from COSMIC and Jason-2 show errors larger than the actual signal at corresponding altitudes thus having little practical value for this study. The results demonstrate that this method is applicable to data from a variety of missions and can provide useful total neutral density measurements for atmospheric study up to altitude as high as 715 km, with precision and resolution between those derived from traditional special orbital perturbation analysis and those obtained from onboard accelerometers.
Terrestrial, Atmospheric and Oceanic Sciences, 2000
As our civilization becomes more dependent on space based technologies, we become more vulnerable... more As our civilization becomes more dependent on space based technologies, we become more vulnerable to conditions in space weather. Accurate space weather specification and forecasting require proper modeling which account for the coupling between the sun, the magnetosphere, the thermosphere, the ionosphere and the mesosphere. In spite of the tremendous advances that have been made in understanding the physics behind different space weather phenomena, the ability to specify or predict space weather is limited due to the lack of continuous and extensive observations in these regions. Placing a constellation of GPS receivers in low-Earth orbit, such as the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), provides an extremely powerful system for continuously and extensively measuring one of these regions, the ionosphere. COSMIC , by use of GPS occultations, will make it possible to obtain continuous and global 3-dimensional images of electron density, irregularities and TIDs in the ionosphere and plasmasphere. COSMIC would provide nearly 5600 globally distributed occultations per day suitable for ionospheric sensing. Occultations can be processed individually to obtain vertical profiles of electron density, with vertical resolution of ~1km, or collectively by means of tomography or data assimilation to obtain 3-D images of electron density or irregularity structure. In this paper we describe the GPS observables for ionospheric sensing and the occultation geometry. Our presentation evolves from discussing simple to more complicated inversion techniques starting with the Abel inversion, gradient-constrained Abel inversion, tomography, and finally data assimilation. In each of these techniques, the accuracy is assessed either via examination of real data from GPS/MET or
Journal of Atmospheric and Solar-terrestrial Physics, Feb 1, 1999
We summarize the main features of the ionospheric F region as observed bythe Sondrestrom, Millsto... more We summarize the main features of the ionospheric F region as observed bythe Sondrestrom, Millstone Hill, Arecibo, and Jicamarca incoherent scatter radars during the 1-5May, 1995 CEDAR Storm Study interval. This paper apparently represents the first study of amajor storm interval using the current incoherent scatter radar chain supported by the U.S.National Science Foundation. We focus most attention on 2-3 May, and include additional datafrom IMP-8, the St. Johns magnetometer, SuperDARN, and global total electron content (TEC)maps from GPS. Three intervals of likely penetration of magnetospheric electric field from high tolow latitude are identified on 2 May. A unique feature of this storm are the strong daytimeequatorward wind surges in the neutral meridional wind observed at Millstone Hill. The first ofthese (at 14 UT on 2 May) is apparently due to a travelling atmospheric disturbance launched byintense frictional and Joule heating as observed at Sondrestrom. An evening enhancement in NmF2 (the dusk effect) is typically seen only on the first day of a geomagneticstorm. However, during this storm a strong dusk effect is seen at Millstone Hill on 2, 3, and 4May, associated with the equatorward wind surges. A penetrating eastward electric field alsocontributed to the dusk effect on 2 May. A large rise in hmF2 at Arecibo near0000 UT on 3 May is due to the same eastward electric field, which penetrates to the equator,causing a strong upward plasma drift at Jicamarca. This apparently results in a polewardexpansion of the equatorial anomaly zones as seen in GPS total electron content, and an increasein NmF2 at Arecibo to the largest value seen at midnight in several years.
We are in the midst of a revolution in ionospheric remote sensing driven by the illuminating powe... more We are in the midst of a revolution in ionospheric remote sensing driven by the illuminating powers of ground and space-based GPS receivers, new UV remote sensing satellites, and the advent of data assimilation techniques for space weather. In particular, the COSMIC 6-satellite constellation launched in April 2006. COSMIC will provide unprecedented global coverage of GPS occultations (~5000 per day),
The Federal Aviation Administration (FAA) is implementing a satellite-based navigation system for... more The Federal Aviation Administration (FAA) is implementing a satellite-based navigation system for aircraft using the Global Positioning System (GPS). Positioning accuracy of a few meters will be achieved by broadcasting corrections to the direct GPS signal. These corrections are derived using the wide-area augmentation system (WAAS), which includes a ground network of at least 24 GPS receivers across the Continental US (CONUS). WAAS will provide real-time total electron content (TEC) measurements that can be mapped to fixed grid points using a real-time mapping algorithm. These TECs will be converted into vertical delay corrections for the GPS L1 frequency and broadcast to users every five minutes via geosynchronous satellite. Users will convert these delays to slant calibrations along their own lines-of-sight (LOS) to GPS satellites. Uncertainties in the delay calibrations will also be broadcast, allowing users to estimate the uncertainty of their position. To maintain user safety without reverting to excessive safety margins an empirical model of user calibration errors has been developed. WAAS performance depends on factors that include geographic location (errors increase near WAAS borders), and ionospheric conditions, such as the enhanced spatial electron density gradients found during ionospheric storms.
2011 IEEE International Geoscience and Remote Sensing Symposium, 2011
The InSAR Scientific Computing Environment (ISCE) is a flexible, extensible software tool designe... more The InSAR Scientific Computing Environment (ISCE) is a flexible, extensible software tool designed for the end-to-end processing and analysis of synthetic aperture radar data. ISCE inherits the core of the ROI_PAC interferometric tool, but contains improvements at all levels of the radar processing chain, including a modular and extensible architecture, new focusing approach, better geocoding of the data, handling of multi-polarization data, radiometric calibration, and estimation and correction of ionospheric effects. In this paper we describe the characteristics of ISCE with emphasis on the ionospheric modules. To detect ionospheric anomalies, ISCE implements the Faraday rotation method using quadpolarimetric images, and the split-spectrum technique using interferometric single-, dual-and quad-polarimetric images. The ability to generate co-registered time series of quad-polarimetric images makes ISCE also an ideal tool to be used for polarimetricinterferometric radar applications.
36th COSPAR Scientific …, 2006
ABSTRACT
The first of NASA's Earth Science Decadal Survey missions, the Soil Moisture Active-Passive ... more The first of NASA's Earth Science Decadal Survey missions, the Soil Moisture Active-Passive (SMAP) mission, is planned to launch in 2014, Its primary objective is to measure global surface soil moisture and freeze/thaw conditions using a scanning L-Band radar/radiometer system. In this paper, we show how a technique derived to correct SMAP soil moisture measurements for Faraday rotation effects caused by propagation through the ionosphere, can be used to map Total Electron Content (TEC) variations at unprecedented spatial scales of 3-10 km. The technique relies on robust characteristics of polarimetric scattering at the surface, the signatures of which can be used to estimate distortions due to 2-way Faraday rotation. The Faraday rotation effect depends on the angle the radar look direction makes with the Earth's magnetic field and the TEC along the path of propagation. It has been validated using much higher resolution L-band radar data from JAXA's PalSAR system. App...
Journal of Geophysical Research, 2005
Model (JPL/USC GAIM) uses two data assimilation techniques to optimally combine ionospheric measu... more Model (JPL/USC GAIM) uses two data assimilation techniques to optimally combine ionospheric measurements with the physics model: a sparse, traditional Kalman filter to estimate the three-dimensional density state, and a four-dimensional variational approach (4DVAR) to estimate ionospheric drivers such as the equatorial E Â B drift or neutral winds. In this paper we study a specific implementation of the JPL/USC GAIM Kalman filter (single ion, low-resolution, and input data from 200 ground GPS sites) and validate its global accuracy over 137 days by comparisons to independent GPS slant total electron content (TEC) observations (''missing site'' tests) and independent JASON vertical TEC observations. The assimilation accuracy is robust with a slant TEC spatial prediction RMS error of 4 TECU (Total Electron Content Unit, 1 Â 10 16 e-/m 2) on average and a vertical TEC JASON RMS error of 7 TECU. Removing what appears to be a positive %4.4 TECU bias from the JASON observations, we obtain an improved performance of 5.3 TECU over the oceans. Comparisons with a single, thin shell global ionospheric map model and the International Reference Ionosphere and Bent ionospheric models are also provided.
Journal of Geophysical Research, 2006
There is great interest in understanding how the thermosphere-ionosphere system responds to geoma... more There is great interest in understanding how the thermosphere-ionosphere system responds to geomagnetic storms. New insights are possible using the new generation of fully coupled three-dimensional models, together with extensive ionospheric databases. The period of postsolar maximum geomagnetic storms in October and November 2003 were some of the largest storms ever recorded. In this paper, we explore how the thermosphere-ionosphere system responded to the onset of the 20 November 2003 geomagnetic storm, using the NCAR TIMEGCM. The model simulates dramatic changes in the thermospheric equatorward winds, O/N 2 , and corresponding ionospheric electron densities. The model is used as a framework to interpret an increase in the observed ionospheric total electron content, and F region electron density, in the European and North African sector, in terms of changes in the neutral gas. Corresponding compositional effects observed by the GUVI instrument on the TIMED satellite lend credence to the model results. We describe some of the important physical processes that will affect planning for the utilization of measurements from the Geospace investigations in NASA's Living With a Star Program. The study illustrates the value of measuring both the neutral and ionized gases, of obtaining quasi-global views from imaging instruments, and the synergy between satellite data, ground-based measurements, and models.
Journal of Geophysical Research, 2011
Journal of Atmospheric and Solar-Terrestrial Physics, 1999
The accuracy of single-frequency ocean altimeters benefits from calibration of the total electron... more The accuracy of single-frequency ocean altimeters benefits from calibration of the total electron content (TEC) of the ionosphere below the satellite. Data from a global network of GPS receivers provides timely, continuous, and globally well-distributed measurements of ionospheric electron content. For several months we have been running a daily automatic Global Ionospheric Map process which inputs global GPS data and climatological ionosphere data into a Kalman filter, and produces global ionospheric TEC maps and ocean altimeter calibration data within 24 hours of the end-of-day. Other groups have successfully applied this output to altimeter data from the GFO satellite and in orbit determination for the TOPEXPoseidon satellite. Daily comparison of the global TEC maps with independent TEC data from the TOPEX altimeter is performed as a check on the calibration whenever the TOPEX data are available. Comparisons of the global TEC maps against TOPEX data will be discussed. Accuracy is best at mid-to-high absolute latitudes (Ilatitudel > 30 degrees) due to the better geographic distribution of GPS receivers and the relative simplicity of the ionosphere. Our highly data-driven technique is relatively less accurate at low latitudes and especially during ionospheric storm periods, due to the relative scarcity of GPS receivers and the structure and volatility of the ionosphere. However, it is still significantly more accurate than climatological models.
Journal of Atmospheric and Solar-Terrestrial Physics, 1999
The Global Positioning System GPS satellites and global as well as regional networks of hundreds ... more The Global Positioning System GPS satellites and global as well as regional networks of hundreds to thousands of ground-based permanent GPS receiver stations have become an unprecedented space weather observation system since the beginning of the new millennium The GPS observables namely carrier phase and pseudorange as well as signal power can be processed to retrieve ionospheric measurements along the receiver-to-satellite line-of-sight such as total electron content TEC and scintillation indices Acquired simultaneously in various directions towards multiple satellites from a single site such data provides ionospheric weather information in an area extending up to 2000 km distance from the station Combining these global simultaneous measurements to produce time sequences of images such as Global Ionospheric Maps GIM makes it possible to observe and understand space weather effects in the ionosphere on global-scales This space weather observation system has also been augmented with...
Geophysical Research Letters, 1996
For the first time, measurements from the Global Positioning System (GPS) worldwide network are e... more For the first time, measurements from the Global Positioning System (GPS) worldwide network are employed to study the global ionospheric total electron content (TEC) changes during a magnetic storm (November 26, 1994). These measurements are obtained from more than 60 worldwide GPS stations which continuously reccivc dualfrequcncy signals. Based on the delays of these signals, we have generated high resolution global ionospheric TEC maps at 15 minute intervals. Using a differential method comparing storm time maps with quiet time maps, we find that the ionosphere during this storm has increased significantly (the percent change relative to quiet times is greater than 150%). IIuring this particular storm, there is almost no negative phase. A possible traveling ionospheric disturbance (TID) event is identified that propagates from high latitudes (subauroral region) to low latitudes covering a large latitude range (-300). This "1'111 is coincident with increases in the THC and the, peak density of the F region (NnlF). Two regions of strong "J%C enhancement occurred in the prc-dawn and noon sectors in the northern hemispheric subauroral latitudes, immediately after the storm main phase onset. This prc-dawn peak then migrated into a subauroral (-60"N) enhancement region during nighttime, while the daysidc peak. which is caused by the "J']]> shifted equatorward down to 30°N with a speed of-460 rids. "l'he TEC increase in the subauroral region is also noted in the southern hemisphere, but the positive effect at other latitudes is weak. l'he global TEC increases lasted for about 2 days, and then a weak decrease followed, These preliminary results indicate that the differential mapping method, which is based on GPS network measurements, appears to be a useful tool for studying the global pattern and evolution process of the entire ionospheric perturbation.
Geophysical Research Letters, 2008
The global ionospheric maps (GIMs) produced by JPL are used to investigate the longitudinal struc... more The global ionospheric maps (GIMs) produced by JPL are used to investigate the longitudinal structure of the low latitude ionosphere. As a proxy of the ionization parameter at low latitudes, the latitudinally integrated total electron content (ITEC) is first extracted from low latitude GIMs and then Fourier filtered to obtain the wavenumber-4 components. We then study in detail the diurnal, seasonal and solar cycle variations of the wave patterns. It is found that the wavenumber-4 patterns are intense and well developed in boreal summer and early boreal autumn, but quite weak in boreal winter. This seasonal variation is consistent with that of the zonal wind of the non-migrating tide mode DE3. We also found that the wavenumber-4 patterns shift eastward with a shifting speed that is smaller in daytime than at night. This is attributed to the contribution of both the eastward propagation of DE3 in E-region and the zonal E Â B ion drifts in F-region. Our results support the suggestion that the longitudinal wavenumber-4 structure of the low latitude ionosphere should be originated from the non-migrating tide mode DE3.
Three magnetic storms occurred during April 17-23, 2002 as an important SEC effect driven by seve... more Three magnetic storms occurred during April 17-23, 2002 as an important SEC effect driven by several solar flares and CMEs. FUV auroral data from the IMAGE spacecraft have-85% coverage for the double main phases of the second magnetic storm that occurred during April 19-20, and have 57% coverage for the main phase of the third magnetic storm that occurred on April 23. During the second storm, when the IMF Bz was strongly southward, northsouth auroras, auroral torches and double auroral ovals occurred within substorm expansion phases. Simultaneously, observations of energetic particles at the geosynchronous altitude showed the feature of "sawtooth injections" at every substorm expansion onset. During the third storm, when
Annales Geophysicae, 2004
A new ionospheric sounding station using a Canadian Advanced Digital Ionosonde (CADI) was establi... more A new ionospheric sounding station using a Canadian Advanced Digital Ionosonde (CADI) was established for routine measurements by the "Universidade do Vale do Paraiba (UNIVAP)" at São José dos Campos (23.2 • S, 45.9 • W), Brazil, in August 2000. A major geomagnetic storm with gradual commencement at about 01:00 UT was observed on 31 March 2001. In this paper, we present and discuss salient features from the ionospheric sounding measurements carried out at S. J. Campos on the three consecutive UT days 30 March (quiet), 31 March (disturbed) and 1 April (recovery) 2001. During most of the storm period, the foF2 values showed negative phase, whereas during the two storm-time peaks, large F-region height variations were observed. In order to study the longitudinal differences observed in the F-region during the storm, the simultaneous ionospheric sounding measurements carried out at S.
Advances in Space Research, 2014
We present a method to estimate the total neutral atmospheric density from precise orbit determin... more We present a method to estimate the total neutral atmospheric density from precise orbit determination of Low Earth Orbit (LEO) satellites. We derive the total atmospheric density by determining the drag force acting on the LEOs through centimeter-level reduceddynamic precise orbit determination (POD) using onboard Global Positioning System (GPS) tracking data. The precision of the estimated drag accelerations is assessed using various metrics, including differences between estimated along-track accelerations from consecutive 30-h POD solutions which overlap by 6 h, comparison of the resulting accelerations with accelerometer measurements, and comparison against an existing atmospheric density model, DTM-2000. We apply the method to GPS tracking data from CHAMP, GRACE, SAC-C, Jason-2, TerraSAR-X and COSMIC satellites, spanning 12 years (2001-2012) and covering orbital heights from 400 km to 1300 km. Errors in the estimates, including those introduced by deficiencies in other modeled forces (such as solar radiation pressure and Earth radiation pressure), are evaluated and the signal and noise levels for each satellite are analyzed. The estimated density data from CHAMP, GRACE, SAC-C and TerraSAR-X are identified as having high signal and low noise levels. These data all have high correlations with anominal atmospheric density model and show common features in relative residuals with respect to the nominal model in related parameter space. On the contrary, the estimated density data from COSMIC and Jason-2 show errors larger than the actual signal at corresponding altitudes thus having little practical value for this study. The results demonstrate that this method is applicable to data from a variety of missions and can provide useful total neutral density measurements for atmospheric study up to altitude as high as 715 km, with precision and resolution between those derived from traditional special orbital perturbation analysis and those obtained from onboard accelerometers.
Terrestrial, Atmospheric and Oceanic Sciences, 2000
As our civilization becomes more dependent on space based technologies, we become more vulnerable... more As our civilization becomes more dependent on space based technologies, we become more vulnerable to conditions in space weather. Accurate space weather specification and forecasting require proper modeling which account for the coupling between the sun, the magnetosphere, the thermosphere, the ionosphere and the mesosphere. In spite of the tremendous advances that have been made in understanding the physics behind different space weather phenomena, the ability to specify or predict space weather is limited due to the lack of continuous and extensive observations in these regions. Placing a constellation of GPS receivers in low-Earth orbit, such as the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), provides an extremely powerful system for continuously and extensively measuring one of these regions, the ionosphere. COSMIC , by use of GPS occultations, will make it possible to obtain continuous and global 3-dimensional images of electron density, irregularities and TIDs in the ionosphere and plasmasphere. COSMIC would provide nearly 5600 globally distributed occultations per day suitable for ionospheric sensing. Occultations can be processed individually to obtain vertical profiles of electron density, with vertical resolution of ~1km, or collectively by means of tomography or data assimilation to obtain 3-D images of electron density or irregularity structure. In this paper we describe the GPS observables for ionospheric sensing and the occultation geometry. Our presentation evolves from discussing simple to more complicated inversion techniques starting with the Abel inversion, gradient-constrained Abel inversion, tomography, and finally data assimilation. In each of these techniques, the accuracy is assessed either via examination of real data from GPS/MET or
Journal of Atmospheric and Solar-terrestrial Physics, Feb 1, 1999
We summarize the main features of the ionospheric F region as observed bythe Sondrestrom, Millsto... more We summarize the main features of the ionospheric F region as observed bythe Sondrestrom, Millstone Hill, Arecibo, and Jicamarca incoherent scatter radars during the 1-5May, 1995 CEDAR Storm Study interval. This paper apparently represents the first study of amajor storm interval using the current incoherent scatter radar chain supported by the U.S.National Science Foundation. We focus most attention on 2-3 May, and include additional datafrom IMP-8, the St. Johns magnetometer, SuperDARN, and global total electron content (TEC)maps from GPS. Three intervals of likely penetration of magnetospheric electric field from high tolow latitude are identified on 2 May. A unique feature of this storm are the strong daytimeequatorward wind surges in the neutral meridional wind observed at Millstone Hill. The first ofthese (at 14 UT on 2 May) is apparently due to a travelling atmospheric disturbance launched byintense frictional and Joule heating as observed at Sondrestrom. An evening enhancement in NmF2 (the dusk effect) is typically seen only on the first day of a geomagneticstorm. However, during this storm a strong dusk effect is seen at Millstone Hill on 2, 3, and 4May, associated with the equatorward wind surges. A penetrating eastward electric field alsocontributed to the dusk effect on 2 May. A large rise in hmF2 at Arecibo near0000 UT on 3 May is due to the same eastward electric field, which penetrates to the equator,causing a strong upward plasma drift at Jicamarca. This apparently results in a polewardexpansion of the equatorial anomaly zones as seen in GPS total electron content, and an increasein NmF2 at Arecibo to the largest value seen at midnight in several years.