Plasmaspheric Total Electron Content Research Articles

The Middle and Low Latitude Ion Composition Variations Observed by the DMSP Satellites on 20–31 August 2018

AbstractThe coupling between the ionosphere and plasmasphere is not known well during the quiet and storm times. The topside ionosphere ion composition reflects the exchange between the ionosphere and plasmasphere. We used the DMSP satellite observations to show the changes of the 840 km altitude H+ and O+ ions at ∼3–6 LT in the mid‐ and low‐latitudes on 20–31 August 2018 where a storm occurred on 25 August. The H+ was the main ion at 3–4 LT, and its concentration presented two peaks around 40°N/S magnetic latitudes on 20–24 August. The H+ concentration strongly lessened for more than five days around 40°N/S magnetic latitudes in the recovery phase of the storm on 26–30 August. Further, the enhancement and reduction of the H+ concentration persisted mainly in the Pacific and Asian sectors. The O+ density increased in Northern Hemisphere at 6:36 LT, and the H+ concentration showed the maximum around the magnetic equator during the quiet times, while the H+ concentration still mainly reduced around 40°N/S and stronger at 40°S in the recovery phase. The plasmaspheric total electron content showed a peak around the magnetic equator on 20–24 August, and the peak was weakened or disappeared on 26–30 August. The plasmasphere might be the principal sources for the mid‐latitude H+ enhancement along the geomagnetic field lines during the quite times and the storm‐time suppressed plasmasphere plasma density would weaken the supply of the H+ at 840 km altitude.

Daily Plasmaspheric TEC Variations From COSMIC GPS Observations Based on RBF Neural Network‐Kriging Method

AbstractThe low Earth orbit (LEO) satellite provides valuable direct observations for scientific investigation of the plasmasphere, while the plasmaspheric total electron content (PTEC) with a high temporal resolution cannot be precisely estimated due to fewer LEO satellites. In this paper, a novel joint method of radial basis function neural network—Kriging (RBF‐Kr) method is designed to construct the daily PTEC model using the Constellation Observing System for Meteorology, Ionosphere and Climate Global Positioning System observations during the low (2009) and high (2013) solar activity years. Compared with the original RBF method, the RBF‐Kr method reduces the mean absolute error and root mean square error from 0.77 to 0.60 TEC unit (TECU) and 0.99 to 0.80 TECU, respectively. The correlation coefficient (Corr) increased from 0.90 to 0.94. Furthermore, daily PTEC variations show that the PTEC at low latitudes is evenly distributed during equinox periods. The South American‐Atlantic Ocean sector has a peak and trough in PTEC during the December and June solstices. A certain symmetrical distribution of PTEC is observed in the latitudinal direction, and the symcenter moves toward the summer hemisphere. The duration of extremal PTEC at 60°W is observed, which lasted up to more than 80 days around the December solstice. An obvious correlation between the solar flux and PTEC is found with up to 0.86, indicating that daily PTEC variations are mainly related to solar activities.

Validation of GNSS-derived global ionosphere maps for different solar activity levels: case studies for years 2014 and 2018

Ionosphere Associate Analysis Centers (IAACs) of the International GNSS Service (IGS) independently produce global ionosphere maps (GIMs) of the total electron content (TEC). The GIMs are based on different modeling techniques, resulting in different TEC levels and accuracies. In this study, we evaluated the accuracy and consistency of the IAAC GIMs during high (2014) and low (2018) solar activity periods of the 24th solar cycle. In our study, we applied two different evaluation methods. First, we carried out a comparison of the GIM-derived slant TEC (STEC) with carrier phase geometry-free combination of GNSS signals obtained from 25 globally distributed stations. Second, vertical TEC (VTEC) from GIMs was compared to altimetry-derived VTEC obtained from the Jason-2 and Jason-3 satellites and complemented for plasmaspheric TEC. The analyzed GIMs obtained STEC RMS values reaching from 1.98 to 3.00 TECU and from 0.96 to 1.29 TECU during 2014 and 2018, respectively. The comparison to altimetry data resulted in VTEC STD values that varied from 3.61 to 5.97 TECU and from 1.92 to 2.78 TECU during 2014 and 2018, respectively. The results show that among the IAACs, the Center for Orbit Determination in Europe global maps performed best in low and high solar activity periods. However, the highest accuracy was obtained by a non-IGS product—UQRG GIMs provided by Universitat Politècnica de Catalunya. It was also shown that the best results were obtained using a modified single layer model mapping function and that the map time interval has a relatively small influence on the resulting map accuracy.

Long-Term Variations of Plasmaspheric Total Electron Content from Topside GPS Observations on LEO Satellites

The plasmasphere is located above the ionosphere with low-energy plasma, which is an important component of the solar-terrestrial space environment. As the link between the ionosphere and the magnetosphere, the plasmasphere plays an important role in the coupling process. Therefore, it is of great significance to study the electron content variation of the plasmasphere for the solar-terrestrial space environment. Nowadays, the topside global positioning system (GPS) observations on Low Earth Orbit (LEO) satellites provide a unique opportunity to estimate and study variations in the plasmasphere. In this paper, the plasmaspheric total electron content (PTEC) is estimated, and its long-term variations are studied from topside GPS observations onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC). The PTEC in the daytime is higher than that in the nighttime, with the peak between 14:00 and 17:00 in the magnetic local time, while the minimum value of PTEC in the belt appears between 3:00 and 6:00 in the magnetic local time before sunrise. For seasonal variations, the PTEC is the highest in spring of the northern hemisphere and the lowest in summer of the northern hemisphere regardless of the state of the solar activity. The long-term variation in PTEC is further analyzed using 11-year COSMIC GPS observation data from 2007 to 2017. A high correlation between PTEC and the F10.7 indices is found. Particularly in the geomagnetic high-latitude region during the daytime, the correlation coefficient reaches 0.93. The worst case occurs during the nighttime in the geomagnetic middle-latitude region, but the correlation coefficient is still higher than 0.88. The long-term variations of plasmaspheric TEC are mainly related to the solar activity.

Estimation of differential code biases with Jason-2/3 onboard GPS observations

With the continuous deployment of Low Earth Orbit (LEO) satellites, the estimation of differential code biases (DCBs) based on GNSS observations from LEO has gained increasing attention. Previous studies on LEO-based DCB estimation are usually using the spherical symmetry ionosphere assumption (SSIA), in which a uniform electron density is assumed in a thick shell. In this study, we propose an approach (named the SHLEO method) to simultaneously estimate the satellite and LEO onboard receiver DCBs by modeling the distribution of the global plasmaspheric total electron content (PTEC) above the satellite orbit with a spherical harmonic (SH) function. Compared to the commonly used SSIA method, the SHLEO model improves the GPS satellite DCB estimation accuracy by 13.46% and the stability by 22.34%, respectively. Compared to the GPS satellite DCBs estimated based on the Jason-3-only observations, the accuracy and monthly stability of the satellite DCBs can be improved by 14.42% and 26.8% when both Jason-2 and Jason-3 onboard observations are jointly processed. Compared with the Jason-2 solutions, the GPS satellite DCB estimates based on the fusion of Jason-2 and Jason-3 observations have an improved consistency of better than 18.26% and 9.71% with the products provided by the Center for Orbit Determination in Europe (CODE) and Chinese Academy of Sciences (CAS). Taking the DCB products provided by the German Aerospace Center (DLR) as references, there is no improvement in accuracy of the GPS satellite DCB estimates based on the fusion of Jason-2 and Jason-3 observations than the Jason-2 solutions alone. A periodic variation is found in the time series of both the Jason-3 and Jason-2 onboard receiver DCB estimates. Preliminary analysis of the PTEC distribution based on the estimated SH coefficients are also presented.

A global empirical orthogonal function model of plasmaspheric electron content

Based on the method for establishing a global plasmaspheric model using observations from COSMIC and MetOp-A orbit determination GNSS receivers, Chen et al. (2017) obtained a global plasmaspheric total electron content product with a spatial resolution of 2.5° × 5° and a time resolution of 4 h. In this paper, we use those global plasmaspheric electron content product in 2008, 2010, 2011, 2012, 2014 for 1446 days to establish a global plasmaspheric empirical model based on empirical orthogonal function (EOF). The model can well characterize the spatiotemporal variation of plasmaspheric electron content (PEC) and the influence of solar radiation on it. Only the first four orders of EOF sequences can characterize the 98.43% features of the original PEC dataset. The principal component coefficient Pk is decomposed twice during modeling, and the combination of trigonometric function and linear function is used to model Pk to characterize the solar cycle, annual cycle, semi-annual cycle and quarter-cycle variation. We compare the PEC model values with the actual observation data, the results show that the empirical PEC model values are highly correlated with the actual observations. The correlation between the two is above 0.96, and the RMS maximum of the difference between the PEC model values and the observed values are 0.70 TECU, and the average of the difference between the PEC model values and the observed values are −0.18 TECU, respectively. In addition, we validate the reliability of the global plasmaspheric model established by two empirical orthogonal function decomposition method using actual observation data, according to the global distribution of the differences between the PEC model values and the observed values in low solar activity and high solar activity, it can be seen that under low solar activity and high solar activity conditions, the model has good adaptability.

Middle‐Latitudinal Band Structure Observed in the Nighttime Ionosphere

AbstractIn this study, the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) electron densities (Ne) from 200 to 800 km during 2006–2017 are used to investigate the middle‐latitudinal band structure of the nighttime ionosphere. The main results are as follows. (1) In the nighttime, the Ne at about ±40° geomagnetic latitudes is generally greater, especially in the topside ionosphere. (2) Double bands of relatively high Ne are seen at all longitudes during the equinoxes, while only one latitudinal band is clearly visible at specific longitudes in the winter hemisphere during the solstices. (3) The middle‐latitudinal band structure of the topside ionosphere is more pronounced at later local times (LTs) at night and under lower solar activity. (4) The middle‐latitudinal band structure mainly occurs at 23‐05 LT. (5) The seasonal/longitudinal variations of the middle‐latitudinal band structure are consistent with those of the plasmaspheric total electron content at low latitudes and upward field‐aligned winds at middle latitudes. (6) At the middle‐latitudinal bands, the downward plasma diffusion from the plasmasphere provides stable plasma source to the topside ionosphere, while the upward plasma motion due to the neutral winds is the primary source for the F2 peak region. (7) The plasma sources for the middle‐latitudinal band structures at different altitude regions are probably different, so that the resulting measurements might be due to a coalescing of these two completely different mechanisms. (8) The modulated recombination rates due to the thermospheric dynamics could also reinforce this structure.

Nighttime Enhancements in the Midlatitude Ionosphere and Their Relation to the Plasmasphere

AbstractIn situ electron density measurements by the CHAllenging Minisatellite Payload and the Defense Meteorological Satellites Program F17 satellites show that the midlatitude ionization at altitudes of ∼350 and 850 km is enhanced in the late evening. The enhancements increase to maximum around midnight and are clearly observed till early morning as the equatorial ionization decays to minimal level. They appear in the winter hemisphere during June and December solstices and in both hemispheres during equinox. The enhancements are well confined between ±30° and ±50° magnetic latitude, with the magnetic flux tubes of L = 1.3 − 2.4 connecting to the plasmasphere. Furthermore, coincident longitudinal variations exist in both the ionospheric enhancements and the plasmaspheric total electron content, especially during the solstice months. The coincidence may suggest essential plasma transport between the ionosphere and the plasmasphere. These facts support the idea that the plasmasphere provides extra plasma to the midlatitude ionosphere through downward plasma influx along the magnetic field lines to form the nighttime ionization enhancements when the sunlight is absent.

Reconstruction of Plasmaspheric Density Distributions by Applying a Tomography Technique to Jason‐1 Plasmaspheric TEC Measurements

AbstractWe have developed a tomography algorithm and applied it to a plasmaspheric total electron content data set from Jason‐1 satellite (~1,336 km), which has a Global Positioning System receiver onboard. To invert the measured plasmaspheric total electron content into vertical distribution of electron density, we adopted a multiplicative algebraic reconstruction technique with an assumed initial density distribution. The reconstruction of the plasmaspheric density distribution was performed on Indian (70°E − 90°E), Pacific (200°E − 220°E), and Atlantic (320°E − 340°E) longitudinal planes during the periods of high solar activity (F10.7 > 100) and low geomagnetic activity (Ap < 12) from 2002 to 2005. The reconstructed density distribution displays general climatological characteristics of the plasmasphere. For all three longitudinal sectors, the reconstructed density distributions show weak diurnal variations being greater during daytime (09–15 LT) than nighttime (21–03 LT). In Atlantic sector, the reconstructed plasmaspheric density exhibits an annual anomaly (higher density in December than in June), which was not apparent in other longitude sectors. By fitting the reconstructed density profiles, we derived empirical functions of equatorial plasmaspheric density profiles for 4 months (March, June, September, and December) in the three longitude sectors. The parameters associated with these functions were found to exhibit diurnal variation and annual anomaly characteristics.

Underutilized Spaceborne GPS Observations for Space Weather Monitoring

AbstractMany of currently operated low Earth orbit satellites, in particular Earth observing meteorological missions, are equipped with dual‐frequency Global Positioning System (GPS) receivers with a zenith‐looking antenna for tasks of precise orbit determination and timing. The already existed databases with accumulated raw GPS measurements, as a by‐product of satellite missions, can arouse specific interest for ionospheric and space weather community too. We demonstrate potential possibilities and advantages of involving underutilized spaceborne GPS measurements for Space Weather activity monitoring by specification of storm‐induced ionospheric plasma density irregularities at different altitudinal domain of the topside ionosphere. We have analyzed a dynamical response of the high‐latitude topside ionosphere to the intense geomagnetic storm of 19–21 December 2015 using up‐looking GPS measurements on board Swarm and Meteorological Operational satellite (MetOp) missions, flying at altitudes of ~500 km and ~835 km, respectively. For the first time, GPS observations on board the meteorological mission MetOp were used to reveal an occurrence of plasma irregularities at altitudes above 835 km. Our results demonstrate that during strong geomagnetic storms the intense plasma density irregularities can occur in the topside ionosphere near ~500 km altitude and can be still persisted above 835 km. Joint analysis of the Super Dual Auroral Radar Network global convection patterns, ground‐based GPS total electron content (TEC) observations, and MetOp‐derived topside TEC observations confirmed that plasma irregularities above ~835 km coincided with the plasmaspheric/magnetospheric part of the storm‐enhanced density and the polar tongue of ionization structures, which is the first direct observation of the storm‐enhanced density/tongue of ionization structure in the plasmaspheric TEC.

Longitudinal variations of topside ionospheric and plasmaspheric TEC

AbstractThe upward looking ionospheric total electron content (TEC) from the MetOp‐A and TSX satellites during 2008–2015 has been used to systematically study the longitudinal variations of the topside ionosphere and plasmasphere. The results of this study are summarized as follows: (1) There are significant longitudinal variations in the topside ionosphere and plasmasphere at low latitudes. The TEC maximum during the June solstice over the Western and Central Pacific Ocean corresponds to a TEC minimum at the same location during the December solstice, but the opposite behavior occurs over South America and the Atlantic Ocean. (2) During the solstices, the relative longitudinal variations in the geomagnetic equatorial region do not have a strong dependence on local time and solar activity. (3) The TEC in the winter hemisphere decreases with increasing solar activity, especially at higher altitudes and at night. The topside TEC depletion with solar activity depends on longitude. (4) The solstice‐like longitudinal pattern lasts much longer than the equinox‐like patterns, with the June solstice pattern lasting the longest. Furthermore, the equinox‐like longitudinal patterns occur in March when expected, whereas they extend from the autumnal equinox until the end of October. (5) The longitudinal variations of upward looking TEC are different from the corresponding longitudinal variations of electron densities around the F2 peak and orbital altitudes. This indicates that the topside ionosphere structure is strongly influenced by the physical processes in the topside region, rather than being a pure reflection of the ionospheric F2 peak structure.

Climatology of plasmaspheric total electron content obtained from Jason 1 satellite

AbstractWe used more than 40 million total electron content (TEC) measurements obtained from the GPS TurboRogue Space Receiver receiver on board the Jason 1 satellite in order to investigate the global morphology of the plasmaspheric TEC (pTEC) including the variations with local time, latitude, longitude, season, solar cycle, and geomagnetic activity. The pTEC corresponds to the total electron content between Jason 1 (1336 km) and GPS (20,200 km) satellite altitudes. The pTEC data were collected during the 7 year period from January 2002 to December 2008. It was found that pTEC increases by about 10–30% from low to high solar flux conditions with the largest variations occurring at low latitudes for equinox. During low solar flux condition, pTEC is largely independent of geomagnetic activity. However, it slightly decreases with increasing geomagnetic activity at low latitudes during high solar flux. The seasonal variations such as the annual and semiannual anomalies in the ionosphere also exist in the low‐latitude plasmasphere. In particular, the American sector (around 300°E) shows strong annual asymmetry in the plasmaspheric density, being larger in December than in June solstice.

Characteristics of global plasmaspheric TEC in comparison with the ionosphere simultaneously observed by Jason‐1 satellite

AbstractWe compared the global plasmaspheric total electron content (pTEC) with the ionospheric TEC (iTEC) simultaneously measured by Jason‐1 satellite during the declining phase of solar cycle 23 (2002–2009) to investigate the global morphology of the plasmaspheric density in relation to the ionosphere. Our study showed that the plasmaspheric density structures fundamentally follow the ionosphere, but there are also significant differences between them. Although the diurnal variations are very similar to each region, the plasmasphere shows much weaker variations, only approximately 1 TECU day‐night difference. By analyzing the day‐night differences in the plasmasphere, we found that the plasmaspheric contribution to the nighttime ionosphere does not increase with solar activity and the largest contribution occurs during June solstice. The plasmasphere shows similar seasonal variations to the ionosphere, except for the semiannual variation, which is essentially absent in the plasmasphere. There is also an important difference in the annual variation: although the annual variation in the ionosphere exists regardless of longitude, it occurs only at American sector in the plasmasphere. As solar activity increases to moderate level, the pTEC substantially enhances from approximately 2 to 4 TECU at the initial increase of solar activity below F10.7p = 100 and then quickly slows down while the iTEC almost linearly enhances. Although it is well known that magnetic storms are the major source of plasmaspheric density depletion, pTEC does not show this aspect of the plasmasphere probably due to the relatively small Kp values for high magnetic activity (Kp > 2.5) in the current study.

GPS TEC and the plasmasphere: Some observations and uncertainties

Plasmaspheric total electron content (PTEC) remains an elusive component of GPS TEC, even in middle latitude regions. PTEC is significant in measuring and modeling the ionosphere to altitudes much lower than GPS, such as for ground‐based transionospheric radars that must detect and track orbital and ballistic objects. In addition, for GPS TEC observations to be useful for today's assimilative models, each TEC measurement needs to have an associated uncertainty. It is clearly beneficial if such uncertainty figures are not constant upper bounds, but rather are as data‐specific and determined as possible. This paper presents results from a study using a chain of GPS stations on the west coast of North America. Ionospheric TEC, PTEC, and uncertainties are derived, and validated against Jason 1 TEC. We suggest that the methodology presented has potential to improve midlatitude ionospheric TEC accuracy for modeling, system impact and forecasting applications, and for monitoring PTEC.

Global plasmaspheric TEC and its relative contribution to GPS TEC

The plasmaspheric electron content is directly estimated from the global positioning system (GPS) data onboard JASON-1 Satellite for the first time. Similarly, the ground-based GPS total electron content (TEC) is estimated using about 1000 GPS receivers distributed around the globe. The relative contribution of the plasmaspheric electron content to the ground-based GPS TEC is then estimated globally using these two independent simultaneous measurements; namely ground-based GPS TEC and JASON-1 GPS TEC. Results presented here include data from 3 months of different solar cycle conditions (October 2003, May 2005, and December 2006). The global comparison between the two independent measurements was performed by dividing the data into three different regions; equatorial, mid- and high-latitude regions. This division is essential as the GPS raypaths traverse different distances through the plasmasphere at different latitudes. The raypath length through the plasmasphere decreases as latitude increases. The relative contribution of the plasmaspheric electron content exhibits a diurnal variation that depends on latitude with minimum contribution (∼10%) during daytime and maximum (up to 60%) at night. The contribution is also maximum at the equatorial region where the GPS raypath traverses a long distance through the plasmasphere compared to its length in mid- and high-latitude regions. Finally, the solar cycle variation of plasmaspheric contribution is also reported globally.

Autonomous estimation of plasmasphere content using GPS measurements

The plasmasphere (also denoted as the protonosphere) is a large toroidal domain of light ionized particles situated above the ionosphere and confined by the Earth's magnetic field. While plasmaspheric charge densities are considerably less than those of the ionosphere, the large extent of the plasmasphere can produce significant charge column densities, or total electron content (TEC), for lines‐of‐sight passing through the plasmasphere. A method for Self‐Calibration of Range Errors (SCORE) has been developed previously both to determine combined bias calibration values for GPS receivers and satellites and to calculate absolute TEC values for the ionosphere. An enhanced SCORE process, described here, retains the “self‐calibration” feature of the original SCORE process, by not requiring any measurements beyond those performed by the GPS receiver system being calibrated. The enhanced SCORE process also determines a characteristic plasmasphere amplitude parameter, thus providing an autonomous determination of both the ionospheric and plasmaspheric TEC. Case studies for a near‐equatorial site are presented, with model parameters derived from 1998 data being applied to determine ionospheric and plasmaspheric TEC for measurements made in 1999.