The International GNSS Service (IGS) Working Group on Ionosphere was created in 1998. Since then, the Scientific community behind IGS, in particular CODE, ESA, JPL and UPC, have been continuosly ...contributing to reliable IGS combined vertical total electron content (VTEC) maps in both rapid and final schedules. The details on how these products are being generated, performance numbers, proposed improvement as far as VTEC evolution trends during near one Solar Cycle, are summarized in this paper. The confirmation of (1) the good performance of the IGS combined VTEC maps, and (2) the characteristic VTEC variability periods, are two main results of this work.
In this work we present a simple technique to estimate the medium‐scale traveling ionospheric disturbances (MSTIDs) characteristics (such as occurrence, velocity, vertical propagation) with periods ...lower than 20 min and its application to a set of GPS data both temporally and spatially representative (near one solar cycle and four local networks in the Northern and Southern Hemispheres, respectively). Some of the main results presented in this paper are the MSTIDs which occur at daytime in local winter and nighttime in local summer, related to the solar terminator and modulated by the solar cycle. They present equatorward (from ∼100 to 400 m/s) and westward (∼50 to 200 m/s) horizontal propagation velocities, respectively. The corresponding periods are compatible (higher) with the theoretical prediction, which is given by the neutral atmosphere buoyancy period associated with the Brunt‐Väisälä frequency (about 600 s). Moreover, higher TIDs productivity is mainly associated with the downward vertical propagation. Finally, the results obtained in this study suggest the possibility of developing future MSTID models to mitigate its impact in applications like precise satellite navigation.
The dual-frequency satellite-based measurements from Global Positioning System (GPS) may provide feasible ways of studying and potentially detecting of earthquake (EQ) related anomalies in the ...ionosphere. In this paper, GPS based Total Electron Content (TEC) data are studied for three major M > 7.0 EQs in Nepal and Iran-Iraq border during 2015–2017 by implementing statistical procedures on temporal and spatial scale. Previous studies presented different time interval of pre-seismic ionospheric anomalies, however, this study showed that EQs ionospheric precursors may occur within 10 days. Furthermore, the ionospheric anomalies on the suspected day occurred during UT = 08:00–12:00 h before the main shock. The Global Ionospheric Map TEC (GIM-TEC) data retrieved over the epicenter of M7.8 (Nepal EQ) showed a significant increase of 6 TECU on April 24, 2015 (one day before the main shock), which is recorded by the ground GPS station data of Islamabad (station lies within the EQ preparation zone). Furthermore, the spatial GIM-TEC result imply significant anomalies over the epicenter during the time interval between UT = 08:00–12:00 h (LT = 13:00–17:00). For M7.3 (Nepal EQ), the TEC anomalies were detected on May 10, 2015 (2 days before the event) in the temporal data. The spatial TEC data imply the huge clouds over the epicenter at about UT = 08:00–12:00 h on May 10, 2015, that may be associated with this EQ in the quiet geomagnetic storm conditions. Similarly, temporal and spatial TEC showed anomaly on November 3, 2017, during UT = 08:00–12:00 (9 days before the Iran-Iraq border EQ) after implementing the statistical method on it. Conversely, there exists a short-term but low magnitude TEC anomaly synchronized with a geomagnetic storm on November 7–8, 2017 (4 to 5 days prior to M7.3 Iran-Iraq border EQ). The diurnal and hourly GIM-TEC and VTEC data also imply the execution of ionospheric anomalies within 10 days prior to all events. All these positive anomalies in TEC may be due to the existence of a huge energy from the epicenter during the EQ preparation period.
In this paper, different aspects of the application of the second‐order ionospheric term (abbreviated as I2) and its impact on geodetic estimates are studied. A method to correct the GPS observations ...from this effect is proposed. This method provides a more accurate correction to the GPS measurements (in some cases, it can even be 50% better) with respect to other ways of computing such effect. Moreover, this method can be applied routinely to estimate geodetic parameters. Applying the I2 correction to subdaily differential positioning, several relationships between the deviation of the parameter estimates and the I2 term are derived in the context of a new global approach to the problem. In particular, it is shown that the effect in receiver position mainly depends on the differential value of this term between GPS receivers, while the satellite clocks are directly affected by the undifferenced values. Data from the International GNSS Service (IGS) global network of receivers have been gathered over a period of 21 months. These data have been used to study the I2 effect on the geodetic estimates, such as receiver positions, satellite clocks, and orbits. The most important effect appears for the satellite clocks, and it can be greater than 1 cm depending on the geographical location, comparable to the IGS nominal accuracies. The effect on orbits consists of a global contribution of several millimeters (which confirms the geocenter displacement detected by other authors) plus a subdaily contribution, also of several millimeters, that is geographically dependent, also comparable to the IGS nominal accuracies. As for the position of receivers, the obtained shifts are, in general, at submillimeter level and are directed southward for low‐latitude receivers and northward for high‐latitude receivers. These results will be explained in detail since they are not completely in agreement with the ones presented in previous works.
This paper reports the results of ionosphere and plasmasphere observations with the Kharkiv incoherent scatter radar and ionosonde, Defense Meteorological Satellite Program, and Arase (ERG) ...satellites and simulations with field line interhemispheric plasma model during the equinoxes and solstices of solar minimum 24. The results reveal the need to increase NRLMSISE‐00 thermospheric hydrogen density by a factor of ~2. For the first time, it is shown that the measured plasmaspheric density can be reproduced with doubled NRLMSISE‐00 hydrogen density only. A factor of ~2 decrease of plasmaspheric density in deep inner magnetosphere (L ≈ 2.1) caused by very weak magnetic disturbance (Dst > −22 nT) of 24 December 2017 was observed in the morning of 25 December 2017. During the next night, prominent effects of partially depleted flux tube were observed in the topside ionosphere (~50% reduced H+ ion density) and at the F2‐layer peak (~50% decreased electron density). The likely physical mechanisms are discussed.
Plain Language Summary
Our planet is surrounded by an extensive envelope of hydrogen gas that stretches a quarter of the way to the moon. It is called the geocorona because it can be seen in ultraviolet light analogous to the corona surrounding the sun during a total eclipse. This hydrogen gas is the source of ionized hydrogen that forms the plasmasphere, which is important because it affects radio wave propagation and therefore the accuracy of global positioning systems. The ultimate source of the hydrogen is the dissociation of water vapor near 100‐km altitude. Both the geocorona and plasmasphere have their source from the atomic hydrogen near 500 km in the thermosphere. For almost half a century, scientists have been using hydrogen density deduced from the observations of Atmospheric Explorer satellite missions. Our study with Kharkiv incoherent scatter radar shows that the hydrogen density is actually ~100% higher than the earlier measurements. This result is supported by independent observations with satellites. Our finding means that many of calculations related to the important aspects of space weather influence need to be revisited. And, in a broader sense, our result points the way to better understanding of long‐standing unresolved problems of solar‐terrestrial interaction.
Key Points
The NRLMSISE‐00 model underestimated thermospheric hydrogen density by ~100% during 2016‐2018
An unusually strong response to the minor storm of 24 December 2017 was observed from the inner plasmasphere to the F2‐layer peak region
The Kharkiv IS radar results are consistent with ionosonde, DMSP, and Arase (ERG) satellite measurements
A comprehensive study of the response of the ionosphere‐plasmasphere system at mid‐latitudes to weak (Dstmin > −50 nT) magnetic storms is presented. For the first time, it is shown that weak magnetic ...disturbances can lead to significant modulation of ionosphere‐plasmasphere H+ ion fluxes. It is found that this modulation is caused by the enhancements/reductions of the topside O+ ion density, which is induced by F2‐layer peak height rise and fall during the storms. The F2‐layer motion is caused by thermospheric wind changes and by a penetration electric field. Both drivers are closely related to the changes in the Bz component of interplanetary magnetic field. The most prominent manifestation of the H+ ion flux modulation is strong changes in H+ ion fraction in the topside ionosphere. This study also indicates that the NRLMSISE‐00 model provides the correct relative changes of neutral H density during weak magnetic storms and also that there is a compelling need to include geomagnetic activity indices, in addition to solar activity (F10.7), as input parameters to empirical topside ionosphere models.
Key Points
Weak magnetic storms cause notable modulation of the ionosphere‐plasmasphere H+ ion fluxes and prominent effects in the topside ionosphere
The modulation is caused by the topside O+ ion density changes induced by F2‐layer peak up/down lifting
F2‐layer peak vertical motion is related to the changes in the Bz component of interplanetary magnetic field
Global Navigation Satellite Systems (GNSS) radio occultations allow the vertical sounding of the Earth's atmosphere, in particular, the ionosphere. The physical observables estimated with this ...technique permit to test theoretical models of the electron density such as, for example, the Chapman and the Vary‐Chap models. The former is characterized by a constant scale height, whereas the latter considers a more general function of the scale height with respect to height. We propose to investigate the feasibility of the Vary‐Chap model where the scale height varies linearly with respect to height. In order to test this hypothesis, the scale height data provided by radio occultations from a receiver on board a low Earth orbit (LEO) satellite, obtained by iterating with a local Chapman model at every point of the topside F2 layer provided by the GNSS satellite occultation, are fitted to height data by means of a linear least squares fit (LLS). Results, based on FORMOSAT‐3/COSMIC GPS occultation data inverted by means of the Improved Abel transform inversion technique (which takes into account the horizontal electron content gradients) show that the scale height presents a more clear linear trend above the F2 layer peak height, hm, which is in good agreement with the expected linear temperature dependence. Moreover, the parameters of the linear fit obtained during four representative days for all seasons, depend significantly on local time and latitude, strongly suggesting that this approach can significantly contribute to build realistic models of the electron density directly derived from GNSS occultation data.
Key Points
Results suggest that the scale height is correlated with height in the topside of the ionosphere
The scale height and its gradient depend on local time and latitude
Imbalance between heating and cooling time of the plasma may yield such a linear scale height
Although vertical total electron content (VTEC) forecasting is still an open subject of research, the use of predictions of the ionospheric state at a scale of several days is an area of increased ...interest. A global VTEC forecast product for two days ahead, which is based exclusively on actual Global Positioning System (GPS) data, has been developed in the frame of the International Global Navigation Satellite Systems (GNSS) Service (IGS) Ionospheric Working Group (IGS Iono‐WG). The UPC ionospheric VTEC prediction model is based on the Discrete Cosine Transform (DCT), which is widely used in image compression (for instance, in JPEG format). Additionally, a linear regression module is used to forecast the time evolution of each of the DCT coefficients. The use of the DCT coefficients is justified because they represent global features of the whole two‐dimensional VTEC map/image. Also, one can therefore introduce prior information affecting the VTEC, for instance, smoothness or the distribution of relevant features in different directions. For this purpose, the use of a long time series of final/rapid UPC VTEC maps is required. Currently, the UPC Predicted product is being automatically generated in test mode and is made available through the main IGS server for public access. This product is also used to generate two days ahead preliminary combined IGS Predicted product. Finally, the results presented in this work suggest that the two days ahead UPC Predicted product could become an official IGS product in the near future.
Key Points
Two‐day ahead prediction model of global VTEC maps
Based on the Discrete Cosine Transform (DCT) and a linear regression module
Tests against Final UPC products and JASON VTEC measurements
In this work, an extension in latitude range and time span with respect previous studies on Medium Scale Traveling Ionospheric Disturbances (MSTID) propagation, is presented. So far they have been ...basically studied at mid latitude and for limited periods (less than few years) at solar maximum conditions. This extension has been possible due to the availability of local Global Positioning System (GPS) networks at mid‐north hemisphere (California), mid‐south hemisphere (New Zealand), high and low latitudes (Alaska and Hawaii), for the last 13, 11, 7 and 4 years respectively. Optimal algorithms specially suitable for mass data processing have been used, such as the Single Receiver Medium Scale Traveling Ionospheric activity index (SRMTID) and the phase difference method for MSTID propagation estimation. The results reveal that several of the main MSTID climatological trends at mid latitude are also shared at low and high latitude, also modulated in intensity also by the Solar Cycle. This is the case for local fall/winter day‐time equatorward propagated MSTIDs with typical velocities and wavelengths of 150–250 m/s and 100–300 km respectively. Moreover the comparison of MSTID propagation estimation using different techniques, and their implications in terms of potential origins of MSTIDs, are also discussed.
Key Points
Characterization of MSTID propagation at low, mid and high latitudes
Characterization of MSTID propagation for up to more than one solar cycle
Comparison of GPS MSTID characterization with other techniques
This paper presents the variations of the ionospheric Vertical Total Electron Content (VTEC) observed over Pakistan at the verge of low- to mid-latitude regions during the years 2016–17 of the ...descending phase of the solar cycle. The study is conducted by considering the ionospheric measurements from dual frequency Global Navigation Satellite System (GNSS) receivers permanently installed at Islamabad (geomagnetic Lat. 25.44°N, Long. 148.83°E), Multan (geomagnetic Lat. 22.13°N, Long. 146.91°E) and Quetta (geomagnetic Lat. 22.50°N, Long. 142.73°E). The diurnal, seasonal and annual variations of VTEC over Pakistan are examined in the context of geomagnetic storm during 2016. This study shows high values during the March and September equinoctial months and lower values during the summer and winter solstices from VTEC estimations. Furthermore, higher, moderate and lower VTEC variations are recorded during the seasonal analysis in the equinoxes, summer solstice and winter solstice, respectively. The maximum seasonal VTEC values are observed during the post-sunrise hours between 11:00–17:00 LT and the minimum values are recorded during the post-midnight hours between 02:00–05:00 LT during each season at all the stations. Moreover, the effect of geomagnetic storm is detected in the ionospheric VTEC of the three different stations, which occurred on 13 October 2016. The initial phase of the storm caused no prominent effect on VTEC, while an enhancement in VTEC is registered in the main and recovery phases of the storm. The recorded VTEC from three different stations is correlated with the indices of the geomagnetic storm.
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