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PPP-RTK based on undifferenced and uncombined observations: theoretical and practical aspects

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

A synthesis of two prevailing global navigation satellite system positioning technologies, namely the precise point positioning and the network-based real-time kinematic, results in the emergence of the PPP-RTK, enabling single-receiver users to achieve high positioning accuracy with reasonable timeliness through integer ambiguity resolution. The realization of PPP-RTK needs to accomplish two sequential tasks. The first task is to determine a class of corrections including, among others, the satellite phase biases (SPBs) at the network level. With these corrections, the second task, then, is to solve for the ambiguity-fixed, absolute position at the user level. In this contribution, we revisit three variants (geometry-free, geometry-fixed and geometry-plus-satellite-clock-fixed) of the undifferenced and uncombined PPP-RTK network model and then point out their implications for practical use. We also carry out a case study using multi-day, dual-frequency global positioning system data from the crustal movement observation network of China stations, aiming to figure out what are the most appropriate linear combinations of the SPBs to be transmitted to the users from the viewpoint of decorrelation, and to assess the static and kinematic positioning performance.

Shipborne gravimetry in the Baltic Sea: data processing strategies, crucial findings and preliminary geoid determination tests

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

Shipborne gravimetry is an essential method to measure the Earth’s gravity field in the coastal and offshore areas. It has the special advantages of high-accuracy and high-resolution measurements in coastal areas compared to other techniques (e.g., satellite gravimetry, airborne gravimetry, and altimetry) used to obtain information about the gravity field. In this paper, we present the data processing strategies of shipborne gravimetry in GFZ. One key point is that the most suitable filter parameters to eliminate disturbing accelerations are determined by studying the GNSS-derived kinematic vertical accelerations and the measurement differences at crossover points. Apart from that, two crucial issues impacting on shipborne gravimetry are the seiches in some harbors and the squat effect in the shallow water. We identified that inclusion of GNSS-derived kinematic vertical accelerations can help to improve the shipborne gravimetry results at these special cases in the Baltic Sea. In the absence of the GNSS-derived vertical accelerations, the cutoff wavelength of the low-pass filter should be large enough to filter out these disturbing acceleration signals which causes a coarser spatial resolution of the gravity measurements. Therefore, the GNSS-derived kinematic vertical accelerations are very useful for optimum shipborne gravimetry. Finally, our shipborne gravimetry measurements are successfully used to verify the previous gravimetry data and improve the current geoid models in the Baltic Sea.

Ionosphere response to three extreme events occurring near spring equinox in 2012, 2013 and 2015, observed by regional GNSS-TEC model

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

Recent Solar Cycle 24 is characterized by the occurrence of three strong disturbances near an equinox. In this contribution, the responses of the ionosphere to the equinox storms in 2012, 2013 and 2015 driven by coronal mass ejections were analyzed. Due to the dynamic nature of changes in the ionosphere, the accuracy and resolution of existing global ionosphere models are often insufficient to reflect the storm-time effects in detail. Bearing this in mind, a new highly accurate and high-resolution regional ionosphere model was applied to study the response of this layer to severe geomagnetic storms over Europe. New regional total electron content (TEC) maps were derived exclusively from precise global navigation satellite systems (GNSS) carrier phase data. Although this study is based on space-geodetic technique, it was also carried out in relation to the observations provided from European ionosondes. In addition, the regional maps were compared to the final IGS Global Ionosphere Maps, where the new solution showed a better detail level. The results of storm-time temporal TEC changes provided by GNSS data were confirmed by NmF2 changes derived from ionosondes. Both data sources confirmed their high compatibility for studying the disturbed ionosphere. The magnetic storms that occurred on 7, 9, 12 and 15 March 2012 were different in nature. The largest change in the total electron content was observed during the storm of 9 March. This storm was associated with an interplanetary coronal mass ejection on 7 March that arrived on Earth 2 days later. The other analyzed events in 2013 and 2015 occurred on the same day of year—17 March. They were triggered by coronal mass ejections, which also hit the Earth magnetosphere at the same time of day. However, again the observed response of the ionosphere to these events was different.

Processing of GNSS constellations and ground station networks using the raw observation approach

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

This article describes the raw observation approach as implemented at Graz University of Technology to determine GNSS products like satellite orbits, clocks, and station positions. To assess the performance of the approach, 15 years (2003–2017) of observations from a network of 245 globally distributed IGS stations to the GPS constellation were processed on a daily basis using the IGS14 reference frame and antenna calibrations. The resulting products are evaluated against those determined by IGS analysis centers. Orbit fit quality relative to the IGS combination is comparable to the best-fitting solutions used for evaluation. Starting from early 2017, when the IGS switched to IGS14, the determined orbits fit better to the IGS combination than any other considered solution. Midnight discontinuities show good internal orbit consistency and no noticeable satellite block-dependency. Satellite clocks are comparable to the considered IGS analysis center solutions. Station positions differ from the IGS combination on a similar level to the solutions they were evaluated against. The temporal repeatability of station positions is slightly better than that of the IGS combination. The quality of resulting GNSS products confirms that the raw observation approach is well suited for the task of determining satellite orbits, clocks, and station positions. It provides an alternative to well-established approaches used by IGS analysis centers and simplifies the introduction of additional observables from new and modernized GNSS.

Toward global instantaneous decimeter-level positioning using tightly coupled multi-constellation and multi-frequency GNSS

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

Autonomous driving represents one of the emerging applications that require both high-precision positions and highly critical timeliness to reach stringent safety standards. We develop a method to potentially achieve global instantaneous decimeter-level positioning by virtue of tightly coupled multi-GNSS triple-frequency observations contributing to precise point positioning (PPP). Inter-system phase biases (ISPBs) for two wide-lane observables are first computed for each station to form inter-GNSS resolvable ambiguities, and then correspondingly two wide-lane fractional-cycle biases (FCBs) are computed for each satellite to recover the integer property of single-station ambiguities. With both ISPB and FCB products, we can accomplish tightly coupled multi-GNSS PPP wide-lane ambiguity resolution (PPP-WAR) using only a single epoch of triple-frequency observations on a global scale. To verify this method, we used 1 month of GPS/BeiDou/Galileo/QZSS data from 107 globally distributed stations and 1 h of such multi-GNSS data collected on a vehicle moving in an urban area. We found that both ISPB and FCB products could be estimated every 24 h with high precisions of around or below 0.1 cycles; 83–98% of their day-to-day variations fell within 0.1 cycles, facilitating their precise predictions for real-time applications. Using these corrections, we achieved both instantly and reliably ambiguity-fixed solutions at 91.2% of all epochs at the 107 stations on average; the resultant single-epoch positions reached a mean accuracy of 0.22 m, 0.18 m and 0.63 m for the east, north and up components, respectively, in case of abundant triple-frequency observations from over 15 satellites. Similarly, for the vehicle-borne test, we obtained instantaneous PPP-WAR solutions at 99.31% of all epochs and achieved a positioning accuracy of 0.29, 0.35 and 0.77 m for the east, north and up components, respectively, which improved significantly the identification of road lanes as opposed to other single-epoch solutions. Finally, we expect that the prospect of instantaneous PPP-WAR in aiding driverless vehicles can be more promising if, whenever possible, integrated with inertial sensors and/or smoothed through multi-epoch data.

Squared M split(q) S-transformation of control network deformations

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

Identification of stable potential reference points (PRPs) is the most critical stage of computations in conventional deformation analysis of geodetic control networks. An appropriate matching of two adjusted networks at stable PRPs plays a key role in this task. Unfortunately, the geodetic control networks are free networks suffering from datum defect and can realize infinitely many possible matchings at PRPs. Therefore, accurate estimation of PRP displacements and later efficient identification of stable PRPs is a quite difficult task. This study makes some step forward in this field and presents a new approach to deformation analysis, including the identification of stable PRPs. The idea behind this approach is inspired by the theory of squared Msplit(q) estimation and lies in the non-conventional assumption that estimated displacements of PRPs can be the realizations—not of one but—of many congruence models, which simultaneously realize many different matchings. Displacements of unstable PRPs in such a multi-split congruence model do not have such a negative effect on expected matching at stable PRPs as in the conventional robust S-transformation. Here, these displacements can be realizations of other congruence models and their attention can be absorbed by other, unexpected, matchings. Thanks to this, the robustness of the suggested approach can be relatively high. To establish what the number of congruence models is in a given case, which model is the one expected and whether the chosen model is valid, the statistical hypothesis tests were proposed. The experiments performed on 1D and 2D simulated control networks showed that the presented approach can provide more accurate values of estimated displacements than conventional approaches, and in consequence, more efficient results of stable PRPs identification, especially when there exist more unstable PRPs than stable ones. In light of the above, the correct identification of stable PRPs and, in consequence, the correct final estimation of controlled object point displacements are possible in cases when it has not been possible so far.

Multi-dimensional particle filter-based estimation of inter-system phase biases for multi-GNSS real-time integer ambiguity resolution

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

In multi-GNSS integration, fixing inter-system double-difference ambiguities to integers is still a challenge due to the existence of inter-system biases (ISB) when mixed types of GNSS receivers are used. It has been shown that when ISB is known, the inter-system ambiguities can be fixed and the reliability of ambiguity fixing can be improved significantly, especially under poor conditions when the number of observed satellites is small. In traditional methods, the intra-system ambiguity is fixed first; then, the ISB is estimated to ultimately fix the inter-system ambiguity. In our work, we use the particle filter-based method to estimate the ISB parameter and fix the inter-system ambiguities to integers at the same time. This method shows higher reliability and higher ambiguity fixing rate. Nevertheless, the existing particle filter approach for ISB parameter estimation is a one-dimensional algorithm. When satellites from three or more systems are observed, there are two or more ISB parameters. We extend the current one-dimensional particle filter approach to multi-dimensional case and estimate multi-ISB parameters in this study. We first present a multi-dimensional particle filter approach that can estimate multi-ISB parameters simultaneously. We also show that the RATIO values can be employed to judge the quality of multi-dimensional ISB values. Afterward, a two-dimensional particle filter approach is taken as an example to validate this approach. For example, in the experiment of GPS L5, Galileo E5a and QZSS L5 integration with 6 satellites using the IGS baseline SIN0-SIN1, only three ambiguities are resolved to integer when the ISBs are unknown. The integer ambiguity fixing rate is 41.0% with 53% of the ambiguity-fixed solutions having positioning errors larger than 3 cm. However, when our approach is adopted, the number of integer ambiguity parameters increases to five. The integer ambiguity fixing rate increases to 99.7% with 100% of ambiguity-fixed solutions having positioning errors smaller than 3 cm.

Modeling the VLBI delay for Earth satellites

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

Very-long-baseline interferometry (VLBI) observations of satellites orbiting the Earth and emitting an artificial radio signal have the potential of becoming an important technique for improving the frame ties between celestial and terrestrial reference frames. Modeling the delay of the signal reception at one station with respect to the other station of a baseline is a fundamental step for correlation and parameter estimation. The near-field VLBI delay models developed so far include numerical computation, which may become expensive in terms of computation time. This applies especially when partial derivatives are to be computed, which is the normal case for least squares adjustments. Furthermore, all the models are formulated in the barycentric celestial reference system requiring large numbers. Here we present an analytical expression for the VLBI delay for the special case of satellites orbiting the Earth, observed by ground-based radio telescopes. We analytically solve the light time equation for each signal propagation path from the source to receiver one and to receiver two under the simplification of linearizing the trajectory of the satellite. By approximating the motion of the Earth as uniform during the short signal travel times we are able to work in the geocentric celestial reference system. We investigate differences between numerical and analytical solutions by simulating VLBI observations of Earth satellites. These tests reveal that delays computed with the analytical formula are consistent with those computed with the numerical solution below the detection level of VLBI but at less computational cost.

The improvement in integer ambiguity resolution with INS aiding for kinematic precise point positioning

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

Despite the benefits of integer ambiguity resolution (IAR) in precise point positioning (PPP), observation outages and harsh signal environments still impact float ambiguity estimation in kinematic surveying, consequently resulting in ambiguity-fixed failure. The inertial navigation system (INS) is an autonomous and spontaneous positioning one, which could provide continuous and superior positioning accuracy over short time. Thus, the INS attains more accurate position than code solution. Moreover, the tight integration of INS and PPP is capable of continuous operation where there are less than four satellites available. These advantages can improve float ambiguity estimation and assist in re-initializing the interrupted ambiguity and PPP solution. Based on the good quality of float ambiguity, the ambiguity dilution precision (ADOP) and the size of integer ambiguity search space are reduced, and then, the IAR-PPP is improved. In this work, the INS aiding effect on IAR-PPP was revealed by the sufficient theoretical analysis and performance assessment. A ring laser gyroscope-based navigation-grade IMU and a fiber optic gyroscope-based tactical-grade IMU were utilized to conduct experiments in an open-sky environment and urban area. The assessment adopted the following aspects of ADOP, bootstrapping success rate, time to fix and position errors. It is found that IAR-PPP with INS aiding achieves an enhanced performance during GPS outage when INS could deliver a superior accurate position. For the navigation- and tactical-grade IMU, the INS-aided ambiguity re-fixing performance can be classified as three levels: significant improvement for the outage duration less than 10 s, moderate improvement for the outage duration from 10 to 60 s and a little or zero improvement for the outage duration longer than 60 s. From the viewpoint of the INS-predicted position domain, an accuracy better than 0.1 m and 1.0 m is required for the significant and moderate improvement, while one can only achieve a little or zero improvement if the position error is larger than 1.0 m. Besides, we also performed the INS-aided IAR-PPP in real urban environment. For the urban environments, the span of clean data is often shorter than 30 min due to intermittent signal interruptions; thus, ambiguity re-fixing for PPP always fails. INS-aided information could bridge the data gaps and achieve fast ambiguity re-fixing. In summary, INS aiding information is capable of improving IAR-PPP performance significantly over a short GPS outage.

Multi-GNSS real-time clock estimation using sequential least square adjustment with online quality control

Journal of Geodesy - Mon, 07/01/2019 - 00:00
Abstract

Real-time satellite orbit and clock product is a key prerequisite for real-time precise positioning service based on precise point positioning (PPP). With the rapid development of the multiple global navigation satellite systems (Multi-GNSS), about 120 satellites will be processed for Multi-GNSS real-time clock estimation. Unfortunately, the computation is very time-consuming, especially for quality control since problematic observations are inevitable. Taking advantage of computer technology, sequential least square adjustment with an adapted online quality control procedure is developed to rapidly estimate Multi-GNSS real-time clocks, although various filtering estimators are commonly used now. A globally distributed network including 70 stations tracking mostly satellites of GPS, GLONASS, BDS, and Galileo is employed for experimental validation. The results show that the computation time per epoch is less than 3 s in average and can meet the 5 s update rate of the IGS real-time clock product. Compared to the GeoForschungsZentrum MGEX (GBM) final clock product, the averaged STD values of the estimated clocks of the four GNSS systems are 0.089 ns and 0.153 ns, respectively, for the clock solutions with and without the online quality control, which also confirms the importance of the quality control procedure. The Multi-GNSS kinematic PPP experiment using the estimated clocks with quality control shows that the positioning RMS is about 4 cm and generally 2 cm in vertical and horizontal components, respectively, and the corresponding convergence time is about 15 min.

IAG Newsletter

Journal of Geodesy - Mon, 07/01/2019 - 00:00

Validation of the IRI 2016, IRI-Plas 2017 and NeQuick 2 models over the West Pacific regions using the SSN and F10.7 solar indices as proxy

Publication date: Available online 5 June 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Yekoye Asmare Tariku

Abstract

This paper investigates the validation of the performance of the latest versions of the International Reference Ionosphere (IRI 2016), the IRI extended to the plasmasphere (IRI-Plas 2017) and NeQuick 2 models in the estimation of the variation of the Total Electron Content (TEC) over the West Pacific regions during the 2015–2017 years. This has been performed employing the GPS-derived TEC data obtained from stations located at Observation Rock, OBSR (geog 46.90°N, 238.18°W, Geom. 52.46°N), Husband, HUSB (44.12°N, 238.15°W, Geom. 49.73°N). It has been revealed that both the GPS-derived VTEC and modelled (IRI 2016, IRI-Plas 2017 and NeQuick 2 VTEC) variations attain their minima at about 13:00 UT (05:00 LT) and maxima at about 20:00 UT (12:00 LT). Moreover, because of the enhancement of photo-ionization process in the region as a result of exposure of the ionosphere for direct radiation in the summer months, large measured VTEC values are seen in the June solstice months during 2015–2017. It has also been shown that, because of the variation of the Sunspot number (SSN) and solar radio flux 10.7 cm (F10.7), the VTEC variations for both the GPS-derived and models show decrease at transition from 2015 to 2017, with some exceptions observed in the June solstice months. In addition, the root-mean-square deviations (RMSD) between the GPS-derived and modelled diurnal VTEC variations are generally less than 0.5TECU in using both the SSN and F10.7 indices. This shows that all the three models are good in TEC estimation with the IRI-Plas 2017 and NeQuick 2 perform the best on most of the hours, in comparison with the IRI 2016 model with IRI2001 option, especially in using the F10.7 index when the solar activity increases. However, when the solar activity decreases, utilizing the SSN for all the three models (especially for the IRI-Plas 2017 and NeQuick 2) shows the better performance. Moreover, during the geomagnetic storm condition, both the IRI 2016 and IRI-Plas 2017 models with the storm option “on” do not adequately reflect to the sharp increase or decrease of the GPS-derived VTEC values.

Atmospheric anomalies associated with M<sub>w</sub>>6.0 earthquakes in Pakistan and Iran during 2010–2017

Publication date: 15 September 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 191

Author(s): Munawar Shah, M. Arslan Tariq, Najam Abbas Naqvi

Abstract

Recent advances on satellite-based measurement of different atmospheric constituents at different heights provide sufficient evidences of short and long-term earthquake (EQ) precursors. In this paper, atmospheric anomalies are investigated related to three large magnitude Mw > 6.0 EQs in Pakistan and Iran during 2010–2017 (i.e., January 18, 2011 (Mw 7.2, Southwestern Pakistan), April 16, 2013 (Mw 7.7, East of Khash Iran) and February 07, 2017 (Mw 6.3, Pasni, Pakistan)). For this purpose, satellite based Outgoing Longwave Radiation (OLR), Surface Temperature (ST), Aerosol Optical Depth (AOD) and NO2 are investigated by statistical bounds of median and standard deviation for two months before and one month after the occurrence of each event. We study the spatial OLR anomaly from National Oceanic and Atmospheric Administration/National Center for Environmental Prediction (NOAA/NCEP). Evidences suggest that abnormal atmospheric anomalies occur within one month before the main shock. For example, in case of Mw 7.2 southwestern Pakistan EQ, significant perturbations in daytime OLR are detected up to 21 days before the main shock, justifying the existence of huge amounts of energy over the tectonic lineaments. The OLR anomaly correlated with ST, AOD and NO2 during 15–21 days before the main shock, which may be attributed to the same event. Similarly, the OLR, ST, AOD and NO2 show irregularities before the 2013, Mw 7.7 East of Khash Iran EQ, where all the anomalies occur 9–10 days before the main shock. The anomalous OLR over the epicenter suggests the authenticity of all the temporal perturbations in the atmospheric parameters related to Mw 7.7 (Iran event). Furthermore, the atmospheric parameters are analyzed temporally by the statistical bounds before the Mw 6.3 (Pasni, Pakistan) EQ. All the parameters behave abnormally during 10–15 days following the Mw 6.3 Pakistan EQ and similarly subsequent spatial OLR enhancement over the epicenter may lead to the conclusion of an extensive energy emanation. The anomalies detected are consistent with the processes of stress activation of proxy defects at the Lithosphere-Atmosphere interface in the seismic breeding zone.

Observed response of stratospheric and mesospheric composition to sudden stratospheric warmings

Publication date: 15 September 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 191

Author(s): M.H. Denton, R. Kivi, T. Ulich, C.J. Rodger, M.A. Clilverd, J.S. Denton, M. Lester

Abstract

In this study we investigate and quantify the statistical changes that occur in the stratosphere and mesosphere during 37 sudden stratospheric warming (SSW) events from 1989 to 2016. We consider changes in the in-situ ozonesonde observations of the stratosphere from four sites in the northern hemisphere (Ny-Ålesund, Sodankylä, Lerwick, and Boulder). These data are supported by Aura/MLS satellite observations of the ozone volumetric mixing ratio above each site, and also ground-based total-column O3 and NO2, and mesospheric wind measurements, measured at the Sodankylä site. Due to the long-time periods under consideration (weeks/months) we evaluate the observations explicitly in relation to the annual mean of each data set. Following the onset of SSWs we observe an increase in temperature above the mean (for sites usually within the polar vortex) that persists for >∼40 days. During this time the stratospheric and mesospheric ozone (volume mixing ratio and partial pressure) increases by ∼20% as observed by both ozonesonde and satellite instrumentation. Ground-based observations from Sodankylä demonstrate the total column NO2 does not change significantly during SSWs, remaining close to the annual mean. The zonal wind direction in the mesosphere at Sodankylä shows a clear reversal close to SSW onset. Our results have broad implications for understanding the statistical variability of atmospheric changes occurring due to SSWs and provides quantification of such changes for comparison with modelling studies.

Linkage of meteorological parameters and anomalous radio propagation profile over Nigeria

Publication date: 15 September 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 191

Author(s): I. Emmanuel

Abstract

The vertical distribution of refractivity gradient is important in determining anomalous propagation condition. Thirty five years of meteorological parameters, obtained from Era-Interim archive of European Centre for Medium-Range Weather Forecasts has been used to analyze and investigated surface meteorological data linked with refractivity gradient aloft altitude across Nigeria. Spatial distribution of surface temperature, relative humidity and refractivity gradient at 0.1 km, 0.5 km, 1.0 km and 1.5 km over Nigeria were plotted. Vertical distribution of temperature, relative humidity, refractivity and refractivity gradient were obtained for some locations across Nigeria. Similarly, spatial distribution of coefficient of determination between surface temperature, relative humidity and refractivity gradient at four different height were estimated. Linear regression were developed to investigate the relationship between surface data and refractivity gradient at different altitude. The result revealed existence of sub refractive, super refractive, and ducting across the country at 0.1 km and 0.5 km however occurrence of ducting and sub refractive disappeared at 1.0 km and 1.5 km. Likewise, the existence of temperature inversion was noticed between surface and 100 m across all the locations except in Lagos. Values of refractivity across the observed locations converged around 0.5 km. Through result of correlation coefficient and statistical parameters, significant linked have been established between surface data and refractivity gradient at different height.

Vertical wavenumber spectral characteristics of temperature in the stratosphere-mesosphere over tropical and subtropical regions

Publication date: 15 September 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 191

Author(s): Priyanka Ghosh, Som Sharma

Abstract

The vertical wavenumber spectra over tropical location, Gadanki (13.5° N, 79.2° E) and sub-tropical location, Mt. Abu (24.5° N, 72.7° E) is studied using the temperature measurements from ground based Rayleigh Lidar and space borne satellite observations. The slope values are lesser over Gadanki than at Mt. Abu for almost all the altitudes except for 40–50 km where it is nearly same and 60–70 km exhibiting opposite nature. Unusual spectral slope of −6.97 (Mt. Abu) and −0.09 (Gadanki) is seen at the altitude of 40–50 km in satellite temperature. Characteristics of wave oscillations perceived over both the stations are described.

Monitoring the global evolution of the storm ring current and storm indices from confined ground geomagnetic observatories

Publication date: 15 September 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 191

Author(s): G. Zeng, C. Shen, Z.J. Rong, X. Li, T. Chen, Z.Q. Chen, Y.H. Ma

Abstract

For the first time, the global structure of the geomagnetic disturbance field around the Earth as well as the magnetic storm indices have been deduced from magnetic field measurements by ground observatories in a confined range of longitude. The spatial gradient of the H component of the geomagnetic disturbance field was obtained from ground geomagnetic observatories only in the Eastern Hemisphere, provided the geomagnetic disturbance field varies approximately linearly in space. Furthermore, the storm symmetric and asymmetric indices were derived and the spatial distribution and temporal evolution of the storm ring current was investigated. It was found that the storm indices derived from observatories in the Eastern Hemisphere are consistent with the officially published Kyoto standard indices which are derived from six globally distributed observatories. We also calculated the storm indices for 1941–1956 by using data from three observatories (HER, KAK and SJG). The correlation coefficient between the symmetric index deduced from three observatories and the one from the global six observatories is 0.98, and the correlation coefficient between the two kind of asymmetric indices is 0.79. Those results suggest that our approach is reasonable and significant when global ground observatories are not readily available.

Feed forward neural network based ionospheric model for the East African region

Publication date: 15 September 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 191

Author(s): A. Tebabal, S.M. Radicella, B. Damtie, Y. Migoya-Orue’, M. Nigussie, B. Nava

Abstract

In this paper, a neural network based regional ionospheric model is developed using GPS-TEC data from 01 January 2012 to 31 December 2015. For this purpose, nine GPS station TEC data in the time intervals 2012 to 2014 were used to determine model parameters. TEC data obtained in various years and geographical locations which are excluded in the training time are used to validate the performance of the model. For the first case, TEC data from each station in the year 2015 is used to validate the performance of the model. In the second case, GPS observations at Metu, Robe, and Serb stations are used to investigate the model’s performance in the year 2012, 2014 and 2013–2014, respectively. In both cases, to validate the accuracy and quality of the model, GPS-TEC values were compared with the predicted TEC. The results indicate that the proposed model can capture most of the spatio-temporal variations of the regional TEC. The present model reproduces the observed hourly TEC with RMS values that lie around 3 to 6.05 TECU at different geographical locations for both one hour and one day ahead prediction. For one day ahead prediction, a comparison of the NN method using NeQuick 2 model outputs with GPS derived measurements have also been conducted. The results indicate that the NN TEC model proposed has a good performance in representing TEC variations compared to climate NeQuick 2 model.

Assessments of the night-time and daytime radiative fluxes balance on seasonal timescale over West Africa

Publication date: 15 September 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 191

Author(s): O.S. Ojo, B. Adeyemi, E.O. Ogolo

Abstract

The main aim of this paper is to assess the impact of climate change on variations and trends of the net radiation flux over West Africa during night and day time. West Africa was sub-divided into six climatic zones as classified by the World Meteorological Organization such as Hyper-Arid (HAR), Arid (ARD), Semi-Arid (SAR), Semi-Humid Dry (SHD), Semi-Humid Humid (SHH) and Humid (HUM) zones. To achieve this aim, thirty-six years' surface data of shortwave and longwave radiations between 1980 and 2015 were obtained from the Archives of the Modern-Era Retrospective Analysis for Research and Application, Version 2 (MERRA-2) database. Analyses showed that the maximum values of net radiative fluxes during the nighttime and daytime have the magnitude in watt per square metres of −70.00 and 225.82 in HAR zone, −62.46 and 248.99 in ARD zone, −51.71 and 304.88 in SAR zone, −40.55 and 334.58 in SHD zone, −34.32 and 352.62 in SHH zone and −30.49 and 362.68 in HUM zone respectively. The effect of population density, emission of greenhouse gas and surface albedo on net radiation was investigated over the climatic zones using the multivariate linear regression analysis. The results of the regression analysis showed that they have significant effects on net radiation. Also, the monotonic trend analysis between 1980 and 2015 was carried out using the non-parametric Mann- Kendall statistical test. The results of the trend test revealed that net radiation showed decreasing trends mainly in the humid zones at over 95% significance level while population density, emission of greenhouse gas and surface albedo showed significant increasing trends at the 99.9% level of significance. The analyses showed that the humid zones have higher values of net radiation, radiative cloud forcing, carbon-dioxide emission, population density and lower surface albedo than arid zones. Therefore, as signatures of climate change, it can be concluded that increase in population density, cloud amount and anthropogenic activities such as land use/land cover and emission of greenhouse gas have contributed greatly to the significant decreasing trends of the radiative flux balance especially in the humid zones of West Africa.

Simulations of the inner magnetospheric energetic electrons using the IMPTAM-VERB coupled model

Publication date: 15 September 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 191

Author(s): Angelica M. Castillo, Yuri Y. Shprits, Natalia Ganushkina, Alexander Drozdov, Nikita Aseev, Dedong Wang, Stepan Dubyagin

Abstract

In this study, we present initial results of the coupling between the Inner Magnetospheric Particle Transport and Acceleration Model (IMPTAM) and the Versatile Electron Radiation Belt (VERB-3D) code. IMPTAM traces electrons of 10−100 keV energies from the plasma sheet (L=9 Re) to inner L-shell regions. The flux evolution modeled by IMPTAM is used at the low energy and outer L∗ computational boundaries of the VERB code (assuming a dipole approximation) to perform radiation belt simulations of energetic electrons. The model was tested on the March 17th, 2013 storm, for a six-day period. Four different simulations were performed and their results compared to satellites observations from Van Allen probes and GOES. The coupled IMPTAM-VERB model reproduces evolution and storm-time features of electron fluxes throughout the studied storm in agreement with the satellite data (within ∼0.5 orders of magnitude). Including dynamics of the low energy population at L∗=6.6 increases fluxes closer to the heart of the belt and has a strong impact in the VERB simulations at all energies. However, inclusion of magnetopause losses leads to drastic flux decreases even below L∗=3. The dynamics of low energy electrons (max. 10s of keV) do not affect electron fluxes at energies ≥900 keV. Since the IMPTAM-VERB coupled model is only driven by solar wind parameters and the Dst and Kp indexes, it is suitable as a forecasting tool. In this study, we demonstrate that the estimation of electron dynamics with satellite-data-independent models is possible and very accurate.

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