New advantages of the combined GPS and GLONASS observations for high-latitude ionospheric irregularities monitoring: case study of June 2015 geomagnetic storm
© The Author(s) 2017
Received: 17 August 2016
Accepted: 2 May 2017
Published: 12 May 2017
The techniques based on the transionospheric radio waves propagation, in particular satellite navigation signals, are effectively used for monitoring and investigation of the main parameters of the ionospheric plasma irregularities. It is known that radio signals passing through the ionosphere suffer varying degrees of rapid variations of their amplitude and phase signal fluctuations, referred to as scintillations, created by random fluctuations of the medium’s refractive index, caused by plasma density gradients inside the ionosphere (e.g., Tsunoda et al. 1985; Basu et al. 1988; Aarons 1997; Prikryl et al. 2012). At high latitudes, these gradients are mostly caused by plasma processes associated with dynamic auroral processes, such as energetic particle precipitation and high-speed plasma convection (Keskinen and Ossakow 1983). Many researchers have used GPS signals to study ionospheric processes (e.g., Pi et al. 1997; Aarons and Lin 1999; Valladares et al. 2004; Jakowski et al. 2008, 2012; Tiwari et al. 2013; Prikryl et al. 2014; Cherniak and Zakharenkova 2015; van der Meeren et al. 2014; Jacobsen and Andalsvik 2016). Recently, Cherniak and Zakharenkova (2016a) applied data of the GPS receivers onboard five low earth orbit satellites to examine the occurrence of the topside ionospheric irregularities under the geomagnetic storm conditions and to compare them with effects registered concurrently in the ground-based GPS data.
The number of the ground-based receivers within the global and regional networks grew significantly from several hundreds worldwide in the 1990s to more than 6000 stations today. These networks provide continuous measurements of navigation signals parameters and open access to their databases. In addition to the increase in the number of the ground-based stations, the GPS constellation was modernized by the addition of satellites in the GPS-IIF series. Other GNSS like the Russian GLONASS, the European GALILEO and the Chinese Beidou systems increased the number of satellites placed into orbit. Further development of the multi-system GNSS constellations and modernization of the ground-based receivers to be able to track multi-frequency and multi-system GNSS signals provide more opportunities for ionospheric research in the near future.
At the moment, the second fully deployed GNSS is the Russian system—GLONASS (GLObal Navigational Satellite System) (see Hofmann-Wellenhof et al. 2008; ICD-GLONASS 2008; Jeffrey 2015). The full orbital constellation consists of 24 satellites into three orbit planes. The orbit altitude is ~19,100 km above the Earth’s surface. A significant advantage of the GLONASS, as compared to the GPS, is that the GLONASS has an orbit inclination of ~65°, that is ten degree higher than the GPS orbit inclination. This feature is important for the high-latitude region, where a multi-system GNSS receiver can track the GLONASS navigation signals for much longer time and with higher elevation angles than GPS ones. The number of the ground-based receivers able to track both GPS and GLONASS signals has been increased significantly in last years. In the present paper, we demonstrate the advantages of the multi-constellation measurements for high-latitude ionospheric irregularities monitoring for the case study of the June 2015 geomagnetic storm.
Case study: the summer solstice 2015 geomagnetic storm
The 3-h mid-latitude geomagnetic index Kp (not shown here) reached the value of 8+ at 18-21 UT on 22 June and at 03-06 UT on 23 June. This geomagnetic storm is the second largest to date in the 24th solar cycle after the St. Patrick’s Day storm that occurred on 17–18 March 2015 (e.g., Cherniak and Zakharenkova 2015).
Data and methodology
For the given research, we made use of all available permanent ground-based GNSS stations, which are able to track the GPS signals only and combined GPS and GLONASS signals. We use ~5800 ground-based GNSS stations gathered separately from several global and regional GNSS networks: the International GNSS Service (IGS), the University NAVSTAR Consortium (UNAVCO), the Continuously Operating Reference System (CORS), the Scripps Orbit and Permanent Array Center (SOPAC), the EUREF Permanent GNSS network (EPN), the Federal Agency for Cartography and Geodesy (BKGE) in Germany, Institut Geographique National in France (IGN), the Swedish geodetic network (SWEPOS), the Finnish Reference Network (FGI-FinnRef), the NOANET GNSS Network in Greece, the Spanish GNSS Reference Stations Network (ERGNSS), the Natural Resources Canada’s Canadian Geodetic Survey, the Canadian High Arctic Ionospheric Network (CHAIN), the Brazilian Network for Continuous Monitoring (RBMC), the Red Argentina de Monitoreo Satelital Continuo (RAMSAC CORS), the Australian Regional GNSS Network (ARGN) and the New Zealand Government GNSS CORS.
Figure 2b demonstrates an example of the data coverage accumulated during 1 h by all available stations tracking the GPS signals; blue dots show location of the ionosphere pierce points (IPPs) on links from a ground-based receiver to a GPS satellite. Figure 2c presents the same maps of the registered GPS data with the superimposed GLONASS measurements (red color dots). It is clearly seen that the GLONASS data coverage is very good and dense over the regions where the ground-based stations can track both systems simultaneously, e.g., the USA, Europe, Australia and Argentina. Thus, in these regions the use of the GLONASS can potentially increase the number of available measurements by a factor of 1.5–2 as comparing with using the GPS only. But impact of the GLONASS is more valuable in the regions with the sparse ground-based stations, particularly, at high latitudes of both hemispheres, the Asian sector, as well as rare islands with GNSS stations. Figure 2d shows the percentage contribution of the GLONASS measurements to each data cell. Here, the dark blue color indicates cells with the GPS measurements only, while orange and red colors depict cells where GLONASS can contribute more than 60–80% to total GNSS measurements. It is clearly seen that the dense networks, which support both types of signals (GPS and GLONASS), can significantly gain in a total number of measurements per a spatial cell/bin by adding the GLONASS system.
We processed GPS and GLONASS measurements derived from more than 6000 ground-based GNSS stations. These data are freely available for users and distributed in the raw RINEX (The Receiver Independent Exchange Format) format (Gurtner 1994). Sampling rate of the raw data is usually 30 s for the majority of stations. For a case of high-rate measurements (e.g., 1 s), the data were resampled to 30 s for uniformity.
After determination of sTEC values along LOS for all visible GPS and GLONASS satellites during 24 h, we applied algorithms for detection and correction of cycle slips and loss-of-lock and removed outliers. The elevation cutoff mask of 25° is used here to minimize the multi-path effects in the sTEC variations. The carrier phase measurements can be also affected by cycle slips which are sudden changes in the integer phase ambiguity due to the phase tracking loop within the receiver. The cycle slip may be as small as one or a few cycles, or contain millions of cycles. Here, we use two approaches for cycle slip detection—the widelane Melbourne–Wübbena linear combination (Melbourne 1985; Wübbena 1985) and method of differencing geometry-free phase observations with estimation of the rate of TEC changes similar to that of Horvath and Crozier (2007). Then, all derived sTEC values are geolocated using a single-layer model approach, when sTEC along the LOS is referred to the point of LOS intersection (IPP) with the thin ionospheric layer located at 350 km altitude.
As an ionospheric irregularity can be characterized by measuring its impact on the phase of the received GPS signal, Pi et al. (1997) introduced into usage for ground-based GPS observations two GPS-based indices: ROT and ROTI. Rate of TEC change (ROT) is the time-derivative of TEC and is considered as a measure of the phase fluctuation activity. Rate of TEC Index (ROTI) represents a standard deviation of the ROT over a selected time interval. The ROTI characterizes the severity of the GPS phase fluctuations and detects the presence of the ionospheric irregularities, which can be characterised by the TEC spatial gradient. Today, the ROT/ROTI indices, derived from the ground-based GPS data, are widely used in near-real-time services of space weather monitoring (e.g., NICT 2016; SWACI 2016 and described by Jakowski et al. 2006; Miyake and Jin 2010) and in investigations of the ionospheric irregularities occurrence at high- and low-latitude regions (e.g., Cherniak and Zakharenkova 2015). Here, we propose to extend the standard GPS database by the combined GPS and GLONASS observations.
Further, to analyze the high-latitude ionospheric irregularities occurrence and temporal development, we construct, from the multi-site GNSS database, the ROTI maps in the geographic coordinate frame. These ROTI maps with a polar view projection were constructed with high spatial resolution for the latitudinal range of 30°–90° in both northern and southern hemispheres. All ROTI values derived from the GPS and GLONASS data along all visible satellite passes were averaged and binned into cells of 1° × 1° resolution in geographic latitude and longitude. No interpolation was used here; the empty cells with no data or with less than 50 points per cell were marked as blank ones. The temporal interval was selected as 1 h.
Results and discussion
Comparison of GPS and GLONASS measurements in polar region
Two-dimensional combined GPS and GLONASS ROTI maps
We should note that the North American and European sectors have an essentially better data coverage than other regions in the northern and southern hemisphere (see Fig. 2a, e), that is why the hourly ROTI maps reveal their best data coverage and higher resolution over these regions. Overall, the mid- and high latitudes of the northern hemisphere exhibit proper coverage by the GPS and GLONASS observation within a wide longitudinal range of 140°W–50°E. Apart from GNSS, there is no other radio-based instrument able to provide such data coverage from the ground.
These hourly ROTI maps demonstrate the dynamics of the ionospheric irregularities in a geographic coordinate frame. The ROTI values marked by dark blue color (ROTI below 0.2 TECU/min) represent very weak or an absence of the ionospheric irregularities. The ROTI values marked by orange and red colors (ROTI >0.8–1.0 TECU/min) correspond to the occurrence of the intense ionospheric irregularities in this sector. Analysis of the ROTI maps for a quiet day of June 20, 2015 (Figs. 4a, 5a) revealed the very quiet situation over the polar regions in both hemispheres with rather weak irregularities occurring in the vicinity of the geomagnetic poles.
The first noticeable changes in the irregularities distribution pattern appeared after 07–08 UT on June 22, 2015, initiated by the second CME arrival and the first intensification of auroral activity (see Fig. 1). The most intense irregularities in both hemispheres were observed after 16 UT on 22 June. Very high ROTI values (>0.8–1 TECU/min) were found to form an oval-like structure around the northern geomagnetic pole. Further, the GNSS-derived irregularity oval expanded equatorward during several hours, and its equatorial edge was detected in the North American sector at ~45°N–50°N geographic latitude for more than 2–3 h. The highest ROTI intensity values in this oval-like feature occurred mainly over Northern Europe. We should also emphasize that the intense ionospheric irregularities were observed over Southern Europe at ~25°N–40°N geographic latitude during the main phase of the storm at 20-04 UT (Figs. 4; Additional file 2: S2, Additional file 3: S3). These irregularities were associated with the occurrence of plasma bite-outs and equatorial plasma bubbles in the postsunset sector (20-04 UT) over low latitudes of Western Africa after the prompt penetration electric fields at 18-20 UT on June 22, 2015 (for more details see Cherniak and Zakharenkova 2016b).
The ionospheric irregularities occured during the June 2015 geomagnetic storm and depicted by the combined GPS and GLONASS observations have impact on the navigation system performance. The WAAS System Performance Analysis Report indicated that during the June 22–23 it was observed a reduction in Localizer Performance with Vertical Guidance (LPV) and Localizer Performance with Vertical Guidance to 200 ft decision height (LPV200) coverage provided by WAAS in the continental US (CONUS), Alaska, and Canada (Wanner 2015). In these regions, there were observed the strong ionospheric irregularities related with the auroral particles precipitations, more detailedly described in next subsections. Moreover, the highly intense irregularities lead to a performance degradation of the European Geostationary Navigation Overlay Service (EGNOS). It is very interesting to note that an impact of the ionospheric irregularities occurrence on the GNSS performance in the European sector was observed not only at high latitudes (irregularities related with particles precipitations and ionospheric patches formation), but also at Southern Europe and the Mediterranean region (irregularities related with the storm-time plasma depletions of equatorial origin, i.e., plasma bubbles development) (Cherniak and Zakharenkova 2016b).
At high latitudes, generation and evolution of the ionospheric irregularities were associated with auroral particle precipitation after the CMEs arrival and further development of the main phase of this geomagnetic storm.
Figure 5 presents the evolution of ionospheric irregularities over the southern hemisphere. Here, it is also possible to estimate differences in the occurrence, intensity and location of the ionospheric irregularities. We note the occurrence of the high ROTI values close to the geomagnetic pole, which can be associated with the ionospheric irregularities generated by particle precipitation to the dayside cusp (e.g., Kelley et al. 1982; Weber et al. 1984). Ionospheric irregularities of such origin are usually developed even under the quiet geomagnetic conditions (see Fig. 5a).
One can recognize the pronounced intensifications and equatorward expansion of the irregularity zone. We should note that due to an essentially poorer coverage by the GNSS data over the southern hemisphere (due to ocean area predomination), such effects were observed in the limited longitude range of 30°E–170°E (mainly over GNSS stations in Antarctica, as well as in the New Zealand and Australia networks and islands in the Pacific Ocean). This limited coverage in the southern hemisphere does not allow to depict the whole pattern of the ionospheric irregularities behavior using the ROTI maps with 1 h resolution in such detail as in the northern hemisphere. Despite this limitation, the 1-h ROTI maps revealed clearly an evolution of the ionospheric irregularities zone with time. Figure 5b demonstrates the occurrence of a narrow oval-like or ring-like structure around the geomagnetic pole at 16 UT, and then, this zone expanded and covered the whole Antarctica continent (20 UT). Further, the irregularities zone expanded equatorward and reached New Zealand and Southern Australia with much smaller ROTI values near the south magnetic pole (Fig. 5c, 04 UT). In general, the evolution of the irregularities oval is rather similar to the evolution observed in the northern hemisphere. However, we should take into account the seasonal (winter to summer) differences between the hemispheres. Laundal and Østgaard (2009) explain this asymmetry in terms of inter-hemispheric currents related to seasons—the difference in ionospheric conductivity is expected to give rise to different auroral intensities in the two hemispheres as well as when the IMF has a significant Bx and By component. All those conditions were observed during the 22–23 June geomagnetic storm.
Meridional slices of the combined GPS and GLONASS ROTI maps
For the quiet day of June 20, 2015, the meridional slices of the northern hemisphere ROTI maps shown in Fig. 6b–e revealed an occurrence of the ionospheric irregularities at high latitudes only within 70°–80° MLAT (close to cusp region) in the American and Australian sectors, probably induced by soft particles precipitation. The first noticeable peak in the ROTI-derived irregularities distribution was recognized after ~06 UT on June 22, 2015, in all considered latitudinal sectors. This period corresponded to the second CME arrival at 05:45 UT, rapid changes of the SYM-H index and the first intensification of the auroral activity, represented by an AE index increase of ~1300 nT (see Fig. 6a). The next peak in ionospheric irregularities at high latitudes was observed at 15-17 UT. These processes were initiated by the IMF Bz southward turn and further increase in the auroral activity when AE rose to ~1340 nT and SYM-H dropped to −70 nT. During this period, ionospheric irregularities were also registered simultaneously as equatorward as 70° MLAT in North America and 65° MLAT in Europe (Fig. 6b, c).
The most intense irregularities in the high and mid-latitudes were found to occur at 18-22 UT on 22 June, which were associated with a new period of the increased auroral activity with two peaks of the AE index of ~2180 and ~2700 nT, observed at 18:49 and 20:10 UT, respectively. During this period, the SYM-H increased to +88 nT and dropped rapidly to the value of −139 nT with dramatical rate of change of about −130 nT/h. As a result, during this period the high-latitude irregularities were detected as equatorward as 54° MLAT in North America and 45° MLAT in Europe. In the southern hemisphere, their signatures were found to extend equatorward to −55° MLAT in South America and −50° MLAT in the Australian sector (Fig. 6d, e). Additionally, we found that images from the SSUSI instrument onboard four DMSP satellites (available at http://ssusi.jhuapl.edu/data/edr-aur-anim//years/2015/173/EDR-AUR_LBHS_2015173.gif and placed as Additional file 4: S4) revealed an increase of the auroral activity on June 22, 2015, and an equatorward expansion of the aurora zone up to 50° MLAT during 18-22 UT.
During the development of the second main phase (01:50–05:40 UT on 23 June), the intense ionospheric irregularities were continuously registered for a longer period (4–5 h) and they covered a latitudinal range from the polar region to 55° MLAT in both sectors of the northern hemisphere (Fig. 6b, c) and to −50° MLAT in the southern hemisphere (Fig. 6d, e). Thus, signatures of the ionospheric irregularities, which were registered by the GPS and GLONASS signals and were analyzed by use of the meridional slice approach, reveal a strong linkage of their intensity and equatorward spatial expansion with auroral activity intensification, in particular represented by the AE and SYM-H indices. Such kind of analysis in the time-latitudinal domain allows us to estimate the principal dependencies of the onset of the ionospheric irregularities and their further development and evolution on space weather drivers. Future studies based on these approaches will allow to formalize these dependencies in the form of an empirical model of the ionospheric irregularities.
We can summarize that despite the unprecedented high number of stations deployed worldwide during the last 5–10 years, the high-latitude regions (above 60° MLAT) in both hemispheres depict a rather sparse coverage by the GPS and GLONASS ground-based observations compared to mid-latitudes. On the other hand, today the ground-based GNSS segment is the only data source able to provide multi-site ground-based observations with the best global coverage.
In this paper, we extend the use of the ROTI maps for analyzing ionospheric irregularities distribution. We demonstrate that the meridional slices of the ROTI maps can be effectively used to study the occurrence and temporal evolution of the ionospheric irregularities over selected geographical regions in quiet and especially geomagnetically disturbed periods. The meridional slices of geographical sectors characterized by a high density of the GPS and GLONASS measurements can represent spatio-temporal dynamics of the intense ionospheric plasma density irregularities with high resolution and they can be used for detailed studies of the space weather drivers on the processes of the ionospheric irregularities generation, their evolution and lifetimes.
We should emphasize that combination of the GPS and GLONASS signals allows to increase significantly the number of the transionospheric measurement links globally. As a result, it allows to improve the performance of the ionospheric irregularities monitoring in both the regions with sparse or dense permanent GNSS network coverage. In case of sparse networks (e.g., Northern Canada and Russia, Antarctica region and coastal zone in polar regions), the adjunction of the GLONASS-based measurements, due to the different constellation configuration as compare to the GPS one, allows to noticeably extend areas covered by the GNSS measurements and essentially increase a number of the available ionospheric piercing points. Particular benefits of GLONASS data at high latitudes can be earlier or better detection of the ionospheric disturbances related to the physical processes in the auroral region and polar cap, in particular through the combination with other instruments such as colocated magnetometers, all-sky cameras and coherent radars. As it is seen on Fig. 4, high and midlatitude areas in the American and European sectors are well covered by the combined GPS and GLONASS measurements without any significant “no data” gaps. For the regions with the dense GNSS networks, the extra use of the GLONASS data would increase a number of the available measurements by a factor of 1.5–2 as comparing with GPS only—for example, for the European region we can get ~1,700,000–1,800,000 IPPs per 1 h. So, we can potentially construct the regional ROTI maps with an unprecedentedly high resolution up to 0.5° × 0.5° in geographic latitude and longitude. Such detailed ROTI maps had been already successfully used for detection of the ionospheric irregularities related with the storm-induced plasma depletion signatures in Europe (Cherniak and Zakharenkova 2016b).
Using a representative database of ~5800 ground-based GNSS stations located worldwide, we have investigated the occurrence of the high-latitude ionospheric plasma density irregularities during the geomagnetic storm of June 23–23, 2015. For the first time, the high-resolution two-dimensional maps of ROTI perturbations were made using not only GPS but also GLONASS measurements.
We note that the current status of the GPS (US) system includes 32 satellites, while the GLONASS (Russia) system includes 24 satellites. The ongoing expansion of the GNSS system includes an increase (and/or renewal) in a satellite number for GPS and GLONASS, development of the European system Galileo (currently 15 satellites in orbit) and the Chinese system BeiDou (currently 18 satellites in orbit), as well as development of the regional Navigational Satellite Systems. Thus, more than 100 GNSS satellites could be available in the near future. This expansion will increase the number of GNSS radio signal ray passes simultaneously scanning the Earth’s ionosphere to an unprecedentedly high value! Signal diversity and redundant measurements, together with better geometry from multiple GNSS satellites, greatly improve the ability to refine the temporal and spatial resolution of the transionospheric measurements, as well as empirical and assimilative ionospheric models . It provides new opportunities to study the space weather impact on the ionosphere and GNSS navigation performance at a new level. Here, we presented the first results demonstrating the advantages of using several independent but compatible GNSS systems like GPS and GLONASS for improvement of the permanent monitoring of the high-latitude ionospheric irregularities.
IC designed this study, analyzed the data and wrote the manuscript. IZ developed software for data processing and helped in interpretation of the data. All coauthors contributed to the revision of the draft manuscript and improvement of the discussion. Both authors read and approved the final manuscript.
We acknowledge use of the raw GPS and GLONASS data provided by IGS (ftp://cddis.gsfc.nasa.gov), UNAVCO (ftp://data-out.unavco.org), NOAA CORS (ftp://geodesy.noaa.gov/cors), SOPAC (ftp://garner.ucsd.edu), EPN (ftp://olggps.oeaw.ac.at), BKGE (ftp://igs.bkg.bund.de/euref/obs), IGN (ftp://rgpdata.ign.fr), SWEPOS (swepos.lantmateriet.se), FGI-FinnRef (euref-fin.fgi.fi), NOANET (www.gein.noa.gr), Natural Resources Canada (webapp.geod.nrcan.gc.ca), CHAIN (ftp://chain.physics.unb.ca/gps/), RBMC (ftp://geoftp.ibge.gov.br/RBMC/), RAMSAC CORS of NGI of Argentina (www.igm.gov.ar/NuestrasActividades/Geodesia/Ramsac/), ARGN (ftp://ftp.ga.gov.au) and NZ CORS (ftp://geonet.org.nz) GNSS networks. We also thank IGS and CODE for providing GPS products (orbits, biases). The authors thank the NASA/GSFC’s Space Physics Data Facility’s OMNIWeb service, for providing OMNI data (http://omniweb.gsfc.nasa.gov/ow_min.html). We gratefully acknowledge the JHU/APL team for providing the DMSP SSUSI data products (http://ssusi.jhuapl.edu).
The authors declare that they have no competing interests.
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