Comparison of GPS and GLONASS measurements in polar region
Figure 3 presents two examples of GPS and GLONASS measurements for two ground-based stations PFRR (65.1°N; 147.4°W) and MAC1 (54.5°S; 158.9°E) located in the polar regions of the northern and southern hemispheres, respectively. Left panels show the geometry of the IPPs distribution of the GPS (blue points) and GLONASS (red points) observations over these stations (black dot) over 24 h on June 22, 2015. It is clearly seen that GLONASS observations are able to cover a wider area in a corresponding poleward direction than that of the GPS coverage ability. As we have explained above, it is due to the higher inclination of the GLONASS satellite orbits (65° vs. 55° of GPS). That is why the GLONASS satellites can be tracked by the same GNSS operational receiver at much higher latitudes than the GPS ones at the same time. Middle and right panels of Fig. 3 show the ROT and ROTI values calculated separately from GPS and GLONASS measurements. The left axis of these graphs indicates the PRN (pseudo-random noise)—satellite number. We note an intensification of the ionospheric irregularities activity started at ~07 UT at both polar GNSS stations. These increases correlate well with three periods of the increased AE index of 06–11, 15–17 and 18–21 UT on June 22, 2015 (see Fig. 1d). An important feature to note is that behavior of the ROT/ROTI values and their amplitude are very similar between GPS and GLONASS measurements. In other words, they are affected in the same way. That is why contribution of the GLONASS data with different geometry and spatial location can significantly complement the GPS observations. Thus, the fluctuation measurements (ROT/ROTI) from GPS and GLONASS are compatible and consistent with each other and can be combined into a composite result such as a ROTI map.
Two-dimensional combined GPS and GLONASS ROTI maps
Figures 4 and 5 show the hourly ROTI maps constructed in a polar geographic projection over the northern and southern hemispheres correspondingly for a quiet day of June 20, 2015, and two disturbed days of June 22 and 23, 2015 . Based on the combined GPS and GLONASS observations, these ROTI maps were constructed with high spatial resolution (1° × 1° in geographic latitude and longitude) and time interval of 1 h. The map for 00 UT means that here we averaged data from 00:00 to 00:59 UT. Full set of the hourly ROTI maps is available in the supporting information (Additional file 1: S1, Additional file 2: S2, Additional file 3: S3).
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
In order to compare the time evolution of the storm-induced ionospheric irregularities during the geomagnetic storm of June 22–23, 2015, we selected the most representative and data covered longitude sectors in both hemispheres and analyzed the meridional slices of the GPS and GLONASS ROTI maps. To increase temporal resolution, we calculated the ROTI maps with 15 min sampling rate instead of 1 h as presented in “Two-dimensional combined GPS and GLONASS ROTI maps” section. Figure 6 shows a comparison of the SYM-H (1-min resolution Dst) and auroral electrojet (AE) indices with the meridional slices of ROTI perturbations, evaluated along the following longitudes: 85°W in North America, 20°E in Europe, 70°W in South America and 150°E in Australian sector during quiet day of 20 June and two disturbed days of June 22–23, 2015. The meridional slices are constructed as an average of the ROTI perturbations within the band of ±5° around a selected geographical longitude and plotted as a function of geographic latitude and time. We consider the geographic latitudinal range of 30°–90° in both hemispheres. The left vertical axis in Fig. 6b–e shows geographic latitudes, and the right axis shows the corresponding corrected geomagnetic latitudes. It is necessary to note that due to the difference between the geomagnetic and geographic poles, the meridional slices in Fig. 6b, e crossed the latitude of the geomagnetic pole.
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).