During the geomagnetic storm, the solar wind energy is impulsively or continually injected into the earth polar region, making the atmospheric and ionospheric states deviate greatly from their background levels (Fuller-Rowell et al., 1994; Mendillo and Narvaez, 2009). In general, the response of the ionosphere to the geomagnetic storm is classified by a variety of different features, one of which being the large-scale traveling ionospheric disturbance (LSTID) that is the wave-like perturbation mainly propagating equatorward from high latitudes. Traveling ionospheric disturbances (TIDs) are classified into LSTIDs and medium-scale TIDs, and they are considered to be the ionospheric manifestation of the presence of atmospheric gravity waves (AGWs) stimulated by different sources. LSTIDs are mainly caused by joule heating or Lorenz drag forcing in the auroral regions during a geomagnetic storm period (Hines, 1960; Richmond and Roble, 1979; Hocke and Schlegel, 1996).

In earlier years, the acquisition of the continuous evolution of LSTIDs on a global scale was limited by the availability of the ionospheric observations. In order to obtain the propagation characteristics of LSTIDs, researchers needed to organize their findings from limited ionospheric observations, for example, the foF2 data from sparsely distributed ionosondes. In the 1980s, the GPS (Global Positioning System) method was introduced into the ionospheric study (Klobuchar, 1986; Lanyi and Roth, 1988; Coster and Gaposchkin, 1989). With the dense and worldwide distributed GPS receivers, some characteristic ionospheric phenomena, like TIDs (Saito et al., 1998; Tsugawa et al., 2004; Ding et al., 2007), ionospheric storms (Ho et al., 1996), and ionospheric responses to solar flares (Afraimovich, 2000; Zhang and Xiao, 2005), were revisited frequently and new results were obtained.

The propagation characteristics of LSTIDs are always topics of great research interest (Hunsucker, 1982; Ho et al., 1996; Balthazor and Moffett, 1999; Afraimovich et al., 1998, 2000; Shiokawa et al., 2002; Tsugawa et al., 2003, 2004; Ding et al., 2008, 2014; Borries et al., 2009, 2017; Habarulema et al., 2015, 2016, 2018; Zakharenkova et al., 2016; Figueiredo et al., 2017; Pederick et al., 2017; Cherniak and Zakharenkova, 2018; Lyons et al., 2019). Based on limited GPS stations measurements, Afraimovich et al. (1998) proposed a radio interferometry method to roughly estimate horizontal propagation velocities and phase front angles of TIDs. Furthermore, the worldwide or local dense distribution of the GPS receivers networks facilitates the acquisition of the global or regional total electron content (TEC) perturbation maps with high spatial and temporal resolutions to reveal the detailed propagating characteristics of TIDs (Ho et al., 1996; Saito et al., 1998; Tsugawa et al., 2004; Borries et al., 2009; Ding et al., 2012). With more than 60 GPS receivers distributed worldwide, Ho et al. (1996) studied the global distribution of TEC perturbations during a magnetic storm. They identified a TID propagating from the northern sub-auroral region to lower latitudes at a speed of about 460 m s-1. The GPS Earth Observation Network (GEONET) in Japan is one of the densest GPS receiver networks on the Earth, and two-dimensional (2-D) TEC perturbations over Japan can be mapped with the GEONET observations. With these high-resolution TEC perturbation maps, the spatial structures and temporal evolutions of a TID in the nighttime midlatitude ionosphere over Japan were revealed clearly (Saito et al., 1998). Since then, with this dense GPS network, the characteristics of LSTIDs over Japan are carefully studied through case and statistical analysis, and some propagation features of TIDs in this region are revealed (Saito et al., 2001; Shiokawa et al., 2002; Tsugawa et al., 2003, 2004, 2006).

For the LSTID with scales of thousands of kilometers, the extensive spatial coverage of ionospheric observations is undoubtedly useful for capturing its propagation features. In recent years, the GPS data from densely distributed GPS stations in China were used to study LSTIDs in this region (Ding et al., 2012, 2013, 2014; Song et al., 2013). Based on the GPS data from the Crustal Movement Observation Network of China (CMONOC), Ding et al. (2012) obtained temporal continuous 2-D imaging of ionospheric disturbances during the geomagnetic storm on 28 May 2011 and found two LSTIDs moving southwestward with the front width of at least 1600 km during different storm stages. In addition, through the comparative climatological study of LSTIDs over North America and China, the different time dependencies of LSTID occurrence over two longitudinal sectors were revealed statistically (Ding et al., 2014). These studies further emphasize the effectiveness of the large-coverage, high-resolution ionospheric observations from GPS networks in revealing the structures of the ionospheric disturbances.

The propagating direction of the LSTID during the geomagnetic storm has always been focused on for the LSTID studies. From case and statistical studies about LSTIDs during geomagnetic storms over the eastern Asia region conducted by Chinese and Japanese scientists independently, the dominant propagating direction of LSTID in China and Japan is a little different. It mainly propagates southwest in the Chinese region (Ding et al., 2014), while it mainly propagates southeast in the Japanese region (Tsugawa et al., 2004). Although the geomagnetic declination is considered to be one of the main factors responsible for the propagation direction of LSTID based on different LSTID studies, the LSTID studies concerning the same geomagnetic storm using both Chinese and Japanese GPS networks together have not been reported yet.

During the period of 17–18 March 2015, the strongest geomagnetic storm in the 24th solar cycle occurred, and LSTIDs were detected and analyzed in different longitudinal sectors (Ramsingh et al., 2015; Borries et al., 2016; Zakharenkova et al., 2016; Habarulema et al., 2018). Meanwhile, two high-frequency (HF) Doppler sounders in Chinese midlatitudes operated by the China Meridional Project (Wang, 2010) recorded large ionospheric HF Doppler shifts after 10:00 UT (universal time), which seem to indicate the LSTIDs in the Asian region between 09:00 and 12:00 UT that were reported by Habarulema et al. (2018). In this study, the multi-network of densely distributed GPS receivers, the HF Doppler sounder stations, and an ionosonde network are used to conduct a more comprehensive study on the propagating characteristics of the disturbances in the eastern Asian region, especially on the characteristics of the dominant propagating direction over China and Japan.

Figure 1 illustrates the locations of ground-based receivers used in this study from four Global Navigation Satellite Systems (GNSS) networks, distinguished by colors. They are the Chinese Meteorological GNSS Network (CMGN), CMONOC, GEONET, and International GNSS Service (IGS). These receivers are selected through data quality checking and regional restriction (10–60∘ N, 70–150∘ E), and the numbers of used stations are 259, 220, 1300, and 31 for CMGN, CMONOC, GEONET, and IGS, respectively. The sample rate of all GPS data is 30 s. Combining the carrier phase and pseudo-range measurements in two L-band frequencies of GPS, the vertical TEC (VTEC) can be obtained. In the calculation, the height of the ionospheric thin shell is set to be 400 km, and the cutoff elevation angle is 30∘. The detailed process of the TEC calculation from GPS data can be found in our previous studies (Zhang et al., 2009, 2010).

Different methods were used for extracting the TEC perturbations related to LSTIDs in previous research (Wan et al., 1997; Afraimovich et al., 2000; Shiokawa et al., 2002; Nicolls et al., 2004; Tsugawa et al., 2004; Ding et al., 2007). Afraimovich et al. (2000) suggested that the LSTID characteristics in TEC can be determined by removing the trend with three- to five-order polynomials, which is introduced by the motion of satellites and background variations in ionosphere. For a similar purpose, Shiokawa et al. (2002) subtracted a running average of TEC over 1 h from the raw TEC, and a series of 2-D TEC perturbation maps was obtained with more than 1000 GPS receivers in Japan. Ding et al. (2007) developed another method of obtaining the 2-D TEC perturbation maps by expressing the VTEC as a one-order function of local time and latitude. According to their argument, this method is sufficient for removing background trends for continuous observation of a GPS receiver–satellite pair without introducing artificial perturbations. After comparing the results of these methods, a method similar to Ding et al. (2007) is conducted in this study in which the VTEC is treated as a function of UT, longitude (Long), and latitude (Lat), i.e., 1VTEC0=C0+C1UT+C2Long+C3Lat,2VTECP=VTEC-VTEC0, in which VTEC0 is the background change and VTECP is VTEC perturbation. Then, the obtained VTECP data are reorganized into pixels which are bounded by 10–60∘ N, 70–150∘ E, and with a spatiotemporal resolution of 1∘ longitude × 1∘ latitude × 10 min. The pixel value is the average of all VTECPs in this pixel. After these steps, the featured ionospheric disturbances are expected to appear in a series of 2-D VTECP maps.

As a comparison, the VTEC map from the Madrigal database of the MIT Haystack Observatory is used to reveal the ionospheric disturbances in the European sector (30–70∘ N, 10–20∘ E). This database provides worldwide VTEC values in 1∘ latitude × 1∘ longitude pixels with a temporal resolution of 5 min (Rideout and Coster, 2006) and has good data coverage in the European sector. VTEC maps with such a high spatiotemporal resolution are suitable for revealing the structures of traveling ionospheric disturbances (Zhang et al., 2017).

The Doppler shift data observed at two HF Doppler sounder stations in China are collected, which are MDT (40.4∘ N, 116.9∘ E) and SZT (22.6∘ N, 114.1∘ E). The sounding system continuously receives electromagnetic waves with a stabilized frequency of 10 MHz transmitted by the National Time Service Center (NTSC; 35.7∘ N, 109.6∘ E) to detect the ionospheric disturbances through the Doppler shifts of this standard frequency. These shifts are considered to be caused by ionospheric variations, mainly around the reflecting point of the electromagnetic wave in the ionosphere. According to the geometrical relationships, the locations of the reflecting point for MDT and SZT are 38.0∘ N, 113.2∘ E, and 29.2∘ N, 111.8∘ E, respectively. These stations are marked in Fig. 1 with colored stars.

In this study, ionograms from eight ionosonde stations in Chinese middle latitudes are used to derive the iso-frequency lines, which vary as a function of universal time and virtual height. The sample rate of the ionograms is 15 min. These ionosondes belong to the China Research Institute of Radio Wave Propagation (CRIRP), and their locations are marked with green triangles in Fig. 1. The virtual height data are manually scaled by us from these ionograms, with professional scaling software provided by CRIRP to reduce possible errors of auto-scaling (Krankowski et al., 2011; Habarulema and Carelse, 2016). During the scaling, we limited the frequency to less than 7 MHz. In addition, the space environment data for this event are from OMNI dataset, which is downloaded from the FTP service of the NASA Goddard Space Flight Center.

Our results show that the propagation parameters of the LSTID in the eastern Asian sector during the St. Patrick's Day storm are longitudinally dependent. Among these parameters, the longitudinal dependence of the propagation azimuth of an LSTID receives much attention in previous works. In general, earlier studies suggested that there are four main factors that affect the direction of a polar-originating LSTID, including the velocity of the background neutral wind (Hines, 1960; Morton and Essex, 1978; Maeda and Handa, 1980), the structure and evolution of the source region in the auroral oval (Maeda and Handa, 1980; Hunsucker, 1982; Ding et al., 2007), the Coriolis force (Maeda and Handa, 1980; Balthazor and Moffett, 1999; Afraimovich et al., 2000; Tsugawa et al., 2004; Ding et al., 2013), and the declination of the geomagnetic field (Tsugawa et al., 2004; Borries et al., 2009).

The Coriolis force effect is generally believed to contribute to the clockwise shift of the propagation direction of the LSTIDs (Afraimovich et al., 2000; Tsugawa et al., 2004; Ding et al., 2013), The observations of the shift (10–20∘ on average) are consistent with the calculation by Maeda and Handa (1980) and the model simulation by Balthazor and Moffett (1999). However, in our study, the shift of the propagation direction is not systematic westward, which means the variability in the LSTID azimuth in our observation cannot be attributed to the Coriolis force, at least not to it alone.

The structure and movement of the source region for the LSTID in the auroral oval are another candidate for explaining the longitudinal dependence of the propagation direction of the LSTID. Previous studies suggested that the westward movement of enhanced electrojets in the auroral arc is an important cause of the westward shift of the LSTID propagation direction at high latitudes (Hunsucker, 1982; Ding et al., 2007). The change of the propagation direction of LSTIDs as they move from high to middle latitudes during the superstorm of 29 October 2003 over North America was explained by Ding et al. (2007) as being related to a change in the position of the electrojet enhancement area near the auroral oval. Nevertheless, since the structure and the evolution process of the source region during storm period are complicated, more cases and modeling studies are needed to find a clear connection between it and the propagation direction of LSTIDs.

In general, the velocity of the neutral wind is much less than that of the LSTIDs, and the thermospheric wind velocity in the same latitudinal belt with a limited longitudinal extension should exhibit little variance. Thus, the contribution of the background wind on the change of the propagating direction would be limited in the absence of the geomagnetic field. However, a combined effect of magnetic declination and zonal wind can cause F-region electron density differences between two sides of the zero declination (Zhang et al., 2011). During storm periods, the enhanced zonal winds (Fuller-Rowell et al., 1994) can intensify these differences (Thomas et al., 2016). As a result, the geomagnetic declination is considered to be an important factor that affects the propagation direction of the LSTID. Some researchers studied the predominant propagation direction of LSTIDs during storm periods in different longitudinal sectors and suggested that, statistically speaking, the predominant directions of LSTIDs in Europe, China, and Japan are primarily southward, south to west, and south to east, respectively (Nicolls et al., 2004; Tsugawa et al., 2004; Borries, 2009; Ding et al., 2013). These results are all consistent with the corresponding geomagnetic declination in each sector.

In the longitudinal region of 70–150∘ E, the geomagnetic declination angles change from northeast in the western side to northwest in the eastern side. This characteristic seems to show some kind of consistency with the azimuth results in Table 1. To illustrate such connection quantitatively, Fig. 9 depicts the geomagnetic declination on the wavefront in different longitudes in Fig. 7 (Fig. 9a) and the propagation direction (azimuth – 180∘) of the LSTID at the same spot (Fig. 9b). The connection between these two parameters is quite obvious in this event. This result proves that the propagation of LSTIDs in different longitudes is probably influenced by the orientation of the geomagnetic field lines in the eastern Asian sector. In addition, the tendency of field-aligned propagation of the LSTID indicates that it is driven by the neutral winds, since the winds push the plasma up and down along the magnetic field lines. There is no evidence, such as simultaneous perturbations at all latitudes in other cases (Borries et al., 2016; Zakharenkova et al., 2016), to show that the LSTID in the Chinese–Japanese sector is affected by the prompt penetration electric field (PPEF) during the same period. Besides, considering the relatively low data coverage in the eastern and western sides of the studied region, it should be noted that our speculation needs to be verified with more observational data and numerical simulation to reduce uncertainty in our propagation estimation and to figure out the detailed physical processes.

During the 2015 St. Patrick's Day storm, LSTIDs in the European–African, American, and Asian–Australian sectors were detected and analyzed with TEC observations (Borries et al., 2016; Zakharenkova et al., 2016; Habarulema et al., 2018). It shows clearly in their results that the European sector also exhibits LSTIDs around 11:00 UT. As a comparison, we also analyzed these LSTIDs, but with VTEC data from the Madrigal database of the MIT Haystack Observatory. To derive the VTECP, a narrow longitudinal band (30–70∘ N, 10–20∘ E) is selected, and the VTEC data with the same latitude at the same time are averaged. At each latitude bin, the averaged VTEC forms a time series, and the temporal resolution is set to 12 min with bin averaging. Then, a running mean with a 1.5 h window is conducted for each time series, and their difference is taken as the VTECP. The result is plotted in Fig. 10 as a TLP. The fitting lines are obtained with the same method as those in Fig. 8.

Figure 10 is basically consistent with previous results (Borries et al., 2016; Zakharenkova et al., 2016), such as the synchronous perturbations around 04:45 and 09:15 UT and the LSTID structures between 10:00 and 17:00 UT. Moreover, our result shows that the VTECP′ behavior between 60 and 70∘ N is quite different from that between 30 and 60∘ N. The pattern around 10:00 UT seems to represent a TID with smaller speed. Considering that the physical processes are more complex in such high latitudes (Foster et al., 2014), we only focus on the perturbations below 60∘ N. The speeds estimated from the most distinct crest and trough are ∼500±51 and ∼427±55 m s-1, respectively, and the estimated period is ∼4.0±0.2 h. It is clear that the appearances of the LSTIDs are different in the European and eastern Asian sectors during the same period for the same storm event. Borries et al. (2016) presented a detailed study on the LSTID in Europe during this storm. It is suggested that the perturbation occurring around 11:00 UT is special, since it is impacted by PPEF and wind at the same time. Comparatively, the LSTID in the Chinese–Japanese sector seems to be only driven by winds. This may partly account for the difference of LSTIDs between the eastern Asian and European sectors. Besides, such a difference may also be related to the location or structure of the joule heating source in the auroral oval or the difference of the background TEC in the two sectors. For better understanding this difference, more studies on the joule heating source are needed.

Using data from four GPS receiver networks (CMGN, CMONOC, GEONET, and IGS), together with recordings of two HF Doppler sounders and eight ionosondes, we provide comprehensive and detailed observation results of the LSTIDs in the eastern Asian sector during the 2015 St. Patrick's Day storm. The GPS receiver networks in China and Japan are combined together to produce 2-D VTEC perturbation maps in order to give a wider image of the LSTID structures in eastern Asia. As a comparison, the ionospheric disturbances in the European sector are also studied with VTEC data from the Madrigal database. The propagation parameters of the LSTIDs are estimated. The main results can be summarized as follows:

An LSTID occurs as a result of possible AGWs propagating from high to lower latitudes around 09:40–11:40 UT, which spans over 60∘ in longitude, and the crest of this LSTID is characterized by a clear tendency to dissipate starting from the eastern side. These features are in good agreement with observations by HF Doppler sounders and ionosondes.