Ionosphere response to three extreme events occurring near spring equinox in 2012, 2013 and 2015, observed by regional GNSS-TEC model
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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.
KeywordsTotal electron content Ionospheric electron density GNSS Ionosonde
Investigations of ionosphere dynamics and disturbances during geomagnetic storms are still of high importance due to influence of the ionosphere on the space weather. The ionosphere, as an active layer, plays an important role in the space weather due to its sensitivity to the solar activity. In turn, the space weather affects satellite systems, energy transport, air traffic and, in consequence, the economy (Schrijver et al. 2015). Therefore, the studies of the ionosphere behavior play a very important role in understanding and forecasting of the space weather. The ionosphere is a layer of the atmosphere consisting mostly of ionized particles, which causes the satellite electromagnetic signals to be delayed, fluctuations in signal strength and other adverse effects. The magnitude of these ionospheric effects largely depends on the state of the varying ionosphere, its total electron content (TEC) and the frequency of electromagnetic waves. The dynamic changes of the ionosphere are most easily divided into regular and sudden ones. Regular changes are directly related to the periodicity of the factors influencing them, such as the solar cycle lasting 11 years or seasonal variations. These regular variations are somewhat easy to model (Bilitza and Reinisch 2008; Maruyama et al. 2009).
Events such as coronal mass ejections could induce a series of disturbances in the ionosphere driven by a magnetic storm. This results in sudden disturbances in the ionosphere. This phenomenon is often called an “ionospheric storm.” The behavior of ionospheric storms can be different, as the storms can be characterized by positive or negative changes in the electron density. Generally, ionospheric storms are a global phenomenon. They happen because, due to a magnetic storm, the density of electrons increases, leading to the creation of regions of storm-enhanced plasma density (SED). This phenomenon is closely related to daytime positive disturbances and manifests several or even a 20-fold increase in the electron density in comparison with the normal sunset conditions. Therefore, permanent monitoring of the ionospheric state is required to mitigate the possible effects this may have on navigation and communication systems and other space and ground infrastructure (Maruyama et al. 2009).
All geomagnetic storms are characterized by an intensification of the ring current, usually resulting in a decrease in the geomagnetic activity Dst index below a set threshold. Furthermore, prolonged periods of southward interplanetary magnetic field (IMF) play a major role in the generation of geomagnetic storms. According to Wu et al. (2016a), a large southward IMF can be associated with an interplanetary shock wave (sheath) (Wu and Lepping 2016), a magnetic cloud (MC) or an interplanetary coronal mass ejection (ICME) (Wu and Lepping 2011) or a combination of these interplanetary structures.
As it was presented by Belehaki et al. (2017), the correlation between southward interplanetary magnetic field and the Earth’s magnetic field leads to a strong electric field during the day. It can cause significant changes in the Earth’s ionosphere, which are especially pronounced at low latitudes. However, as pointed out by Tsurutani et al. (2004), plasma enhancements are often observed during intense storm-time eastward electric fields. Then, the range of this plasma reaches middle latitudes. This phenomenon is called “super-fountain effect.” The increases of dayside TEC at these latitudes can amount to up to 80% compared to a quiet day.
In recent decades, the electron density perturbations are often monitored based on the analysis of observations from satellite navigation systems, which have become kind of ionosphere scanners, and are excellent supplements of classical sounding methods. For over 20 years, space-geodetic techniques such as global navigation satellite systems (GNSS) data from ground permanent networks governed by International GNSS Service (IGS) and EUREF Permanent Network (EPN) have allowed to determine the ionospheric TEC in regional and global scales (Afraimovich et al. 1998; Hernandez-Pajares et al. 2006; Tsurutani et al. 2004). Earlier, the ionosondes were principal source of the ionosphere sounding, providing valuable information about the analytic function of electron density with height, and also the propagation of perturbations at different altitudes (Hajkowicz 1992). However, these facilities have limitations arising from their number and information delivery which has reference only to the conditions in the lower part of ionosphere over limited areas. In recent years, we can observe examples of TEC modeling with the additional use of terrestrial ionosondes, and the obtained results provide an excellent, reliable source of information, supporting the satellite systems in the ionosphere modeling (Belehaki et al. 2012, 2015). Nevertheless, most of the observation-based ionosphere models are still based on the observations from GNSS. Most of them are developed using carrier phase-smoothed pseudorange data, which is characterized by low accuracy and requires strong smoothing of the results (Ciraolo et al. 2007). The recent analyses of these models evaluate their absolute accuracy at the level of 4–8 TEC units (TECU) and relative accuracy at only 20–30% (Hernandez-Pajares et al. 2017). Therefore, these models can be useful to the general overview of the ionosphere response to a geomagnetic storm. However, due to the dynamic nature of changes in the ionosphere, not only during geomagnetic storms, the resolution of existing models is often insufficient to analyze the storm-time effects in detail (Astafyeva et al. 2015). Therefore, in order to have more detailed information about the ionospheric changes over the selected area, especially in the studies of disturbed conditions, it is necessary to use more accurate ionosphere maps based on improved ionosphere modeling techniques, which are also derived with higher temporal and spatial resolution. For this reason, two major approaches to detailed ionospheric TEC estimation based on carrier phase data were developed. The first one applies precise point positioning technique, where carrier phase ambiguities and interfrequency biases are estimated in the geometry-based positioning model (Ren et al. 2016; Liu et al. 2018; Nie et al. 2018). The second one is based on a direct carrier phase bias estimation using geometry-free model, and this approach is applied in this research (see Sect. 3.1).
This paper describes our studies on the response of the ionosphere layer to the largest geomagnetic storms during Solar Cycle 24. Three extreme events having immense importance to space weather were discussed. For analyses, we applied the new highly accurate and high-resolution regional ionospheric TEC model based on multi-GNSS carrier phase data. Our European TEC maps are characterized by 2-min time interval and 0.2° spacing in both latitude and longitude. The accuracy of the vertical TEC maps is assumed to be better than 1 TECU (Krypiak-Gregorczyk et al. 2017b). In addition, critical frequency foF2 data from two ground-based ionosondes located in Europe was used for comparisons with GNSS estimates.
2 Overview of major disturbances of the Solar Cycle 24
The current Sunspot Cycle 24 is the lowest one since Cycle 14, in which the maximum smoothed sunspot number occurred in February 1906. The Sunspot Cycle 24 has two peaks: the first reached the smoothed sunspot number maximum in March 2012 and the second peak, which reached the highest number of sunspots, took place in April 2014 and was larger than the first one. In this paper, three very strong geomagnetic storms that took place near the spring equinox were selected for a study on regional ionosphere response to the geomagnetic storm.
It is well known that the geomagnetic storms take place as a result of the transfer of solar energy into the magnetosphere following magnetic reconnection between the southward IMF Bz component and the antiparallel geomagnetic field at the magnetopause (Dungey 1963; Gonzalez et al. 1994; Okpala and Ogbonna 2017). An interplanetary CME reaching to Earth with a preceding shock usually leads to classic three steps of geomagnetic storm phase: a sudden commencement generated by the shock, the main phase and then a recovery phase. The studies conducted by many authors have shown that 95% of strong geomagnetic storms were associated with coronal mass ejection (Tripathi and Mishra, 2006; Liu et al. 2015). Moreover, Gonzalez and Tsurutani (1987) indicated that two structures: interplanetary field strength and directional components (mainly negative Bz) are essential for the development of the storm.
2.1 Storms taking place in 2012, 2013 and 2015
According to the universal definition of the storm as used in practice, when the intensities reach Dst < − 50 nT, it signifies the occurrence of a magnetic storm. Thus using this definition, during the first of the analyzed periods four storms were identified: on 7, 9, 12 and 15 March with the peaks intensity of Dst: − 98 nT, − 148 nT, − 67 nT, − 79 nT, respectively. The first magnetic storm on 7 March (DOY 67) was initiated by southward directed interplanetary sheath fields IMF Bz observed before the first interplanetary shock, which occurred at 03:28 UT on 7 March. The main phase of the magnetic storm began when solar wind energy was transferred to the magnetosphere due to the magnetic reconnection between the southwardly sheath fields and the northwardly magnetopause fields, as pointed out by Tsurutani et al. (2014). This phase started at ~ 2:00 UT on 7 March and reached the maximum at ~ 5:15 UT. The Dst peak intensity was − 98 nT. The intense southward IMF component amounted to ~ 17 nT in the sheath behind the shock and correlated well with the Dst decrease, but with a slight delay. According to Belehaki et al. (2017), the magnetic storm initiated on 8 March (DOY 68) was characterized by a different nature and a different interplanetary cause. At 11:30 UT on 8 March, as a result of shock compression of the magnetosphere, an intense sudden impulse occurred (Joselyn and Tsurutani 1990; Araki et al. 2009; Belehaki et al. 2017). As it can be seen in Fig. 1, AU index reflecting the directly driven component of auroral activity and AL index containing significant contributions from directly driven and substorm expansion phase activity, increased almost symmetrically. The reading of the AU index achieved slightly higher values than the AL index. During the whole directly driven phase, the IMF Bz parameter reached positive values, and after the intense sudden impulse, all IMF components presented strong pulsations. On the next day, the IMF Bz returned to the south, the AL index fell suddenly and the AU index stayed at very low values. Due to the magnetic reconnection between the southwardly sheath fields and the northwardly magnetopause fields, the solar wind energy was transferred to the magnetosphere. Thus, the loading–unloading mechanism in the magnetosphere was fully developed (Tsurutani et al. 2014). The main phase of the storm began and reached a peak intensity of Dst = − 131 nT at 8:00 UT on 9 March (DOY 69). The next event on 12 March (DOY 72) was a “double-shock” event: the first one at ~ 12:28 UT on 11 March and a second one at ~ 08:30 UT on 12 March. In the first case, the values of magnetic field doubled across the shock. The second shock on 12 March exhibited an enhanced magnetic field. The cause of this magnetic storm was the southward component of the high-intensity magnetic field. The shock occurring on this day triggered a “supersubstorm.” The AE index was above 1500 nT. The last storm during this period took place on March 15 (DOY 75) and was characterized by the Dst index peaking at around 20:00 UT. The interplanetary shock occurred at ~ 23:3 UT. Then, the IMF Bz amounted to ~ − 5 nT, and the Dst index did not exceed − 30 nT.
The first super geomagnetic storm of Solar Cycle 24 occurred on 17 March 2015 (DOY 76) and was called the St. Patrick’s storm. It should be emphasized that it was a two-step storm. Namely, the solar event on 15 March 2015 can be considered the initial cause of the storm. The storm intensified with the arrival of a coronal mass ejection (CME) at 04:45 UT (Wu et al. 2016a). The interplanetary magnetic field (IMF) turned northward. The Bz component increased from + 10 to + 26 nT, and an increase in the Dst index amounted from + 10 to + 60 nT (Fig. 1). At the same time, the AE magnetic index reached a small peak of 300 nT. The Bz component turned southward at around 06:00 UT, reaching the value of − 22 nT at 06:20 UT. At this time, the compression phase had ended. The AE magnetic index grew reaching the maximum value of 1016 nT around 09:30 UT, and the Dst index decreased and reached the minimum value of − 97 nT at around 10:00 UT. From ~ 09:30 UT until 12:00 UT, there was a partial recovery phase. The Bz was again northward, the AE index decreased to ~ 50 nT and the Dst increased from − 97 to − 38 nT. Then a large MC field caused the second storm intensification. The main phase of this storm began when the IMF Bz started turning southward after 12:00 UT, and Dst index began gradually decreasing. At around 13:00 UT, the Bz component changed direction for a very short period, and at this time the AE index reached the maximum value of 2298 nT. Then the Bz became southward again and remained southward until 24:00 UT. During this main phase of the storm that lasted 18 h, the Dst kept falling, reaching the minimum value (Dst = − 223 nT) at 22:45 UT. During the recovery phase, the Dst was increasing to − 50 nT and the magnetic index AE exhibited a few peaks larger than 800 nT (Liu et al. 2015; Singh et al. 2015).
The last of the analyzed storms was the one of 17 March 2013 (DOY 76). This event, like the previous one, took place on St. Patrick’s Day. This storm was triggered by a coronal mass ejection that occurred on 15 March (Wu et al. 2016b). The changes in IMF parameters (Bx, By and Bz) indicate the shock arrival at 06:00 UT on 17 March, which caused the sudden commencement (SC) of the storm. With the beginning of the storm, there was a sudden drop in the Dst values. The double-phase minimum Dst ~ − 107 nT at around 10:30 UT and 12:00 UT was observed. The first southward turning of Bz occurred almost immediately after the shock arrival at ~ 06:00 UT and stayed southward throughout the main phase of the storm. However, at this time (06:00–24:00 UT), there were fluctuations with several northward turnings. During the storm, the By component changed amplitude from − 10 to 10 nT, while an immediate onset of intense auroral activity is seen in the AE index. At about 15:30 UT on 17 March, a MC occurred indicated by intense magnetic fields with a general lack of Alfvén waves and discontinuities (Tsurutani et al. 1988; Verkhoglyadova et al. 2016). Then, the IMF Bz turned southward in the MC. The Dst index noted the second decrease, reaching − 132 nT at around 20:30 UT on 17 March, creating the second main phase storm (Fig. 1).
2.2 Data and methods used
The highly accurate and high-resolution model was analyzed for the three tested periods representing the disturbed ionosphere conditions: 1–16 March 2012 (DOYs 61–76), 10–19 March 2013 (DOYs 69–78) and 10–19 March 2015 (DOYs 69–78). For TEC calculation, the data processing from over 200 stations of ground GNSS networks: EUREF Permanent Network (EPN) and European Position Determination System (EUPOS), with 30-s sampling interval and an elevation mask of 30°, was carried out. The mapping of the vertical TEC at the ionospheric pierce point (IPP) locations is based on a single-layer model approach and its associated mapping function (Schaer 1999). Next, a thin-plate splines interpolation is applied for accurate ionospheric TEC modeling and providing the vertical regional TEC maps. These maps are characterized by a 2-min interval and the region limited by − 10° and 38° in geographic longitude and 36° and 64° in geographic latitude, with 0.2° spacing in both directions. The previous results show that the accuracy of these regional TEC maps is better than 1 TEC unit (Krypiak-Gregorczyk et al. 2017b). Note that TEC derived from GNSS data is often denoted as GNSS-TEC. It should be mentioned that since 1 January 2017, we have been providing our results of this European ionosphere model on the Web site http://ginpos.uwm.edu.pl/iono/index_en.php (Krypiak-Gregorczyk et al. 2014).
The quality of the presented highly accurate and high-resolution ionosphere model was tested by a comparison to the broadly used Global Ionosphere Maps (GIMs) provided by the IGS. This commonly available product offers 2.5° by 5.0° spatial resolution and temporal resolution of 2 h. IGS GIMs are developed as an official product of the IGS Ionosphere Working Group by performing a weighted mean of the various Analysis Centers (AC) TEC maps: CODE, ESA, JPL, UPC and NRCan. According to Hernández-Pajares et al. (2011, 2017), IGS maps are characterized by estimated accuracy ranging from a few TECU to approximately 10 TECU.
The GNSS data have been downloaded from the following servers ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily and the IGS GIMs from ftp://cddis.gsfc.nasa.gov/gnss/products/ionex/. The ionosonde data are available online at http://dias.space.noa.gr.
3 Analysis on the ionosphere response to the analyzed storms
The three selected events of the Solar Cycle 24 are characterized by the duration of the shock–sheath–ICME passage from ~ 4 to 9 h. According to Verkhoglyadova et al. (2016), the study of temporal ionosphere responses in global scale needs ionosphere data with temporal resolution of at least 1 h. The study presented in this paper concerns the changes of the ionosphere in a regional scale; hence, an even higher temporal and spatial resolution of the ionosphere maps is necessary. For comparison, the ionospheric TEC variations obtained from well-established IGS GIMs are also presented. For clarity, from this point on, TEC derived from UWM maps is denoted as UWM-TEC, whereas TEC derived from IGS maps is denoted as IGS-TEC.
3.1 Space weather events on March 2012
During the disturbed day of 9 March (DOY 69), the highest UWM-TEC was observed at the 42°N latitude. With the increase in the latitude, the UWM-TEC values decreased, reaching the 58°N latitude, a 2.5-fold decrease. In addition, at 50°N latitude, a clear evolution from the negative to the positive phase of the storm is visible. This negative phase of the storm is observed at all analyzed longitudes and latitudes above 50°N. This different nature of the storm on 9 March was associated with an ICME on 7 March that arrived on Earth 2 days later (Belehaki et al. 2017). On the next day, at higher latitudes, the recovery phase followed, and a clear increase in UWM-TEC was visible, while at the lower latitudes, for which the positive phase was observed, UWM-TEC decreases. The reduction of UWM-TEC deepens on 11 March (DOY 71), equating the state of the ionosphere in the quiet period. The event on 12 March (DOY 72) caused a significant 1-day UWM-TEC increase in relation to both the previous day and preceding quiet ionosphere period. The last storm was preceded by increased UWM-TEC, followed by its reduction, bringing the negative phase lasting from sunrise to the afternoon. Then, the positive phase started, which remained until midnight. The next day was characterized by negative changes in the UWM-TEC values, which have reached half of the values of the reference/background period.
The bottom panel presents the differential map providing the information about the IGS-TEC. The general nature of the observed changes during the analyzed period shows compatibility with UWM-TEC. However, the level of IGS-TEC variations during these storms is clearly lower, and its maximum differences occur at lower latitudes than in the case of the new model.
As it is well known, GNSS-TEC is the integrated number of electrons along the radio wave propagation path from the GNSS satellites to the receiver. Hence, it is clear that this parameter depends to a large extent on the peak electron density in F layer (NmF2). Therefore, the response of the ionosphere to the disturbances perceived in terms of the NmF2 and GNSS-TEC should be to some extent similar. It is a commonly accepted assumption that TEC and NmF2 are highly correlated, and thus, TEC can be used for studies of F2 layer dynamics (Mendillo 2006; Prolss 2006; Zolotukhina et al. 2017). However, it should be emphasized that approximately two-third of TEC comes from the topside ionosphere. This is a region above the ionization maximum (Mendillo 2006; Liu et al. 2016). According to Yizengaw et al. (2006) and Astafyeva et al. (2015), the F layer and the topside ionosphere do not always react in the same manner during geomagnetic storms. Many authors have proven that the TEC variation depends on ionospheric peak density variability, topside electron density and temperature fluctuations, and changes in the plasmasphere (Klimenko et al. 2015; Liu et al. 2016).
3.2 Space weather event on March 2013
The top panel in Fig. 8 shows the variations in the F-layer maximum electron density measured by Juliusruh ionosonde. The positive phase of the storm started at 8:00 UT and lasted until 16:00 UT. Then, the UWM-TEC values fell below the reference level and the negative phase lasted for the next 2 days. The observed disturbances correspond to the changes of UWM-TEC presented in Fig. 7, in the panel describing the disturbances at 54°N latitude and the 12°E longitude.
Then, in the case of IGS map, TEC had been increasing from 16:00 UT, reaching the second peak around 19:00 UT. The UWM map observed this increase a little later. However, this second peak also occurred at around 19:00 UT. For both models, this peak was characteristic only at latitudes below 44°N, and its maximum value occurred at latitudes below 36°N. In addition, the second peak in TEC amounted to 50% of its level observed during the first peak. Despite the overall compatibility of the changes in the ionosphere observed by both TEC maps, there is a clear difference in the character of the second peak, with TEC fluctuations seen in UWM-TEC.
The next day was characterized by the negative phase at all latitudes, and the largest negative changes were observed at 12:00 UT, for both models. On this day, there are significant differences in TEC variations presented by both models. These negative changes also continued on the following day, but the level of TEC changes was much lower. In the case of UWM-TEC, the storm recovery phase is clearly visible, while IGS-TEC almost does not reflect this phase of the storm. Figure 9 shows the clear differences in the ionosphere response to the analyzed geomagnetic storm.
3.3 Space weather event on March 2015
With regard to the changes in NmF2 measured by the Juliusruh ionosonde, a significant increase is visible after 10:00 UT on the disturbed day, reaching its peak before noon. Then, the NmF2 values decrease and the negative phase of the storm began, which lasted for the next 2 days. A significant 50% decrease in NmF2 was observed. Only at sunrise on 18 and 19 March, there was a short-lived increase in the analyzed NmF2, approaching the level from the quiet period. The discussed variations of the electron density observed by the Juliusruh ionosonde correspond with the changes of UWM-TEC for the location of this ionosonde.
Correlations between NmF2 variations and both UWM-TEC and IGS-TEC during all analyzed periods
7–16 March 2012
17–19 March 2013
17–19 March 2015
4 Discussion and summary
The three discussed geomagnetically active periods occurred during the same solar illumination conditions. All of them took place near the spring equinox, and moreover, the events in 2013 and 2015 occurred on the same day of the year. This leads to similar ionospheric conductivity profiles; however, these storms are associated with very different IMF conditions.
Figure 17 shows maps of local time–latitude UWM-TEC distributions for four events on 7, 9, 12 and 15 March 2012. Figure 17a presents a typical behavior of the ionosphere during stormy days. Again, as a reference, a quiet period of the ionosphere from 1 to 6 March 2012 was selected. The middle panel (b) presents the UWM-TEC distribution during stormy days on March 2012. Note that the maps showing the quiet and disturbed ionosphere are presented in the same scale. The maps of differences between an active and quiet ionosphere are presented in the bottom panel, and a different scale is used.
It should be emphasized that at 4:30 UT on 7 March there was an increase in auroral activity, and according to Prolss (1995) this should have immediately triggered a negative storm phase. However, in this case it did not happen. The rising TEC reached its peak at about 11:00 LT at latitudes above 40°N, and then, the highest values in daytime were achieved at noon at latitudes lower than 40°N. This effect may indicate the dayside super-fountain effect. According to Tsurutani et al. (2014), the absorption of the electric field contributes to TEC decrease at the magnetic equator during dusk local times, and to its rise and considerable enhancement at middle latitudes during daytime (Tsurutani et al. 2004; Mannucci et al. 2005; Tsurutani et al. 2014). After 14:00 LT, as in Belehaki et al. (2017), the bulge of the composition is visible, starting in the auroral zone. It can be presumed that this is the effect of substorm intensification. On this day, there was the second peak of TEC level at around 18:00 LT.
The differential UWM-TEC map provides more information about the occurred disturbances, clearly showing the above-mentioned TEC peaks. As presented in Fig. 17, the storm on 9 March had a completely different nature than on 7 March. A clear TEC increase is observed during daytime, the largest one in this period. The TEC values began to grow at sunrise, reaching the highest level at around 11:30 LT at latitudes below 40°N. The differential map confirmed a significant increase in UWM-TEC on this day, emphasizing more than a two-time increase in TEC at low latitudes (below 36°N).
The next disturbed day (12 March) was characterized by a sudden increase in TEC at about 10:30 LT, reaching peak values at about 11:00 LT. A rapid decrease in TEC was observed from 15:00 LT. This phenomenon is perfectly reflected by the differential map. The last disturbance, characterized by the smallest TEC changes, presents a completely different response of the ionosphere. The map showing this disturbed day clearly indicates two TEC peaks: the first peak occurred after 12:00 LT and the second one at around 18:00 LT. The analysis of the differential map confirms the occurrence of these anomalies. In addition, Fig. 17 reveals that the increase in the TEC value was preceded by a reduction of TEC after 6:30 LT at all observed latitudes. At around 13:00 LT, at latitudes below 42°N, there was a slight increase in TEC values in relation to its average for quiet days. While the second peak at around 18:30 LT is clearly visible, the increased TEC lasted until the end of the day.
UWM-TEC disturbance dependency on time and latitude for two St. Patrick storms in 2013 and 2015 is presented in Fig. 18. The top panel shows the typical behavior on quiet days as reference for March 2013 and 2015. This reference is derived from average ionosphere for quiet days from 10 to 16 March for each year. At latitudes lower than 40°N, the second TEC peak appeared at around 16:00 LT. In order to study the storm-time response of the ionosphere, the TEC variations during the magnetic storm on 17 March 2013 are presented in the middle panel. Morning TEC did not show any pronounced storm-driven effects. During a sheath passage, there is an increase in TEC at midlatitudes. Its maximum was achieved at 11:00 LT in the north (60°N), and just before noon in the south (34°N). As presented in Verkhoglyadova et al. (2016), the second TEC increase occurred during the MC passage. This second peak is observed at around 18:00 LT at the latitudes lower than 50°N. However, its maximum values reached 45 TECU and thus about 15 TECU less than during the first peak. The presented differential map in the bottom panel confirms these events. In addition, it can be seen that the increase in TEC values relative to the quiet days during the first peak amounted to 22 TECU and 13 TECU during the second peak.
In the top panel of Fig. 18, the quiet-time reference ionosphere is presented. During the background quiet period (10–16 March), TEC presented a gradual increase with its maximum reaching up 40 TECU. In the middle panel, a very clear TEC increase starting from 8:00 LT is presented. From 10:00 LT, the TEC values oscillate at the level of 60 TECU. This increase in TEC values corresponds to the sheath passage, as it has been noted by Verkhoglyadova et al. (2016). These observed TEC variations correspond to the coronal loop passage and auroral activity seen in the AE index. At about 11:00, the first TEC peak is visible, later on TEC decreases. After 14:00 LT, TEC decreases at all latitudes and then reaches its peak at 17:00 LT. A sharp drop in TEC is visible during a MC passage, and then, a pronounced negative storm signature occurred in the evening. The differential map presented in the bottom panel (Fig. 18b) allows to clearly indicate the positive phase of the storm. In addition, two peaks of this positive effect were clearly visible in a transparent manner. It should be emphasized that the TEC increase during the second peak was twice as large with respect to the first one; moreover, this peak corresponds to the coronal loop passage and auroral activity seen in AE index.
In this paper, the dynamics of the ionosphere during geomagnetic storm periods in March 2012, 2013 and 2015 was analyzed. The strong geomagnetic storms occurred near an equinox and were driven by coronal mass ejections. The largest change in the total electron content during the stormy period of 2012 was observed on 9 March and was associated with an ICME from 7 March, which arrived on Earth 2 days later. The two events in 2013 and 2015 took place on the same day of year and were triggered by coronal mass ejections which hit the Earth at the same time of day. However, the response of the ionosphere to these two events had a completely different nature.
The impact of the space weather events during the selected periods of GNSS-TEC variations was analyzed using new regional ionosphere maps developed at UWM in Olsztyn. The quality of the highly accurate and high-resolution regional ionosphere model was evaluated by a comparison to the widely used IGS GIMs. In addition, temporal TEC variations were compared with the variations of the NmF2 parameter observed by European ionosondes. This approach made it possible to assess the capability of the new high-resolution regional ionosphere model to reproduce the complex behavior of the ionosphere. The analysis of the temporal TEC changes provided by UWM maps in relation to NmF2 changes derived from ionosondes confirmed the high compatibility of these techniques for the disturbed ionosphere. In general, in all cases, UWM-TEC better reflects the changes in the disturbed ionosphere than IGS-TEC, as compared to the ionosonde data. This is also proved by the correlations between variations of NmF2 and UWM-TEC that are higher than between NmF2 and IGS-TEC during all analyzed periods.
It should be emphasized, as stated by Astafyeva et al. (2015), that widely available ionosphere maps with a time resolution of 1–2 h and low spatial resolution of several degrees may not be sufficient to analyze the storm-time effects in detail. In order to have more detailed information about the storm-time spatial and temporal TEC variations, more accurate and high-resolution ionosphere maps should be used. The presented results showed that the regional ionosphere model based on phase observations provides useful information on the response of the ionosphere to the geomagnetic disturbances. Therefore, the application of this new high-resolution model can provide more complete information about the ionosphere response to magnetic disturbances than the currently available TEC maps.
The research is supported by Grant No. UMO-2013/11/B/ST10/04709 from the Polish National Center of Science.
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