In this section, several examples of the response of the ionosphere to geomagnetic storms occurring in different seasons of the year will be presented. On the basis of the methods described above, maps of the relative deviation of TEC were obtained, illustrating the spatial distribution of ionospheric anomalies. Different examples are presented for the detailed visualization of the positive anomaly observed in each of the events. For this purpose, the distribution of the amplitudes and the phases of the approximation of the longitudinal distribution of the relative TEC by SPW1 are shown. By definition, SPW1 is a sinusoidally distributed quantity dependent on longitude with a period of 360°. On the basis of the presented maps, information is obtained about the maximum positive response of TEC by time and location.
3.1. Geomagnetic Storm 23–24 April 2023
In order to trace the behavior of the geomagnetic disturbance, the indices describing the storm are presented in
Figure 1. As seen in
Figure 1 (top panel), the Kp-index gradually reaches values above 8 in the hours just before 18 UTC on 23 April 2023. According to the NOAA Space Weather Scales, the observed event is of Class G4 (Severe) [
30]. According to the classification, a storm of Class G4 can cause some problems related to: (i) power systems—voltage control problems; (ii) spacecraft operations; and (iii) other systems: HF radio propagation, satellite navigation, low-frequency radio navigation. The time period in which Kp ≥ 5 is from 12 UTC on 23 April to 13 UTC on 24 April. The behavior of the geomagnetic indices and solar wind parameters (see
Figure 1) shows that the storm has two sharply distinguished stages.
During the first stage of the considered event until 18 UTC on 23 April, the Bz component of the IMF is negative (southward) and has the opposite direction to the Earth’s magnetic field, leading to changes and the reconnection of IMF and the Earth’s magnetic field. The parameter indicating the speed of the solar wind clearly illustrates a sharp increase in its values from about 350 km/s (quiet conditions speed) to over 700 km/s in the hours before 20 UTC, which is another indication of the geomagnetic storm occurring. At around 21–22 UTC, there is about a four-hour period of positive Bz values in which the Power index decreases to a value close to that before the onset of the storm. The next phase of the storm begins around 01 UTC on 24 April and ends around 12 UTC on 24 April when the Bz component becomes positive again and the coupling of the Earth’s magnetosphere to the solar wind ceases.
To investigate the ionospheric response, the latitudinal and longitudinal distribution of the TEC response during the storm, represented by the values of the relative deviation from the running hourly medians, is considered.
The results are shown in
Figure 2 and
Figure 3 in the form of spatial maps plotted in the longitude-modip. It can be seen from
Figure 2 that on 23 April at 18 UTC, two characteristic regions of positive anomaly began to appear in the Southern Hemisphere. The two positive anomalies in TEC are located around 60° S, 60° E and 30° S, 60° W. These positive responses increased until around 00 UTC on the next day (24 April), after which they began to decrease.
An interesting fact about the region 60° S, 60° E is that it coincides with the night sector and the southern polar oval, which gives us reason to assume that this is the region of particle precipitation in the Southern Hemisphere. Symmetrical to the resulting Southern Hemisphere anomalies, a weak positive response was also observed in the Northern Hemisphere at 19–22 UTC on 23 April around 60° N, 90° E. In both cases, the probable reason for the positive response in TEC is direct ionization under the action of solar wind particles entering the auroral ovals. From the results shown in
Figure 2 and
Figure 3, it can be seen that the relative influence of additional ionization is stronger in the Southern Hemisphere where conditions approach winter conditions—i.e., observed a decreased electron density.
The second observed region of positive anomaly located around 30° S, 60° W has a complex structure where a split is observed around the 60° W meridian where the two parts move in opposite directions. In addition, an almost symmetric but weaker response is observed in the Northern Hemisphere at the latitude 30° N, especially distinct around 21–23 UTC on 23 April.
A similar positive ionospheric response was obtained in another paper [
31]. In this study of the response of the equatorial ionosphere during geomagnetic storm on 15 July 2000 for the point FORT (Fortaleza, Brazil) with coordinates 3.85° W, 38.45° S, the authors explain the obtained positive response of vertical TEC with the theory of ionospheric disturbance dynamo. The use of point data from Jicamarca radar observations of F region vertical plasma drifts and indices of the auroral electrojet allowed them to determine the storm time dependence of the equatorial disturbance dynamo zonal electric fields [
32].
An analogous result of an increase in the ionospheric electron density values was also obtained for the Halloween geomagnetic superstorm that occurred on October 29–30 2003 over the American longitude sector (~70° W). The results in this paper show that the observed TEC enhancement at higher latitudes is the result of the expansion of the equatorial ionization anomaly, while the obtained TEC enhancement at low- and mid-latitudes is the result of storm-generated equatorward neutral winds in addition to the eastward penetration electric field [
33].
The papers described above in which analogous positive ionospheric responses were obtained provide possible explanations for the observed response in
Figure 2 and
Figure 3.
The secondary particle precipitation in the early morning hours (in UTC) coincides with the intensification of the positive response at 120° W in the Southern Hemisphere. Until the end of the geomagnetic storm, this positive response remained at the same longitude and latitude (see
Figure 3, in the hours between 19 and 22 UTC).
For comparison of the obtained result of a negative response of TEC in the Northern Hemisphere, a study of the same geomagnetic storm, but based on single-point station measurements, is considered [
34]. In this study, variations in the critical frequency foF2 were examined for the period from April 23 at 10 UTC to April 24 at 06 UTC from the ionograms, received at the Leibniz-Institutfür Atmosphären Physik (IAP) with ionosonde coordinates 52.21° N, 21.06° E (access available at:
https://www.iap-kborn.de, accessed on 23 April 2023). The results of the foF2 behavior show a decrease in the critical frequencies from 8 MHz to 2 MHz (negative response) according to the IAP ionospheric station data. The results obtained for a specific IAP station coincide with the obtained negative anomaly for the Northern Hemisphere from the global TEC data presented in
Figure 2 and
Figure 3 [
34].
Figure 4 shows in more detail the maps for 21 UTC on 23 April and 05 UTC on 24 April. The selected hours coincide with the two maxima of the Power Index, i.e., of the energy entering the auroral regions.
Figure 4 (top panel) shows that the positive response at 60° S occurs at 1 h in local time, i.e., close to midnight. The positive response at about 30° S occurs at local time between 17 and 19, which can be considered the approximate time of sunset in winter conditions. The presented map at 05 UTC on 24 April (see
Figure 4, bottom panel) shows a weak positive response at 60° S at the zero meridian and corresponding local time at 5. The most significant positive response shown in
Figure 4 (bottom panel) is located at 30° S and 120° W. The analogous positive response in the Northern Hemisphere is around 40° N and 120° W.
From the results presented in
Figure 4, it can be seen that the behavior of the relative TEC in the two auroral regions gives us reason to assume the location of the areas of strongest thermal impact of the particle precipitation in the both hemispheres. The first particle precipitation probably causes heating of the region around 60° E where the strongest positive TEC response occurs (
Figure 4, top panel). The second particle precipitation is weaker and is manifested by additional ionization, although it is energetically stronger and can be assumed to be localized around the zero meridian (see
Figure 4, bottom panel). A possible reason for its weak manifestation in TEC is the fact that the area of particle precipitation was already significantly heated by the previous precipitation, in which the change in the O/N
2 ratio caused an increase in recombination and correspondingly a decrease in the effect of additional ionization. Evidence for this assumption is the presence of an area of extreme negative response in the Northern Hemisphere (see
Figure 4, bottom panel).
Additional investigation of the positive anomaly observed in the Western Hemisphere is presented in
Figure 5. The plot illustrates the variations in the relative critical frequency foF2, whose data are received from the ground-based ionospheric station for vertical sounding Cachoeira Paulista (23.2° S modip, 50° W) marked in a blue color. The same figure also shows the behavior of the relative TEC at the closest point by coordinates to those of the selected station (22.5° S modip, 50° W) from the map of the TEC data (marked in red in
Figure 5). The positive responses of both quantities with a maximum around 22 UTC on 23 April show a complete coincidence (see
Figure 2).The latitudinal location of the positive anomaly in the Western Hemisphere, which has symmetry with respect to the magnetic equator, suggests that this anomaly is due to a forced fountain effect. A possible explanation for the observed positive response during the night hours (unlike quiet conditions) may be due to the reverse fountain. One possible hypothesis for the action of this important mechanism is related to the increase in ionization at equatorial anomaly latitudes in the night hours, with some contribution from the prereversal strengthening of the forward fountain [
35]. An additional argument for the stated hypothesis is the appearance of a region around the magnetic equator where the TEC response is negative. It should be expected that the plasma transfer from the equator to the mid-latitudes will be accompanied by the reduction in plasma around the equator. Assuming that the observed anomaly in the latitudinal range between 30° S and 30° N is due to DDEF (Disturbed Dynamo Electric Fields) for the selected ionospheric storm, a number of peculiarities are noted.
In order to obtain a more detailed view of the place where the maximum response was obtained in relative TEC for the period 23–25 April, spatial maps of the amplitudes and phases of SPW1 are presented. The amplitude map (see
Figure 6a) presents the two activations of the response at latitudes 30° S and 30° N coincident in time with the two particle precipitations according to the solar and geomagnetic parameters illustrated in
Figure 1. From
Figure 6b, it can be seen that the SPW1 phases (i.e., the location of the positive maxima) at these two latitudes during the storm occur in the Western Hemisphere.
Figure 7 presents a comparison of the behavior of the amplitude and phase of SPW1 for the locations in the two hemispheres, where a positive response occurs in the ionospheric TEC. The upper panel shows the Power Index, which indicates the moment of increase in incoming solar radiation coinciding in time with the onset of the geomagnetic storm according to the considered parameters in
Figure 1.
The asymmetry of the response in the two hemispheres, northern and southern, can be explained by two hypotheses.
Due to the fact that the conditions in the Southern Hemisphere are close to winter, it should be expected that the temperature of the neutral air is lower than that in the Northern Hemisphere; therefore, particle precipitation can cause a stronger warming, therefore a relatively more significant disturbed wind in the Southern Hemisphere. It should be emphasized that the relative TEC deviation used is relative to quiet ionospheric conditions, which condition also includes the condition of the neutral atmosphere during the given month of the year. Additionally, in the present work, selected ionospheric storms that occurred in the months of June and December (summer and winter solstice), which show the same dependence on the season, are considered.
The second essential peculiarity of the ionospheric response is the longitudinal distribution of the response. The maximum positive response is observed only in the Western Hemisphere, although particle precipitation continues (with a short interruption) throughout the day. An illustration of this feature is the behavior of the amplitudes and phases of the approximation of the longitudinal distribution of the relative TEC by SPW1, which is an approximation of this distribution by the cosine with a spatial period of 360 geographic degrees shown in
Figure 6. The maps illustrated in
Figure 2,
Figure 3 and
Figure 4 show that this representation reflects the main peculiarities of the longitudinal distribution of the relative TEC response. As mentioned in
Section 3.2. Methods and methodologies, and more specifically in the “Method of stationary amplitudes and phases”, the amplitude reflects the maximum deviation from the zonal mean value of the relative TEC, and the SPW phase indicates the latitude of the positive maximum of the response at a given hour. To follow in detail the positive anomaly in TEC for the considered storm in April 2023,
Figure 7 shows the amplitudes and phases of SPW1 at the latitudes 30° N and 30° S. The results show that the phases show an apparent stationarity. In the Northern Hemisphere during the activation of SPW1, they appear in the limits around 90° W–120° W, and in the Southern Hemisphere around 90° W–150° W, respectively. This result coincides with the spatial distribution of relative TEC shown in
Figure 2 and
Figure 3.
3.2. Geomagnetic Storm 22–24 June 2015
The purpose of studying this event is the opportunity to investigate the storm effects in ‘opposite’ seasons—summer and winter.
According to the behavior of the geomagnetic indices Dst and Kp, shown in
Figure 8 upper panel, 18 UTC on 22 June 2015 is determined as the beginning of the geomagnetic storm. This storm is occurring in several stages. In the first stage, the Kp-index reaches almost 9 while the Dst-index has a value of about −120 nT in the hours around 19 UTC. According to the classification of the Kp-index, this storm is similar to the above mentioned event in April 2023 both in terms of the value of the index and in its biphasic manifestation (with two expressed maxima in the Kp-index). According to the classification, the investigated storm is of the Strong class because it corresponds to the cases when −100 > Dst > −200 nT [
36,
37]. The second stage, which is clearly visible from the indices in
Figure 8 upper panel, has a maximum manifestation at around 04 UTC on 23 June 2015. According to the value of the Kp-index, which in this case is almost 8, the considered geomagnetic storm is of the Strong type. The difference compared to the first geomagnetic event is that in the first case the Kp-index reacts more strongly, while in the second case according to the Dst-index (which has values around −210 nT) the storm is of the Severe class. As seen in
Figure 8 bottom panel, three consecutive periods of negative Bz values (i.e., interaction of the solar wind and the Earth’s magnetosphere) are observed during the geomagnetic storm. The first period occurs around 18 UTC on 22 June 2015 and coincides with the sudden increase in solar wind speed. The second moment of the negative Bz component of the IMF is around 02 UTC on 23 June 2015, and the third (which is the weakest) is around 08 UTC on 23 June 2015. The active phase of the storm can be considered to end around 12 UTC on 23 June, when the Bz component of the IMF has a stable positive value and the Kp-index has values below 3.
In order to make an analogous study of the maximum manifestation of the TEC response in
Figure 9, the amplitude and phase maps of SPW1 are presented. The maps show a strongly increased modip amplitude at the 60° S latitude with similar time but weaker activation at the 45° N latitude. Similarly, in this storm, a strong response is observed in the winter hemisphere. The SPW1 activation on 23 June 2015 has maxima at 40° S and at the equator, with a larger amplitude in the Southern Hemisphere, confirming the well-known seasonal response in winter conditions [
28,
29].
Figure 10 shows the amplitude and phase variability of SPW1 at three characteristic latitudes. At the latitude 60° S, the amplitude is maximal around 23 UTC on 22 June 2015, when the first particle precipitation is caused by the negative values of the Bz component. The phase in LT until the end of the storm is close to zero, which means that the impact on the ionosphere is shifted along with the local midnight region, which means that the positive maximum of the ionospheric response is related to the ionization of the particle precipitation region. The positive maxima of the response in the equatorial region and in southern midlatitudes, however, have quite different behavior. The phase, i.e., the positive maximum, is almost stationary and is located in the Western Hemisphere at the beginning of the geomagnetic storm. At both latitudes, the phase tends to decrease, i.e., westward direction change, but this variation is too slow compared to the movement of the midnight meridian over time. As in the previous storm considered, the low and mid-latitude anomaly has two maxima, suggesting that it is caused by a “fountain effect”. Compared to the April 2023 storm, there is a tendency for the response to shift to southern latitudes (i.e., in the winter hemisphere).
The considered event is well studied and described in a significant number of papers, and the results of the type of ionospheric response are analogous to the presented amplitudes and phases of SPW1 illustrated in
Figure 9 and
Figure 10.
In the first paper about the geomagnetic storm on 22–23 June 2015, the authors use different types of ground-based instruments and data of multiple satellite missions to study equatorial and low-latitude electrodynamic and ionospheric disturbances. Several conclusions are drawn from the results in this investigation. PPEF have considerable impact on the equatorial electrojet and the equatorial zonal electric at the beginning of the storm. The significant ionospheric uplift and positive ionospheric storm on the day side, and downward drift on the night side observed by the authors, they explain, is due to the eastward direction of the PPEF. Some variations in the equatorial electrojet at the end of the main phase were also obtained, which the authors conclude are the result of the disturbance dynamo effect was already in effect, competing with the PPEF and reducing them. The authors of this paper associate the observed second positive storm with the influence of a disturbed thermosphere [
38].
The second global study of the response of the ionospheric TEC during the geomagnetic storm on 22 June 2015 shows the following results: (a) before the main phase of the considered geomagnetic storm, increases in TEC at high latitudes were obtained; (b) there was a difference in the TEC response at mid and low latitudes in the Northern Hemisphere and the Southern Hemisphere and decreases at equatorial latitudes. The author explains the observed ionospheric response of the equatorial and low latitudes during the main phase of the storm with the influence of these areas by Prompt Penetration Electric Fields [
39].
Another investigation of the ionospheric effect of the same event in June 2015 for the South American region is based on a comparison between different measurements. Based on the large number of different types of data used, the authors illustrate and explain the observed expansion of the crest of EIA (Equatorial Ionization Anomaly) at mid-latitudes and high latitudes mainly due to PPEF during the main phase and the recovery phase of the geomagnetic storm during the day [
40].
3.3. Geomagnetic Storm 14–16 December 2006
The selected geomagnetic storm is close to the winter solstice in the Northern Hemisphere. In order to examine in detail the behavior of the geomagnetic storm,
Figure 11 shows the geomagnetic indices Kp and Dst (top panel) and the solar parameters of the Bz component of IMF and the solar wind speed (bottom panel).
The behavior of the Kp-index shows a sudden increase after 12 UTC on 14 December, 2006, when the quantity has values above 5. At around 00 UTC on 15 December, maximum Kp values of almost 9 were also recorded. According to the classification of this index, the event under consideration is of Class G4 (Severe), analogous to the events discussed above. The recovery of index values to quiet conditions was observed a few hours before 00 UTC on 16 December. According to the behavior of the Kp- and Dst-indexes, the considered geomagnetic storm had a sudden onset, occurring around 23 UTC on 14 December 2006. In this particular case, the Dst values reach about −160 nT over a period of about 7–8 h, which is also the time of the main phase of the storm. The recovery phase of Dst continues until 17 December 2006.
An analogous representation of the behavior of the solar wind parameters shows a sharp increase in the solar wind speed to about 950 km/s around 14 UTC on 14 December 2006. After that, the speed gradually decreases (see
Figure 11 bottom panel).
Special attention should be paid to the behavior of the Bz component of the IMF for the considered time interval (see
Figure 11 bottom panel). The moment of increase in the speed of the solar wind coincides in time with the short-term interaction of the Earth’s magnetosphere and the solar wind, described by the small negative values of Bz (which indicates the moment of interaction between the solar wind and the Earth’s magnetosphere). The considered parameter has positive values between 18 UTC on 14 December and in the hours between 20–21 UTC on 14 December, when the Bz component reaches negative values. The minimum of the Bz component of the IMF is at 00 UTC on 15 December 2006, coinciding in time with the most significant responses and in the geomagnetic indices presented in
Figure 11 top panel.
Again, an investigation of the ionospheric response is suggested by the resulting SPW1 amplitude and phase maps shown in
Figure 12. It can be seen from the figure that the maps presented in
Figure 12 show a visible similarity to those illustrating the geomagnetic storm in June 2015 (see
Figure 9), with the amplitudes of the ionospheric response again being higher in the winter hemisphere (in this case for geomagnetic storm on 15 December 2006—this is the Northern Hemisphere).
Between 18 UTC and 24 UTC on 14 December, an activation of SPW1 amplitude was observed at modip latitudes of 60° N and 60° S (modip latitudes close to the northern and southern auroral ovals). At lower latitudes, the response maximizes on 15 December around 06 UTC, being stronger in the Northern Hemisphere.
The investigated geomagnetic storm in December 2006 is of interest to scientists and has been studied by a large number of authors [
41,
42,
43]. In the first discussed paper in which the response of the ionosphere was studied, a considerable positive response was obtained over the Atlantic sector after the onset of considered storm. The authors explain the observed positive anomaly during the initial phase with changes in the electric fields, also mentioning a possible influence as a result of neutral winds and composition changes. An observed result in this study is that the reduction in electron densities in the two hemispheres is different, being much more rapid in the winter hemisphere. The authors explain the differences in the positive ionospheric response for the two hemispheres with the presence of electric fields and their influence on the electron density [
41].
The second work, the result of which confirms the results seen in the present investigation for the geomagnetic storm on 15 December 2006, is based on a combination of different types of measurements, allowing the study of the ionospheric positive response by revealing the storm time response in different altitude regions [
42]. The results of this study show an analogous response to that in [
41], characterized by electron density enhancements at low latitudes to mid-latitudes during the main phase of the storm. The positive anomalies, analogous to those obtained in the present study, which the authors obtain for the Pacific Ocean region remain present in the equatorial ionization anomaly crest regions at the hours 12:00 UTC on 15 December. The authors explain the observed positive responses with the enhanced eastward electric field and equatorward neutral wind [
42].
The third case, which is used for comparison, is a study of observations related to the generation of equatorial ionospheric anomalies including ionospheric plasma bubbles and variations in the ionospheric F-region in the South American sector during the same geomagnetic storm in December 2006. The authors investigated the anomalies in the F-region based on two ionospheric stations during the night hours of 14–15 December, namely São José dos Campos (SJC, 23.2° S, 45.9° W; dip latitude 17.6° S), and Port Stanley (PST, 51.6° S, 57.9° W; geom. latitude 41.6° S), which show strong oscillations due to the propagation of traveling ionospheric disturbances by the Joule heating in the auroral region. Unlike the F-region response, the anomaly for VTEC obtained by the authors is both positive and negative, and the explanation of the observed response for the South American sector proposed by the authors is related to changes in the O/N
2 ratio in the Southern Hemisphere [
43].
Shown in
Figure 13, the behavior of the amplitudes and phases at three characteristic latitudes is very similar to the analogs of the two storms discussed above (see
Figure 7 and
Figure 10). It can be seen from the figure that the response phase at 65° N expressed in local time is close to 0 LT. At latitudes of 30° S and 30° N, the phase is again stationed in the Western Hemisphere, in this case near the longitude 180°.