Extended study of extreme geoelectric field event scenarios for geomagnetically induced current applications

Transcription

1 SPACE WEATHER, VOL. 11, , doi:1.12/swe.221, 213 Extended study of extreme geoelectric field event scenarios for geomagnetically induced current applications Chigomezyo M. Ngwira, 1,2 Antti Pulkkinen, 1,2 Frederick D. Wilder, 3,4 and Geoffrey Crowley 4 Received 19 September 212; revised 11 December 212; accepted 13 December 212; published 29 March 213. [1] Geomagnetically induced currents (GIC) flowing in man-made ground technological systems are a direct manifestation of adverse space weather. Today, there is great concern over possible geomagnetically induced current effects on power transmission networks that can result from extreme space weather events. The threat of severe societal consequences has accelerated recent interest in extreme geomagnetic storm impacts on high-voltage power transmission systems. As a result, extreme geomagnetic event characterization is of fundamental importance for quantifying the technological impacts and societal consequences of extreme space weather. This article reports on the global behavior of the horizontal geomagnetic field and the induced geoelectric field fluctuations during severe/extreme geomagnetic events. This includes (1) an investigation of the latitude threshold boundary, (2) the local time dependency of the maximum induced geoelectric field, and (3) the influence of the equatorial electrojet (EEJ) current on the occurrence of enhanced induced geoelectric fields over ground stations located near the dip equator. Using ground-based and satellite-borne Defense Meteorological Satellite Program measurements, this article confirms that the latitude threshold boundary is associated with the movements of the auroral oval and the corresponding auroral electrojet current system, which is the main driver of the largest perturbations of the ground geomagnetic field at high latitudes. In addition, we show that the enhancement of the EEJ is driven by the penetration of high-latitude electric fields and that the induced geoelectric fields at stations within the EEJ belt can be an order of magnitude larger than that at stations outside the belt. This has important implications for power networks located around the electrojet belt and confirms that earlier observations by Pulkkinen et al. (212) were not isolated incidences but rather cases that can occur during certain severe geomagnetic storm events. Citation: Ngwira, C. M., A. Pulkkinen, F. D. Wilder, and G. Crowley (213), Extended study of extreme geoelectric field event scenarios for geomagnetically induced current applications, Space Weather, 11, , doi:1.12/swe Introduction 1 Department of Physics, Catholic University of America, Washington, DC, USA. 2 NASA Goddard Space Flight Center, Code 674, Greenbelt, Maryland, USA. 3 Laboratory for Atmospheric and Space Physics, The University of Colorado, Boulder, Colorado, USA. 4 Atmospheric and Space Technology Research Associates, Boulder, Colorado, USA. Corresponding author: C. M. Ngwira, Department of Physics, Catholic University of America, Washington, DC 264, USA. (chigongwira@yahoo.co.uk) [2] Geomagnetically induced currents (GIC) are the lowfrequency currents (.1 1 Hz) driven by adverse space weather events. During a geomagnetic storm, intense timevarying currents are produced in the magnetosphere and ionosphere, which result in rapid variation of the geomagnetic field. A geoelectric field is induced at the Earth s surface as defined by Faraday s law of induction. This electric field then drives currents in man-made technological conductor networks, such as power transmission systems, oil and gas pipelines, and telecommunication cables [e.g., Pirjola, 2]. [3] In electric power transmission systems, which constitute the most critical technological infrastructures concerning GIC effects today, GIC flowing through transformer windings can saturate the core leading to transformer malfunctions. In worst cases, this core saturation can even extend to permanent damage of transformers or a collapse of the whole system [e.g., Molinski, 22; Bolduc, 22; Pulkkinen et al., 25; Wik et al., 29]. The March 1989 geomagnetic storm demonstrated the need for power companies to assess the space weather risk to transmission system 213. American Geophysical Union. All Rights Reserved. 121

2 reliability. Since then, a wide range of GIC studies on power transmission systems have been undertaken [see, e.g., Viljanen and Pirjola, 1994; Thomson et al., 25; Trichtchenko and Boteler, 26; Ngwira et al., 28; Liu et al., 29; Watari et al., 29, and references therein]. [4] Society today has become highly reliant on electricity to meet essential needs. Therefore, the threat of severe societal consequences has accelerated recent interest in extreme geomagnetic storm impacts on high-voltage power transmission systems, as evidenced by the rise in the number of studies during the last few years. As a result, extreme geomagnetic event characterization is of fundamental importance for quantifying the technological impacts and societal consequences of extreme space weather (e.g., [Thomson et al., 211; Riley, 212; Pulkkinen et al., 212]). [5] Recently, Thomson et al. [211] used 28 years of 6 s geomagnetic data from Europe to quantify extreme behavior of geomagnetic activity. They found that there was a marked maximum in the extreme levels of the horizontal geomagnetic field time derivative at about 55 6 geomagnetic latitude. A more recent study by Pulkkinen et al. [212] on the generation of 1 year GIC scenario also showed that for extreme geoelectric fields, the maximum amplitude of the horizontal geomagnetic component and its time derivative all displayed a characteristic latitude threshold boundary. The threshold boundary is a middle to high-latitude transition region marked by about an order of magnitude increase of the three geophysical parameters. In addition, the study revealed for the first time a potential threat of GIC for power network systems along the dip equatorial region as a consequence of enhanced induced electric fields in this region associated with the equatorial electrojet (EEJ) current. However, a marked limitation of this study was that the data set it used only comprised of two geomagnetic events on October 23 and 13 March 1989, whereas in the case of Thomson et al. [211], the data set only covered Europe. In our more general approach, we extend the study by Pulkkinen et al. [212] by including a collection of 12 severe geomagnetic storm events that occurred during the period [6] This article reports on an investigation of the global behavior of the ground horizontal geomagnetic field and the geoelectric field fluctuations during severe/extreme geomagnetic events. This includes (1) an extended investigation of the latitude threshold boundary, (2) the local time (LT) dependency of the maximum induced geoelectric field, and (3) the influence of the EEJ on the occurrence of enhanced induced geoelectric fields over ground stations located near the dip equator. [7] In section 2 of the article, we describe the data sources used and the processing methods applied. Presented in section 3 are the results for the study, which include the latitude threshold boundary investigation in section 3.1, with the LT dependence of the maximum induced geoelectric field appearing under section 3.2, while the solar wind and magnetosphere control of the EEJ amplitude is handled in section 3.3. We provide a discussion of the various results in section 4 and draw the conclusions of this work in section Data and Methodology [8] Twelve severe geomagnetic storm events that occurred between 1989 and 25 were considered for this study. The storms were selected based on the minimum Dst value category less than 245 nt. The events are listed in Table 1 together with the periods of primary interest for this study, the minimum Dst index values, and the approximate auroral equator-ward boundary locations (given in geomagnetic latitude [MLAT]) as inferred from Defense Meteorological Satellite Program (DMSP) satellite observations. Earlier studies have argued that 6 s temporal resolution magnetic data is sufficient for global investigations of extreme geoelectric field amplitudes. Therefore, global 6 s magnetic field data were obtained from INTERMAGNET ( for the present study. The number of stations for which data were available varied between 4 and 9, with the fewest stations typically during the period A mathematically determined quiet-time baseline (mean) was then removed from the observations. The data were further checked for alias values, and stations with erroneous data were removed from the analysis. Short data gaps no longer than a few minutes were patched using linear interpolation. We then determine the geoelectric fieldateachgroundstation using the plane wave method [see, e.g., Cagniard, 1953; Pirjola, 1982] while applying the Quebec ground conductivity model [see Pulkkinen et al., 212]. In principle, the plane wave method presumes that the electric and magnetic fields have Table 1. Selected Extreme Geomagnetic Storm Events With the Study Period of Interest, Their Minimum Dst Index, and the Approximate Low-Latitude Auroral Equator-ward Boundary Locations a Storm date and the time period considered for each storm event Minimum Dst Auroral equator-ward boundary location (degrees of MLAT) 13 March 1989, 2: UT March 1989, 5: UT 24 March 1991, 2: UT March 1991, 8: UT 8 November 1991, 6: UT November 1991, 12: UT 1 May 1992, 1: 22: UT April 2, 16: UT April 2, 1: UT 15 July 2, 14: 23:3 UT March 21, : 23: UT November 21, 1: 2: UT October 23, 5: 23: November 23, 7: 23: November 24, 1: UT November 24, 12: UT 15 May 25, 2: 15: UT a The maximum equator-ward extent is based on electron precipitation data from the DMSP satellites and given in geomagnetic latitudes (MLAT). 122

3 no spatial dependence in the area considered. In the absence of ground conductivity anomalies, this is usually an acceptable assumption at mid latitudes. However, [Viljanen et al., 24] have shown that this method can be applied to infer the GIC at a specificsitefromlocalvaluesoftheelectricfield, magnetic field and the site-specific surface impedance, even when the fields and impedance may vary from one GIC site to another. [9] To determine auroral boundaries, the present study uses precipitating particle flux from the DMSP SSJ/4 particle detectors. Each spacecraft is capable of detecting the flux of precipitating ions and electrons using 19 channels within the.3 to 3 kev range [Hardy et al., 28]. The primary interest in this study is the influence of precipitating particle flux on ionospheric conductivity, and as such, regions of higher-energy (>1 kev) particle flux with the ability to penetrate into the lower F region and E-region will be investigated [Robinson et al., 1987; Hardy et al., 1984]. Following Hardy et al. [1984], one can calculate integrated energy flux over all channels, called J ETOT, given by equation (2): J ETOT ¼ J E ðe 1 ÞðE 2 E 1 Þþ X18 i¼1 J E ðe i Þ E iþ1 E i 1 2 þ J E ðe 19 ÞðE 19 E 18 Þ (1) [1] where J E (E i ) is the differential energy flux measured by the ith channel and E i is the energy detected by the ith channel. In the present study, we will emulate [Redmon et al., 21] and determine auroral boundaries using the total energy flux over the nine high-energy channels between 1.39 and 3 kev (channels 11 19), which we refer to as J E,HE. [11] Figure 1 shows a stack plot of particle precipitation data observed by the DMSP F13 spacecraft between 17:43 and 18:5 UT during the 6 November 21 geomagnetic storm event. Electron and ion spectrograms are given, as well as a time series of J ETOT and J E,HE. It can be seen that the trend in the two integrated energy values is fairly similar; however, for passes with enhanced precipitation of electrons with energies less than 1 kev, using J ETOT alone can lead to a misidentification of the main auroral oval. At the start and the end of the pass, there is a region with enhanced flux of electrons and ions with energies greater than 1 kev, which is the auroral oval [Newell and Meng, 1992]. Between the two regions, the ion flux drops out and the electron flux is largest in the channels with energies less than 1 kev. This region is typical of open field lines, and the precipitation is largely thought to be due to the polar rain [Newell et al., 28]. [12] In this study, we are interested in the equatorward auroral boundary, which corresponds to a large increase in ionospheric conductance in the poleward direction. However, the poleward boundary is included for completeness. The equator-ward boundary of the auroralovalisdeterminedbyvisualinspectionofboth the electron spectrogram and the trend in J E,HE. First, the electron spectrogram is visually inspected to determine the two auroral zone regions of enhanced >1 kev particle precipitation. Then, the equator-ward edgeoftheregionwithasteepslopeinj E,HE is chosen as the equator-ward boundary. In particular, the region of steep slope is chosen, which corresponds to the Figure 1. Data from the DMSP F13 SSJ/4 particle detector on 6 November 21. From top to bottom: (1) the total electron energy flux, J ETOT, over the satellite pass; (2) the total energy flux of electrons with energies greater than 1.39 kev; (3) a spectrogram of the electron differential energy flux; and (4) a spectrogram of the ion differential energy flux. 123

4 region where a visual inspection of the spectrogram would determine a steep drop off in electron precipitation. Because precipitation is often highly structured, sometimes it is difficult to precisely determine the equator-ward auroral boundary. In general, there is an uncertainty of 1 to 2 geomagnetic latitude, which is within the uncertainty of the overall study. A similar analysis can be used to determine the poleward auroral boundary. Both the equator-ward and the poleward boundary are given by the vertical red bars in Figure 1. In this article, passes by the DMSP spacecraft throughout each storm are investigated using the methodology above to find the lowest magnetic latitude extent of the equator-ward edge of the auroral oval. [13] Besides individual satellite passes, high-energy electron precipitation can be investigated in a statistical sense. To do this, values of J E,HE from each satellite pass during each storm are organized into a spatial grid with 2 spacing in magnetic latitude and longitude. The maximum value of J E,HE is then taken in each spatial grid cell. [14] In addition to high-latitude currents such as the auroral electrojet, extreme geoelectric field events have been observed in the equatorial zones in the EEJ. The daytimeeejisanarrowband(2 )ofstrongeastward ionospheric current flowing at E-region altitude along the dip equator [e.g., Rastogi, 1974; Anderson et al., 24, and references therein]. Anderson et al. [24] demonstrated that a direct way to measure the strength of the daytime EEJ is by computing the difference in the horizontal H-field (ΔH) between two magnetometers, i.e., one placed directly at the dip equator and another placed 6 9 away. However, in this study, we define the measure of the strength of the EEJ as the difference (ΔH) between the observed horizontal H-field and the nighttime baseline of the H-field for stations located at/ near the dip equator [see e.g., Rastogi, 1974; Yizengaw et al., 211]. A positive ΔH bay is a proxy for the eastward electrojet or electric field, and a negative ΔH bayisa signature of the equatorial counter-electrojet (CEJ) or westward electrojet, which is associated with a westward electric field. 3. Results 3.1. Maximum Induced Geoelectric Field Latitude Threshold Boundary [15] As mentioned earlier, recent observations indicate that the maximum amplitude of the horizontal geomagnetic field, its time derivative, and the induced geoelectric field all display a similar latitude threshold boundary. From these observations, it has been presumed that the threshold at about 5 55 of MLAT could be a universal feature of most major or extreme geomagnetic storms. In this section, we present a follow-up study to evaluate these recent observations by considering a larger number of extreme geomagnetic storm events. [16] Shown in Figure 2 are the results of (top) the maximum computed geoelectric field, (middle) the maximum time derivative of the horizontal geomagnetic field, and (bottom) the maximum amplitude of the horizontal geomagnetic field for all the storms. The red dashed line is the 5 MLAT marker, and the thick solid curve is a sixth-order polynomial fit. It is interesting to note that the fitted curve captures the equatorial enhancement, which will be our topic of discussion in section 3.3. In any case, it is clear that there are fewer observations available from the Southern Hemisphere due to the low distribution of geomagnetic stations. Nevertheless, as clearly seen in all the panels, there is a distinct transition region in both Hemispheres, which is identified by a marked increase of all the three parameters as one moves toward the high latitudes from the midlatitudes. This region is the latitude threshold boundary, which typically tends to occur between 5 and 55 of MLAT. [17] Pulkkinen et al. [212] suggested that the threshold geomagnetic latitude be investigated by means of observations of the equator-ward boundary from auroral emissions, which would provide an indication of the general location of the auroral oval. The determination of the auroral oval equator-ward boundary location is discussed in the previous section. Figure 3 depicts the auroral oval poleward and equator-ward boundary locations derived for multiple DMSP satellite passes with each dot representing the location during a particular satellite pass. In addition, maps of the DMSP maximum energy flux showing the range of extent of the poleward and equator-ward boundaries for all storms are given in Figures 4a 4b. The regions of increased precipitation particle flux in Figure 4 will have an increased ionospheric conductivity [Robinson et al., 1987; Ahn et al., 25] and can be associated with auroral electrojet currents. The range of the visually determined equatorward boundaries for the individual storms are summarized in Table 1. Interestingly, these boundaries are consistent with the observations in Figure 2 in that the threshold boundary location falls within the auroral equator-ward expansion zone. Furthermore, these findings are in agreement with earlier observations by [Thomson et al., 211] and more recently by [Pulkkinen et al., 212], which estimated the location to be within 5 62 of MLAT. [18] Table 2 lists the extreme storm events and displays in the last two columns the geomagnetic storm phase associated with the maximum induced geoelectric field for each storm. The analysis was performed using geomagnetic data recorded at European and North American high-latitude ground stations. Close examination reveals that the maximum induced geoelectric field was associated with the storm main phase (six events), the sudden impulse (one event), and the remaining five events were associated with the main phase and/or sudden impulse. Figure 5 is a plot of the same parameters as in Figure 2, but exclusively for the March storm event. The peak geoelectric field values for this storm are associated entirely with a sudden impulse driver as revealed in Table 2. However, the position of the latitude threshold boundary 124

5 (a) (b) Max E field Geomagnetic latitude [deg.] Geomagnetic latitude [deg.] (c) Max H field Geomagnetic latitude [deg.] Figure 2. Geomagnetic latitude distributions of the (top) maximum computed geoelectric field, (middle) the maximum time derivative of the horizontal magnetic field, and (bottom) the maximum amplitude of the horizontal magnetic field for all the geomagnetic storm events listed in Table 1. is still consistent with Figure 2 and is also consistent with the location of the auroral equator-ward edges in Table 1, despite the driving mechanism LT of the Maximum Geoelectric Field [19] One of the concerns from the power utility industry viewpoint has been the issue of the LT of occurrence of large GIC amplitudes. The general concern is that during periods of maximum loading on power systems, GIC may increase the probability of problems in operating the system [Molinski, 22]. Figure 3. Auroral poleward and equator-ward boundary locations determined from multiple DMSP satellite passes for all storm events. Each dot represents the location for a particular satellite pass. Figure 4. Maps of the maximum energy flux showing the extent of the auroral poleward and equator-ward boundaries for all storms. The flux is in units of kiloelectron-volt. 125

6 Table 2. The List of Selected Extreme Geomagnetic Storm Events Where the Last Two Columns Display the Geomagnetic Storm Phase Associated With the Maximum Induced Geoelectric Field a Year Month Day Storm phase Europe North America 1989 March 13 Main phase Main phase 1991 March Sudden impulse Sudden impulse 1991 November 8 Main phase Main phase 1992 May 1 Late main phase Main phase 2 April 6 7 Main phase/sudden impulse Main phase/sudden impulse 2 July 15 Main phase Main phase 21 March 31 Main phase/sudden impulse Late main phase/sudden impulse 21 November 6 Main phase/sudden impulse Main phase/sudden impulse 23 October 29 Main phase Main phase/sudden impulse 23 November 2 Main phase Main phase 24 November 7 8 Main phase/sudden impulse Main phase/sudden impulse 25 May 15 Main phase Main phase a The analysis was performed using geomagnetic data from European and North American high-latitude ground stations. [2] In an attempt to address this concern, we examined the occurrence of the maximum geoelectric field with respect to LT. The distribution of the LT dependence of the maximum induced geoelectric field at individual geomagnetic recording sites considered in this study is depicted in Figure 6. Each data point represents a value of the maximum electric field at a particular station, during a particular storm and specific time interval given in Table 1. The distribution shows a particular clustering between 45 and 8 of geomagnetic latitudes in the Northern hemisphere, but this is just a result of the station distribution with respect to the latitude Solar Wind and Magnetosphere Control of the EEJ Amplitude [21] The relationship between the electric field at the dip equator and the orientation of the interplanetary magnetic field (IMF) is a complex phenomena that has been a subject of intense studies for many decades [see e.g., Matsushita, 1977; Patel, 1978; Kikuchi et al., 2; Wei et al., 29; Huang, 212]. The IMF Bz component particularly has a significant effect where rapid reversals from south to north are sometimes correlated with reversals of the equatorial east-west electric current during both daytime and nighttime. However, it should be emphasized that the Bz component may sometimes reverse without any apparent effect at the dip equator. To examine the solar wind and magnetosphere influence on the equatorial electric current, we used geomagnetic measurements recorded at ground stations located over the dip equator in the American and African sectors. [22] Figure 7 shows a comparison of the interplanetary and ground measurements during the storm event on 7 November 24. The figure displays from top to bottom Max. E Max. db/dt [nt/s] Max. H Geomagnetic latitude [deg] Figure 5. Same as Figure 2 but exclusively for the March 1991 storm event. This storm was associated with a strong sudden impulse driver. Geomagnetic latitude [deg] Local time [Hour] Figure 6. LT distribution of the maximum geoelectric field at individual geomagnetic recording sites for all the 12 geomagnetic storm events. 126

7 the IMF Bz-component, the interplanetary electric field, the horizontal geomagnetic field (Δ H), the time derivative of the horizontal component (db/dt), and the computed induced geoelectric field components E x and E y. It should be noted that in this particular case, the OMNI interplanetary data were shift by 6 min to best match the ground observations, which accounts for the propagation error. The match Bz IEF [mv/m] ΔH db/dt [nt/s] E x E y Bz IEF [mv/m] ΔH db/dt [nt/s] E x E y November Time [UT hour] 7 November Time [UT hour] Figure 7. Comparison of the interplanetary and the ground geomagnetic and geoelectric field measurements. The ground measurements are for two equatorial stations at (left) Huancayo and (right) Addis Ababa during the storm event on 7 November 24. Displayed from top to bottom are the IMF Bz-component, the interplanetary electric field, the horizontal geomagnetic field (ΔH), the time derivative of the horizontal component (db/dt), and the induced geoelectric field components E x and E y. Note that: LT = UT + 5 and LT = UT + 3 at Huancayo and Addis Ababa, respectively. was defined by the instance of sharp IMF Bz reversal. The ground measurements in Figure 7 are for the equatorial stations at (left) Huancayo (Geo: 75.3 W, 12.1 S, dip latitude: 1.9 N) and (right) Addis Ababa (Geo: 38.8 E, 9. N, dip latitude:.5 N). It is obvious from the figure that the time of peak geoelectric field and db/dt (183 UT) corresponds to a period of sudden northward IMF Bz component (at the time of a shock arrival: other solar wind parameters not shown here), as identified by the red vertical dashed line. At the same instance, ΔH is subjected to a sudden strong enhancement that leads to an amplified positive bay at Huancayo (changing from about 9 to 18 nt) and a reduced negative bay at Addis Ababa (changing from about 98 to 5 nt). Both cases are associated with a decrease of the CEJ strength, with the former turning from westward (CEJ) to eastward (EEJ). The detailed mechanisms responsible for the observed differences between the two regions are out of the scope of this study. [23] In contrast to Figure 7, results in Figure 8 display a different response pattern where a strong ΔH negative bay perturbation corresponds to the sudden reversal of the Bz component in both cases. These negative bays are associated with strong CEJ. Kikuchi et al. [23] studied the negative magnetic bay associated with substorms and found that there was a marked enhancement of the bay at stations located at the daytime dip equator. They also found that the amplitude of the negative bay was amplified at the dip equator by a factor of 2.5 compared with the low-latitude negative bay. Furthermore, they were able to associate the latitudinal dependence of the negative bay strength with the disturbance polar (DP) ionospheric currents, in addition to the three-dimensional magnetospheric current system. [24] Shown in Figure 9 are the auroral upper and auroral lower geomagnetic indices corresponding to the three storm events presented in Figures 7 and 8. The red dashed line in each panel is correlated to the time of EEJ enhancement for each storm event, respectively. Each panel, respectively, reveals that the time of the EEJ enhancement is associated with increase of high-latitude geomagnetic activity. Kikuchi et al. [23] report of observational evidence that showed that a strongly enhanced negative bay at the daytime dip equator, which was associated with substorm activity, was related to the negative bay observed at afternoon high-latitude stations. 4. Discussion [25] The results presented in section 3.1 confirm that the latitude threshold boundary is actually correlated with the movements of the auroral oval under strong forcing from the solar wind energy input, and the associated auroral current system, which is the main driver of the largest perturbations of the ground geomagnetic field. We have major to extreme driving (in terms of Dst) and the boundary seems to sit approximately in the same place and definitely not going below about 5 latitude for all 127

8 Bz IEF [mv/m] ΔH db/dt [nt/s] E x E y Bz IEF [mv/m] ΔH db/dt [nt/s] E x E y March Time [UT] November Time [UT] Figure 8. Same as Figure 7, but for the stations at (left) Addis Ababa during the storm event on 31 March 21 and (right) Huancayo during the storm event on 2 November 23. the storms. As showed in Figure 5, the position of the latitude threshold boundary remains consistent with Figure 2 even for the maximum induced geoelectric fields associated with a sudden impulse event. Consequently, the location of the threshold geomagnetic latitude could probably be related to the maximum possible expansion of the auroral electrojet current system. [26] It is well established that the center of the auroral electrojet tends to expand equator-wards with increasing geomagnetic activity. Ahn et al. [25] utilized an extensive magnetometer database for the North American auroral region to determine quantitatively the equator-ward AU/AL AU/AL AU/AL 31 March November November Time [UT] Figure 9. Auroral upper and auroral lower indices for three selected geomagnetic storm events presented in Figures 7 and 8. The red dashed lines correspond to the time of EEJ enhancement for each storm event, respectively. expansion of the auroral electrojet during the year They noted that regardless of the geomagnetic activity levels, the westward electrojet expansion appeared to have a lower limit, which was established to be around 6 MLAT. The auroral electrojet equator-ward expansion limit was attributed to the saturation of the cross-polar cap potential (CPCP) in response to increasingly southward IMF [see e.g., Weimer et al.,199;russell et al., 21]. The specific cause of this nonlinear response of the CPCP is still largely unknown, but there are several hypotheses that are successful in predicting it [Wilder et al., 211].In addition, the study by Ahn et al. [25] also found that the most intense auroral electrojet currents were associated with less luminous regions of the auroral oval just slightly poleward of the bright auroral luminosity, with the current center appearing to be the region where both ionospheric conductivity and electric field became sufficiently high. They attributed this to be partly caused by high ionospheric conductivity, which can even cause a short-circuit of the electric field. Because the saturation level of the PCP is lowered as the auroral conductivity enhances, which is associated with bright aurora regions, this may explain why the electrojet current does not expand continuously but reaches a certain limiting latitude, even during periods that aurora can be seen at much lower latitudes [Ahn et al.,25]. [27] From the GIC perspective, the proximity to the auroral zone is not the only criterion for quantifying the power system s response and GIC risk. Instead, both the amplitudes of GIC and the ability of a system to live through disturbances depend to a large extent on the structure, resistances, and other technical details of the network and its equipment. For example, GIC-induced failures have been reported on power systems in midlatitude locations [e.g., Gaunt and Coetzee, 27], a region well away from auroral zone influence and previously considered to have low GIC risks. In addition, it is known that the ground conductivity can have a significant impact on the amplitude of 128

9 the GIC flowing on a system [see e.g., Ngwira et al., 29; Pulkkinen et al., 212]. All these factors demonstrate that a complete understanding of the coupling of geomagnetic disturbances with specific power transmission systems is required for detailed assessment of the risk GIC poses on the system. [28] The LT distribution in Figure 6 shows no distinct pattern to suggest any preference of maximum induced geoelectric field occurrence, i.e., any clustering with respect to the LT. This is interesting because it was generally expected that the maximum induced geoelectric field would tend to occur in the vicinity of the local midnight at highlatitude locations in association with substorm activity, which is known to drive large auroral electrojet currents [e.g., Pukkinen et al., 23;Viljanen et al., 26].Substormactivity in the night sector of the auroral zone is related to dynamical processes in the unstable magnetospheric tail region that takes place in response to the reconnection between the IMF and the Earth s magnetic field at the dayside magnetopause. These changes are associated with a thinning of the near-earth plasma sheet and an intensification of the cross-tail current [Farrugia et al., 1993].Thefields and plasmas in the magneto-tail near this current sheet go through a chain of dramatic changes signifying the expansion phase of the substorm, which drives large currents in the nighttime auroral zone. [29] Bolduc [22] reports that the March 1989 Hydro- Quebec blackout (2:45 a.m. LT) was preceded by an abrupt large geomagnetic field variation that caused the entire system to become unstable, leading to the eventual collapse. The large geomagnetic field variations during this period were attributed to the rapid westward expansion of the westward electrojet as the auroral oval expanded considerably over Canada [Boteler et al., 1998, and references therein]. However, in the present case, it appears that the maximum induced geoelectric distribution is dependent on the storm timing and phase but does not show a distinct LT preference. This could be associated with the magnetospheric ring current whose development and decay depends on the phase of the storm [Tsuji et al., 212]. During storm periods, the ring current system, which becomes strongly asymmetric during the main phase, largely influences the energy content of the inner magnetosphere and therefore is the primary factor that controls the electric and magnetic fields in the neargeospace environment [Ganushina et al., 212]. This means that power systems are susceptible to large induced geoelectric fields at any given time of the day. [3] Earlier, it was noted in Figures 2a 2c that all the three geophysical parameters exhibited an enhancement at the dip equator latitudes. This enhancement was initially reported by Pulkkinen et al. [212] in relation to GIC applications. The results presented in this article show that this phenomena can be associated with high-latitude electrodynamics and that the induced geoelectric fields at stations around the EEJ belt can be an order of magnitude larger than that at stations outside the belt. It is important to note that this phenomena is always present but is expected to be strongly amplified during certain severe/ extreme geomagnetic storm events. The phenomena can be explained by the unique orthogonality orientation of the electric field and magnetic field and the vertical plasma drift in the ionospheric E-region over the dip equator, which results in a large enhancement of the conductivity (i.e., Cowling conductivity) in the EEJ belt [Rastogi, 26; Muralikrishna and Kulkarni, 26]. This leads to a larger current over the dip equator with the same E-region electric field as at other low-latitude stations outside the EEJ belt [Baker and Martyn, 1953]. [31] Generally, EEJ perturbations are known to occur normally during the onset of substorms in the highlatitude region or at the beginning of a substorm recovery phase, but not both. According to Fejer et al. [1979], the first case is closely coupled to periods of southward Bz turning, whereas the second case is normally related to significant northward Bz recoveries after steady periods of southward Bz for more than 1 h. Furthermore, these authors concluded that the EEJ perturbations were not directly associated with the IMF but rather were a consequence of the magnetospheric convection changes and the high-latitude substorm phenomena which can be initiated by IMFdriven changes. Kelley et al. [1979] proposed the penetration of magnetospheric electric fields to the low-latitude ionosphere to explain the EEJ perturbations. The field lines in the inner magnetosphere map onto the low and midlatitude ionosphere. The penetration phenomena is associated with a change in high-latitude convection pattern. Since then, it has become clear that sudden decrease/increase of the convection electric field can lead to a breakdown of the shielding mechanism in the inner magnetosphere. A report by Sastri et al. [21] found that the amplitude of the positive and negative bay-type perturbations showed a clear enhancement at locations inside the EEJ belt in comparison with stations outside the electrojet belt. This feature is considered to be a consequence of penetration of electric fields associated with the rapid changes in the magnetospheric convection due to the sudden reversal of the IMF Bz, before shielding by the ring current becomes effective. Therefore, the convection electric field penetrated into low-latitude ionosphere, drives global DP2 currents, which at the dayside EEJ region are composed of the ionospheric Pedersen currents amplified by the Cowling effect [e.g., Rastogi, 26; Tsuji et al., 212, and references therein]. [32] In the present day understanding of electrodynamic perturbations at low and equatorial latitudes, it is generally accepted that the short-lived electric fields are associated with penetration of magnetospheric or highlatitude electric fields, which results from the undershielding and overshielding effects. The penetration of magnetospheric electric fields to lower latitudes occurs in response to large and rapid IMF-driven changes in the strength of magnetospheric convection, when there is a momentary lapse of equilibrium between the convection-related charge density and the charge density in the Alfvén layer at the corotation/convection separatrix 129

10 [Kikuchi et al., 23; Fejer et al., 27]. These electric fields are attributed to changes in the strength of the region 1 and region 2 Birkeland (field-aligned) current systems, which are required for shielding the inner magnetosphere, and the low and midlatitude ionosphere from the complete effect of the dawn-dusk magnetospheric convection electric field [Fejer et al., 27; Maruyama et al., 27]. However, when convection suddenly increases/decreases, mainly driven by the reversal of the IMF Bz-component, which in turn leads to an increase/decrease in the cross-pcp drop, the shielding mechanism fails to protect the mid- and lowlatitude ionosphere from the effects of the convectiondriven electric field [Maruyama et al., 27; Kikuchi et al., 2]. Therefore, the high-latitude electric fields can perturb the ionosphere nearly simultaneously ( promptpenetration ) from middle to equatorial latitudes. According to our knowledge, this is the first time it has been shown that penetration electric fields can cause large induced geoelectric fields on the ground at dip equatorial regions, which raises concern for power transmission systems located in this region. [33] We would like to note that the enhancement shown in Figure 7 occurred at the time of a sharp northward intensification of Bz component and a compression of the magnetosphere caused by a sudden increase of the solar wind dynamic pressure (not shown here). A corresponding check of the SYM-H index shows an increase of only about 45 nt at that time, but ΔH measurements at Huancayo and Addis Ababa show increase of about 26 nt and 93 nt, respectively. Therefore, it is possible that the enhancement may have been driven by sudden compression and a penetration electric field. However, the details of the contribution of the different driving mechanisms are out of the scope of this work. 5. Conclusions [34] In this article, we studied the global behavior of the ground horizontal geomagnetic and the induced geoelectric field variations by considering 12 severe/extreme geomagnetic storm events. Specifically, we explored the latitude threshold boundary, the maximum induced geoelectric field LT distribution, and the enhanced EEJ current system. It has been established that the latitude threshold boundary is located at about 5 55 of MLAT. Furthermore, we have shown that the boundary location can be attributed to the movement of the auroral oval. Also, it has been established that the maximum equatorward expansion of the threshold boundary location could be associated with the auroral electrojet current system and therefore is determined by the saturation of the polar cap potential drop. [35] In addition, this article showed that the enhancement of the induced ground geoelectric field is associated with the penetration of high-latitude electric fields. In other words, the Pedersen currents associated with the global DP2 current system alter the ground geomagnetic field variations at the dayside dip equator, which is enhanced by the cowling effect. The implication is that the induced ground geoelectric fields around the EEJ belt can be an order of magnitude larger than locations outside the belt. Therefore, power systems located around the electrojet belt can be exposed to higher levels of electric currents compared with midlatitudes, which increases their susceptibility to GIC risks. It is important to reemphasize here that a power system s response to geomagnetic disturbances is governed by several factors, and being exposed to higher levels of geoelectric field does not necessarily put it at higher risk than other systems. However, it should be noted that repeated exposures to GIC can lead to cumulative damage that may shorten the operational life of transformers [see, e.g., Albertson et al., 1992; Gaunt and Coetzee, 27]. 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