We investigate the formation and development of a large-scale current system in the dawn sector during intense geomagnetic storms. Four events are selected based on the historical ranking of the westward deflections of the morningside midlatitude magnetic field. For each event the polar distribution of equivalent currents indicates a significant intensification of the westward electrojet (WEJ), which initially takes place at postmidnight and then extends eastward covering the entire dawn sector. The longitudinal confinement of the enhanced WEJ suggests that it closes with downward and upward field-aligned currents (FACs) at its eastern and western ends, respectively, and therefore, the entire system may be envisioned as a wedge current. It is noted, however, that the primary closure of FACs is meridional, and those upward and downward FACs are considered to be unbalanced parts of the R1 and R2 currents. In one event dipolarization was observed in the dawnside plasma sheet near the magnetic conjugate point of the enhanced WEJ, and in another event a major auroral expansion was observed in the entire dawn sector. It is therefore suggested that the formation of the wedge current system and the subsequent expansion toward dayside is an ionospheric projection of the tail current reduction extending toward the dawnside flank. This wedge current system is similar to the substorm wedge current system except that it is centered at dawn. The recurrent formation of this dawnside current wedge in intense storms suggests that this is another distinct constituent of the storm time current system.

Key Points

During geomagnetic storms a wedge current system forms recurrently in the dawn sector as the westward auroral electrojet (WEJ) intensifies
Whereas the associated upward FAC stays at postmidnight, the intensified WEJ extends eastward presumably along with the downward FAC
The dawnside wedge current system is accompanied by a major auroral intensification and the reduction of the cross-tail current

1 Introduction

The present study is motivated by a geomagnetically induced current (GIC) event, which took place during a storm of 14–15 December 2006. Watari et al. (2009) examined GIC measurements made near Memambetsu (MMB; GLat: 43.9; GLon: 144.2; MLat (Altitude Adjusted Corrected Geomagnetic (AACGM) Coordinate system): 35.7°; local time, LT = UT + 9.6), Japan, in collaboration with Hokkaido Electric Power Company for December 2005 to December 2007. They identified this event as the largest GIC event during this 2-year period. Figure 1 shows the GIC intensity along with the northward (B_N), eastward (B_E), and vertically downward (B_Z) magnetic components measured at MMB in this event. The GIC intensity was well correlated with B_E but not very clearly with B_N. The correlation with B_E is expected to some extent since the power line was oriented in the northeast-to-southwest direction, and similar correlations were found for other events (Watari, 2015; Watari et al., 2009). The fact that the GIC intensity is correlated with the magnetic perturbation, rather than with its time derivative, may be attributed to the locality of ground conductivity (Pulkkinen et al., 2010); in other areas of the world, GICs may be better characterized by the rate of the change of the local geomagnetic field (e.g., Rodger et al., 2017, Thomson et al., 2005)

Details are in the caption following the image — **Figure 1**
Open in figure viewer PowerPoint

Geomagnetically induced current (GIC; brown-green) and the northward B_N (blue), eastward B_E (red), and vertically downward B_Z (green) magnetic components measured at Memambetsu on 14–15 December 2006.

The source current of this midlatitude B_E disturbance is not obvious. Its timescale was of the order of 1 hr, which is comparable to that of substorms. However, this event was observed in the late morning sector, LT = 9.6, where the effect of the nightside substorm current system, if at all, is generally considered insignificant. The magnetopause current is another source of midlatitude geomagnetic disturbances, especially on the day side. However, its effect should be more pronounced in the north-south component rather than in the east-west component. The same remark also applies to the ring current. In general, the meridional component of ionospheric current and the field-aligned current (FAC) are the two main causes of geomagnetic deflections. In either case this GIC event suggests that a certain magnetosphere-ionosphere (M-I) coupling process in the morning sector possibly prevails various processes in other LTs, including nightside substorms, in the magnitude of midlatitude magnetic deflection.

As will be shown soon, the extreme magnetic deflection observed at MMB is a recurrent feature of intense storms, and therefore, the responsible M-I coupling process is apparently a fundamental constituent of the storm time current system. The goal of this study is to identify the basic structure of this previously undocumented current system and present an initial description of its formation and development. In section 2 we statistically examine the characteristics of azimuthal magnetic disturbances at MMB to establish the prevalence of large westward magnetic deflections on the morning side during storms. In section 3 we examine four storm events, including the December 2006 storm, to characterize this current system in the context of global storm electrodynamics. We discuss the results in section 4, and section 5 summarizes this study.

2 Statistical Characteristics

In this section we statistically examine the occurrence of large magnetic deflections using 1 min averages of magnetic field measurements made at MMB for 30 years from 1987 to 2016; here we use the data provided by the MMB observatory originally in geographic coordinates. The amplitudes of diurnal, annual, and secular variations of the horizontal magnetic field are of the order of 100 nT, which are comparable to those of largest hourly disturbances. To remove these trends, we took the median of each magnetic component for each hour of UT (e.g., from 0000 to 0059 UT for UT = 0) of each day and adopted as the reference (for that component of that particular 1-hr UT range) the medians of the 31-day interval centered at that day. We define ΔB_D as the eastward horizontal deviation (in nT) from the reference magnetic field.

Figure 2 shows the occurrence distribution of ΔB_D as a function of LT (= UT + 9.6). The bright green line shows the median, which is very close to zero at each LT bin. The pairs of orange, red orange, and red lines show the central 90%, 99%, and 99.9% ranges of data points, respectively. The purple dashed lines show the minimum and maximum values. For each 1-hr bin there are about 11,000 data points, and accordingly, there are 5 points outside of each red line. Therefore, the corresponding range represents events that occur, on average, once or twice during one solar cycle. The extremes in ΔB_D are highly asymmetric with respect to sign. Whereas the top 0.05, 0.5, and 5 percentiles vary with LT within rather narrow ranges, the corresponding bottom percentiles, especially the bottom 0.05 percentile, are the most negative in the late morning. That is, largest magnetic deflections tend to be directed westward, and they tend to take place in the late morning sector. The December 2006 event (Figure 1), which took place at magnetic local time (MLT) ~ 9, is actually representative of a consistent pattern in extreme disturbances.

We selected the minimum hourly value of ΔB_D for 7 < LT < 11 of each day, and Table 1 lists the largest (in magnitude) 10 events of ΔB_D for 1987–2016. All of these events took place during magnetospheric storms, and some of them were particularly intense storms. The March 1989 storm, which is widely known for the collapse of Hydro Quebec system, ranks first, and the Halloween storm, which took place in October 2003, ranks second. The December 2006 storm (Figure 1) ranks fourth. It is therefore suggested that the responsible current system corresponds to a recurrent storm time process, which to our best knowledge, has not been identified previously.

Table 1. Ten Largest Hourly ΔB_D Measured at Memambetsu for 1987 to 2016

Rank	Year	Month	Day	Hour	ΔB_D	Dst	IMF B_Y
01:	1989	3	14	00	−197.0	−583	—
02:	2003	10	29	23	−173.2	−350	—
03:	1991	11	9	00	−147.2	−291	—
04:	2006	12	15	00	−136.3	−146	2.9
05:	2003	11	20	22	−131.2	−405	−19.9
06:	2015	3	17	23	−126.8	−214	−13.9
07:	2005	1	7	23	−123.6	−71	−0.1
08:	1989	11	17	22	−120.4	−266	—
09:	1992	5	10	22	−113.2	−196	—
10:	2001	10	22	00	−112.5	−177	−17.9

Finally, we note that if those westward ΔB_D disturbances are caused by an ionospheric current flowing above MMB, the current has to be directed poleward. However, in the morning sector, the midlatitude electric field is usually directed equatorward, especially when the external driving is intense, and therefore, the required ionospheric current has to be a dynamo current flowing in the direction opposite to the electric field. It is highly unlikely that such a dynamo process takes place repeatedly in a particular LT sector during geomagnetic storms. Instead, it is more likely that the observed westward ΔB_D disturbances are remote effects of FACs. For a 100-nT midlatitude magnetic deflection, the total intensity of the net downward current is estimated at 1–2 MA based on simple geometrical assumptions (e.g., vertical currents and flat ionosphere), which comprises less than 10% of the total FAC during intense storms, ~20 MA (e.g., Anderson & Korth, 2007). Other magnetospheric currents, especially the ring current, probably also contribute to ΔB_D since at MMB the local magnetic field is not exactly parallel to the terrestrial dipole axis; we will address this issue later in the paper.

3 Event Studies

In this section we examine four of the events listed in Table 1, the December 2006, March 2015, October 2001, and November 2003 events. We first examine the polar distribution of equivalent currents during the December 2006 storm, which motivated the present study. For each of the other three events, an additional and equally critical observation is available, and we consider the results of these events in combination to understand the storm time M-I coupling process that the observations reflect. For examining geomagnetic disturbances, we use the SuperMAG data set (Gjerloev, 2012), which consists of magnetometer data provided by numerous networks of ground stations. For each station, data are given, after subtracting the baseline, in the NEZ local magnetic coordinate system. For any practical purpose the N and E components can be regarded as the H (northward) and D (eastward) components, respectively, and Z is directed vertically down; see Gjerloev (2012) for the details of the data processing.

3.1 December 2006 Event: Event 1

The December 2006 event ranks fourth according to hourly ΔB_D. Figure 3e shows again the three magnetic components at MMB. The three vertical dashed lines mark the approximate times of the start, peak, and the end of the westward B_E excursion. Figure 3a shows the time-shifted interplanetary magnetic field (IMF) data from the OMNI data product. To roughly account for the propagation from the subsolar point of the bow shock to the dayside cusp region in the ionosphere, the data are shifted additional 5 min. After large variations with multiple sign changes, IMF B_Z became less than −10 nT at 2310 UT and remained around −15 nT for the rest of the interval. Responding to this intense driving, Sym-H started to decrease sharply (Figure 3b), and both Asy-H and Asy-D increased significantly starting around 2330 UT, which suggests the formation of a wedge-like FAC system (Iyemori et al., 1989).

Both the eastward and westward electrojets, hereafter denoted by EEJ and WEJ, respectively, intensified after 2310 UT as shown by the increases of SMU and SML in magnitude (Figure 3c). This suggests the development of the two-cell global convection. SMU and SML are derived basically in the same way as the official AU and AL indices but from the SuperMAG database of ground magnetometer data from stations distributed at 40–80° in magnetic latitude, and each index has supplementary indices, SM#S and SM#D (# is U or L), which are derived from ground stations in the sunlit (solar zenith angle, SZA, < 90°) and dark hemispheres (SZA > 90°), respectively (Gjerloev et al., 2010). Whereas SMUD and SMUS remained comparable, SMLD dominated SMLS in magnitude indicating that the WEJ was more intense in the dark hemisphere than in the sunlit hemisphere continuously. Figure 3d shows the three magnetic components observed at Norilsk (NOK: 69.40°N, 88.10°E; LT = UT + 5.9) in the auroral zone. B_N decreased to below −2,000 nT, indicating a significant intensification of the WEJ in the dawn sector. The interval of the WEJ enhancement, 2230 UT on 14 December 2006 to 0100 UT on 15 December 2006, matched that of the B_E disturbance at MMB.

We also found that B_E variations similar to those observed at MMB were observed at Beijing Ming Tombs (BMT; 40.30°N, 116.20°E; MLat: 34.7°; LT = UT + 7.7) and Lanzhou (LZH; 36.09°N, 103.85°E; MLat: 30.6; LT = UT + 6.9) (not shown), although those stations were located near the terminator, and therefore, the local ionospheric conductance was significantly different from that at MMB; the SZA was 74.2° at MMB, 86.1° at BMT, and 92.2° at LZH at 0000 UT. This result also supports the interpretation that the observed midlatitude B_E disturbances were a remote effect of a downward FAC at high latitudes, rather than an effect of a local ionospheric current.

Figure 4 shows the polar map of equivalent currents at selected times, for which the horizontal ground magnetic disturbance at each station is rotated by 90° clockwise, and its magnitude is given in the unit of nanotesla. In general, various magnetospheric currents contribute to geomagnetic disturbances, and accordingly, the equivalent currents, especially those outside of the auroral zone, very often do not represent local ionospheric currents. During geomagnetic storms, for example, the enhanced ring current reduces the horizontal magnetic component, which is represented by a global pattern of equivalent currents circulating clockwise if viewed from above the northern pole. The FAC also makes an important contribution away from its ionospheric footprint. If the magnetic field is vertical and the ionospheric conductance is uniform, it does not cause any magnetic disturbance on the ground (Fukushima, 1969). If, however, upward and downward FACs are closed through a highly conductive channel (e.g., auroral oval), the associated equivalent currents are represented by a dipole pattern with the divergence and convergence at the footprints of the upward and downward FACs, respectively.

By 2325 UT the equivalent currents intensified globally; compare Figures 4a and 4b. The intensification of the WEJ at postmidnight was apparently confined in longitude suggesting the development of a substorm. In the Scandinavian sector, a sharp reduction of the horizontal magnetic component started around 2318 UT (not shown). At lower latitudes in the same sector, equivalent currents were directed equatorward (i.e., eastward B_E), which can be attributed to an upward FAC in the auroral zone. The pattern lasted for the rest of the event, suggesting that this upward FAC was sustained and stayed in the same sector. These observations suggest the formation of a wedge-like current system, which is also consistent with the enhancements of Asy-H and Asy-D around the time (Figure 3b). The WEJ did not enhance yet at the two auroral zone stations to the east, one of which is NOK (as marked in Figure 4a), and therefore, the current system was confined in the postmidnight sector.

Thereafter, the intense WEJ extended both poleward and eastward by 2340 UT (Figure 4c). It also intensified in the dawn sector and extended further eastward (Figures 4d and 4e). At Tixie (TIK; 71.6°N, 129.0°E; MLat: 66.2°; LT = UT + 8.6; see Figure 4a), the auroral zone station at MLT ~ 8, the intensification of the WEJ was obvious in Figure 4e; the primary disturbance was actually an increase in the Z component (not shown), suggesting that the WEJ was located mostly equatorward of this station. In contrast, at Chokurdakh (CHD; 70.6°N, 147.9°E; MLat: 65.1°; LT = UT + 9.9), the next auroral station to the east, the magnetic disturbance was less clear in any component (not shown). We therefore conclude that a downward FAC fed the enhanced WEJ between TIK and CHD. In the next hour the WEJ weakened around the terminator (Figures 4f and 4g), although it (re)intensified in the postmidnight sector. By 0130 UT (Figure 4h) the equivalent currents further weakened globally.

In summary, the evolution of global equivalent currents suggests that a wedge current system, which consisted of downward and upward FACs at its eastern and western ends closing with a WEJ, formed in the postmidnight sector and then intensified. Whereas the upward FAC stayed at postmidnight, the sector of the intense WEJ extended eastward presumably along with the downward FAC at its eastern edge. The unusually large westward magnetic disturbances observed at morningside midlatitudes are interpreted as a remote signature of this downward FAC.

3.2 March 2015 Event, Event 2

The storm event of March 2015, Event 2, is widely known as the 2015 St. Patrick's Day storm. This event ranks sixth according to hourly ΔB_D at MMB (Table 1), and it is the only event in the top 10 list later than January 2010, when data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE; cf. Anderson et al., 2014) became available. Figure 5 shows the IMF data, geomagnetic indices, and ground magnetic disturbances for the 4-hr interval centered at 0000 UT on 17 March 2015. To examine the dawnside WEJ, we use data from Amderma (AMD; 69.5°N, 61.4°E; MLat: 65.4°; LT = UT + 4.1), which was located in the predawn sector.

At MMB B_E was already ~−80 nT but became even more negative during 2310 to 0016 UT (Figure 5e). This interval is marked by the two vertical dashed lines in Figure 5. IMF B_Z was around −18 nT before this interval, and then it increased (became less negative) by ~10 nT within 40 min (Figure 5a). During the same interval IMF B_Y became more negative, and IMF B_X changed from negative to positive. Sym-H reached −234 nT, the minimum of this storm period, at 2247 UT, and then it started a gradual recovery (Figure 5b). Asy-H increased by ~60 nT within 10 min after B_E at MMB started to decrease further, suggesting the formation of a wedge-type current system. Both SMUS and SMLS were elevated when B_E was further negative at MMB, suggesting the enhancement of dayside convection (Figure 5c). In the dark hemisphere, in contrast, the WEJ dominated the EEJ: SMLD reached −1,500 nT. The intensification of the WEJ was observed at AMD as indicted by the negative excursion of B_N (Figure 5d), which was roughly confined in time during the B_E reduction at MMB. Large fluctuations of B_E and B_Z at AMD might reflect a wandering of the WEJ.

Figures 6a–6d show the equivalent currents for this event. The equivalent currents were already intense at 2305 UT (Figure 6a). In the polar region the dawn cell with clockwise rotation was dominant, but we can also find a counter clockwise rotation confined in the late afternoon sector. A pattern with a dominant dawn cell is expected for negative IMF B_Y as observed (Figure 5a). The equivalent currents at midlatitudes rotated clockwise reflecting an intense ring current near the peak of the storm intensity as measured by Sym-H (Figure 5b), which might also affect high-latitude equivalent currents to some extent.

The equivalent currents significantly intensified on both dawnside and duskside by 2315 UT (Figure 6b) as expected from the SMU and SML enhancements (Figure 5c). The intensification of the WEJ in the midnight-to-dawn sector is especially noticeable. By contrast, in the late morning-noon sector, the WEJ had not intensified significantly, suggesting that the enhanced WEJ closed with a downward FAC somewhere in the dawn-to-morning sector. In the postmidnight sector, the midlatitude equivalent currents had large equatorward components, which are indicative of an intense upward FAC in the auroral zone. Along with the downward FAC in the morning sector, this postmidnight upward FAC presumably completed a wedge current system, which is consistent with the concurrent enhancement of Asy-H (Figure 5b). The overall current system enhanced further by 2345 UT (Figure 6c), which might be attributed to the enhancement of IMF B_Y (Figure 5a). Thereafter, the equivalent currents weakened significantly (Figure 6d) probably responding to the less southward IMF.

Figures 6e–6h show original magnetic disturbances from AMPERE for the 10-min intervals centered at the epoch times of the corresponding polar maps of equivalent currents (Figures 6a–6d). Measurements made by different satellites are shown in different colors except that the data from the satellites that span the locations in gray were not of sufficient quality for analysis, and data are inserted from the most recent prior 10-min interval from a satellite yielding valid data. On the dawnside, the perturbation magnetic fields were directed approximately westward and were confined mostly in the auroral zone. Those magnetic perturbations can be attributed to a pair of downward and upward FACs on the poleward and equatorward sides, respectively: the former is the morningside R1 current and the latter is the R2 current. The associated electric field is directed equatorward, which drives a westward Hall current, that is, the WEJ. Thus, the AMPERE magnetic perturbations in the auroral zone reflect the distribution of the auroral electrojets.

The AMPERE perturbations are consistent with the WEJ extent inferred from the equivalent currents. The polar map at 2300–2310 UT suggests that the WEJ extended from prenoon to premidnight, whereas the EEJ was confined in the dusk sector (Figure 6e). In the next 10 min, the magnetic disturbances enhanced in the dawn sector (Figure 6f), suggesting the intensification of the WEJ. In the following 30 min, the WEJ noticeably expanded both poleward and equatorward (Figure 6g). In both Figures 6f and 6g, the westward magnetic disturbance appeared to peak at MLT = 6–9, and it decreased in both directions but more sharply toward noon.

In Figures 6f and 6g, far equatorward of the auroral zone along the orbital planes at MLT ~ 08 and 10, magnetic disturbances were as large as a few hundred nanoteslas and were directed westward. They are considered to be a remote effect of the net downward FAC in the auroral zone. Along the two orbits in the postmidnight sector, in contrast, magnetic disturbances were directed eastward at midlatitudes as expected for the net upward FAC. The fact that we observed magnetic deflections in the same direction below and above the ionosphere again precludes the possibility that the midlatitude disturbances were caused by ionospheric currents.

The panels in the third row show the polar distributions of FAC density along with fitted magnetic disturbances derived from observed magnetic disturbances for the corresponding intervals in the second row. We also calculated the total upward (positive, red) and downward (negative, blue) currents as well as their balance (i.e., net current) for each 2-hr MLT bin, and the results are shown in the bottom row. For those calculations we set the lower cutoff current density threshold, σ_J, at 0.15 μA/m². We found that the threshold does not significantly affect the calculation of the total currents, especially the net currents; for example, for Figure 6o near the peak of the WEJ enhancement, the maximum downward net current, which took place at MLT = 7–9, is 1.44 MA for σ_J = 0.15 μA/m², and it is 1.43 MA, only 0.01 MA less, if we change σ_J to 0.20 μA/m². Similarly, the upward net current at MLT = 1–3 are 0.681 and 0.677 MA for σ_J = 0.15 and 0.20 μA/m², respectively.

Figure 6i shows that a pair of downward R1 and upward R2 currents was the primary structure of FACs in both morning and evening sectors as expected from triangle-shaped magnetic disturbances along each orbital plane. For 2310–2320 UT the FACs were significantly more intense both in the morning and evening sectors (Figures 6j and 6n). The net current was directed upward and downward in the postmidnight and dawn-to-late morning sectors, respectively, as expected from the closure of the enhanced WEJ in the dawn sector. Both net upward and downward currents were noticeably more intense for 2340–2350 UT (Figures 6k and 6o). Whereas the net upward FAC was distributed widely in MLT from evening to postmidnight, the net downward FAC was confined in the morning sector, and it exceeded 1 MA at MLT = 7–9. The net downward FAC for MLT = 5–11 was about 2.1 MA in total, which is consistent with an estimate from the midlatitude westward magnetic deflection (Section 2). The net currents were reduced along with the total upward and downward currents at 0000–0010 UT (Figures 6l and 6p) as expected from the weakening of the dawnside WEJ.

3.3 October 2001 Event, Event 3

The third event we examine, Event 3, took place in October 2001, which ranks tenth in terms of ΔB_D at MMB (Table 1) but is of key importance because of the availability of global auroral imagery for this event. Figure 7 shows the IMF data, geomagnetic indices, and ground magnetometer data from TIK and MMB for the 4-hr interval centered at 0000 UT on 22 October 2001. The minimum of B_E at MMB took place at 0008 UT as marked by the dashed line (Figure 7e); the other two dashed lines mark 2319 and 0107 UT, as guides of the start and end of the enhancement of the westward magnetic deflection at MMB.

Sym-H was around −150 nT for the entire 4-hr interval (Figure 7b). This event took place in the middle of the gradual recovery of an intense storm, for which the minimum of Sym-H, −219 nT, took place more than 1 day before, at 2118 UT on 21 October 2001. The start of the enhanced westward magnetic deflection at MMB was apparently preceded by the reduction of a large southward IMF B_Z and a negative turning of IMF B_Y (Figure 7a). The magnitudes of SMUS and SMLS started to increase around 2300 UT suggesting the enhancement of dayside convection. SMLD enhanced simultaneously, but more sharply, reaching below −1,100 nT at 2325 UT (Figure 7c). Asy-H started to increase around the start of the negative B_E excursion at MMB (Figure 7b), suggesting the formation of a wedge current. At the same time B_N and B_Z started to decrease and increase, respectively, at TIK (Figure 7d), which can be attributed to the intensification of the dawnside WEJ south of the station. The sequence of B_E at MMB apparently followed the intensification and reduction of the WEJ as we found for Events 1 and 2.

Figure 8 shows the polar distributions of equivalent currents at four epoch times along with global auroral images taken by the IMAGE/UVI imager (top) and by the Polar/VIS imager (bottom). The two image data sets show the same sequence of auroral development, but they complement each other by covering areas that were either missing or somewhat distorted by projection in one of the data sets. By 2315 UT, when B_E at MMB and B_N at TIK started to decrease, an auroral bulge was already formed in the late evening sector (Figures 8a and 8e), which developed from a bright spot that formed at 2302 UT (not shown). Although this initial auroral enhancement appeared to be a substorm, the bulge expanded more eastward than westward by 2330 UT (Figures 8b and 8f), and the peaks of auroral emission and WEJ moved to slightly after midnight. Interestingly, the auroral emission also enhanced in the entire morning sector. In the nightside polar cap, equivalent currents rotated clockwise, which was likely a manifestation of the substorm wedge current system with net upward and downward FACs before and after midnight, respectively, near the poleward boundary of the auroral oval (Gjerloev & Hoffman, 2014).

The area of intense aurora expanded further poleward and more noticeably dawnward. The center of auroral emission moved to near the dawn flank by 0005 UT (Figures 8c and 8g), and the WEJ enhanced at TIK at MLT ~ 9; see also Figure 7d. In the midday sector, the enhancement of the WEJ was not obvious, which suggests that the enhanced WEJ was fed by a downward FAC in the prenoon sector. In the postmidnight subauroral zone, the equivalent currents were directed equatorward, which can be attributed to a net upward FAC in the auroral zone. It is suggested that this net upward FAC, along with the net downward FAC at prenoon, formed a wedge current closing with the enhanced WEJ. A comparison of the auroral images at 0005 UT (Figures 8c and 8g) with the earlier ones indicates that the polar cap noticeably shrank in size as the WEJ enhanced, suggesting that the enhancement of the WEJ reflects the unloading of magnetic energy stored in the magnetotail lobes. The subsequent decay of this current system was accompanied by the decay of auroral activity in the entire dawn sector (Figures 8d and 8h).

3.4 November 2003 Event, Event 4

The last event we examine, Event 4, took place on 20 November 2003, which ranks fifth according to hourly ΔB_D at MMB (Table 1). Figure 9 summarizes this event showing various measurements made for 2100–2400 UT on that day. This event took place during the early recovery phase of a severe storm (Figure 9b), when IMF B_Z, as well as IMF B_Y, was strongly negative (Figure 9a). The minimum of Sym-H was −490 nT, which took place at 1815 UT on the same day. B_E at MMB was negative throughout this 3-hr interval, but it made an additional reduction (Figure 9e). The three dashed lines mark the approximate start and end of this B_E reduction along with its minimum. The WEJ intensified during the same interval as indicated by the enhancement of SML (Figure 9c). The sequence of SMLD was well followed by B_N at NOK (Figure 9d), which was located at LT ~ 4, except that the start of its reduction somewhat delayed from that of SMLD.

Figure 10 shows the polar maps of equivalent currents at every 10 min after 2220 UT. The global clockwise rotation of equivalent currents, which is indicative of an intense ring current, was prominent throughout this event; Sym-H was less than −300 nT (Figure 9b). Nevertheless, we can clearly see the formation and development of the dawnside wedge current system. By 2230 UT the WEJ started to intensify in the postmidnight sector; compare Figures 10a and 10b. In the Scandinavian sector B_N started to decrease around 2215 UT (not shown). The enhancement of the WEJ already reached NOK (LT ~ 4.5) at 2230 UT; see also Figure 9d. The WEJ further intensified in the next 10 min and extended farther eastward reaching prenoon (Figure 10c). By 2250 UT it already started to weaken in the dawn sector. Thus, the overall sequence of the spatial development of equivalent currents was very similar to what we found for the previous three events.

The orange circle in Figure 10c marks the magnetic footprint of the Geotail satellite, which was located at (−8.0, −8.9, and 1.7) R_E in Geocentric Solar Magnetospheric (GSM) coordinates in the dawn sector in the near-Earth magnetosphere. For the field line tracing, we used the T96 model magnetic field (Tsyganenko, 1996). The footprint should be considered only as a guide. Figure 11 shows three magnetic components observed by Geotail for 2130–2300 UT. The satellite was initially in the plasma sheet boundary layer as indicated by the magnetic orientation almost parallel to the X-Y plane. B_Z tended to decrease until 2215 UT suggesting an enhancement of the tail flaring angle, which was likely caused by an intensification of the tail current. Then in the next 30 min, B_Z increased by ~50 nT. Simultaneously, B_X decreased and became comparable to B_Z in magnitude. That is, the local magnetic field changed from a stretched to a more dipolar configuration. Note that the interval of this dipolarization roughly matched that of the enhancement of the WEJ in the dawn sector (Figure 9c). It is therefore suggested that the eastward expansion of the WEJ enhancement was an ionospheric manifestation of the dawnward expansion of the dipolarization region in the magnetosphere. In this sense the dawnside wedge current system may be considered as a substorm wedge current system but significantly skewed toward dawn and extending wide in longitude.

4 Discussion

4.1 Dawnside Wedge Current System

For each of the four major storm events examined in section 3, the observations indicate that a large-scale wedge current system formed in the dawn sector. We infer that this current system consists of downward and upward FACs in the morning and postmidnight sectors, respectively, closing with the enhanced WEJ. From the polar distributions of equivalent currents, as well as magnetic disturbances observed by AMPERE for Event 2, we inferred that these downward and upward FACs result from the unbalance between the R1 and R2 currents.

Crooker and Siscoe (1981) proposed, based on the current continuity at the poleward boundary of the auroral oval, that there exist downward and upward FACs around noon and midnight, respectively. More specifically, the Hall current corresponding to the global two-cell convection flows from the polar cap to the auroral oval on the day side, where it closes with a downward FAC, and vice versa on the nightside. Although this idea was originally proposed as a possible explanation of the dawn-dusk asymmetry of the midlatitude H reduction during geomagnetic storms, those net FACs are expected irrespective of storm activity, since the two required factors, two-cell convection and conductance difference between the polar cap and auroral oval, are rather general. Nakano et al. (2002) reported, based on a statistical analysis of midlatitude magnetic deflections, that even at nonstorm time there exist net downward and upward FACs on the dayside and nightside, respectively, and their intensities increase as the AE index increases. They also found that those net FACs are more intense in winter than in summer suggesting that the ionospheric conductance plays a critical role in the current closure as expected from Crooker and Siscoe's model.

Since the events that we examined took place during intense geomagnetic storms, it is plausible that those net FACs also enhanced and contributed to the observed geomagnetic disturbances. However, this conductance related process must be secondary, even if it plays a role, in the formation of the dawnside wedge current system. This is so because the wedge current system initially formed at postmidnight and then expanded toward dayside. It should be more reasonable to consider that the formation and development of this current system reflects a magnetotail process. In Event 4, Geotail observed the local magnetic field to dipolarize as the intensification of the WEJ extended to the sector of the Geotail footprint, which supports our interpretation.

One might question why the corresponding downward FAC is located in the late morning sector if the enhanced WEJ is a manifestation of the tail current reduction. However, this is actually consistent with field line mapping since the magnetotail is magnetically connected to a broad LT range in the ionosphere that extends to the dayside. For a closed magnetosphere, the dayside cusp is a singular point, and the field lines that originate from there cover the entire magnetopause. Therefore, as an equatorial point moves toward the magnetopause at any X distance, whether X is positive (i.e., dayside) or negative (i.e., nightside), its ionospheric footprint moves toward the dayside cusp. This tendency should also hold for a more realistic magnetospheric configuration (Kaufmann et al., 1993; Stasiewicz, 1991). In other words, the eastward extension of the enhanced WEJ to the late morning sector suggests that the region of the tail current reduction extends toward the dawnside flank of the magnetosphere.

For Event 3 the global auroral images show that the polar cap shrank as the dawnside WEJ enhanced. Also for the other three events, the poleward boundary of the dawnside WEJ, which we infer from magnetic disturbances either on the ground or above the ionosphere, moved poleward. Therefore, the shrinkage of the polar cap size seems to be a common feature for the development of this dawnside WEJ, which suggests that reconnection in the magnetotail plays a certain role in the formation of the dawnside wedge current system.

This idea does not exclude the possibility that the reduction of the tail current, or the ring current, also takes place closer to Earth and extends eastward. In fact, in a similar event that took place during the 18 January 2005 storm, the Cluster satellites observed dipolarizations inside geosynchronous orbit; the equivalent currents are not shown, but Cluster magnetometer and IMAGE/UVI data are shown by Ohtani et al. (2010). However, such a near-Earth process alone would not cause poleward auroral expansion as observed in Event 3, in which the intense auroral region reached 75° in MLat at dawn.

Interestingly, Gjerloev and Hoffman (2014) suggested based on an empirical model of nightside equivalent currents that the substorm current system consists of two current wedges with the R1 polarity, one centered at premidnight and another at postmidnight, which overlap in the midnight sector. The premidnight one is considered to be the conventional substorm current wedge, and it has a WEJ at higher latitudes than the postmidnight one. For each event we examined, the equivalent currents in the subauroral zone suggested that the net upward FAC stayed at postmidnight. We also note that in Event 3, the wedge current system developed along with a large-scale auroral form on the dawn side although the auroral bulge in the premidnight sector had already started to fade (Figure 8). Therefore, it appears that in those events, the postmidnight wedge of Gjerloev and Hoffman's two wedge system intensified independently from the premidnight one and expanded dawnward in longitude.

This dawnward expansion still remains to be understood physically and phenomenologically. Whereas in each event we examined, both B_E at MMB and B_N at the dawnside auroral zone station peaked in magnitude 20–60 min after the WEJ enhancement around midnight, they actually started to change with a much shorter time delay. In Event 4 the negative B_E excursion at MMB and B_Z increase at NOK (as a manifestation of the WEJ enhancement south of the station) started immediately following the reduction of SMLD. In Event 3 the auroral intensification propagated quickly first along the auroral oval covering more than 6 hr in LT within 10 min, and then the poleward expansion extended later from midnight (Figure 8). The complexity of the dawnward expansion may be attributed to different processes in different regions such as the change of the ionosphere potential distribution, the dawnward drift of injected electrons in the inner magnetosphere and the subsequent enhancement of precipitation and ionospheric conductance, and the spatial expansion of the responsible process in the plasma sheet. Those processes are probably coupled with each other, and untangling them is a key to understanding the formation of the dawnside wedge current system.

4.2 Statistical Characteristics and Global Current Configuration

One outstanding issue concerns with the eastward midlatitude magnetic deflection that is expected for the nightside upward FAC. According to the occurrence probability of ΔB_D at MMB (Figure 2), it is far less prominent than the westward deflection in the morning sector. We can find this contrast not only for the minimum and maximum of ΔB_D but also for the 99.9% and 99% ranges, which should be statistically more significant. However, for the events we examined, the midlatitude equivalent currents in the postmidnight sector are directed equatorward corresponding to large eastward magnetic deflections. For example, in Event 4, B_E became +241 nT at 2245 UT at LAquila (AQU; 42.4°N, 13.3°E; MLat: 36.2°; LT = UT + 0.9); the B_E peak at MMB was −171 nT in that event (Figure 9e). Nakano and Iyemori (2005) also reported that in the postmidnight sector the magnetic field is often deflected eastward in association with storm time substorms.

This apparent discrepancy can be attributed mostly to the locality of the geomagnetic field. At MMB the local magnetic field is directed ~16° west from the geomagnetic north. Therefore, if the ring current enhances causing a geomagnetic disturbance pointing to the geomagnetic south, it has a finite component perpendicular to the local magnetic field. This magnetic deflection is westward irrespective of the LT of MMB, and it should increase in magnitude with increasing ring current intensity. For an intense storm with Dst = −200 nT, its magnitude is expected to be several tens of nanoteslas (see Appendix A). If the intensity of the dawnside wedge current system is correlated with the ring current intensity, the occurrence probability of westward ΔB_D extends in a wider range than the range expected solely for the downward FAC, whereas that of the eastward deflection is confined in a narrower range. This is consistent with the observed tendency of ΔB_D (Figure 2). We also note that in Event 2 (Figure 5), for which Sym-H was ~−200 nT, B_E at MMB apparently had a negative offset of −70 to −80 nT. This offset may be explained in terms of this ring current effect. However, for the other events, it is rather difficult to identify the contribution of the ring current to the B_E variations.

Another important issue concerns with the lack of the eastward midlatitude magnetic deflection in the afternoon sector (section 2); this feature cannot be attributed to the aforementioned orientation of the local magnetic field relative to the dipole axis as is shown in Appendix A. The auroral expansion is usually more pronounced on the evening side during substorms (e.g., Akasofu, 1964). It is therefore expected that in general the area of tail current reduction expands more duskward than dawnward, and if it reaches close to the dusk flank, the ionospheric footprint of the associated upward FAC should move to the late afternoon sector causing positive B_E variations at midlatitudes, which, however, cannot be identified in Figure 2. One possibility is that such B_E variations are partially canceled by the aforementioned downward FAC in the midday sector that arises from the conductance gradient. For the dawnside current wedge, in contrast, the same downward FAC can augment the negative B_E disturbance in the morning sector. Therefore, the preferred occurrence of extreme negative ΔB_E disturbances in the morning sector may not be the effect of the dawnside current wedge system alone.

There are two more factors that may affect this conductance-related downward FAC. The first one is the season. Table 1 may give an impression that extremely large ΔB_D preferably takes place in winter. In winter, the ionospheric conductance inside the polar cap is lower, and accordingly, the latitudinal conductance gradient on the dayside is steeper, which probably enhances the downward FAC. Another factor is IMF B_Y. Table 1 may also suggest that negative IMF B_Y is favorable. For negative IMF B_Y the dayside throat region shifts dawnward in the northern polar region (e.g., Reiff & Burch, 1985, Weimer, 2005), and probably so does the downward FAC. However, a more extensive study is required for a definitive assessment. In general, during geomagnetic storms various processes intensify simultaneously, and it is extremely difficult to observationally isolate the effects of individual processes. We may need to wait for future modeling efforts to fully understand the formation of the dawnside wedge current system.

We started this study motivated by the GIC event of December 2006 (Event 1) reported by Watari et al. (2009). In closing we would like to make several points, based on the results of the present study, regarding the possible prediction of such extremely large B_E deflections. First, the external driving has to be intense as is usually the case for extremely geomagnetic disturbances. Second, the probability is highest in the morning sector. Third, if a substorm-like enhancement of the WEJ has started in the postmidnight sector, the magnitude of B_E in the morning sector is expected to peak in 30–60 min. Finally, the probability is possibly higher if the season is winter, and IMF B_Y is negative (in the Northern Hemisphere). Therefore, for a target structure (e.g., power lines) with a north-south orientation, which is subject to azimuthal geomagnetic disturbances, the alert level needs to be elevated if it is in the local morning during a severe geomagnetic storm and the WEJ starts to enhance at postmidnight. Then the risk is expected to become highest in less than an hour. Additional caution may be required in winter and for negative IMF B_Y. We therefore expect that the new insights into the dawnside current wedge that our study provides will help predicting the potential hazard.

5 Summary

Motivated by the GIC enhancement observed at MMB in Event 1, we started this study with the characterization of ΔB_D at MMB. We found that extremely large disturbances tend to be directed westward and observed in the morning sector during intense geomagnetic storms. Those disturbances can be attributed to a downward FAC in the auroral zone, and we sought to identify the associated current system. Based on the ranking of ΔB_D at MMB, we selected four events for a detailed study, which took place during the December 2006 (Event 1), March 2015 (Event 2), October 2001 (Event 3), and November 2003 (Event 4) storms. For each event, the polar distribution of equivalent currents, as well as the AMPERE magnetic field measurements above the ionosphere for Event 2, indicates that the WEJ intensified in the dawn sector. The enhancement of the WEJ started at postmidnight and then extended eastward. The longitudinal confinement of the enhanced WEJ suggests that it closed with downward and upward FACs at its eastern and western ends, respectively, and therefore, the entire system may be envisioned as a wedge current. Whereas the upward FAC stayed at postmidnight, the downward FAC presumably moved toward dayside as the enhanced WEJ extended eastward. We noted, however, that the primary closure of FACs was meridional, and those upward and downward FACs should be regarded as the unbalanced parts of the R1 and R2 currents over certain areas. It is likely that the formation and dawnward extension of the wedge current system is an ionospheric manifestation of tail current reduction as suggested by a gradual dipolarization observed by Geotail in the dawnside magnetotail in Event 4. The overall sequence of the wedge current development is consistent with the global auroral images taken in Event 3, which shows the dawnward and poleward expansion of auroral intensification. Therefore this current system is similar to the substorm current wedge except that it extends primarily eastward covering the entire dawn sector. The formation and development of this dawnside wedge system is a recurrent feature of intense storms, which suggests that this current system is a fundamental constituent of the storm time M-I coupling.

Acknowledgments

SuperMAG indices as well as polar diagrams of equivalent currents were provided through the SuperMAG website (http://supermag.jhuapl.edu/). SuperMAG is an international collaboration with many organizations and institutes funded by National Science Foundation (NSF) grant 1417899. Memambetsu magnetometer data were provided by Meteorological Research Institute in Japan at http://www.kakioka-jma.go.jp/en/index.html. Ground magnetometer data from Norilsk, Amderma, and Tixie stations were provided by the Arctic and Antarctic Research Institute in Russia (http://geophys.aari.ru/index.html) through the aforementioned SuperMAG website. Support for AMPERE has been provided under NSF sponsorship under grants ATM-1420184 and ATM-0739864 to The Johns Hopkins University. Data used in this study are available via http://ampere.jhuapl.edu. (AMPERE, the AMPERE Science Data Center). IMAGE/UVI and Polar/VIS Earth Camera auroral images were provided by Louis A Frank and Stephen Mende, respectively, and are available through the SuperMAG website. The OMNI data were obtained from the GSFC/SPDF OMNIWeb interface at https://omniweb.gsfc.nasa.gov. The Sym-H, Asy-H, and Asy-D indices were provided by the World Data Center for Geomagnetism, Kyoto, and are available at http://wdc.kugi.kyoto-u.ac.jp/. Geotail magnetometer data were provided by S. Kokubun and T. Nagai and are available at https://darts.isas.jaxa.jp/index.html.en. Support for this analysis at the Johns Hopkins University Applied Physics Laboratory was provided by National Aeronautics and Space Administration (NASA) grant NNX16AG74G and NSF grants 1502700 and 1417899 for S.O., by NSF grant 1417899 for J. W. G., and by NASA grant NNX14AF82G for B. J. A.

Appendix A

In section 2 we statistically examined the deflection of the magnetic field at MMB (GLat: 43.9°; GLon: 144.2°), ΔB_D, for which we used as a reference the orientation of the local magnetic field. It is a common practice to interpret the midlatitude magnetic deflection as a remote effect of FACs at high latitudes. However, as we address below, other external currents also cause the deflection of magnetic field especially if the local magnetic field is directed off the orientation of the terrestrial dipole axis. In this appendix we also examine the occurrence probability of the east-west magnetic disturbance at MMB in geomagnetic as well as geographic coordinates in the same way as we did in Figure 2 and interpret the difference from Figure 2 in terms of the ring current contribution to the local magnetic deflection.

Figure A1 shows, as a function of geographic longitude, GLon, the deflection of ground magnetic field D_T96 (in nT) expected from the Tsyganenko 96 (T96) external current model (Tsyganenko, 1996) for three different values of Dst, −200, −100, and 0 nT. The R1 and R2 current modules are unplugged from the T96 model, but other magnetospheric current systems such as the symmetric ring current, tail current, and magnetopause current are included. The geographic latitude is set at 43.9°, GLat of MMB. D_T96 is calculated referring to the International Geomagnetic Reference Field (IGRF) model (e.g., Thébault et al., 2015). Even though the result does not include the effects of the R1 and R2 currents, the peak-to-peak amplitude of D_T96 is almost 100 nT for Dst = −200 nT. The minimum takes place at GLon = 141°, close to the longitude of MMB. In contrast, AQU (42.4°N, 13.3°E), which we mentioned in section 4.2 in the context of the eastward magnetic deflection around local midnight, is located near the positive peak of D_T96.

The minimum of D_T96 around MMB can be attributed to the locality of geomagnetic field in that longitudinal sector. At MMB the geomagnetic field is directed ~16° west from the geomagnetic north; this angle changed <1° for 1987–2016, for the 30-year period that we examined in section 2. The local magnetic field is directed 9° west from the geographic north, whereas the north geomagnetic pole was in the direction 7–8° east from the geographic north according to the IGRF model. If the ring current intensifies during storms, its geomagnetic effect is considered to be parallel to the dipole moment pointing southward. Therefore, it has a finite westward component relative to the local magnetic field. Note that −200 nT × sin(16°) = −55 nT, which roughly matches D_T96 = −49 nT as calculated above for Dst = −200 nT. This westward magnetic disturbance increases with increasing ring current intensity.

Figure A2 shows the probability distributions of eastward magnetic disturbances for three different coordinate systems in the same format as Figure 2. Figure A2a is basically the same as Figure 2, for which we used the orientation of the local magnetic field at MMB as a reference (section 2). For Figure A2b we used the geomagnetic north, the direction of the north geomagnetic pole from MMB, as a reference, for which we expect that the effect of the ring current is minimal. Compared to Figure A2a, the occurrence of negative disturbance is confined in a narrower range; see, for example, the bottom 0.5 percentile (red). The top 0.5 percentile (red) is pushed up along with the maximum (purple dashed) especially on the nightside, which can be attributed to the net upward FAC. In contrast, we still cannot find any clear prevalence of eastward deflection in the early afternoon sector, which would be a counterpart of the enhanced westward deflection in the morning sector (section 4.2). Figure A2c shows the probability distribution of the geographic eastward component, which shows features somewhat intermediate between Figures A2a and A2b as expected from the fact that its reference direction, the geographic north, is in the middle of the local magnetic field (Figure A2a) and geomagnetic north (Figure A2b).

References

Akasofu, S.-I. (1964). The development of the auroral substorm. Planetary and Space Science, 12(4), 273–282. https://doi.org/10.1016/0032-0633(64)90151-5
Anderson, B. J., & Korth, H. (2007). Saturation of global field aligned currents observed during storms by the iridium satellite constellation. Journal of Atmospheric and Solar - Terrestrial Physics, 69(1-2), 166–169. https://doi.org/10.1016/j.jastp.2006.06.013
Anderson, B. J., Korth, H., Waters, C. L., Green, D. L., Merkin, V. G., Barnes, R. J., & Dyrud, L. P. (2014). Development of large-scale Birkeland currents determined from the Active Magnetosphere and Planetary Electrodynamics Response Experiment. Geophysical Research Letters, 41, 3017–3025. https://doi.org/10.1002/2014GL059941
Crooker, N. U., & Siscoe, G. L. (1981). Birkeland currents as the cause of the low-latitude asymmetric disturbance field. Journal of Geophysical Research, 86, 11,201–11,210. https://doi.org/10.1029/JA086iA13p11201
Fukushima, N. (1969). Equivalence in ground geomagnetic effect of Chapman–Vestine's and Birkeland–Alfven's electric current-systems for polar magnetic storms. Report of Ionosphere and Space Research in Japan, 23, 219–227.
Gjerloev, J. W. (2012). The SuperMAG data processing technique. Journal of Geophysical Research, 117, A09213. https://doi.org/10.1029/2012JA017683
Gjerloev, J. W., & Hoffman, R. A. (2014). The large-scale current system during auroral substorms. Journal of Geophysical Research: Space Physics, 119, 4591–4606. https://doi.org/10.1002/2013JA019176
Gjerloev, J. W., Hoffman, R. A., Ohtani, S., Weygand, J., & Barnes, R. (2010). Response of the auroral electrojet indices to abrupt southward IMF turnings. Annales Geophysicae, 28(5), 1167–1182. https://doi.org/10.5194/angeo-28-1167-2010
Iyemori, T., Araki, T., Kamei, T., & Takeda, M. (1989). Mid-latitude geomagnetic indices ASY and SYM (Provisional) No. 1, Data Anal. Center for Geomagn. and Space Magn., Kyoto University Kyoto, Japan, 1992.
Kaufmann, R. L., Larson, D. J., Beidl, P., & Lu, C. (1993). Mapping and energization in the magnetotail: 1. Magnetospheric boundaries. Journal of Geophysical Research, 98(A6), 9307–9320. https://doi.org/10.1029/93JA00304
Nakano, S., & Iyemori, T. (2005). Storm-time field-aligned currents on the nightside inferred from ground-based magnetic data at midlatitudes: Relationships with the interplanetary magnetic field and substorms. Journal of Geophysical Research, 110, A07216. https://doi.org/10.1029/2004JA010737
Nakano, S., Iyemori, T., & Yamashita, S. (2002). Net field-aligned currents controlled by the polar ionospheric conductivity. Journal of Geophysical Research, 107(A5), 1056. https://doi.org/10.1029/2001JA900177
Ohtani, S., Korth, H., Keika, K., Zheng, Y., Brandt, P. C., & Mende, S. B. (2010). Inductive electric fields in the inner magnetosphere during geomagnetically active periods. Journal of Geophysical Research, 115, A00I14. https://doi.org/10.1029/2010JA015745
Pulkkinen, A., Kataoka, R., Watari, S., & Ichiki, M. (2010). Modeling geomagnetically induced currents in Hokkaido, Japan. Advances in Space Research, 46(9), 1087–1093. https://doi.org/10.1016/j.asr.2010.05.024
Reiff, P. H., & Burch, J. L. (1985). IMF by-dependent plasma flow and Birkeland currents in the dayside magnetosphere: 2. A global model for northward and southward IMF. Journal of Geophysical Research, 90, 1595–1609. https://doi.org/10.1029/JA090iA02p01595
Rodger, C. J., Mac Manus, D. H., Dalzell, M., Thomson, A. W. P., Clarke, E., Petersen, T., et al. (2017). Long-term geomagnetically induced current observations from New Zealand: Peak current estimates for extreme geomagnetic storms. Space Weather, 15, 1447–1460. https://doi.org/10.1002/2017SW001691
Stasiewicz, K. (1991). Polar cusp topology and position as a function of interplanetary magnetic field and magnetic activity: Comparison of a model with Viking and other observations. Journal of Geophysical Research, 96(A9), 15,789–15,800. https://doi.org/10.1029/91JA01420
Thébault, E., Finlay, C. C., Beggan, C. D., Alken, P., Aubert, J., Barrois, O., et al. (2015). International Geomagnetic Reference Field—The twelfth generation. Earth, Planets and Space, 67(1), 79. https://doi.org/10.1186/s40623-015-0228-9
Thomson, A. W. P., McKay, A. J., Clarke, E., & Reay, S. J. (2005). Surface electric fields and geomagnetically induced currents in the Scottish Power grid during the 30 October 2003 geomagnetic storm. Space Weather, 3, S11002. https://doi.org/10.1029/2005SW000156
Tsyganenko, N. A. (1996), Effects of the solar wind conditions on the global magnetospheric configuration as deduced from data-based field models, Proc. of 3rd International Conference on Substorms (ICS-3), Versailles, France, 12-17 May 1996, ESA SP-389, 181–-185.
Watari, S. (2015). Estimation of geomagnetically induced currents based on the measurement data of a transformer in a Japanese power network and geoelectric field observations. Earth, Planets and Space, 67(1), 77. https://doi.org/10.1186/s40623-015-0253-8
Watari, S., Kunitake, M., Kitamura, K., Hori, T., Kikuchi, T., Shiokawa, K., et al. (2009). Measurements of geomagnetically induced current in a power grid in Hokkaido, Japan. Space Weather, 7, S03002. https://doi.org/10.1029/2008SW000417
Weimer, D. R. (2005). Improved ionospheric electrodynamic models and application to calculating Joule heating rates. Journal of Geophysical Research, 110, A05306. https://doi.org/10.1029/2004JA010884

Citing Literature

Volume123, Issue11

November 2018

Pages 9093-9109

Dawnside Wedge Current System Formed During Intense Geomagnetic Storms

Abstract

Key Points

1 Introduction

2 Statistical Characteristics