Adv. Polar Upper Atmos. Res., 18, 23–34, 2004
Â 2004 National Institute of Polar Research

Solar activity dependence of two types of east-west
geomagnetic disturbances at mid latitudes
Shin’ya Nakano
Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502(Received December 10, 2003; Accepted May 31, 2004)

Abstract: It has been suggested that auroral electrojets are of two types, and each ofthem is associated with a different current system: the wedge current system and a current
system related with magnetospheric convection. In this paper, solar cycle dependences of
two different current systems were examined. Each of the two current systems is associated with a different field-aligned current system, which generates a different pattern of geomagnetic disturbances at mid latitudes. In order to examine solar cycle dependences of thetwo current systems, we investigated those of mid-latitude geomagnetic disturbances. The
results suggests that the two current systems have different solar cycle dependences. The
activity of the current system associated with convection has a good correlation with Dst
activity, and it tends to be enhanced around a solar maximum. On the other hand, thewedge current system is enhanced in the late declining phase of the solar cycle. It was alsosuggested that relative contributions of the two current systems to auroral electrojet activity has a solar cycle dependence. The contribution of the wedge current system to auroralelectrojet activity is rather small around a solar maximum, while it becomes considerable
around a solar minimum.
key words: solar cycle, magnetic disturbances, magnetic storm, substorm

1. Introduction
Field-aligned currents have a significant effect on geomagnetic disturbances at mid latitudes. For example, it is thought that asymmetrical disturbances at mid latitudes are mainly
caused by field-aligned currents (e.g., Nakano and Iyemori, 2003a). In particular, east-west
geomagnetic disturbances ∆D at mid latitudes are mainly attributed to field-aligned currents(e.g., Sun et al., 1984). Since field-aligned currents are closely associated with geomagnetic
activities at auroral region, east-west geomagnetic disturbances at mid latitudes show agood correlation with intensity of auroral electrojets (e.g., Nakano et al., 2002).
It has been suggested that auroral electrojets are generally divided into two components (Troshichev et al., 1974; Rostoker et al., 1987; Kamide and Kokubun, 1996). One is awestward electrojet around midnight which develops at substorm expansions, and it corresponds to the wedge current system (Clauer and McPherron, 1974). The other is electrojets
which develop when the interplanetary magnetic field (IMF) is directed southward. Since
the two different electrojets are associated with different field-aligned current systems, twodifferent geomagnetic variations are accordingly observed at mid latitudes. At a substorm
expansion, a wedge current signature is observed; that is, an eastward/westward disturbance

23

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S. Nakanoin the pre-midnight and a westward/eastward disturbance in the post-midnight in the northern/southern hemisphere are observed (Clauer and McPherron, 1974). On the other hand,when the IMF is directed southward and magnetospheric convection is developed, an eastward/westward disturbance in the northern/southern hemisphere centered around the midnight or the post-midnight is observed (e.g., Clauer and Kamide, 1985; Nakano and Iyemori, 2003b).
Geomagnetic disturbances are closely related with solar activity. Therefore it is expected that the two auroral current systems depend on the solar cycle. However, different magnetospheric phenomena do not necessarily have the same dependence on solar activity. For
example, it has been pointed out that storms and substorms have different dependences on
the solar cycle (Vennerstrøm and Friis-Christensen, 1996). Thus the two auroral current systems may have different dependences on the solar cycle. However, the solar activity dependence of each of the two different current systems has not been investigated.
The purpose of this paper is to examine the solar activity dependence of each of thetwo current systems. Recently, Nakano and Iyemori (2003b) have introduced two parameters, which roughly monitors the two different current systems. The two parameters arederived from east-west components of geomagnetic data obtained at mid-latitude observatories. The solar activity dependence of each of the two parameters must reflect on that ofeach of the two different current systems. In this paper, the solar activity dependences of thetwo parameters were investigated, and the solar activity dependences of the two current systems were discussed.
2. Parameterization
In this paper, we analyzed the east-west component of geomagnetic data obtained at
mid-latitude observatories listed in Table 1. As mentioned above, the east-west geomagnetic
disturbances at mid latitudes are mainly attributed to field-aligned currents. In order toavoid a mixing of the effect from the ring current, we used the eastward component in the
dipole coordinate system in this paper according to Iyemori (1990). From the data, wederived a local-time profile of the east-west geomagnetic variations at mid latitudes. The
local-time profile reflects the local-time structure of field-aligned currents. From the localtime profile, we derived two parameters, which roughly monitors the two different current
systems. The method is same as that of Nakano and Iyemori (2003b) except that the set ofobservatories used in this study is different. Here we describe the outline of the method.
Table. 1. List of mid-latitude geomagnetic observatories used in this paper.

Fresno (FRN)
Boulder (BOU)
Fredericksburg (FRD)
Chambon-La-Foret (CLF)
Martin De Vivies (AMS)Memambetsu (MMB)
Honolulu (HON)

Geographic (deg.)
Lat. (N)Long. (E)
37.1
240.3
40.1
254.8
38.2
282.6
48.0
2.3
−37.8
77.6
43.9
144.2
21.32
202.00

Dipole (deg.)
Lat. (N)Long. (E)
43.7
304.5
48.7
319.8
48.8
352.9
49.9
85.8
−46.8
143.6
34.9
210.8
21.60
269.45

Std. dev.
of ∆D (nT)
9.58
13.22
11.50
12.34
11.28
8.19
4.42

Solar activity dependence

25

We subtracted the Sq-field for each observatory for each month from the original data
to obtain a eastward disturbance field, ∆D. The Sq-field was estimated from data of quiet
days (in UT) for each month. As the quiet days, we have selected the days when high latitude geomagnetic disturbances (AE and Kp activity) are nearly or completely absent over awhole day. The averaged daily variation of the quiet days for each month for each observatory was fitted by 12 terms Fourier series. These Fourier series were used as the Sq-field andthe geomagnetic main field.
We derived a magnetic local time (MLT) profile of east-west geomagnetic disturbances, ∆D, for each minute at the mid-latitude observatories by linear interpolation. In
deriving the MLT profile of ∆D, we inverted the sign of ∆D at southern hemisphere observatory (AMS) because magnetic effects of field-aligned currents on the east-west component
in the southern hemisphere must be opposite to those in the northern hemisphere. In order
to eliminate latitudinal and longitudinal dependences, ∆D for each observatory was normalized such that its standard deviation within 2100–0300 MLT from 1987 to 2001 is 10.0 nT
before deriving the MLT profile. (The actual standard deviation of ∆D for each observatory
is displayed in Table 1.)
The top panel of Fig. 1 indicates a UT variation of the MLT profile of ∆D at mid latitudes during a storm on 13 October 2000. The ordinate denotes MLT, and the abscissa
denotes UT. Positive represented by red-yellow denote eastward/westward disturbances in
the northern/southern hemisphere, which correspond to the effect of upward field-aligned
currents, and negative represented by blue-cyan denote westward/eastward disturbances in
the northern/southern hemisphere, which correspond to the effect of downward fieldaligned currents. The location (MLT) of each observatory is indicated by a solid line superposed on the panel. This figure also shows the SYM-H index (second panel), the ASY-D and
ASY-H indices (third panel) (Iyemori, 1990) are also displayed. The SYM-H index is a geomagnetic index which essentially represents the Dst field with a 1-min resolution. The signof the ASY-H index is inverted in plotting it. The QL AE indices and the z-component of the
IMF (in GSM coordinates) Bz obtained by the ACE satellite are shown in the fourth panel
and the fifth panel, respectively. Parameters ψ and ζ displayed in the bottom panel will be
explained later. For convenience of comparison with geomagnetic variations, the IMF measured by the ACE is plotted time delayed at the Earth location. The delay time from theACE position to the Earth location is assumed to be 50 min in this paper. Figure 2 indicatesa UT variation of the MLT profile of ∆D during a substorm on 21 January 1999. The data
appended in this figure are the same as those of Fig. 1.
As shown in the top panel of Fig. 1, a positive ∆D disturbance at mid latitudes wasseen on the nightside from about 0100 UT to about 0300 UT and from about 0400 UT to
about 0600 UT on 13 October 2000 (top panel). This positive disturbance means thatupward field-aligned currents developed on the nightside. Such a disturbance is basicallyseen during southward IMF conditions (e.g., Clauer and Kamide, 1985; Nakano and Iyemori, 2003b). On the other hand, Fig. 2 shows that a wedge current signature appeared at
mid latitudes around 1100 UT on 21 January 1999; that is, positive and negative disturbances are observed in the pre-midnight and post-midnight, respectively (Clauer and
McPherron, 1974). These two types of disturbances are commonly seen at mid latitudes.
From the MLT profile of ∆D, we derive two parameters: ψ and ζ. A parameter ψ isdefined as the average value of the MLT profile of ∆D within 2100 ＜− MLT ＜0300, and it

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S. NakanoFig. 1.

Overview plot for a weak storm on 13 October 2000. The top panel shows ∆D at mid-latitudes in the MLT-UT plane. Positive variations (red-yellow) denote eastward/westward disturbances in the northern/southern hemisphere, and negative variations (blue-cyan) denotewestward/eastward disturbances in the northern/southern hemisphere. The location of eachobservatory used here is indicated by a solid line superposed on the panel. In this figure, theSYM-H index (second panel), the ASY-D, H indices (third panel), the QL AE indices (fourth
panel), and the z-component (in GSM coordinates) of the IMF Bz are also displayed. In
addition, ψ and ζ defined in the text are also shown in the bottom panel with the red andblue lines, respectively.

Solar activity dependence

Fig. 2.

27

Overview plot for a substorm on 21 January 1999 in the same format as Fig. 1.

generally represents upward currents centered around the midnight as seen in Fig. 1, which
are accompanied with southward IMF. Another parameter ζ is defined as the average of the
MLT profile of ∆D within 2100 ＜−MLT＜0000 with subtraction of the average of that within 0000 ＜
MLT
＜
0300.
Positiveζ indicates that ∆D in the pre-midnight are larger than−those in the post-midnight. The ζ substantially represents whether Region 1 sense currents
or Region 2 sense currents are relatively intensified. The development of Region 1 sense
currents enhances ζ, and the development of Region 2 sense currents depresses ζ. Since a

28

S. Nakanocurrent wedge around the midnight, which is in a Region 1 sense, would enhance ζ, we canregard the ζ as a rough proxy of wedge current intensity. We must note that the ζ can notalways detect development of current wedges. If the local-time where a current wedge
develops is located far from the midnight, the current wedge can not enhance the ζ. In addition, the seven observatories, which are distributed sparsely, may not detect either or both of
the upward and downward currents which consist in the current wedge. However, the ζ canbe used as a rough proxy of current wedge activity, as far as it is used for statistical analyses. In the bottom panels of Fig. 1 and 2, the variations of ψ and ζ are plotted with red andblue lines, respectively. ψ was enhanced from about 0100 UT to about 0300 UT and fromabout 0400 UT to about 0600 UT on 13 October 2000 according that the positive ∆D disturbance was seen on the nightside (Fig. 1). ζ was enhanced around 1100 UT on 21 January
1999 according that the current wedge signature was observed (Fig. 2). Figure 1 also indicates that ζ was enhanced around 0100 and 0340 UT on 13 October 2000 and that ψ develops after the brief enhancements of ζ, which means the development of the current wedge.
The upward currents centered around the midnight usually develops after substorm onsets,
although it principally depends on southward IMF. This point is further discussed anotherpaper (Nakano and Iyemori, 2003b).
3. Analysis
In order to examine long term variations of ψ and ζ, we calculated minutes of ψ ＞ 20
and of ζ ＞ 20 per day for each month for the period from 1987 to 2001, and we used themas proxies of activities of ψ and ζ, respectively. The minutes of ψ ＞ 20 per day for a monthis defined as a product of probability for ψ ＞ 20 in the month and minutes of one day (i.e.,1440). The probability of ψ ＞ 20 for each month can be calculated by dividing number ofdata when ψ ＞ 20 by total number of available ψ values in the month.
In Fig. 3, the minutes of ψ ＞ 20 per day for each month are shown with dotted lines inthe top panel. In the middle panel, monthly averages of the Dst index are displayed. Solidlines in the top and the middle panel indicate 1-year running average values. The monthly
sunspot number variation is also appended in the bottom panel of this figure for reference.
The ψ activity well correlated with Dst activity. Both the ψ activity and the Dst activity have
two peaks in a solar cycle (1989 and 1991), although the ψ activity was more high at thelatter peak than at the former while the Dst activity at the latter peak was as high as at theformer. The good correlation between the ψ activity and the Dst activity is natural because
both are accompanied with southward IMF (Nakano and Iyemori, 2003b). The ψ activity
also roughly correlated with the solar activity. In the solar minimum (1996), the ψ activity
was reduced to a minimum. When solar activity is high, however, the relation between theψ and the solar activity was rather complicated. The peak of the ψ activity was delayed
behind the solar activity. In addition, the ψ activity dipped in the late of 1990. These features were seen in the variation of the averaged Dst as well.
In Fig. 4, the minutes of ζ ＞20 per day for each month are shown with dotted lines inthe top panel. A solid line indicates 1-year running average values as well as in the top andthe middle panel of Fig. 3. The monthly sunspot number values are also displayed in thebottom panel again. The relation between the ζ activity and the solar activity was different
from that between the ψ activity and the solar activity. The peak of the ζ activity was inSolar activity dependence

Fig. 3.

The minutes of ψ＞20 per day for each month from 1987 to 2001 (top panel). Monthly averages of the Dst index (middle panel) and the sunspot number (bottom panel) for each month
are also displayed for reference. In the top and middle panels, 1-year running average values
are superposed with solid lines.

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S. NakanoFig. 4.

The minutes of ζ ＞ 20 per day for each month from 1987 to 2001 (top panel). The monthly
values of the sunspot number are also displayed as well as in Fig. 3 (bottom panel). In the top
panel, 1-year running average values are superposed with a solid line again.

1994 when solar activity was decaying. Around the solar maximum, the ζ activity was only
slightly enhanced.
Vennerstrøm and Friis-Christensen (1996) have suggested that geomagnetic activity
has three peaks during one solar cycle. The first peak appears slightly prior to a sunspot
maximum. In the first peak, the ring current activity is high while the high-latitude geomagnetic activity is not enhanced. In the second peak, which comes after a sunspot maximum,
both the ring current activity and the high-latitude activity are enhanced. In the third peak,which appears in the late declining phase of the solar cycle, the high-latitude activity areenhanced while the ring current is not so activated.
The three peaks of geomagnetic activity may be seen in the result of this study as well,although our study examined only one solar cycle. In the first peak (around 1989), the Dst
activity and the ψ activity were high, and the Dst was more activated than the ψ. In the second peak (around 1991), both the Dst activity and the ψ activity were high again, and the ζwas slightly activated. In the third peak (around 1994), the ζ activity became high while theSolar activity dependence

31

Dst activity was not so enhanced. Since ψ and ζ correspond to the high-latitude geomagnetic
activity, the result of this study indicates that the high-latitude geomagnetic activity was
mainly enhanced in the second and third peaks as well as the result by Vennerstrøm andFriis-Christensen. However, this study suggests that the high-latitude geomagnetic activity
was associated with different current systems between in the second peak and in the third
peak. The second peak in the high-latitude geomagnetic activity was mainly attributed tothe ψ activity; that is, the development of geomagnetic activity at high latitudes were mainly attributed to magnetospheric convection, which is caused by southward IMF. The third
peak in the high-latitude geomagnetic activity was mainly attributed to the ζ activity; thatis, the development of geomagnetic activity at high latitudes were mainly caused by thedevelopment of the wedge current systems.
This fact suggests that the high-latitude geomagnetic activity is associated with different current systems between around the solar maximum and around the late declining phaseof the solar cycle. In order to compare the contributions of the two different current systems
to the high-latitude geomagnetic activity, we examined relation between ψ and the AE index
and that between ζ and the AE index. Figure 5 indicates correlation coefficients between ψ
and AE and those between ζ and AE. In general, the correlation between ψ and AE is higher
than that between ζ and AE. This indicates that AE activity is mainly attributed to the current system which is associated with magnetospheric convection. Although the correlation
between ψ and AE varies little, the correlation between ζ and AE became gradually higher
from 1990 to 1994. Around the solar maximum (from 1990 to 1992), the correlation
between ζ and AE was low. In the declining phase (1993 and 1994), ζ came to have a correlation with AE. This may indicate that a contribution of the wedge current system to AE
activity has a solar cycle dependence. The wedge current system hardly contributes to AE
activity around the solar maximum, while the wedge current system comes to have a considerable contribution to AE activity after the solar maximum.
Unfortunately, the Provisional AE index is not available for most of periods from 1987
to 2001 when we can obtain ψ and ζ values. In order to investigate long term variation ofrelative contribution of the two current systems to the high-latitude geomagnetic activity,
we compared ψ and ζ with the ASY-H index (Iyemori, 1990). The ASY-H index indicatesFig. 5.

Correlation coefficients between ψ and AE and thosebetween ζ and AE for each year from 1990 to 1994.

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S. Nakanoasymmetrical geomagnetic north-south disturbances at mid latitudes. Since the asymmetrical disturbances at mid latitudes are associated with the high-latitude geomagnetic activity(e.g., Crooker and McPherron, 1972; see also, Nakano and Iyemori, 2003a), we can use theASY-H as a proxy of the high-latitude geomagnetic activity. Figure 6 indicates correlation
coefficients between ψ and ASY-H and those between ζ and ASY-H. In general, ψ is positively correlated with ASY-H as well as AE, and the correlation coefficient varies little. Correlation between ζ and ASY-H is likely to have a solar cycle dependence. Around the solar
maximum, the correlation between ζ and ASY-H was near zero. From the declining phase to
the developing phase of the solar cycle, ζ have a certain correlation with ASY-H. Thus it issuggested that a contribution of the wedge current system to the high-latitude geomagnetic
activity has a solar cycle dependence.

Fig. 6.

Correlation coefficients between ψ and ASY-H and those between ζ and ASY-H for eachyear from 1987 to 2001.

4. Summary and discussions
In this paper, we derived two parameters (ψ and ζ), which roughly monitors the twodifferent current systems, according to Nakano and Iyemori (2003b), and long term variations of the two parameters were investigated during the period from 1987 to 2001. The ψ
generally represents intensity of the current system which is associated with magnetospheric convection. The ζ generally represents intensity of the wedge current system at substorm
expansion. The result suggested that the two current systems have different solar cycle
dependences. One current system, which is associated with convection, closely correlateswith the Dst activity, and it roughly correlates with the sunspot number. The other current
system, the wedge current system, does not have any clear correlation with the sunspot
number. However, it is enhanced in the late declining phase of the solar cycle.

Solar activity dependence

33

The long-term variations of solar wind parameters have been examined by many
authors (e.g., King, 1991). Around solar maxima, the IMF intensities become larger thanaround solar minima. The convection electric field in the magnetosphere and the ionosphereis dependent on the IMF intensities, although it is dependent on the IMF orientation as well(e.g., Boyle et al., 1997). Therefore it is natural that the solar wind condition more favorable for the development of the current system associated with convection in solar maxima.
On the other hand, it is suggested that high speed solar wind streams frequently occur in the
declining phase of the solar cycle. Thus the result of this study might suggest that high solar
wind speed conditions are favorable for the development of the wedge current system.
However, our result is not a conclusive evidence of the relationship between the development of current wedge and solar wind speed. It is necessary to compare the occurrence frequency of current wedge developments with various solar wind parameters to clarify theorigin of the solar cycle dependence of current wedge developments in the future.
Since the two current systems have different solar cycle dependences, relative contributions of the two current systems to auroral electrojet activity has a solar cycle dependence. Around the solar maximum, the contribution of the wedge current system is rather
small, and auroral electrojet activity is mostly attributed to convection. On the contrary, thecontribution of the wedge current system becomes considerable around the solar minimum.
This fact may explain the result by Nakai and Kamide (1999) who have indicated that therelation between the AL index and the Dst index becomes closer in the solar maximum thanin the solar minimum, because geomagnetic storms is associated with the convection.
The wedge current system appears at substorm expansions. It is well known that a substorm expansion is accompanied with a Pi2 pulsation. If the current wedge and the Pi2 pulsation are generated through the same process, the current wedge activity and the Pi2 pulsation activity should have the same solar cycle dependence. However, it is suggested that
occurrence frequency of the Pi2 pulsations inversely correlated with solar activity (Saito
and Matsushita, 1968), while the result of this study has suggested that the current wedge
activity reaches a peak in the declining phase of the solar cycle. Possibly most of the current wedges are so weak that ζ can hardly exceed 20. Otherwise most of Pi2 pulsations
around a solar minimum might not be accompanied with any current wedge activations.
This study examined geomagnetic variations for only one solar cycle. Therefore we
can not actually say whether the suggestion provided by the result of this study is universally valid for other solar cycles. We must make a similar analysis for other solar cycles in the
future to confirm whether the suggestion is generally valid or not.
Acknowledgments
The geomagnetic data and the geomagnetic indices used in this paper were obtained
through Data Analysis Center for Geomagnetism and Space Magnetism, Kyoto University.
I appreciate all the staffs of the magnetic observatories who kindly provided the geomagnetic data. We also thank the ACE Magnetic Field Instrument team and the ACE Science Center for providing the ACE data. I am also grateful to T. Iyemori for valuable comments.
This study was partially supported by a Grant-in-Aid for the 21st Century COE Program (Kyoto University, G3).
The editor thanks Dr. S. Kokubun and another referee for their help in evaluating this34

S. Nakanopaper.
References
Boyle, C.B., Reiff, P.H. and Hairston, M.R. (1997): Empirical polar cap potential. J. Geophys. Res., 102, 111–125.
Clauer, C.R. and Kamide, Y. (1985): DP 1 and DP 2 current systems for the March 22, 1979 substorms. J. Geophys. Res., 90, 1343–1354.
Clauer, C.R. and McPherron, R.L. (1974): Mapping the local time-universal time development of magnetosphericsubstorms using mid-latitude magnetic observations. J. Geophys. Res., 79, 2811–2820.
Crooker, N.U. and McPherron, R.L. (1972): On the distinction between the auroral electrojet and partial ring current systems. J. Geophys. Res., 77, 6886–6892.
Iyemori, T. (1990): Storm-time magnetospheric currents inferred from mid-latitude geomagnetic field variations. J.
Geomagn. Geoelectr., 42, 1249–1265.
Kamide, Y. and Kokubun, S. (1996): Two-component auroral electrojet: Importance for substorm studies. J. Geophys. Res., 101, 13027–13046.
King, J.H. (1991): Long-term solar wind variations and associated data sources. J. Geomagn. Geoelectr., 43,
Suppl., 865.
Nakai, H. and Kamide, Y. (1999): Solar cycle variations in the storm-substorm relationship. J. Geophys. Res., 104,
22695–22700.
Nakano, S. and Iyemori, T. (2003a): Local-time distribution of net field-aligned currents derived from high-altitudesatellite data. J. Geophys. Res., 108 (A8), doi:10.1029/2002JA009519.
Nakano, S. and Iyemori, T. (2003b): Characteristics of upward field-aligned currents on the nightside duringstorm-time inferred from ground-based magnetic data at mid-latitudes: Relationships with the interplanetary magnetic field and substorms. submitted to Ann. Geophys.
Nakano, S., Iyemori, T. and Yamashita, S. (2002): Net field-aligned currents controlled by the polar ionosphericconductivity. J. Geophys. Res., 107 (A5), doi:10.1029/2001JA900177.
Rostoker, G., Akasofu, S.-I., Baumjohann, W., Kamide, Y. and McPherron, R.L. (1987): The roles of direct input
of energy from the solar wind and unloading of stored magnetotail energy in driving magnetosphericsubstorms. Space Sci. Rev., 46, 93–111.
Saito, T. and Matsushita, S. (1968): Solar cycle effects on geomagnetic Pi 2 pulsations. J. Geophys. Res., 73,
267–286.
Sun, W., Ahn, B.-H. Akasofu, S.-I. and Kamide, Y. (1984): A comparison of the observed mid-latitude magnetic
disturbance fields with those reproduced from the high-latitude modeling current system. J. Geophys.
Res., 89, 10881–10889.
Troshichev, O., Kuznetsov, B.M. and Pudovkin, M.I. (1974): The current systems of the magnetic substorm growthand explosive phases. Planet. Space Sci., 22, 1403–1412.
Vennerstrøm, S. and Friis-Christensen, E. (1996): Long-term and solar cycle variation of the ring current. J. Geophys. Res., 101, 24727–24735.