The ionospheric Sq current system obtained by spherical harmonic analysis

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 118, , doi: /jgra.50194, 2013 The ionospheric Sq current system obtained by spherical harmonic analysis R. J. Stening 1 and D. E. Winch 2 Received 4 September 2012; revised 29 January 2013; accepted 20 February 2013; published 27 March [1] An earlier comprehensive analysis of geomagnetic tides by Winch is revisited to display changes in the current systems with season and longitude. The data used come from the quiet Sun years We choose to present some total equivalent current systems, being the sum of the external and internal parts derived in the spherical harmonic analysis. The latitudes and local times of the current system foci are generally in agreement with other workers. The amplitudes of the current system vortices follow an annual variation with maximum in summer. The longitude variations of the vortex amplitudes vary as the inverse of the magnetic fieldstrengthatbothequinoxesbuthavedifferent variations at the solstices. Other longitude maxima which have been reported in the equatorial electrojet intensity were not found. We examine in detail the invasions of the summer current systems across the equator, identifying these as signatures of field-aligned currents (FACs). The tilting of current contours with respect to the equator is interpreted as being due to midday FACs. As others have found, the identification of afternoon FACs is more difficult. The seasonal swap-over of FAC directions occurs not in September but in October November. Citation: Stening R. J., and D. E. Winch (2013), The ionospheric Sq current system obtained by spherical harmonic analysis, J. Geophys. Res. Space Physics, 118, , doi: /jgra Introduction [2] For many years, magnetometers have been running at ground-based stations around the world, yielding daily variations of the three elements of the magnetic field, namely, X, Y, and Z (northward, eastward, and vertical components, respectively). These data have been scaled to get magnetic field values in digital form, most frequently hourly mean values. Data such as these may easily be downloaded from the Edinburgh World Data Center for Geomagnetism website ( [3] The daily magnetic variations found from these magnetometers are primarily due to electric currents flowing in the ionosphere at heights around 110 km. Here we are interested in those currents flowing at magnetically quiet times. Thus, currents associated with magnetic storms, aurorae, and solar disturbances are excluded. The remaining quiet time currents are thought to be driven by tidal motions of the neutral atmosphere interacting with the ionosphere to produce electromotive forces through the dynamo mechanism [Baker and Martyn, 1953; Richmond et al., 1976]. Generally, it is thought that the (1, 2) evanescent diurnal mode is the prime contributor to the generation of the Sq currents [Tarpley, 1 School of Physics, University of New South Wales, Sydney, NSW, Australia. 2 Department of Mathematics and Statistics, The University of Sydney, Sydney, NSW, Australia. Corresponding author: R. J. Stening, School of Physics, University of New South Wales, Sydney, NSW 2052, Australia. (r.stening@unsw.edu.au) American Geophysical Union. All Rights Reserved /13/ /jgra b; Stening, 1970], though this has not been established for certain. Other tidal modes, especially semidiurnal modes, will contribute to the day-to-day variability of the current system. [4] Lunar tides are also present at ionospheric heights, and these drive an associated current system with lunar periodicities [Tarpley, 1970a; Malin, 1973; Winch, 1981; Stening and Winch, 1979]. These will be the subject of a later work. [5] The use of spherical harmonic analysis for analyzing geomagnetic variation data was probably first presented by Schuster [1889] and Chapman and Bartels [1940]. One feature of the spherical harmonic analysis method is that it enables a separation of current sources above and below the observing level. Thus, separate pictures are generated of current systems due to the overhead ionospheric currents, often called the external system, and the induced currents below the Earth s surface, called the internal system. [6] Notable earlier analyses include those of Matsushita and Maeda [1965], Malin [1973], and Winch [1981]. The raw magnetic field data include the much larger Earth s main field. In the aforementioned analyses, a daily mean value is subtracted from the raw data, and this removes the main field, leaving just the daily variation due to the ionospheric currents and the currents induced beneath the Earth s surface by the varying magnetic field. The problem here is that such an analysis yields significant flow of currents during local nighttime when the conductivity of the ionosphere is expected to be very much lower than in daytime (a factor of about 40 less at night). It has become the practice for some to take magnetic values around local midnight as the zero value from which measurements are taken. This is probably a fairly good approximation at solar minimum, less so at solar maximum. This highlights the 1288

2 difficulty in deciding at what time the ionospheric currents reverse direction from east to west. [7] Much of the early work is summarized by Matsushita [1967]. He presented current contours for three different longitude zones for June and December and equinox months and for the external and internal systems. The data were derived from 69 stations during Fifteen magnetically quiet days from each of the three seasons were selected with an even distribution of lunar ages to remove the lunar effect. [8] The analysis of Malin [1973] included a much larger number of days, taking data from 100 stations over the period to About 24 of these stations are at quite high magnetic latitudes. This again is a time around solar maximum. Malin included all data except that from the five international disturbed days of each month. He did not separate data from different seasons. Indeed, the prime focus of Malin s work was an analysis of the lunar contributions, though he does also present results for Sq. [9] Another analysis method takes a slice of data in a region with observatories well distributed in latitude but over a limited longitude range. They then basically equivalence local time with longitude in order to perform a spherical harmonic analysis. This method was pioneered by Campbell [1983], with further work done by Campbell and Schiffmacher [1985, 1988], Campbell et al. [1993], Yamazaki et al. [2011], and others. The works with Campbell as author used a combination slice-mirror method in that only one hemisphere (northern or southern) was analyzed at a time, the other hemisphere being simulated by inserting Fourier components with a 180 phase shift. [10] Takeda [1999, 2002] also presented results of spherical harmonic analysis (SHA) of data from globally distributed observatories. His analysis method differs from ours in that he performed a SHA of data for just 1 day and Universal Time (UT) and then averaged these using only quiet days, i.e., days with the Kp index less than 3 in Takeda [1999] and less than 4 in Takeda [2002]. [11] Winch [1981] used data from 130 stations during the solar minimum years 1964 and A number of these stations (about 36) are at quite high latitudes and thus subject to auroral effects that occur there. Again, he removed the five international disturbed days for each month. Since this is such a high-quality data set, it is desirable to present results from it which highlight the seasonal and longitude variations of the Sq system, so that comparisons can be made with results from other workers which were derived from other methods of analysis and other data sets, some from different years. In particular, we shall highlight the possible contributions of field-aligned currents (FACs) to some of our results. [12] Effectively, Winch analyzed the hourly mean data for X, Y, and Z from each observatory to obtain amplitudes A kn and phases l kn of the form A qn sin nt þ qh þ l qn (1) where n =1 4 andq = 2 to+2,t is the local time, and h is the longitude of the mean Sun (giving variation with season). In this way, there are four diurnal harmonics together with annual and semiannual seasonal variations. Each of these 20 Fourier components is then subjected to spherical harmonic analysis to degree 6 giving a current function R, where R ¼ 10R 4p X a j 2j þ 1 n o R j þ 1 Ak jme cos k þmt0 þ a k jme j;k P k j ðcosθþ: [13] This is Winch [1981, equation (29)]. If the Earth s radius R is expressed in kilometers and the amplitudes A k jme is in nanoteslas, then the current function R is in amperes. The currents are assumed to flow at a distance a from the Earth s centre. t 0 is the Universal Time, is the east longitude, θ is the colatitude, m is the number of cycles per day, and Pj k is an associated Legendre function. The subscript e denotes the external currents. There is a similar, but different, equation for the internal currents induced below the Earth s surface. Winch provided tables (his Tables ) of the amplitudes A k jme and phases ak jme for various values of m, j, k with one table for each value of q in equation (1). Winch also calculated separate values for coefficients for negative values of m in equation (2). These terms represent eastward travelling waves and are often neglected, and we neglect them in the analyses presented here. [14] Both Winch and Malin s analyses still have significant nighttime current flow as the daily mean value is effectively taken as the zero line. We will address this issue in a future communication. 2. Results [15] We start with the coefficients listed in Table 8.10 [q = 0 in equation (1)], Tables 8.11a and 8.11b (q = 1), and Tables 8.12a and 8.12b (q = 2) of Winch [1981]. Table 8.10 gives the solar terms without any seasonal variation. Tables 8.11a and 8.11b give the annual variation terms, and Tables 8.12a and 8.12b give the semiannual terms. Winch presented the current systems related to the annual and semiannual terms separately. We can then use equation (2) to give current function values for a given season h and Universal Time t as a function of colatitude θ and local time t and add all the aforementioned terms together to give a current system for any given season h. [16] While we are able to separately generate results for external and internal current systems, we first choose to display the sum of these as this will correspond most closely to observed geomagnetic variations. In Figure 1, we have chosen to show results for Universal Time UT = 10 h as this centers the current system over Europe and Africa, where there is the largest number of observatories contributing data [see Winch, 1981, Figure 3.1]. The current contours are labeled in kiloamperes. Solid line blue contours enclose counterclockwise flowing currents, and dashed line red contours enclose clockwise flowing currents. The position of the magnetic equator is shown with a solid dashed line. [17] As the daily mean value is taken as the zero datum, currents appear at nighttime, which are probably not real. [18] Whereas many earlier workers analyzed equinox months together, we have shown March and September separately. In March, the two current whorls are similar with similar magnitudes, 47 ka in the north and 50 ka in the south. In September, the northern current system is stronger (50 ka) than the southern ( 36 ka), and the southern focus is about 1.5 h later than that in the north. In June as expected, (2) 1289

3 Figure 1. Combined external and internal current systems at 10 h UT in (a) March, (b) June, (c) September, and (d) December. the northern system is stronger (52 ka) than that in the south ( 21 ka). The amplitude difference in December is much greater with a ratio of 67 to 13 ka. [19] We can also see the invasion of the winter hemisphere current system by the summer system in both June and December. This phenomenon was first described by Mayaud [1965] when he found that the daily variation of the declination D at some low-latitude stations in winter resembled that of the opposite hemisphere. This is illustrated in Figure 2, where the monthly average daily variation of D is shown for Bangui (4.6 N, 18.6 E). Bangui is chosen because it is outside of the equatorial electrojet zone (computed magnetic dip angle 15.1 ) and is south of the magnetic equator. In southern summer (December), there is a typical Southern Hemisphere variation of D with a morning minimum and an afternoon maximum. In winter (June), there is a Northern Hemisphere variation with a morning maximum. The accompanying minimum occurs very early at 11 h LT. The current plot figures show that the invasion occurs predominantly in the morning hours with a lesser effect, if any, in the afternoon. These and similar following plots have been constructed by taking all the mean values for each Figure 2. Average declination variations at Bangui for June and December

4 hour of the magnetic element during a particular month. A zero line is artificially inserted corresponding to midnight values to assist in visualization. [20] While no invasions are apparent in March (Figure 1a), there is a morning invasion into the Southern Hemisphere in September (Figure 1c) similar to that in June. We will show later more evidence for September current systems being frequently similar to those in June Seasonal Variations [21] In Table 1, we show the geographic latitude and local time of the current system foci at three longitude zones. These have been selected to approximate to those of Matsushita and Maeda [1965], where 12 UT centers on Europe/Africa, 4 UT centers on Asia/Australia, and 20 UT on America. Comparison with Matsushita and Maeda is a little difficult since we present results for both external and internal systems taken together and they present them separately. A comparison with their external results seems appropriate. Also, we use geographic latitudes, and they use magnetic dip latitudes. Nevertheless, our results support their conclusions that the local time of the summer focus is earlier in time than that of the winter focus and that the latitude of the summer focus is higher (further poleward) than that of the winter system. [22] Matsushita and Maeda also found that during equinox months, the focus is earlier in time in the Northern Hemisphere than in the Southern Hemisphere, and our results agree with that except for March at 4 UT. [23] We also note that at 12 UT, the time difference between the northern and southern foci is 0.7 h in March but 2.4 h in September. Takeda [1999] also found such differences. [24] In Figure 3, we show the seasonal variation of the amplitudes of the two current systems. There is a clear annual variation with maximum in summer: The Northern Hemisphere variation is rather flatter at 12 UT. Here we also agree with Matsushita and Maeda [1965] in that the equinox values are greater than the mean solstice values. At 12 UT, the mean solstice values are 77 and 78 ka, while the equinox values range from 84 to 110 ka. At 4 UT, the mean solstice values are 72 and 77 ka, while the equinox values range from 78 to 100 ka. [25] To examine seasonal variations, Pedatella et al. [2011] used data from the CHAMP satellite and groundbased data from a longitudinal chain of 14 stations in the Table 1. Geographic Latitudes and Local Times of Combined External and Internal Current Systems Foci in Northern and Southern Hemispheres UT Season NH Latitude SH Latitude NH Time SH Time 12 Mar Jun Sep Dec Mar Jun Sep Dec Mar Jun Sep Dec Figure 3. Seasonal variation of the amplitudes of the (dotted line) northern and (dashed line) southern current systems for Universal Times of 12 and 4 h. European/African sector. Their data came from the solar minimum period They found an annual variation in current system strength in the Northern Hemisphere but a semiannual variation in the Southern Hemisphere. Using data from 21 stations at Japan/Australian longitudes, Yamazaki et al. [2011] constructed an empirical model of the Sq field which revealed an annual variation of Sq amplitude in the Northern Hemisphere at all solar epochs but a semiannual variation in the Southern Hemisphere during higher solar activity years and an annual variation during the lowest solar activity years. The latter result agrees with ours Longitude Variations [26] Longitude variations of the current system amplitudes were found by generating current systems at hourly Universal Times (every 15 of longitude), and these variations are displayed in Figure 4. The behaviors of both hemispheres are very similar in March and September. The opposite behavior in December is quite remarkable: around 240 longitude (20 UT), the north and south amplitudes of the combined systems are similar, while near 0 longitude, the summer amplitude is more than three times that in winter. [27] The unusually large winter current amplitude is illustrated in Figure 5. Large areas of the current systems are located in the Pacific Ocean, where there are very few observatories. There is also a much larger nighttime current, which, when corrected for, may enhance the southern amplitude. Notice also how the northern system seems to be constrained by the magnetic equator. [28] To check the currents in Figure 5, we examined the declination variation amplitudes recorded at Tucson (32 N, 239 E), Apia (14 S, 188 E), and Guam (14 N, 145 E) and show these in Figure 6. The difference in the ranges at Tucson (33), in the north, and Apia (42), in the south, is not so large. The declination variation at Guam is of the Southern Hemisphere type, showing the invasion of the summer system north of the equator. 1291

5 [29] What is special about Northern American longitudes that might give rise to this effect? The position of the north magnetic pole causes the magnetic field lines to be closer to the vertical in this region and so larger emf s may be generated when horizontal winds are crossed with the near-vertical magnetic field. This idea needs to be confirmed by a modeling study. [30] Pedatella et al. [2011], using CHAMP data from the low solar activity years , found maxima in the Sq current function at longitudes near 0,120, and 280.Our results agree at 280 for the equinoxes and for the northern system in June and December. Figure 4. Longitude variations of current system amplitudes for various months. Note different scales. (full line) Southern Hemisphere. (dotted line) Northern Hemisphere. Figure 5. External currents at 18 UT in December Field-Aligned Currents Related to Invasions [31] It has been realized for some time now that the apparent invasions of the current of one hemisphere into the other are in fact the signature of field-aligned currents (FACs). Fukushima [1994] summarized these into three components: (1) currents flowing from the summer to the winter hemisphere at dawn, (2) FACs flowing from winter to summer around noon, and (3) FACs also flowing from winter to summer at dusk. The basic evidence for these comes from changes in the magnetic declination ΔD,sothatpositiveexcursionsof ΔD signify southward flowing FAC and negative excursions hint at northward FAC. [32] Yamashita and Iyemori [2002] were able to compare these ground-based magnetic measurements with those from the rsted satellite and confirmed the presence of all three FACs mentioned above, though the dusk ground-based data were unclear. In addition, they found that the direction of flow did not reverse at the equinoxes but in April May and November. [33] Park et al. [2011] further investigated FACs using CHAMP satellite data. They also found FACs flowing in the directions indicated above at dawn and noon, but that they were always southward at dusk irrespective of season: in particular, the currents flowed southward at dusk in June from summer to winter. [34] Fukushima [1994] mentioned how Price and Stone [1964] had noticed how the northern and southern Sq current systems interpenetrated each other across the equator. With our data, we are able to examine this in a more systematic way. The penetrations of the summer system into the winter system at dawn are very clearly seen for June and December in Figures 1b and 1d, confirming the dawn current directions seen by others. [35] We can compare our results with those of Takeda [2002] for 1985, a year near solar minimum. One aspect he emphasized is that from April to September, the current Figure 6. Average daily variation of declination component during December 1965 at Tucson (TUC), Apia (API), and Guam (GUA). 1292

6 flows across the equator from the morning side in the Southern Hemisphere to the afternoon side in the Northern Hemisphere. This means that during northern summer, the northern current system invades the south in the morning, which we also see in Figure 1b. We also see that at this time, the current contours are tilted with respect to the equator. However, from November to February, he finds the southern system invading the north in the afternoon, while we see this happening only in the morning in Figure 1d. [36] In the following sections, we shall get some idea of longitude differences by examining currents systems at 6, 12, and 18 UT corresponding to systems centering over Asia, Africa, and North America, respectively Noon FACs [37] At noon, such penetrations are not clearly seen, but the presence of FACs may be inferred by the tilt of the current system. This is often more clearly seen in presentations of the external system alone. In Figure 7b, the currents in January at noon are considerably tilted with respect to the magnetic equator. The observed tilted current might be understood as a combination of a southward FAC with the normal eastward current. This then agrees with the noon FAC directions described above. Takeda [2002] showed noon tilts from April to October at both solar maximum and minimum, indicating northward FACs, again broadly agreeing with Fukushima s summary. [38] At 6 and 12 UT, we observed the noon tilt over fewer months than Takeda, from May to September only. At 12 UT, the tilt is sometimes hard to evaluate as the magnetic equator similarly tilts with respect to the geographic equator and the current contours tend to run parallel to the magnetic equator, as shown in Figure 7a. [39] April may be regarded as the changeover month around which the current systems change from one solstice regime to the other. In April, we see clear tilts in the current system at 16 and 18 UT and slight tilts at 20 and 22 UT. At 0 and 2 UT, we see no clear tilt in April Morning Invasions [40] The morning invasions are those most clearly seen in our work. At 6 and 18 UT, the Southern Hemisphere is invaded by the north from April to September. At 2 and 12 UT, the invasions persist from May to September, but not April, in agreement with the 2 UT results of Takeda [2002]. [41] Regarding morning invasions of the Northern Hemisphere by the Southern Hemisphere, we see these from November to February at 6, 10, 12, and 20 UT. Figure 7b shows an example. However, at 16 and 18 UT, the invasion is only seen in December and January. At 14 UT, the situation in November and February is unclear Afternoon Invasions [42] These are rather more difficult to detect. Takeda [2002] presented results at 2 UT. This means that noon falls in eastern Australia and just east of Japan and most of the current system is then over the Pacific Ocean, where, in the north, Honolulu is the sole observatory in both Takeda s and in our analysis for afternoon times. Takeda s results would predict, between about 14 and 20 h local time, first northward, then southward current flow, giving negative and then positive excursions in the magnetic declination results for Honolulu, as shown in Figure 8a. [43] We show in Figure 7b our results for 2 h UT in January for the external currents only (we use January because the Figure 7. External current systems at (a) 12 UT in April and (b) 2 UT in January. afternoon invasion is more prominent for this month in Takeda s work). In Figure 7b, we can see that the clockwise vortex in the evening Northern Hemisphere may be representing the afternoon invasion, rather than spurious nighttime currents as we might otherwise suppose. Southward equivalent currents around 20 h LT (80 W in the figure) correspond to the small positive ΔD at this time at Tucson, as shown in Figure 8b. The crucial point here is the timing of the southward currents in the Northern Hemisphere evening time. If they are very late, e.g., after 20 h LT, then one might decide they are not real but arise because of the use of the mean datum line for the SHA. 1293

7 Figure 8. Average ΔD variations for January 1965 at Honolulu and Tucson and for January 1986 at Lanzhou. [44] Figure 8a is similar in form to the ΔD variation at Vassouras (22.4 S, E) in winter shown by Yamashita and Iyemori [2002], except the Honolulu variation is shifted to a later local time with the afternoon minimum at 13 h LT and the later maximum at 17 h, while at Vassouras, these times are 10 and 15 h, respectively. The positive ΔD in the winter diagrams of Takeda [2002] may be as late as 19 h LT. [45] In Figure 8c, we see how Lanzhou (36.1 N, E) in China has an early morning northward current (negative ΔD), indicating an invasion, followed by a normal Northern Hemisphere positive, then a negative ΔD and an afternoon small positive ΔD, the latter may be due to an afternoon invasion. The variations at Tucson in Figure 8b are actually similar to those at Lanzhou but with different relative amplitudes. [46] So why do we not see an afternoon invasion at 10 h UT for December in Figure 2b? If we examine ΔD at Kanoya (31.4 N, E) for December 1965, it is has the same peaks and troughs as Lanzhou in Figure 8, but the peak at 17 h LT at Kanoya is only 0.4 arc min, enough to indicate a small afternoon invasion but not large enough to generate an observable feature in the contour plot. [47] Maybe this comparison between our results and Takeda s should serve as a warning that unusual shapes of the current systems should be closely examined to test their reality. An example of the sort of test required is that shown in Figure 2, where the declination variation at Bangui verifies the direction of current flow. [48] When these invasions are closely examined, often there are no contributing observatories in the regions where they occur, or, in the case of Takeda s 1985 data, there are data missing from Honolulu for all of January and half of February. Such issues may or may not give rise to significant differences in the overall results. [49] As shown in Figure 7b, the important signature in the Northern Hemisphere is a southward current in the late afternoon or evening. [50] Takeda [2002] found afternoon invasions into the Northern Hemisphere at 2 UT from November to February. We find similar invasions but from October to January from 2to6UT.From10to18UT,thereisnosignofafternoon invasions into the Northern Hemisphere during any month. At 22 UT, invasions are just detectable in January and December and at 0 UT in January only. When the invasion signatures are small, it is hard to differentiate them from the nighttime currents arising from the choice of a mean value for the datum line. [51] Afternoon invasions of the northern into the southern system during northern summer are even harder to detect. The very small northward current off the coast of Western Australia at around 19 h LT shown in Figure 9 would fit Figure 9. External currents in June at 12 UT. Figure 10. Contour plot of ΔD variations at Trivandrum in Amplitudes are 0.1 arc min. Full (dashed) lines enclose positive (negative) ΔD values. the pattern suggested by Fukushima [1994], but examination of the ΔD variation at Gnangara (31.8 S, E) in Western Australia also shows a negligibly small deviation at this time. 1294

8 A southward current, as observed by Park et al. [2011]atthe earlier time of h LT, would be hard to detect because that is the direction of the normal Sq currents at dusk in the Southern Hemisphere Equatorial ΔD Variations [52] Fukushima [1994] pointed out that ΔD variations at a magnetic equatorial station such as Koror (7 N, 134 E geographic) give evidence of FAC flow reversal with season. We have repeated such an analysis for Trivandrum (8.5 N, 76.9 E) using 1965 data. We found it necessary to subtract a different baseline for each month as there was an apparent drift in the original data. Figure 10 shows how the FACs reverse direction around 10 h LT. The northern summer currents are much stronger than those in December. The seasonal switchovers occur in March April and in October. The latter switch is clearly later than the September equinox, agreeing with other workers. 3. Discussion [53] Three basic methods of determining quiet time ionospheric electric currents have been used. The method in this paper uses continuous data from a global array of observatories but suffers from lack of input in some sparsely covered areas of the Earth, particularly in the oceans. Nevertheless, the features discussed in this paper are of sufficiently large scale for our results to be reliable, and, in many cases, we validate them by examination of records from a relevant observatory. [54] Overhead data, such as those collected by the CHAMP and other satellites, give a more global coverage, but it is not continuous. For example, Pedatella et al. [2011] binned their CHAMP data with a 40 day window to obtain global coverage. This averaging process leads to a lesser contrast between the summer and winter solstice systems in their work. [55] Campbell et al. [1993] found that the Sq current whorl completely disappeared in winter over India and Siberia. Our results (Figure 11a) verify that this occurred Figure 11. (a) External currents at 6 UT in December. (b) External currents at 18 UT in June. for December and January only. We also found that the southern vortex seems to disappear in June (Figure 11b) at American longitudes, though this was not found by Campbell and Schiffmacher [1988], possibly due to their different computation method. [56] The longitude maxima of the Sq current function found by Pedatella et al. [2011] line up to some extent with those found in the equatorial electrojet by workers such as Lühr et al. [2008, 2012], Maute et al. [2012], and earlier workers. The electrojet maxima center on longitudes of 0, 90, 180, and 270 and are generally attributed to the influence of nonmigrating tides, particularly the diurnal, eastward propagating DE3 mode. On the other hand, the longitudinal variation of the electrojet strength found by Doumouya et al. [2003] is quite similar to our equinox results in Figure 4 with a maximum near 280 and a minimum near 80 : these results fairly closely follow the inverse of the strength of the Earth s main magnetic field. Yet the maximum at 280 is some distance west of the center of the South Atlantic magnetic anomaly at 315, where ionospheric conductivities are higher than average. It is noteworthy that the simulation of Doumbia et al. [2007], using the Thermosphere-Ionosphere-Electrodynamics General Circulation Model, also found a dominant maximum in the equatorial electrojet amplitude at 280 longitude, though their simulation had no nonmigrating tide input. The simulation by these authors also generally failed to correctly reproduce the observed ΔD variations in the vicinity of the equator. [57] We are not aware of any established close correlation between equatorial electrojet strength and Sq current vortex intensity on a day-to-day basis. Yamazaki et al. [2010] showed a correlation between electrojet intensity and the mean strength of the eastward electric currents flowing between the north and south foci, but this may be a different quantity from the vortex strength determined by spherical harmonic analysis. [58] Takeda [2002] mentioned how the noon FACs will also cause the Sq focus times to be displaced. Referring to Figure 7b, the southward FAC at noon will displace the northern focus eastward and the southern focus westward, causing the southern focus to be at an earlier time than the northern. In Table 1, this happens only in December at all UTs displayed and in March at 4 UT. These December results further support the presence of a southward FAC at noon in that season. [59] One question to be answered is whether the afternoon FAC in June flows northward as suggested by Fukushima [1994] or southward as found by Park et al. [2011]. Our Figure 10 suggests a small northward flow around 20 LT, but this and other evidence examined are unconvincing. [60] It is noticeable in Figure 10 that the positive ΔD, southward flow, in December is centered around 15 LT, rather than noon, and this closely agrees with the results of Park et al. [2011]. [61] Generally, the pattern of current flows typical of June extends for about 6 months, while the December pattern lasts for a shorter time, i.e., 2 4 months. In particular, the morning current invasions over America are seen only in January and December. It is possible that the effects of the FACs are obscured when the northern and southern current systems have similar amplitudes as is the case at American longitudes around the December solstice. By contrast, at these 1295

9 longitudes in June, the southern current system virtually disappears. Campbell et al. [1993] attributed such effects to the relative positions of the magnetic and geographic poles: in the case of India/Siberia, there is a smaller area of the ionospheric dynamo system exposed to solar radiation. This explanation awaits further verification. [62] Takeda [2002] noted that the later October/November turnaround time relates to changes in the semidiurnal tides at 90 km heights [Tsuda et al., 1988; Xu et al. 2011]. The latter workers identified a northern summer regime and a winter regime where the tides had different phases and vertical wavelengths in these two parts of the year with the transition in October/November. This in turn may be related to the turnaround time of stratospheric winds as discussed by Belmont et al. [1975]. [63] The presence of FACs in September, as indicated in Figure 1c, can be also seen in the results of Park et al. [2011] and of Yamashita and Iyemori [2002]. In particular, Park et al. [2011, Figure 3] showed a FAC in September at 315 longitude as large as any other they detected. 4. Conclusions [64] 1. Latitude variations of the strength of the currents systems follow an annual variation in both hemispheres as found by Yamazaki et al. [2011] at low solar activity. [65] 2. The longitude variations of the current system strengths are similar to ground-based observations of the equatorial electrojet amplitude but different from satellite results. Relations between current system strength and electrojet strength need further investigation. [66] 3. Seasonal changes in the inferred field-aligned currents agree with the model of Fukushima [1994] in the morning and noon. Noon FACs are deduced from the tilt of the current system. Afternoon/dusk results are harder to interpret: it seems that our results agree more closely with the Fukushima model than with the satellite results of Park et al. [2011], though the satellite currents are earlier in local time. [67] 4. The late seasonal regime swap-over in October November, rather than September, agrees with other workers. [68] 5. The seasonal variations of field-aligned currents roughly follow Figure 10 with southward currents at dawn and northward currents at noon between May and September, and sometimes in April. The morning northward currents flow from November to February at some UTs and only in December and January at UT when the current system centers over America. [69] 6. Changes in focus positions with season and Universal Time find broad agreement with the earlier determinations by Matsushita and Maeda [1965]. Differences between March and September are noted. [70] Acknowledgments. Some of the data used were downloaded from the Edinburgh World Data Center for Geomagnetism website. References Baker, W. G., and D. F. Martyn (1953), Electric currents in the ionosphere. II. The atmospheric dynamo, Philos. Trans. R. Soc. 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