Day-to-day changes in the latitudes of the foci of the Sq current system and their relation to equatorial electrojet strength

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2005ja011219, 2005 Day-to-day changes in the latitudes of the foci of the Sq current system and their relation to equatorial electrojet strength Robert Stening and Tamara Reztsova School of Physics, University of New South Wales, Sydney, New South Wales, Australia Le Huy Minh Institute of Geophysics, Vietnamese Academy of Science and Technology, Hanoi, Vietnam Received 4 May 2005; revised 11 July 2005; accepted 15 July 2005; published 28 October [1] The latitudes of the foci of the Sq current systems over Australia and Japan are determined on a daily basis during the period from December 1989 to June The availability of a dense network of observatories in Australia during that time enabled a better determination in that region. Latitudinal movements of the foci are compared with the strength of the equatorial electrojet, and generally an increase in electrojet strength is accompanied with a poleward movement of the focus, especially in Japan. Some examples are noted where the foci in the two hemispheres move poleward or equatorward together from one day to the next, but this relationship was not found to be statistically significant. It is hard to disentangle effects due to other current systems such as Sq p from changes related to atmospheric tides. If some of the observed effects are due to tides, then the (2,3) and (2,4) semidiurnal modes are more likely contributors than the (2,2) mode. Citation: Stening, R., T. Reztsova, and L. H. Minh (2005), Day-to-day changes in the latitudes of the foci of the Sq current system and their relation to equatorial electrojet strength, J. Geophys. Res., 110,, doi: /2005ja Introduction [2] During times when the level of magnetic disturbance is low, the main current system flowing in the ionosphere has come to be known as the Sq system (or solar quiet ). The main driver of this system is thought to be the (1, 2) atmospheric tidal mode, generated in situ in the ionospheric E region by absorption of solar radiation [Tarpley, 1970; Stening, 1971]. The Sq system has two current whorls, one in each of the northern and southern hemispheres, which together feed into an equatorial electrojet flowing eastward along the magnetic equator. There are conspicuous seasonal changes in the current system [Matsushita and Maeda, 1965; Stening, 1971] and, while some features are similar between one quiet day and the next, noticeable changes may also be observed on similarly quiet days. Some of the most remarkable changes seen are variations in the strength of the equatorial electrojet with occasional reversals in direction of the electrojet, commonly known as a counterelectrojet. Another remarkable change occurs in the position of the center or focus of the current whorl in each hemisphere. It is thought that such changes on magnetically quiet days must be due to changes in the (tidal) winds systems in the ionosphere. It seems unlikely that changes in the diurnal (1, 2) mode could produce the changes observed in the currents or rather in the magnetic fields generated by the currents. It is more likely that Copyright 2005 by the American Geophysical Union /05/2005JA semidiurnal tides, known to also be present in the E region, will be responsible for these day-to-day changes [Stening, 1991]. Knowledge of how the focus positions relate in each hemisphere may enable us to at least deduce whether these tidal changes responsible are predominately symmetric or asymmetric about the equator. [3] During a dense network of magnetic observatories were set up on the Australian mainland [Chamalaun and Barton, 1993]. This network, known as AWAGS (Australia-Wide Array of Geomagnetic Stations), enabled a much more accurate pinpointing of the Sq focus position over Australia than is usually available. 2. Background [4] One of the more thorough examinations of this problem was performed by Schlapp [1976]. He concluded that the latitudinal positions of the northern and southern foci were only weakly related but their tendency was to move poleward and equatorward together. The correlation with electrojet strength was also weak but the tendency was for a stronger electrojet to correlate with foci more poleward. Schlapp used values of DH at an hour near noon, which he suggests is nearly the same as using DH at the time when DY is zero. He omitted all days for which the magnetic disturbance index C p > 0.5. He used data from the IQSY and from the IGY. Correlations from IGY data were rather higher than for the IQSY data since, as Schlapp suggests, disturbance current systems... tend to fluctuate synchronously over wide areas. His 1of6

2 Figure 1. Current vectors over Australia at 3 h UT on 5 May most significant correlations came from Spanish and African stations and from a Japanese and an Australian station with Koror (7.3 N, E) as the electrojet station. [5] Takeda and Araki [1984] followed the form of the Sq current system through 18 consecutive days in March 1980 when magnetic disturbance was low. They noted day-to-day changes. Some of these they attributed to currents flowing outside of the ionosphere. The others represented increases or decreases in the overall current amplitude or the addition of an apparent semidiurnal effect. None clearly demonstrated a change in focus latitude. Further similar studies were performed by Takeda [1984] using data from March Here changes in shape rather than intensity were noted but these were attributed to disturbances, as measured by the AE index, or, again, semidiurnal tides. [6] Kane [1974] examined data in the Indian region during 1964 and found that the Sq focus latitude shifted equatorward when the overall Sq current strength was larger and also when the equatorial electrojet strength was larger. This is opposite to the relation found by Schlapp [1976]. (Kane estimated the Sq strength from values of DY at Indian and Russian stations near the focus and used Trivandrum DH data to give the electrojet amplitude). His conclusions were reached from qualitative inspection of average curves during equinox in [7] So why did Kane and Schlapp reach different conclusions? First, it is not clear that Kane would have obtained a negative correlation if he had evaluated it, though his data do seem to indicate that this would be likely. Second, the Sq current system over India/Russia has some peculiarities. It almost disappears in winter [Rastogi et al., 1996], though Kane s results were restricted to equinox. Kane [1990] also discusses the variability of the focus position in South America in 1958, as evidenced by changes in DH at Trelew (43.3 S, 65.3 W). 3. Method [8] Different methods used in determining the latitude of the Sq system focus were discussed by Stening et al. [2005]. The preferred method was that in which the time when the declination variation DD changed sign was found first. This was the time when DD changed from negative to positive in the southern hemisphere and from positive to negative in the northern hemisphere. For the Australian data the eastern magnetic element DY was used. The horizontal element DH or northward element DX was then evaluated at the time when DD =0. [9] We calculated the focus positions in both north and south hemispheres each day for all months from December 1989 to June 1990, except February The latter month had so many disturbed days that it was not possible to obtain a useful plot. [10] The southern hemisphere focus was determined from an array of Australian stations which were operating during that period. In some cases we checked the focus position over Australia by drawing a full map of the current vectors from the AWAGS network as in Figure 1. The current vectors were obtained by rotating the magnetic field vectors clockwise through 90 while the magnetic vectors were derived from magnetometer data at the individual observatory sites. The strength of the electrojet at Baclieu (9.3 N, E, geographic) is estimated from the horizontal magnetic field variation DH. The mean of the preceding and succeeding midnight values are subtracted from the maximum value to determine the DH value plotted. There are occasional gaps in the Baclieu data. 2of6

3 Table 1. Observatory Geographic Coordinates Observatory Code Latitude Longitude Guam GUA 14N 145E Lunping LNP 25N 121E Kanoya KNY 31N 131E Kakioka KAK 36N 140E Memambetsu MMB 44N 144E Weipa WEI 13S 142E Cooktown CKT 15S 145E Robinson River ROB 17S 137E Mount Isa ISA 21S 139E Winton WTN 22S 143E Alpha ALP 24S 147E Birdsville BIR 26S 139E Quilpie QUI 27S 144E Etadunna ETA 29S 139E Bourke BUK 30S 146E Menindee MEN 32S 142E Port Augusta PTA 32S 138E Condobolin CDN 33S 147E Portland POL 38S 141E [11] For the northern hemisphere focus, usually determined from data from Japanese observatories, we were only able to determine the local time of the focus from changes in DD (we did not have a magnetometer array like that in Australia). This causes some uncertainty. In addition we found that during northern winter, the DD variation at the stations nearest to the equator sometimes did not exhibit the usual daily variation with a morning maximum followed by a later minimum. Instead a southern hemisphere type of variation appeared with a morning minimum. Such occurrences of this invasion phenomenon [Mayaud, 1965] led to obviously incorrect values. If the program yielded a time for DD = 0 outside of the range of 7 to 14 h LT, we replaced it with the zero time most commonly seen for that month at that station. 4. Results [12] Table 1 gives a listing of the geographic coordinates of the observatories used. Those used vary a little from month to month on account of breaks in the availability of data. We choose the best set of observatories for each month. In fact the same northern hemisphere observatories were used for all months but January, namely LNP, KNY, KAK, and MMB. In January we added GUA (Guam) for a better result. [13] It is difficult to decide at what level of magnetic disturbance we should start to reject data. Unfortunately, 1990 is near the maximum of the solar cycle, so a fairly high level of disturbance frequently occurs. On some disturbed days there is an additional westward current flow at high latitudes which extends on to the Australian mainland, or at least its magnetic effect extends there. This will result in pushing the observed focus to a lower latitude than normal. It is also well known [Onwumechili et al., 1973; Reddy et al., 1979] that the amplitude of the equatorial electrojet is often diminished on disturbed days, so a disturbance will lead to a positive correlation between focus latitude and electrojet strength. Days with a Kp disturbance index greater than 3 + in the 6 hours around local noon are marked with an asterisk on the bottom of the diagram. In February 1990, 18 out of the 28 days are disturbed according to this criterion, so we omit consideration of that month. [14] We look first at the plot for January 1990 in Figure 2 where we show the latitudes of the two foci and the strength of the electrojet at Baclieu, measured as described above. The Australian observatories used were CKT, QUI, BUK, CDN, and POL. Several days in this plot can be seen where all three plotted parameters move together, but clearly this does not happen all the time. [15] The day of 25 January 1990 may be an example of the disturbance influence mentioned above as both foci move equatorward together. The equatorward focus movements on 9 and 10 January we would also attribute to magnetic disturbance; Kp values are 4 + and 4 0 during local noon hours in eastern Australia. Another factor that arises on10 January is that the D variation at Lunping is of the southern hemisphere type with a morning minimum followed by a maximum, the invasion phenomenon mentioned above. [16] In March the observatories used were WEI, ISA, BIR, ETA, and PTA, a chain slightly to the west of those used in January, selected because they gave the best data coverage. There are a lot of disturbed periods. Dips in the Baclieu DH record usually occur at these times as can be seen in Figure 3 on 6 March (Kp =5 0 ), 13 March (Kp =6 + ), and 26 March (Kp =6 0 ). On 13 March the foci appear to move to very high latitudes and on 26 March they move to very low latitudes. [17] On 6 March the magnetic records do not show clear signs of disturbance and so, even though the equatorial electrojet amplitude is diminished, the focus positions are probably not much affected by the disturbance. On 13 and 26 March there are strong westward currents all over continental Australia. The declination D variation is also irregular and this invalidates the method we use to find the focus. [18] Two other notable changes might be noted in March. From 6 to 7 March the focus over Japan moves poleward by 12 and the focus over Australia moves poleward by 6. (The day of 6 March has a Kp of 5 0 but there is little clear evidence of disturbance on the records). From 17 to 18 Figure 2. Variations with the day of the month, in January 1990, of the latitude of the northern hemisphere focus (full lines and open squares), the latitude of the southern hemisphere focus (dotted lines and filled squares), and of the strength of the equatorial electrojet (dashed lines and crosses) at Baclieu. The actual value of DH at Baclieu (in nt) is obtained by multiplying the ordinate value by 5. 3of6

4 Figure 3. As for Figure 2 but for March Figure 5. As for Figure 2 but for June March the Australian focus moves poleward by 12 while that over Japan moves poleward by about 4. On these 2 days the Kp values are 2 and 1 0. [19] On 7 and 18 March there are clear afternoon reversed electrojets at Baclieu accompanied by poleward movements in both foci. These were the only clear examples of this phenomenon identifiable during quiet times within the period under examination. [20] The variations of focus positions and electrojet strength for May are shown in Figure 4. The Australian observatories used were CKT, ALP, BUK, and CDN. In a cursory inspection one again may see that all three parameters move up and down together. If the Baclieu electrojet plot is moved back (left) by one day, the correspondence may seem even more remarkable. Yet there are other times when no correspondence can be noted. The days of 11, 26, and 27 May had significantly large disturbances which would have influenced the results. [21] On 12 May 1990 the equivalent currents over Australia look quite strange. If there is an identifiable focus in Eastern Australia, it is certainly at quite a low latitude, less than 15 S and north of Cooktown. DH is positive at Port Moresby (9.4 S) so a focus exists between 15 S and 9 S. The Kp value is 3 + but the disturbance does not look severe. The day of 20 May is similar (Kp is 4 ) with a focus near 15 S. [22] On 19 and 22 May, magnetic disturbance obliterates the Sq system. On 20 May there is some evidence of a focus near the latitude of Cooktown but this is too far north to show a complete whorl. [23] The days of 13 to 17 May are a group of five quiet days where Kp <= 1 + near noon hours. We checked the large 10 poleward change in the focus latitude over Australia between 14 May and 15 May and found this to be correct (see Figure 1 for all current vectors on 15 May). The focus over Japan also moves poleward, by 6. [24] From 16 May to 17 May the focus over Japan moves poleward by 10 while that over Australia hardly moves at all. We mention these particular changes at quiet times, which have been checked by examining original data, as in Figure 1, to show that sometimes there is some degree of correlation and sometimes there is not. This is often what is found when dealing with geophysical data like these. [25] In June 1990 (Figure 5) we used CKT, QUI, BUK, MEN, and POL for the Australian chain. The day of 19 June is an interesting day in that the focus is clearly north of the Australian continent. DH is negative at all the east coast observatories after 13 h LT, while at Port Moresby (9.4 S) DH is positive. There is an eastward to westward transition in DH earlier in the day at 11 h LT, but this never becomes a focal point on the continent. There is an equatorward movement of the focus in Japan also on this day. [26] The December 1989 Australian observatory data were taken from CKT, ISA, QUI, MEN, and POL and the focus latitude variations are shown in Figure 6. The days of 10, 11, and 12 December are a group of really quiet days. On 11 December (Kp is 0 + ) the focus is off the map south of the continent and so at a latitude greater than 40. Both the northern and southern foci show a large poleward movement from 10 December to 11 December. [27] On 22 December the focus appears to be north of Memambetsu (43.6 N) in Japan. While the behavior is not uniform on disturbed days, there are several such days where the foci are more equatorward than usual. Figure 4. As for Figure 2 but for May Figure 6. As for Figure 2 but for December of6

5 Table 2. Correlations Between Focus Latitudes and Between Focus Latitudes and Electrojet Strength Australian focus Japanese focus Australian focus electrojet strength Japanese focus electrojet strength On 20 December, DH is indeed negative at Lunping (25.2 N) so that the focus is equatorward of that latitude. 5. Correlations Correlation Coefficient Number of Data Confidence Level % % [28] We evaluated the correlation coefficients between the latitudes of the northern and southern foci and between the latitudes of the foci and the electrojet strength as given by DH at Baclieu. We used data from 19 November 1989 to the end of June 1990, omitting all of February Japanese data also omitted the few days in November. We removed all days with a Kp value greater than 3 + during the UT interval 21 h to 06 h covering the middle of the day in the Australian longitude sector in local time. We also removed any data which were clearly incorrect. We see that the only significant correlation found was between the northern hemisphere focus latitude and the electrojet strength. 6. Discussion [29] Matsushita [1975] has shown that the direction of the east-west component of the interplanetary magnetic field affects the latitude of the Sq focus by about 4 of latitude. This seems to come about because the shape of the polar Sq p system is different for the two directions of the interplanetary field (IMF). The daytime part of the Sq p system moves from mostly in the late afternoon in the away phase of the IMF to earlier in the afternoon in the toward phase [Matsushita et al., 1973]. Since Sq p provides mainly westward currents at lower latitudes near the focus, the toward phase of the IMF lowers the DX component of the field at these latitudes by about 5 nt and so moves the focus equatorward. [30] It should be emphasised that Sq p is a magnetically quiet current system. The analysis of Matsushita [1975] required Kp 2+. The relative strength and position of this system will almost always have some influence on the Sq focus position and so will make it even harder to detect any effect due to changes in the atmospheric tides. Nevertheless, we are carrying out a similar analysis using data from the solar minimum year 1997 to compare with the present results. [31] We have made no corrections for ring current effects using the Dst index. We note that generally the Dst effect is larger at lower latitudes. A negative Dst is therefore likely to move the latitude at which currents reverse from east to west (the focus) equatorward and simultaneously cause a reduction in equatorial electrojet current, giving a negative correlation to compare with the results in Table 2. [32] As the correlation between northern hemisphere focus latitude and electrojet intensity is the most significant, we should first address possible mechanisms behind such an effect. Overall, the effect of magnetic activity and the Sq p system, just mentioned, may be the main contributor. However, we have also identified a few cases where these parameters move together during magnetically quiet conditions. This does not necessarily mean that the Sq p system is still responsible, but we will consider other possibilities. [33] It has been suggested [Stening, 1989, 1991] that changes in the electrojet strength and form, such as counterelectrojets, may be due to variations in the semidiurnal tidal modes driving the currents by means of the dynamo mechanism. The (2,2) tidal mode, which has a similar form for both the solar and lunar tides, yields current whorls with foci near 20 dip latitude. The superposition of this current system over the average Sq system, with a phase such that the electrojet strength is increased, would lead to a diminution of current strength around 25 dip latitude and so to an equatorward movement of the focus, opposite to what we observe. The diagram of Figure 7 gives some idea of how this might work. This also seems to disqualify lunar tidal effects from contributing to the observations described. The current whorls generated by a (2,4) mode wind system have their foci at a higher latitude than that of the average Sq system [Stening, 1977]. Thus an additional (2,4) mode wind could generate the observed correlation between electrojet strength and focus latitude. Stening [1989] investigated current systems arising from the asymmetric (2,3) mode. The results become complicated because geographic coordinates will govern the latitude structure of the tidal wind while the dynamo response is influenced by the geographic latitude of the magnetic equator in the longitude zone under consideration. It is possible that this mode may yield the observed effects, but the exact calculation in the Australian longitude zone has not been done. Figure 4 of Stening [1989] is suggestive in that a weaker electrojet is accompanied by an equatorward movement of the focus in one hemisphere and a somewhat lesser poleward movement of the focus in the other hemisphere. Such a (2,3) mode system could explain the presence of a correlation of the electrojet with the Japanese focus latitude but no correlation with the focus over Australia. On the other hand it may simply be Figure 7. Diagrammatic representation of current flows due to different tidal systems. The diagram represents the southern hemisphere daytime with latitudes indicated at the left. The full line flow is the dominant flow pattern as produced by the (1, 2) mode tide. The dotted line flow represents that additional flow produced by a (2,2) mode system with its focus at a lower latitude and a phase such that the equatorial electrojet is reinforced. 5of6

6 due to the Japanese focus being closer to the dip equator than the Australian focus, on the average. The poleward movement of the foci on the afternoon counterelectrojet (CEJ) days suggests that at least on these days, the (2,2) mode is not responsible for the CEJ. 7. Conclusions [34] (1) Large day-to-day changes are often observed in the latitudes of the foci of the Sq current system. (2) Some of these changes, but not all, may be attributed to the influence of higher latitude current systems such as Sq p and this may also account for simultaneous reductions in the equatorial electrojet amplitude. (3) On several occasions an increase in the latitude of the foci in both hemispheres occurs from one day to the next accompanied by an increase in equatorial electrojet strength. (4) If additional semidiurnal tides are responsible for some of the relations noted here, and this is by no means certain, then the (2,3) and (2,4) modes are more likely contributors than the (2,2) mode. [35] Acknowledgments. The Australian AWAGS data were kindly made available by Francois Chamalaun and Charles Barton. The generation of maps like Figure 1 was programmed by Jon Turner. The Japanese data were downloaded from The World Data Center C. [36] Arthur Richmond thanks Subramanian Gurubaran and another reviewer for their assistance in evaluating this paper. References Chamalaun, F. H., and C. E. Barton (1993), Electromagnetic induction in the Australian crust: Results from the Australian-wide array of geomagnetic stations, Explor. Geophys., 24, Kane, R. P. (1974), Relation between the strength of the Sq current system and its focus position, Proc. Ind. Acad. Sci., 80(1), Kane, R. P. (1990), Variability of the Sq focus position in the South American continent, Proc. Indian Acad. Sci., 99, Mayaud, P. N. (1965), Analyse morphologique de la variabilité jour-à-jour de la variation journalière régulière S R du champ magnétique terrestre, II. Le système de courants C M (Régions non-polaires), Ann. Geophys., 21, Matsushita, S. (1975), IMF polarity effects on the Sq current focus location, J. Geophys. Res., 80, Matsushita, S., and H. Maeda (1965), On the geomagnetic solar quiet daily variation field during the IGY, J. Geophys. Res., 70, Matsushita, S., J. D. Tarpley, and W. H. Campbell (1973), IMF structure effects in the quiet geomagnetic field, Radio Sci., 8, Onwumechili, A., K. Kawasaki, and S.-I. Akasofu (1973), Relationships between the equatorial electrojet and polar magnetic variations, Planet. Space Sci., 21, Rastogi, R. G., H. Chandra, and M. E. James (1996), On the disintegration of the vortex structure of ionospheric current system along the Asian longitude sector, J. Geomagn. Geoelectr., 48, Reddy, C. A., V. V. Somayajulu, and C. V. Devasia (1979), Global scale electrodynamic coupling of the auroral and equatorial dynamo regions, J. Atmos. Terr. Phys., 41, Schlapp, D. M. (1976), Day-to-day variability of the latitudes of the Sq foci, J. Atmos. Terr. Phys., 38, Stening, R. J. (1971), Longitudinal and seasonal variations of the Sq current system, Radio Sci., 6, Stening, R. J. (1977), Ionospheric dynamo calculations with semidiurnal winds, Planet. Space Sci., 25, Stening, R. J. (1989), A calculation of ionospheric currents due to semidiurnal antisymmetric tides, J. Geophys Res., 94, Stening, R. J. (1991), Variability of the equatorial electrojet: its relations to the Sq Current system and semidiurnal tides, Geophys. Res. Lett., 18, Stening, R., T. Reztsova, D. Ivers, J. Turner, and D. Winch (2005), A critique of methods of determining the position of the focus of the Sq current system, J. Geophys. Res., 110, A04305, doi: /2004ja Takeda, M. (1984), Day-to-day variation of equivalent Sq current system during March 11 26, 1970, J. Geomagn. Geoelectr., 36, Takeda, M., and T. Araki (1984), Time variation of instantaneous equivalent Sq current system, J. Atmos. Terr. Phys., 46, Tarpley, J. D. (1970), The ionospheric wind dynamo, 2. Solar tides, Planet. Space Sci., 18, L. H. Minh, Institute of Geophysics, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Str., Cau Giay, Hanoi, Vietnam. T. Reztsova and R. Stening, School of Physics, University of New South Wales, Sydney, NSW 2052, Australia. (r.stening@unsw. edu.au) 6of6