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

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 1990. 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 Sqp 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

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 2005.

[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 1990 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 Sqp

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, A10308, doi:10.1029/2005JA011219.

1 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

iono-spheric 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

hemi-spheres, 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

simi-larly 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

magnet-ically 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

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 1989 – 1990 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 Sta-tions), enabled a much more accurate pinpointing of the Sq focus position over Australia than is usually available

[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 Cp > 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

Copyright 2005 by the American Geophysical Union.

0148-0227/05/2005JA011219

most significant correlations came from Spanish and

African stations and from a Japanese and an Australian

station with Koror (7.3°N, 134.5°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 1970 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 ofDY 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 1964

[7] So why did Kane and Schlapp reach different

con-clusions? 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 inDH at Trelew (43.3°S, 65.3°W)

[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 variationDD changed sign was found first This was the time whenDD 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 elementDY was used The horizontal element DH

or northward element DX was then evaluated at the time whenDD = 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 1990 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, 105.7°E, geographic) is estimated from the horizontal magnetic field variation DH The mean of the preceding and succeeding midnight values are sub-tracted from the maximum value to determine the DH value plotted There are occasional gaps in the Baclieu data

Figure 1 Current vectors over Australia at 3 h UT on 5 May 1990

[11] For the northern hemisphere focus, usually

deter-mined 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

occur-rences of this ‘‘invasion phenomenon’’ [Mayaud, 1965] led

to obviously incorrect values If the program yielded a time

forDD = 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

the disturbance influence mentioned above as both foci move equatorward together The equatorward focus move-ments on 9 and 10 January we would also attribute to magnetic disturbance; Kp values are 4+and 40during 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 fol-lowed by a maximum, the ‘‘invasion’’ phenomenon men-tioned 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 = 50), 13 March (Kp = 6+), and 26 March (Kp = 60) 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 50but 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

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 10

[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

param-eters 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

Aus-tralia 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

inDH 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 move-ment 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 3 As for Figure 2 but for March 1990

Figure 4 As for Figure 2 but for May 1990

Figure 5 As for Figure 2 but for June 1990

Figure 6 As for Figure 2 but for December 1989

On 20 December, DH is indeed negative at Lunping

(25.2°N) so that the focus is equatorward of that latitude

5 Correlations

[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 1990 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

Sqp system is different for the two directions of the

interplanetary field (IMF) The daytime part of the Sqp

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 Sqp

pro-vides mainly westward currents at lower latitudes near the

focus, the toward phase of the IMF lowers theDX

compo-nent of the field at these latitudes by about 5 nT and so

moves the focus equatorward

[30] It should be emphasised that Sqp 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,

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 diminu-tion 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 coor-dinates 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 accom-panied 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

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

[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 Sqp

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

geomag-netic stations, Explor Geophys., 24, 179 – 186.

Kane, R P (1974), Relation between the strength of the Sq current system

and its focus position, Proc Ind Acad Sci., 80(1), 17 – 25.

Kane, R P (1990), Variability of the Sq focus position in the South

Amer-ican continent, Proc Indian Acad Sci., 99, 405 – 412.

Mayaud, P N (1965), Analyse morphologique de la variabilite´ jour-a`-jour

de la variation journalie`re ‘‘re´gulie`re’’ S R du champ magne´tique terrestre,

II – Le syste`me de courants C M (Re´gions non-polaires), Ann Geophys.,

21, 514 – 544.

Matsushita, S (1975), IMF polarity effects on the Sq current focus location,

J Geophys Res., 80, 4751 – 4754.

Matsushita, S., and H Maeda (1965), On the geomagnetic solar quiet daily variation field during the IGY, J Geophys Res., 70, 2535 – 2558 Matsushita, S., J D Tarpley, and W H Campbell (1973), IMF structure effects in the quiet geomagnetic field, Radio Sci., 8, 963 – 972 Onwumechili, A., K Kawasaki, and S.-I Akasofu (1973), Relationships between the equatorial electrojet and polar magnetic variations, Planet Space Sci., 21, 1 – 16.

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, 1481 – 1488.

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, 189 – 201.

Schlapp, D M (1976), Day-to-day variability of the latitudes of the Sq foci,

J Atmos Terr Phys., 38, 573 – 577.

Stening, R J (1971), Longitudinal and seasonal variations of the Sq current system, Radio Sci., 6, 133 – 137.

Stening, R J (1977), Ionospheric dynamo calculations with semidiurnal winds, Planet Space Sci., 25, 1075 – 1080.

Stening, R J (1989), A calculation of ionospheric currents due to semi-diurnal antisymmetric tides, J Geophys Res., 94, 1525 – 1531 Stening, R J (1991), Variability of the equatorial electrojet: its relations to the Sq Current system and semidiurnal tides, Geophys Res Lett., 18,

1979 – 1982.

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:10.1029/2004JA010784 Takeda, M (1984), Day-to-day variation of equivalent Sq current system during March 11 – 26, 1970, J Geomagn Geoelectr., 36, 215 – 228 Takeda, M., and T Araki (1984), Time variation of instantaneous equiva-lent Sq current system, J Atmos Terr Phys., 46, 911 – 915.

Tarpley, J D (1970), The ionospheric wind dynamo, 2 Solar tides, Planet Space Sci., 18, 1091 – 1103.

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)