Magnetic variations at other latitudes during reverse equatorial electrojet

~~~~~~~~0, .~t~osph,wr und ~urcsrr~u~ Physics. Vol. 29. pp. 1071 1077. Pergamon Press 1977. Printed in

Northern Ireland

Magnetic variations at other latitudes during reverse equatorial electrojet R. J. STENING W. S. & L. B. Robinson University College, University of New South Wales, P.O. Box 334, Broken Hill. N.S.W. 2880, Australia Abstract-Several reverse equatorial electrojet events are examined. Magnetic variations from a sample of worldwide stations on the same days are compared with the quiet daily average for the same month. In most cases departures can be found elsewhere at the same time as the reverse jet. Various wind systems in the dynamo region might produce the observed results but different systems are required on different occasions.

INTRODU(JTION The

phenomenon

of the reversed equatorial

electrojet

has been receiving increased attention lately. GOUIN and MAYAUD(1967) drew attention to the fact that the horizontal geomagnetic variation AH on occasion fell below its night-time value on magnetically quiet days at Addis Ababa. Investigations at other equatorial electrojet stations showed a preference for sunspot minimum years, different seasonal variations depending on the station, and a tendency for lunar control (e.g. HU~N and OYINLOYE,1970). ONWUMECHILLIand AKA~OFU (1972) examined some non-equatorial stations at the time of reverse jet occurrence and found no perturbation from the normal at the non-jet stations. They therefore asked the question “Where does the return current of the reverse jet flow?“. The present investigation was aimed at answering this question. On coming to examine the geomagnetic data, the author brought with him a conviction that the reverse jet effects were caused by an additional current system in the dynamo region generated by some thermo-tidal mode. Such a current system would produce effects at other latitudes and these should be searched for. The aforementioned conviction is not entirely substantiated by the following data, but it provided a framework for considering the results. METHOD OF ANALYSIS The days of reverse electrojet occurrence were originally chosen from Huancayo magnetograms for 1964 and 1965. A spread of other magnetic observatories over the globe operating at that time were selected with an emphasis on trying to get lines of stations at the same longitude as electrojet stations, but at different latitudes.

The stations are listed with their coordinates in Tabie 1. One would always wish for more stations to be available in an investigation such as this, but, at least in the American sector, the coverage is not too inadequate. Breaks or absences of records at particular stations on particular days sometimes also occurred. Following the idea that the reverse electrojet is part of a current system which is superposed on the ‘normal’ Sq system, it was felt necessary to compare the magnetogram for the reverse jet day with some ‘normal’ magnetogram from the same station (see captions). The choice of a ‘normal’ magnetogram presents a problem, as what might look like a ‘normal’ day at one station may be found to have abnormalities elsewhere. Where possible the mean variation for the five quiet days of the same month was taken as the normal variation for comparison. In January 1964 two of the reverse jet days themselves are quiet days, making this procedure unsuitable, so a particular day was selected for comparison. It is important to note that all the ‘normal’ curves plotted for comparison have been determined in the same way for the different stations on the same day. All the curves in Figs. l-8 have been plotted from hourly mean values of the horizontal geomagnetic component. The values of the planetary geomagnetic index A, and of the largest K, value for the day are indicated in the captions. An attempt has been made to cause a coincidence between the morning and evening night-time levels of the reverse jet curve and the normal curve. It will be seen that a shift in the ‘Sq baseline’ sometimes occurs during the course of the day so that it is not possible to make both morning and evening levels coincide. It is not a worldwide D,, phenomenon but is restricted to a few stations (e.g. some stations in

1071

1072

R. J. Table 1. Stations used

Abbreviation African group Moscow Odessa Misallat Addis Ababa Moca Nairobi Tananarive Hermanus

Geomagnetic latitude longitude

Dip

Mos Od Mi Ad Ab Mot Na Tan He

50.9 43.1 26.9 5.4 5.7 -4.4 -23.7 - 33.3

120 111 106 109 79 105 112 81

71.2 64.4 42.5 -0.4 - 17.3 - 25.6 - 53.9 -65.8

Fr Da Tu SJ Fu Hu La Q Pi Tr A Is

49.6 43.0 40.4 29.6 16.9 -0.6 - 10.6 - 20.2 -31.7 - 53.8

350 328 312 03 355 354 03 05 03 03

70.2 63.2 59.5 50.7 33.3 1.3 - 14.7 -28.5 -40.8 -58.1

Me Ka Ko Pt M To M Is

34.0 20.5 -3.2 - 18.6 -46.1 -61.0

208 198 204 218 221 243

57.5 44.2 -0.8 - 32.8 - 68.4 -79.1

Tas Sa Al Kod Ke

32.3 20.5 9.5 0.6 -57.3

144 150 144 147 128

60.6 45.2 23.0 2.6 - 67.4

American group

Fredericksburg Dallas Tucson San Juan Fuquene Huancayo La Quiaca Pilar Trelew Argentine Island Pa& group Memambetsu Kanoya Koror Port Moresby Toolangi Macquarie Island Indian group Tashkent Sabhawala Alibag Kodaikanal Kerguelen

STENING

the American Zone in Fig. 8). It would be desirable to have some more sophisticated method of correcting for these level changes, but a deeper understanding of the various current systems present at various times will be needed before this can be done. DlSCUS!SION

OF RESULTS

(a) Migrating semidiurnal tide At the start of this investigation it was hoped that deviations from the normal Sq pattern could be identified as current systems generated by a travelling semidiumal tidal mode. In this case one would expect to see (i) an increase in AH in the morning when there is a decrease in the afternoon, (ii) evidence of a focus of the perturbing current system with deviations of AH from the normal pattern in opposite directions above and below this ‘perturbing focus’, and (iii) similar deviations at similar local times in different longitude zones. Examination shows that the only days which fit this pattern are 13 and 14 January 1964 although a few other days not shown were found. Looking at 13 January 1964 (Fig. 1) in the American Sector, the afternoon northern hemisphere perturbing focus appears to be south of Fuquene. There is a clear deviation at San Juan. Now this may be regarded either as a shift of the ‘Sq focus’ from its normal position which is south of San Juan or the addition of an extra semidiumal current system causing positive AH in the afternoon and negative AH in the morning at higher latitudes. The afternoon

Ka2lN

PtM 19 s To475 +f ,Mls6lS

LMT

Kod0.6N Ke 57s

4

Tan24S

LMT

Fig. 1. Magnetic variations on 13 January 1964 (solid line) and 22 January 1964 (dashed line). Longitude sectors, from left to right, are Pacific, Indian, African and American. A, = 6, K, (max) = 3 -. Scale 2 h = 20 nT except Huancayo: 2 h = 50 nT. Latitudes indicated are geomagnetic.

Magnetic variations during reverse equatorial electrojet

Mos51N Mi 27N Tas32N Al95N

AdAb 5 N

Moc6N

Kod 0.6N

Tan 245 Ke57S He 335 41

6

12 LMT

I8

I.2 LMT

6

f8

12 LMT

6

18

,,.I

I; LMT

6

18

Fig. 2. Magnetic variations on 14 January 1964 (solid line) and 22 January 1964 (dashed line). A,, = 1. K,(max)= l-.

positive deviation may also be regarded as due to the ‘return current’ of the reverse equatoriat electrojet in evidence at Huancayo. In the southern hemisphere the perturbing system focus is probably near Pk. Similar patterns can be found in the Indian sector, but in the African and Pacitic sectors the deviations appear to be below the normal at all latitudes and the in~~~~tion fails. On 14 January 1964 (Fig. 2) the large morning positive deviation at Huancayo returns mostly in the southern hemisphere. The perturbing system focus would be north of La Quiaca in the morning and south in the afternoon. While the afternoon perturbing focus position could fit the ‘2,T mode, the morning situation is some more peculiar effect. Evidence of semidiumal effects can be seen

also in the other longitude sectors. In both of Figs. 1 and 2, Misallat has a negative deviation in the afternoon in the same sense as Addis Ababa, which places the perturbing system focus more than 22” from the dip equator. There does not seem to be any clear evidence of effects associated with the ‘2,4’ tidal mode with a focus more than 30” from the equator and a ~midiumal variation. (b) Other wind systems Although some aspects of the variations on 29 January 1965 (Fig. 3) would support a semidiurnal tide, the variations are not in phase (Koror has an increase in the afternoon, while Huancayo has a decrease). A standing ~~diurn~ mode might be con-

Kod 0.6 N Ke 575 I

6

LMT

12 LMT

18

6

12 LMT

I8

6

I2 LMT

18

Fig. 3. Magnetic variations on 29 January 1965 (solid line) and the 5 Quiet days mean for January 1965 (dashed line). A, = 5, K, (max) = 2+.

R. J.

1074

_+++f

Ka21 N

+,-++.$

PtM19S

-J-t-

To47S

fhENING

AdAb5N

A-

Tas32N Moc6N

w

;;;.:“, No 4 S


MlsGIS

Kod0.6N

~~~r&~

1

I2

6

l.l~!ll 6

18

Fig. 4.

-.

1

I2

I8

6

I2

I8

Tr32S

.- Al5545

I6

LMT

Magnetic variations on 1 February 1965 (solid line) and the 5 Quiet days mean for January 1965 (dashed line). A,, = 4, K, (max) = 3-.

sidered but the universal times of the AH minima at equatorial stations are also different. Data from 1 February 1965 (Fig. 4) have some similar characteristics but again a semidiurnal mode fixed in either local or universal time cannot be fitted. Return currents at higher latitudes can often be found corresponding to equatorial deviations. Likewise 25 November, 1965 (Fig. 5) does not fit into a local or universal time pattern. On 5 December 1965 (Fig. 6) there are decreases at the equator at all longitudes but no increase. In many places it appears that the negative effect lasts

for more than 8 hr. In America the negative deviations extend as far south as 32”, while there is some return current at San Juan in the north. On 7 December (Fig. 7) 2 days later, the pattern is similar but with increased intensity in America but decreased in India and the Pacific. A further 2 days later on 9 December (Fig. 8) some of these characteristics still remain in the American zone but there is a shift to a positive morning deviation at other longitudes. This positive deviation is in the same sense at all latitudes shown in the Indian and Pacific sectors indicating a source with a focus higher than 34”N, maybe even

I -+--’

PtM

19s

pi -

,

^\Y

I2

6

LMT

LMT

LMT

\ \-

1,

Ke 57 S

‘ /d

xYI-

To47S

_-. &

,’

‘\

‘\

/

‘LoOIlS

I ,I’ ti-*

*\ \

Tan 24 S

He 33s

6

12 LMT

16

6

12 LMT

I8

6

12 LMT

I8

6

I2 LMT

I8

Fig. 5. Magnetic variations on 25 November 1965 (solid line) and the 5 Quiet days mean for November 1965 (dashed line). A, = 5, K, (max) = 3 -

Pi 205

Magnetic variations during reverse equatorial electrojet

Mo5 51 N Od 44 N Mi

Iti;;l;-$l

Al

9.5N

AdAb

’

_p’

61 S

TOQ 245

~

Ke 57 S He 33 s

Ll6

5 N

Mot 6 N

Kod 0.6 N ,_ f. . M Is

27N

I2

6

18

12 LMT

LM-

I8

Fig. 6. Magnetic variations on 5 December 1965(solid line) and the 5 Quiet days mean for November 1965 (&shed line). A, = 3. K, (max) = 1 C.

into the aurora1 region. Again the effect does not appear to be semidiumal, at least at the lower latitudes. A small disturbance may be seen in the afternoon in the African Zone and in the morning in the American Zone (Fig. 8). Some AD variations were also examined at Trelew and San Juan. These are usually reckoned as giving some measure of the ‘total Sq current strength’ in their respective hemisphere. On both the days of Figs. I and 2 and two other days not shown AD was larger than normal in the southern hemisphere and smaller than normal in the northern hemisphere. This might indicate the existence of asymmetric tidal modes. Other effects may be due to the operation of a particular tidal mode in one hemisphere only.

(c) Calculations The current systems produced in such cases as those just mentioned now need to be calculated. The results of one such calculation are shown in Fig. 9. Here we show the directions of current flow due to a ‘2,2’ semidiurnal wind system operating in the northern hemisphere only. The calculation was performed using the equivalent circuit method of STENING (1968). It can be seen that a normal two-whorl current system exists in the northern hemisphere. whereas in the southern hemisphere current directions tend to be eastward at all latitudes at 08OOLT and westward at 1600. The southern hemisphere currents are driven by electrostatic fields generated in the northern hemisphere and communicated to the south-

. I--

T .,Jr-TT-rT’ ; Fr TN”1 +~

1 -’

.

* “ -.--\..\

1

Me

34N

Ko

21 N

, &““-

r

T*

Tr---

. _’ -F’-‘. _

Mos51

-[---:-_

k-4

_+

N

Od 44 N Mi 27N

/-iI

x Ml56lS Ke 57 S

L / , LLLJ 6 I2 I6 LMT

6

I2 LMT

50 N

Da 43 N

16

Fig. 7. Magnetic variations on 7 December 1965 (solid line) and the 5 Quiet days mean for November 1965 (dashed line) A, = 3, K,, (max) = 2.

R. J.

1076

STENING

Mos 51 N Od44

N

MI 27

N

--

Tos 32

N AdAb

So

SJ 30 N Fu 17 N

5 N

21 N

Al 9.5 N

Mac 6 N

Kod 0.6 N

Tan 245

No 4 S

Ke57S 6

12 LMT

I8

6

I2 LMT

18

He 33s 6

12 LMT

18

6

I2 LMT

18

Fig. 8. Magnetic variations on 9 December 1965 (solid line) and the 5 Quiet days mean for November 1965 (dashed line). A, = 6, K, (max) = 3 +.

by the magnetic field lines. At 30% the current amplitude is about 50% of that at the same northern latitude, reducing to 30% at 40”. The variations shown in Fig. 5 could possibly be explained by a system such as this. Directions of the fieldaligned current flow are also shown. The amplitudes of the field aligned currents are about four times those found when the winds operate in both hemispheres together. It has been noted that some deviations have the same sign for 8 hr or more and this, on the face of it, would suggest a semidiumal mode is not responsible. Yet calculations, details of which will be separ-

em hemisphere

Fig. 9. Horizontal electric current directions calculated for a ‘2.2’ semidiumal mode operating in the northern hemisphere alone, centred on American longitudes at equinox. Length of arrows has no significance. Q and 8 indicate regions of maximum upward and downward field aligned current flow in the southern hemisphere.

ately reported, show that semidiumal wind systems can give rise to current systems which yield a AH of the same sign for eight or more hours at certain latitudes. (d) General discussion Some of the global patterns of geomagnetic variation presented here are hard to explain. These include (i) when the deviation is confined to a narrow strip round the dip equator, (ii) when the phase of the deviation varies with longitude but cannot be fitted into universal time either, (iii) when the sign of the deviation is the same throughout the hemisphere so that no return current can be found (unless it runs along magnetic field lines into the other hemisphere). The effect of wind and temperature profiles in the mesosphere (GELLER, 1970) may allow some tidal modes to penetrate into the dynamo region at some times and not others. If these wind and temperature profiles vary with latitude and longitude, it would seem possible that tidal energy might only leak through into the upper layers in restricted areas on the globe. These possibilities have yet to be investigated. Other wave numbers of the semidiurnal oscillation might be considered but these all have amplitudes on the ground of less than one tenth that of the wave number 2 (HAURWITZand CCWLEY,1973). In addition to winds associated with tidal modes, the effects due to planetary waves should also be looked at. These waves are at times dominant in a particular longitude zone and the pattern shifts slowly over a period of days. The similar behaviour of the magnetic deviations on 5. 7, and 9 December 1965,

Magnetic variations during reverse equatorial electrojet

for example, may reflect this behaviour. The possibi-

latitudes. They are sometimes

lity of an association

sphere or the other.

and stratospheric gested

between

warming

(STENING, 1977).

mentioned

reverse jet occurrence

events has also been sug-

However

in this report occurred

none

of the days

during a stratwarm

period. FAMBITAKOYE et al. (1976) have shown that a westward high altitude

(above 125 km) wind system can produce effects which concentrate at the dip equator. This wind system, which could form part of a planetary wave pattern, would yield other smaller changes at non-equatorial stations. Patterns such as on 7 and 9 December 1965 may possibly be explained in this way. There must be some special mechanism which accounts for the deviations which are sometimes very much larger in the electrojet than at other latitudes. CONCLUSIONS (1) Return equatorial

currents

electrojet

for deviations

from the normal

can very often be found at higher

IQ17 confined

(2) On occasion the reverse jet seems to be associated with the additional imposition of a current system generated by a semidiurnal tidal mode. (3) Identification of tidal modes added is difficult as return currents often appear in only one hemisphere. A calculation shows that this effect can be reproduced by a wind system operating in one hemisphere only. (4) At other times the wind systems associated with planetary waves seem to be more likely to be responsible for deviations from the normal pattern. (5) Current patterns persist or change slowly over several days. Acknowledgements-The data were obtained during a short stay with the Data Studies Division of the National Geophysical and Solar-Terrestrial Data Center, Boulder. The assistance of all staff of the Center is acknowledged and financial support was received from the U.S. National Academy of Science.

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to one hemi-

1976 1970 1967 1973

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1970

An& Gt!ophys. 26, 927

1972 1968 1977

J. Geomag. Geoelectr. 24, 161.

Planet. Space Sci. 16, 717. J. atmos. terr. Phys. 39, 157.