The day-to-day variability of upper atmosphere dynamo currents

A. PHILLIPS and B. H. BRIGGS

OK-Yl69,‘91 %3.UU+ 00 Pergamon Prrv plc

tidal winds and

Department of Physics and Mathematical Physics. University of Adelaide. Adelaide. SA 5000. Australia

Abstract-A daily measure of the strength of the ionospheric current system near noon for each day of 1985 has been determined from magnetometer records. These daily values are correlated with daily tidal amplitudes in the mesosphere observed by the spaced-antenna wind technique. If upward-propagating tidal modes play any part in inducing dynamo currents some correlation would be expected. but none is found. Also. the day-to-day variations of the diurnal and semi-diurnal tides are uncorrelated with each other. Possible reasons for the results are discussed.

1. INTRODUCTION

Ionospheric dynamo currents, computed from mag-

netometer records, vary considerably in form and amplitude from day-to-day. If longer-term effects due

to solar cycle. solar rotation and seasonal changes are removed, the residual variations seem to be random.

and arc almost uncorrelated from one day to the next (BRICKS. 1984). The amplitudes of these random vari-

ations are quite large, and are of the order of 50%.

The dynamo currents are generally believed to flow

between altitudes of 100 and 150 km and to be induced by tidal winds, which move the conducting air in the

Earth’s main magnetic field (e.g. RICHMOND et ol., 1976). Upward-propagating tidal winds observed by

ground-based radar techniques in the height range 8& 100 km are also found to be variable from one day to

the next (e.g. VINCENT and BALL, 1977). Assuming

that the observed tides continue to propagate upwards

into the dynamo region. some correlation might be expected, on a day-to-day basis, between the currents

and the tides. It is the purpose of the present paper to search for such a correlation using data from the Australian region.

2. DATA

Estimates of the strength of the dynamo currents arc based on the range of H, the horizontal North- South component of the magnetic field measured at

the Earth’s surface. On days which are magnetically quiet. the magnetic effects of the currents arc denoted

by S, (quiet day solar variation). Here we use a method described by HIBBERD (1981, 1983) which

enables currents to be estimated on disturbed days as

well. and denote the variations by S, (regular solar

variation). Hibberd’s method involves using the

difference AH of the H values at two magnetic observ-

atories which have the same longitude but different latitudes. Since the disturbance variations arc very similar at the two places they cancel, but the effects due

to the ionospheric currents remain. For studying the

strength of the daytime current system. a good choice for the two observatories is for one to be north of the

‘focus’ of the current loop, and the other to be to the

south of the focus. The signs of the daily variation

of H are then opposite, but the departures add on subtraction. giving a peak deviation of AH near noon

each day. For the present study. the observatories

used and their geographic coordinates were Canberra (35 S. 149 E) and Port Moresby (9 S, 147 E).

The method is illustrated in Fig. 1. which shows

plots of the hourly mean values of H for October 1985.

The records show irregular variations superimposed

on the regular daily variations. The difference AH is

much less disturbed, and the regular daily variations stand out clearly. A measure of S, was assigned to

each day of the year 1985 by measuring the maximum deviation (in nT) from the mean nighttime level. In

this way it was possible to obtain S,< values for all but

a few days of the year. These S, values arc shown in

Fig. 2 (top). Figure 3 shows the autocorrelation function of the

daily values of S, to shifts of _t60 days. The cor-

relation drops by a large amount for shifts of k I day, and then by much smaller amounts for k:! and f 3 days. The long-term trend is due to seasonal variations. The autocorrelation function for this sun- spot-minimum year of 1985 differs from that for 1972

39

40 A. PHILLIPS and B. H. BRIGGS

OCTOBER 1985 PORT MORESBY - (mean=931.8nT)

50

2 0 X

-50

5d- CANBERRA (mcan=698.9nT)

DIFFERENCE

-50 t 1 2 3 4 ‘5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

UNIVERSAL TIME/days

Fig. I. Plots of hourly mean values of H for Port Moresby and Canberra for October 1985. The lower trace is the difference AH.

Fig. 2. Daily values of various quantities for each local-time day of 1985. S, is the magnitude of AH in nT. ~1,. I’, are the amplitudes of the EW and NS components of the diurnal tide in m s- ‘. 1,:. 13~ are the

amplitudes of the EW and NS components of the semi-diurnal tide in m s- ‘.

Variability of tidal winds and dynamo currents 41

-0 4

1 -60 -40 -20 0 20 40 60

LAG/days

Fig. 3. Autocorrelation of the daily S, values

of higher sunspot activity (BRICKS, 1984. fig. 5) in

the absence of a 27-day solar rotation effect, but is otherwise very similar.

7 7 Tides ___.

An extensive database of ground-based radar observations of winds is held at Adelaide. We have

made use of the winds measured by the spaced-antenna

technique using a 3 MHz radar at Buckland Park

(35 S. 138 E).

Most previous studies of tides have used mean

values determined for data intervals of many days,

in an attempt to minimize local effects. At a recent

international discussion meeting. it was concluded that .‘the term *tide’ should only be used with reference

to oscillations of 12 and 24 h which are globally coher- ent in the form of modes in the classical tidal theory

sense”. An analysis interval of 30 days was recom-

mended to help ensure this (reported by FORBES.

1986). However. for the present investigation it is necessary to determine ‘tides’ on a daily basis. This is

because S,, with which the tides are to be correlated, shows hardly any correlation from one day to the next. It is recognized that the daily tides may be influ-

enced by purely local effects : this point is considered

further in Section 4. The procedure for determining the daily tidal com-

ponents was as follows. To reduce random errors and record gaps, the data were averaged over the height interval 87.55975 km. This is the highest IO km inter- val for which results are available, and therefore is the interval most likely to show correlation with dynamo currents. Also, at these heights results are available for the full 24 h. which is necessary for determining

the diurnal tide (at lower heights results are obtained

in the daytime only). Averaging over the 10 km height

interval should not greatly affect the tidal amplitudes ; at Adelaide the diurnal tide has a vertical wavelength

of 3040 km, and the wavelengths of the semi-diurnal tides are even larger (VINCENT et ul., 1989). Any reductions in amplitude which may occur would not

affect the present results, because absolute values are

not needed in the computation of correlation coefficients, The possible elimination of modes with

short vertical wavelengths could be an advantage,

since such modes would be unlikely to be correlated

with ground-level magnetometer records because of

phase cancellation in the dynamo region.

The data wcrc also averaged in I h blocks to give

hourly means centred on each hour of local time.

These means have adequate time resolution for the

determination of diurnal and semi-diurnal tides. The

tides were determined separately for the EW(U) and

NS(r,) components of the wind by least-squares fitting,

and using 24 h data blocks centred on local noon. The

fitted functions were

I’ = (‘1,+1’, cos (c0tt0,) fl’,COS (?or+O,,

where (11 = 27~124 h- ‘, I+,, p,, = constant components,

V, = diurnal amplitudes, u2, L’, = semi-diurnal

f:mphtudes. $,, 0, = diurnal phases. and 4?, 112 = semi-diurnal phases.

In accordance with a common convention for tides,

the phases were expressed by the local times - 4 ,,iw,

-O,,/(o. - $2/~f~. and -O,/U. at which each com- ponent has its maximum positive value.

Even after the height and time averaging, there were

still gaps in the hourly wind values. Days for which 50% or more of the hourly values were missing were

not used for the determination of the tides. This

resulted in a loss of 120 days out of the possible 36.5.

The amplitudes ~1,. L’,. 112 and L’? are shown in Fig. 2. It will be seen that the day-to-day fluctuations are

quite large. For the diurnal tide, the standard devi- ation is about 40% of the mean, except for a period in March/April when the mean increases; the stan-

dard deviation is then about 30% of the mean. For the semi-diurnal tide, the standard deviation is about 50% of the mean at ail times of the year.

Before proceeding further it is desirable to examine the daily tidal values in order to test their reliability. Also, some statistical analysis of the results will be

made, because they are of some interest in their own right, quite apart from their possible correlation with the dynamo currents.

Tidal modes at 35 S are expected to be elliptically

42 A. PHILLIPS and B. H. BRIGGS

polarized in an anti-clockwise sense, with the

axes of the ellipse aligned NS and EW. Therefore

for each individual day we would expect to find

M, = kp,, u2 = k2uZ (where k,, k, are constants),

(&,-0,)/o = 6 h and (4,-0,)/w = 3 h. Of course, these results will not hold exactly because of random

effects, both geophysical and experimental. Never-

theless, we would expect day-to-day fluctuations of u, to be highly correlated with those of I’ ,, and similarly

for u? and r12. We would also expect a distribution of

the values of (4, - 0 ,)/w to show a peak at 6 h, and a

distribution of the values of (4?-U2)/(f) to show a peak at 3 h. If the data exhibit these features we can have

confidence in the fitting procedure using individual

days Fig. 4 shows cross-correlation functions of I(,

against P, and u2 against zj2. In each case there is the

-0-4 I

-80 40 -io 6 20 4b 60

1’01 I

-io- -60 0 T -7” 20 60

LAG/days

Fig. 4. Cross-correlation functions of 11, against C, and u2 against v2.

expected positive correlation at zero lag; the

maximum correlation is about 0.6. Figure 5 shows

distributions of diurnal and semi-diurnal phase

differences, and these show peaks near 6 and 3 h. It is interesting that both distributions are slightly dis- placed from the expected values; the mean phase differences are 7.0* 0.1 h for the diurnal tide and

3.4kO.l h for the semi-diurnal tide.

It is of interest to investigate whether the random fluctuations of tidal amplitude show persistence from

one day to the next, as this may assist in identifying

the cause of the variations. To study this we have

computed the autocorrelation functions of u,, 11,. 11: and 11~ and these are shown in Fig. 6. The fluctuations

of the amplitude of the diurnal tide appear to be correlated over about four days, at least for the EW

component (the behaviour of the NS component is less clear). The longer-term effects are due to seasonal

changes in the amplitude. The fluctuations of the

semi-diurnal amplitudes show no persistence at all

from one day to the next. It is also of interest to see whether the fluctuations

of the diurnal and semi-diurnal tides are correlated

with each other. Figure 7 shows cross-correlation

functions of ~1, against u,, and L’, against v?. There is no evidence of any correlation.

The significance of these various results is con-

sidered in Section 4.

3. CORRELATION OF TIDES AND

DYNAMO CURRENTS

To search for any relationship between the tides

and the dynamo currents, cross-correlation functions of S, against u,, I’,, U, and c2 were computed to lags

of k60 days. The results are shown in Fig. 8. Any

correlation of the daily variations would show up as a peak near zero lag.

The results are completely negative. There appears to be no relationship between the variations, apart

from some long-term correlations due to seasonal effects. The latter show up particularly in the cor- relation of the dynamo currents with the diurnal

tide. since both of these have significant seasonal changes. The correlation of about fO.2 at zero lag in this case does not imply any correlation of the day- to-day fluctuations, since there is no peak at zero lag.

In case this negative result was due in some way to the use of the AH differencing technique, the cor- relations were also computed using the actual daily departures of H for each station separately. This could be done for quiet days only. The results (not shown) also indicated no correlation with the tidal wind amplitudes.

Variability of tidal winds and dynamo currents 43

-4 -2 0 2 4 6

4 s

lo 12 14 16 18

-

6 7 8 9

PHASE DIFFERENCE/h

Fig. 5. Distributions of the phase differences between the NS and EW tidal components : top-diurnal; bottom-semi-diurnal.

4. DISCUSSION though it seems rather surprising that this effect _ -

The lack of any correlation between the day-to-day alone could reduce the correlations to zero.

fluctuations of the upward-propagating tides and of 2. The observed ‘tides’ may not be tides in a global

the dynamo currents is unexpected. However it is not sense (FORBES. 1986) and the observed fluctuations

difficult to think of a number of possible explanations. may be due to effects which are quite localized. The

Any of the following may be important : dynamo currents, on the other hand. are a large-scale phenomenon and will depend on the average tidal

1. The measured tides in the height range 87.5- driving forces over a large region. Localized effects 97.5 km will be subject to further changes as they which could cause the day-to-day fluctuations of the propagate up to the dynamo region at about 106 tides include : patchy regions of solar insolation (water 150 km. Changes in propagation conditions in this vapour or ozone), local variations of upward propa- height interval may reduce the expected correlations, gation conditions, and localized modulation of depo-

44 A. PHILLIPS and B. H. BKIC;C;S

1.0

0.8

3 0.6 &

& 0.4 0

5 0.2

d 3 0.0 0 U

-0.2

1.0

0.8

f 0.6 ;

g 0.4

i; < 0.2 d g 0.0

8 -0.2

-0.4

-60 -40 -20 0 20 40 I

1’011 0.8

? IO

P -60 -40 -20 0 20 40 f

LAG/days

Fig. 6. Autocorrelation functions of u,. r,. ~1~ and I’?

? 9

5 -60 -40 -20 0 20 40 t

LAG/days

sition of momentum by gravity waves. Any of these

could have day-to-day variations and their spatial scales are unknown. Non-migrating tides may be

important (KATO, 1989). To investigate these possi- bilities further, it is clearly important to observe the tides simultaneously at more than one place. in order

to investigate over what spatial scales they are cor- related [see also FORBES (1984) for a discussion of tidal

variability]. 3. The dynamo currents may be driven entirely by

the in-situ thermospheric (I,-2) diurnal tide, rather than by upward-propagating modes. The (I,-2) mode would be of small amplitude in the height range 87.5-97.5 km [the diurnal tide observed in the present work is probably the (1,l) propagating mode]. While it is generally agreed that the (I.-?) mode is very important in driving dynamo currents, it is also gen- erally thought that this mode could not vary greatly

on a day-to-day basis, because it is not subject to the

vagaries of transmission through a changing atmo- sphere. For this reason SALAH and EVANS (I 977) and

RICHMOND et al. (1976) considered it likely that the turiubility of the dynamo currents arises from currents

driven by the upward-propagating (2.4) semi-diurnal mode. If this is so, it seems unlikely that our inability

to observe the (I .-2) mode can by itself account for

the negative results, since currents which do not vary from day-to-day will not affect the correlation coefficients.

However. GREENER and SCHLAPP (1979) suggest that the (I ,-2) mode [denoted by (I.-I) in their paper] could have day-to-day variations because of spatial variations in the concentration of molecular oxygen. This would result in patchy heating by the ultra-violet radiation and consequently some irregu- larity in the resulting wind patterns.

Variability of tidal winds and dynamo currents

“T 0.8

I

Fig. 7. Cross-correlation functions of U, against U? and r, against vL

1.0

0.6

c 1 0.6 i s z; o’4 i; I y=$ 0.2

-60 -40 -20 0 20 40

LAG/days

1.0-

3 a 0.6. i

g 0.4.

F; -t 0.2-

ri

2 0.0.

8 -0.2-

-O.lL -60 -40 -20 0 20 40 t

‘;: a_ 0.6

rz

g 0.4

F; 4 0.2

4

2 0.0 0 u

-0.2

-0.4

-60 -40 -20 0 20 40 t LAG /days

45

Fig. 8. Cross-correlation of S,. the daily amplitude of the dynamo currents, with the four tidal amplitudes 11,. 1‘1. u2 and 1‘:.

46 A. PHILLIPS and B. H. BRICGS

4. It may be that the results are telling us that the dynamo currents are not tidally driven. Alternative theories have been proposed by MATSUSHITA (1971) and HIBBERD (1989).

Some interesting properties of the tides in the height range X7.5-97.5 km have been revealed by this inves- tigation. The day-to-day fluctuations of the diurnal and semi-diurnal tides arc uncorrelated. and the time scale, or persistence. of the fluctuations is quite differ- ent for the two. These results both suggest that differ- ent factors are producing the fluctuations in the two cases. Possible explanations for this might be :

1. The diurnal and semi-diurnal tides may be driven at different levels in the atmosphere, and therefore the day-to-day variations in the heating rate may be diffcrent. It seems probable that at 3.5 S the diurnal tide is driven by water vapour insolation close to the ground. while the semi-diurnal is driven by ozone insolation in the stratosphere (GROVES, 1982a,b). The variability at the two levels is likely to be unrelated. and the time scales of variations may be different.

2. The tides may be affected differently by back- ground winds as they propagate from their rcspcctive source regions to the region where they are observed.

It should be pointed out that the amplitude of the senli-diurnal tide is small at the latitude of Adelaide (35 ). Therefore, there are large errors in the ampli- tudes for individual days. In spite of the evidence of Fig. 5 (which appears to suggest that the daily determinations are fairly reliable) it may be that the errors arc tending to dominate the results. This could explain the lack of correlation from one day to the next and with the dynamo currents. In this connection it is interesting to note that KAYDALW and PORT- NYAUN (198 I), observing at a higher latitude where the semi-diurnal amplitudes are larger. found that fluctuations were correlated over several days. GLASS

et al, (1978) show band-passed velocity fluctuations, which also appear to suggest that the semi-diurnal tide has persistence over many days. However. as the width of the band-pass is not stated, it is impossible to be sure that this is not an artefact of the filtering.

A conclusion that upward-propagating semi-diur- nal tidal modes are not involved at all in the pro- duction of dynamo currents would contradict many theoretical studies which suggest that the contribution of such modes ought to be significant (e.g. FOREES

and LIN~ZEN, 1976; TAKEDA and YAMADA. 1987;

SXNING, 1987: TAKEDA, 1990). It would therefore be

valuable to repeat the search for correlations using wind data from a station at a higher latitude where the semi-diLlrnal tide has a larger amplitude.

The subject of tidal variabihty and day-to-day per- sistence is of considerable interest in its own right, quite apart from possible correlations with the dynamo currents. Work in this area is being con- tinucd.

5. CONCL.tiSIONS

No correlation was found between day-to-day Ruc- tuations of upward-propagating mesospheric tides and dynamo currents in the Australian region. Poss- ible reasons for this have been discussed.

This paper represents only a first preliminary attempt to relate the fluctuations of measured upward-propagating tides to phenomena in the iono- sphere at greater heights. It is belicvcd that further work along these lines would be valuable.

,It.l;-tzoi~(e~~~~~~~~f.s--This work forms part of a program of upper atmosphere research supported by the Australian Research Council. We are indebted to the Austraiian Bureau of Mineral Resources Geology and Geophysics for the sup- ply of magnetogram data. Helpful discussions with T. J. Harris. W. K. Hocking and R. A. Vincent are acknowledged with thanks,

REFERENCES

BKIIXS B. I-I. FORTES J. M. FOKNS J. M.

FOKMES J. M. and LINDZEN R. S. GLASS M., BEKNAKD R., FELLOUS J. L.

awl MASSEBEUF M.

GREENEK J. G. and SCHLAPP D. M. GKOVES G. v.

@WVES G. V. f~lBBEIW F. H.

HI~~EKD F. H. KATO S. KAYI>AL~)V 0. V. and POKTNYAGIY Yu. I

I984 19x4 19%

J. uIt~Ios. twr. Ph15. 46, 419. J. um~os. wr. PhJ?. 46, 1049. Middle Attnospherc~ Prqrcm Hundhook, Vol. 2 1. p. 2 I.

SCOSTEP Secretariat, Urbana, U.S.A.

Variability of tidal winds and dynamo currents

MATSUSHITA S.

RICHMOND A. D., MATSUSHITA S. and TARPLEY J. D SALAH J. E. and EVANS J. V. STENING R. J.

TAKEDA M. TAKEDA M. and YAMADA Y. VINCENT R. A. and BALL S. VINCENT R. A., TSUDA T. and KATO S.

1971 Radio Sci. 6, 279. 1976 J. geophys. Res. 81, 547. 1977 J. geophys. Res. 82, 2413. 1987 J. geophps. Res. 94, 1525. 1990 J. atmos. terr. Phys. 52, 59. 1987 Ann. Geophysicae SA, 429. 1977 J. utmos. ferr. Phys. 39, 965. 1989 J. utmo.v. terr. PII~T. 51. 609.

R&w~ce is also made to the following unpublished materiul.

HIBBERD F. H. 1989 IAGA Meeting, 24 July4 August, Exeter, U.K

47