On the return current of the equatorial electrojet

Adv. Space Res. Vol. 12, No. 6, pp. (6)33—(6)43, 1992 Printed in Great Britain. All rights i~served.

02731 177/92 $15.00

Copyright © 1991 COSPAR

ON THE RETURN CURRENT OF THE EQUATORIAL ELECTROJET C. Agodi Onwumechili Ananthra State University of Technology, P.M.B. 01660, Enugu, Nigeria

ABSTRACT In practically all cases, investigators have found it compelling to include westward currents on the flanks of the dip equator in order to fit well the observed dip equatorial magnetic variation profiles. There are three sources of the return currents: the geometry of field lines in the dynamo region, the polarization at the boundaries of enhanced conductivity at the dip equator, and the local neutral winds varying with h 8ight. The.. sembime constructively, taking advantage of the peaks of conductivities around 5 dip latitude, to provide for the return currents of practically all the eastward0eleotro.jet c8rrent. The return currents flow on the flanks of the dip equator from about 3.5 to about 20 dip latitude, in any case not extending beyond the Sq focus. The negative correlation between the width and the intensity of the equatorial electrojet has been confirmed with data derived from physical model, indicating its origin in the return currents. The ionospheric current system so far detected by rockets is essentially in two layers. The intense lower layer including the return currents peaking around 5 dip latitude may be associated with the equatorial electrojet; and the weak upper layer that maintains fairly steady altitude characteristics everywhere may be associated with the worldwide part of the Sq currents. INTRODUCTION Shortly after its articulation, Chapman /1/ named the then new phenomenon, the equatorial eleotrojet (EEJ). He gave three empirical models of the EEJ: the line current model, the errent ‘ribbon of constant intensity model, and the current ribbon of parabolic intensity model. He expected the return current of EEJ to flow on the flanks close to magnetic dip equator where it should depress the geomagnetic solar quiet daily variation of the horizontal field sq(H). He urged a search of the return current in that region. As it became clear that none of the three models of Chapman /1/ fitted well, the field of the EEJ observed on the ground, Onwuinechili /2/ introduced a two—dimensional continuous distribution of current density model. This model fitted very well the horizontal field of the EEJ observed on the ground in its entire range, the altitude profile of the EEJ current density observed by rockets, as well as the worldwide part of the observed horizontal field of Sq. However, he was initially surprised that the profiles of the observed field led to the parameter ~ being negative. Onwumechili /3/ also reported that the continuous distribution of current density model fitted the measurements of Forbush and Casaverde /4/ extremely well, with not only ~ being negative but the horizontal field was also negative on both flanks of the dip equator, reaching up to about —19 nT. This led to the realization that the significance of negative ~ was the presence of westward (return) currents on both flanks of the magnetic dip equator. Reporting this in /2,3/, the configuration of current distribution in the EEJ was given as in Figure 1. However, opinion seems to have drifted away from the existence of the return currents of the ~ZJ close on the flanks of the dip equator in spite of relevant evidence reported by various investigators from time to time. Most of them analyzed their observed data with empirical models: the line current model, a delta function; the current ribbon of constant intensity model, a step function; the Current ribbon of parabolic intensity mo4el, divergent but trim— cated; the current ribbon of fourth degree intensity model, the square of the parabolic model function; and the continuous distribution of current density model. Apart from the eontinñous distribution model, all the others end abruptly; have zero thickness and therefore no possibility for contours; are unidirectional and consequently their users have been forced to *

Current address: 18 COhn Close, Cohindale, London NW9 Q~T,United Kingdom

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2 relative to its peak density Jo, at Fig. 1. (a) forContours EEJ densitycurrent j A/lan in solid contours and westward the centre, ~ — —2ofand ~ current 0. Eastward current in broken contours. (b) Variation of EEJ current intensity J A,/km relative to its peak intensity J , with latitude x for ~ — -2. (c) Variation of current intensity J relative to J 0, with°latitudex if u 0 (from /2,6/). introduce ad hoc westward currents in order to fit ~bserveddata. In the continuous distribution of current density model, the density j A/km at the point (x,z) is given by 22 222 2 .a(a + acx)b(b +pz).o .~IJ0 2 22 2 221 (a +x) (b +z) where x is measured northwards, y eastwards and z downwards from the current centre; ~ is unit vector; Jo is the density at x 0, z — 0; a and b are constant scale lengths along x and z respectively; and ~ and p are dimensionless constants controlling the distribution of current along x and z respectively. For expressions of the current and magnetic field parameters and values that can be derived from these 5 primary parameters, see /5,6,7,8,9/. The derived parameters include: the intensity J .A/km, the total current I amperes, the width 2w ha, the thickness 2d ha, the total latitudinal extent 2L1 ha, and the total vertical extent 2L2 km. There is no need for truncating because the current and magnetic fields decrease naturally to zero with increasing distance. It fits current and magnetic fields, observed horizontal and vertical profiles, and contours are possible. Fitting observed data determines the sign of ~. If ~ ~ 0, the current is unidirectional but if ~ ~ 0, westward (reverse) currents flow on the flanks of eastward currents at the magnetic dip equator, Figure 1. INDICATIONS OF RETURN CURRENT ~R0}J SATELLITE OBERVATIONS OF THE ELECTROJET The signature of the P2J observed by P000 satellites was not the expected simple V—shape /10/. This was modified by the existence of high shoulders on the flanks of the magnetic equator. The most natural explanation of the high shoulders is the existence of westward currents on the flanks of the dip equator. In Figure 2 Cain and Sweeney /10/ extrapolated with dashed curve, the course of the Dst event at the time. The high shoulders were always present on quiet days, but Even in this case, the westward currents were able to roll back the decrease engendered by the Det event and succeeded in creating the high shoulders. As the effect of the westward currents wanes, and the profile returns from the high shoulders to the general background cFve, apparent minima are created on the flanks. Although Cain and Sweeney /10/ used the current ribbon of constant intensity model of ELI, they stated, “Again, the very high shoulders on sonic of the ~F profiles indicate that a simple model without some westward currents in nearby latitudes may be unreasonable.. • .The high shoulders on the profiles seem to imply a frequent westward current during these times”.

Equatorial Electrojet Return Current

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Onwumechili and his collaborators /9,11,12,13,14,15,16,17,18,19/,

have studied the parameters of the EEJ relevant to this review using the P000 satellites data and the continuous distribution of current density model, equation (1). These parameters include: the ratio of the peak intensity of the westward current J , on the flanks of the dip equator to the peak intensity of the eastward electrojet J , at tL dip equator; the location of the peak intensity of the westward current x km from tRe dip equator, and its ratio to the half width w kin of the eastward ELI; and the ~‘atitudinalextent L 1 km of the ELI. In the continuous distribution of current density model these parameters are functions of a and a only and are given by ~nI~O xdw

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They analyzed satellite profiles ~n September equinox covering 1100 to 1500 LT. Their grouping into 36 longitudes at 10 intervals gave values in the ranges given in Table 1 row A. The improved analysis of /19/ covered 894 satellite profiles in three seasons from 0900 to 1500 LT. Of these, 865 or 97% gave values in the intervals given in Table 1 row B. All the 894 profiles gave a < —1, with 13 between —1.20 and —1.26, and 16 between —2.00 and —2.63. This parameters show that the westward currents are sufficiently strong to be the return current of the equatorial electrojet (ELI). From the parameters of the EEJ determined from POGO satellites data of December 1968 and June 1969 soistices, Onwumechili and Ozoemena /16/ constructed an elevation of the ELI as would be seen under the sun, Figure 3. This shows how the eastward ELI could return close by both flanks of the dip equator. TABLE 1 Some EEJ Parameters from (A) 510 Satellite Profiles of September Equinox, (B) 865 Satellite Profiles of Three Seasons, (C) 24 Ground Profiles from India, Nigeria and Peru, and (D) 171 Days Ground Data from Central Africa.

~5/J0

—

Mm. Mean A B C D

Max.

—1.99 —1.86 —1.72 —2.00 —1.53 —1.26 —2.20 —1.59 —1.26 —2.00 —1.26

%

x~/w

Mm.

Mean

Max.

—33

—30 —23 —24

—27 —18 —18 —18

—33 —38

—33

Mm. 1.93 1.81 1.81 1.81

Mean 1.96 1.88 1.90

x Max. 2.00 2.00 2.05 2.00

km

Mini. Mean 415 530 520

469 570 680

L1 lan

Max. Mm. Mean 521 600 830

735 810 1080 600

784 1290 1440

Max. 838 2250 3100 1100

INDICATIONS OF RETURN CURRENT OF THE ELECTROJET FROM GROUND—BASED OBSERVATIONS Hesse /20/ found from northeastern Brazil, very clear depressions of the horizontal magnetic field profiles on the flanks of the magnetic equator. These manifest the same phenomenon of westward currents on the flanks as the high shoulders of satellite profiles. The high shoulders above the EEJ are now inverted into depressions below the EEJ as would be expected. As the negative field of the westward current wanes with distance, and the profile recovers from the depression towards the general level of the background field, apparent maxima are created on the flanks making the profile look wavy. Thus, Reese /20/ writes of “permanently appearing undulated shape” and “waviness” of the latitudinal profiles. Describing Figure 4 he stated, Nit can be seen that the phenomenon of waviness is directly connected with the amplitude and direction of the EEJ. At 1000 hr when the enhancement of current density at the magnetic equator is about zero, there is no phenomenon of waviness. With the increasing and later on decreasing ELI also the phenomenon of waviness increases and decreases. And in the early morning, when the ELI changes direction, the shape of the undulated latitudinal profiles is inverted”. He appreciated that one possible explanation of the waviness was the westward return currents on the flanks expected in /1/ and reported in /2,8/. Hesse used the line current model and transformed the results. Because eastward line currents cannot fit the negative field causing the depressions he had to introduce westward line currents on the flanks of the magnetic dip equator. The horizontal field of external origin derived by Forbush and Casaverde /4/, their Figure 12, clearly showed negative field south of the dip equator but was not mentioned. They used the line current and the current ribbon of constant intensity models. Since these models cannot fit negative fields, they would have been forced to introduce westward current if they had pursued getting better fit of their data from Peru. From the data of /4/, Onwwnechili /3/ found a< —l as well as negative horizontal field north and south of dip equator, and attributed it to the return currents of the EEJ close by the flanks of the dip equator. The result of ~utton /21,22/ from South American data showed the waviness reported by Hesse ~2O/. Stening /23/ interpreted the features of /21,22/ as depressions of H peaking at about 5.5 N latitude.

(6)36

C. A. Onwumechii

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Fig. 3. Subsolar elevation of EEJ for the combined solstices of December 1968 and June 1969 from POGO satellites data. 5000 A flow between adjacent lines (from /16/).

Fig. 4. Hourly latitudinal profiles of the northward field variation, AB nT, on 26 Oct 1971 in Brazil (from /20/).

The data of 3 surveys in 2 solstices and one equinox in Nigeria /24,25,26,27/ were analyzed in /8/and each set gave a (—1, indicating the presence of substantial westward currents close by the flanks of the dip equator. Oldenburg /28/ used the equinox data in /27/ and the inversion theory of Bachus and Gilbert to estimate the height integrated current density producing the observations. He confirmed the westward currents on both flanks of the dip equator and stated that they “are well established and are not likely the result of erroneous data because the magnitude of the current density estimate is determined primarily by the magnetic perturbations within a few degrees on either side of the latitude of interest and it is unlikely that errors at the northern and southern stations are such as to produce westward current densities there... .The example of data from the ELI demonstrates the existence of both eastward and westward current flow in the region of the dip equator. These results are similar to those from the analysis by Onwumechili and Ogbuehi /27/”. From each of 21 profiles of Indian observatories data Onwumechili and Ogbuehi /29/ obtained a <—1, confirming /8,28/ and the same interpretations apply. Together, 24 ground—based profiles from India, Nigeria and Peru /3,7,8,27,29,30/ yield HE.! parameters in the intervals given in Table 1 row C where values under columns of “Mean” are medians. Fambitakoye and Maysud /31/ found the depressions of H as in /20/ from their Central African ground data and called then “deformations”. They used the current ribbon of fourth degree model to generate polynomials P and Q for fitting and were forced to introduce westward currents stating, “One can show that the 1330 profiles (their Figure 6) are well simulated with a current distribution resulting from the superposition of two ribbons of current flowing in opposite directions, with the westward ribbon about twice as wide. Because such deformations of profiles are not rare, the analysis is made with a double set of functions P and Q, assuming that both ribbons are at the same height h and have the same centre”. Their superposition of two current ribbons flowing in opposite directions produced a current intensity distribution like Figure lb and thus confirmed /2,5,6,8/. The ratio of the peak westward current to the peak of the eastward current in their superposed current ribbons, provides a translation of their results to equivalent parameters in the continuous distribution of current density model. The equivalent intervals corresponding to results obtained and accepted by them /31/are given in Table 1 row D. These confirm values of our p~rameters. They also got into tune with our ideas when they interpreted the situation when S~, the curve for the worldwide part of Sq intersects and is above S, t~ecurve for the fichE of the RE.! as around the depressions by stating, “T~ismeans that tile undergoes a ~hange of sign and is apparently disconnected from the Si.. . . It can also happen that the currents become negative

SR

SR

Equatorial Electrojet Return Current

whereas S~ currents are still

(6)37

positive”.

Using the global data of /32/, Suzuki /33/ solved the dynamo equation for ~, the current of the HE.!, together with div J — 0 to produce the elevation, Figure 5. He stated, “The field (E — is a polarization l~eldproduced by the mechanism that the currents collected in the morning side of the equator because of the high conductivity, cannot escape from there in the afternoon side against the lower conductivity outside the equator. This is the driving force of the return currents... . It is seen from Figure 5 that the return currents begin to leave the equator gradually after the peak of the jet intensity around 11 hr. Theoutgoing currents are largest between 13 hr and 14 hr and incoming ones are largest around 8 hr. . . .The east— west component of the return currents reveals a decreasing tendency of the intensity with increasing latitude”. Thus the return current is most intense close to the dip equator, and nearly all the eastward currents return before the Sq focus.

~)

INDICATIONS OF RETURN CURRENT OF TIlE EQUATORIAL ELECTROJET FROM PHYSICAL MODELS Sugiura and Poros /34/ ~mproved the physical model of /35/ and found reverse westward currents from about 4.5 and peaking at about 6 dip latitude north and south. They attributed it to the reversal of vertical polarization field. Subsequently, physical models got into the regime of invoking a combination of assumed tidal modes. It is possible to simulate virtually any current or magnetic field by adjusting the amplitudes and phases of freely selected combination of tidal modes as the hypothetical winds driving the dynamo mechanism. This underlines the concern of Stening /36/. Richmond /37/ encountered the result of /34/ and reported, “Mapping the polarization field down the magnetic field lines, we find that the eastward current density (j~) at 110 km minimizes around 60 from the dip equator and increases towards the ‘background”values at greater latitudes. This depression of j~below the background value may be thought of as a partial return current for the large eastward current nearer the equator”, but he found it could account for only a small fraction of the EEJ. He also found that east—west winds varying with height importantly affect the latitude profile of currents and magnetic fields beyond a few degrees dip latitude. With assumed tidal (1,1) mode he si~u1atedthe reverse westward current overlaid by eastward current observed by rocket at 4.5 dip latitude /~s/. Fambitakoye, Mayaud and Richmond /39/ postulated high altitude wind profiles to explain the depressions of H caused by westward currents close by the flanks of the dip equator and they derived a current intensity profile very much like Figure lb. They adjudged, “we conclude that the thermospheric winds are variable not only during the course of a day but also from day to day and month to month. Nevertheless, there seem to be average winds present through the year, which make their presence known by their characteristic effects on the H and Z profiles averaged for the year (in Figure 6 of /31/). In particular, secondary (westward) ribbons are present in the yearly profiles between 1130 and 1530 LT, suggestive of high altitude westward winds during this part of the day”. However, only the arbitrary limits set in /31/ prevented the inclusion of westward ribbons for 0830, 0930 and 1030 LT data. Consequently, their depressions were not fitted and they reappeared in the residuals in Figure 6 of /31/. Therefore their above conclusion on the presence of average annual winds applies to the whole day from 0830 to 1530 LT. Anandarao and Raghavarao /40/ gave a crucial test of the effect of zonal neutral winds with real winds measured in /41/. They found, Figure 6, that, “Substantial negative (westward) current c~hls on either side of the dip equator, centred on 5 dip latitude and extending beyond 10 are formed... .On either side of the equator the contribution of the uniform wind, primarily because of curvature of the field lines, become equally significant, and may even predominate over the wind shear effect”. The peak eastward current density increased by about 5% on 9 Feb 1975 and about 14% on IA Feb 1975, while the width was compressed in latitude to about half its size without the wind. Reddy and Devasia /42/ studied electric fields and currents generated by 10 altitude profiles of local east—west winds, Ul to U8 plus U7R and UER, 4 of which: US, U6, U7 and U8 were real winds measured in /41,43/, and the rest were hypothetical. 0They confirmed /37,39,40/, that the effects are small close to the dip equator but beyond 3 dip latitude the wind generated current predominates an~reverses the total ER.! current into westward current more likely0 observable from 3 to 7 dip latitude; and that ~hiE substantially changes the widths: W1 of the total HE.! current intensity ~T profile and W2 of the total horizontal magnetic field ~ We have analyzed Table 2 given by /42/. The correlation coefficients with AJ% are —O.86t for and —0.899 for W2, both statistically significant with 99.5% confidence level. In Figure 7 solid lines are regressions of W on A..~and broken lines are regressions of A~% on W. Note the parallelisms. It is abundantly clear that the Table 2 eloquently confirms the results of Onwumechili and Ogbuehi /27/ and /12,18/ that the intensity of the ER.! increases as its width decreases along with its other landmark distances. Amazingly, Reddy and Devasia /42/ concluded that Table 2 was “inconsistent with the suggestion of Onwumechili and Ogbuehi /27/”.

(6)38

C. A. Onwumechili

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/44, 45, 46/ the situations in Richmond /37/ and Reddy and Devasia /42/ are special cases They found: (i) On integrating along field lines: Pedersen gonductivity reaches a maximum at about 5.4 geomagnetic0latitude and decreases to about 19% by 20 ; while the ~al1 conductivity reaches maximum at about 4.6 and decreases to about 14% by 200. (ii) The divergence of east-west currents makes a large contribution to electrostatic field in the EEJ region. (iii) Because of strong coupling, a

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large part of the contributions of global wbserved winds, at 1100 Ill’ (from /40/). tidelike winds, fohlsw-ing.hlgher latitude dynamo, come from latitudes higher than 100 but less than midlatitudes. (iv) On the contrary, following dynamo action in the equatorial and low latitude regions, dynamo emfs in these regions do not cause significant current to flow out into the middle and high latitudes, and moat of the transverse currents leaving the generating region of the equatorial ionosphere return along paths extending not far out from the generating region. (v) Since the integrated TABLE 2 Widths of Latitudigal Profiles at Half Maximum Value, W~’for EEJ Current Intensity J , and W2 for ER.! Horizontal Magnetic Field O~IL~, with their Associated Ch~ngesin EL! Current Intensity t0J Per Cent.

Wind Altitude Profile U5 Ul

~m Profi~e Width W1

~H,,,Profi~e Width W2

4.0

5.0

3.4

3.0

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4.6 6.6

Intensity Change AJ% 17.0

16.0

6.4

6.6

9.8

12.2

8.0 7.0 1.1 —1.1 —8.0

117

12.0

15.0

—10.3

US

20.0

20.0

—16.0

U6 112 134 U3

4.0

5.2

I

conductivities decrease considerably before 120 latitude, it is not convenient for these returning current flow paths to lie in high latitude ionosphere. (vi) The equatorial local dynamo action of an east-west wind produces vertical polarization electrostatic field and an associated Hall current in the east-west direction. On the other hand, the higher latitude dynamo action produces the eastward electric field often taken as uniform. (vii) Near the equator the wind dynamo field !x~, is cancelled by the associated polarization field and therefore no significant current is produced. But away from the equator, the vertical polarization field decreases sharply, the dynamo field becomes dominant and generates the westward current. (viii) In the general case, the main eastward HE.! extends to about ,0 latitude and peaks over the dip equator at about 102 km altitude. A weak upper layer exists with a peak

Equatorial Electrojet Return Current

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2. (a) General case of local plus distant dynamo, the eastward field generated by the distant dynamo calculated Fig. 8. with Contours of zonal uniform currentelectric density A/km self—consistently. Contour interval — 0.25, maximum — 1.25, and minimum — —1.25. (b) Special case of local dynamo only, with eastward electric field from the distant dynamo set equal to zero. Contour interval — 0.25, maximum — 0.60, minimum — —1.40. Solid contours for eastward current and dashed contours for westward current (from /46/). current density of about 0.5 A/km2 at an altitude of about 122 km. (ix) “Away from the equator at about 3.5 latitude and 111 km altitude a large peak of westward current appears. The sheets of strong westward currents centred around the above altitude are mainly Hall currents near the peak of Hall conductivity”. The weak eastward upper layer continues above the westward current sheets, Figure 8. INDICATIONS OF RETURN CURRENT OF EQUATORIAL ELECTROJET FROM ROCKET MEASUREMENTS It is seen from /17/a and some of the above that the westward current extends very thinly from about 30 to about 12 dip latitude or evgn beyond. Therefore the only reasonable chance of detecting it is near local noon around 5 dip latitude. There are only two rocket launches in tha~region: Cahill /47/ near Line Islands about 537 km north and Maynard /38/ near Peru about 4.5 N dip latitude. Both detected currents as ~o11ows: in /47/, westward currents from 102 km to 108 km altitude with a peak of about —7 A/km at 106 km, overlaid by eastward currents from 109 km to 124 km altitude peaking probably around 113 and in /38/, westward currents from 104 km to 109 km altitude with a peak of about —0.8 A/km near 107 km~overlaid by eastward currents from 109 km altitude to 129 km with a peak of about 2.3 A/km near 114 km altitude. With hypothetical winds, this altitude structure of currents has been simulated by physical models in /37,42,46,48,49/. From a comprehensive study of rocket measurements Onwumechili /50/ constructed Figure 9, showing: (i) the weak global upper layer, eastward before and westward after the Sq focus; (ii) the lower layer, observed up to 20°latitude, westward around 5°dip latitude; (iii) the crosshatched section of the transition zone in which the lower layer was observed sometimes and at other times the upper layer, as evidenced from /51,52/. This compares Figure 8. It is suggested that the lower layer is the EEJ and the upper layer the worldwide part of Sq. ELECTROJET VIEWS PHOM THEORY, GROUND, ROCKET AM) SATELLITE MEASUREMENTS With the abundant evidences from all sources in Table 3 there is hardly any doubt that reverse westward currents flow on the flanks of the dip equator. Average values of parameters need to be taken with the characteristic variability of ionospheric currents in mind. The values are —l8%~J,,/J 0~ —33%, mean —27+3%; x — 5.3±0.7° or 585±73 lan; 1.8l~x,,/w~2,mean 1.91+0.04; 10 ~ L1~30

,

mean 10±3 or 1113+284 km, but more likely 12

to 20

(6)40

C. A. Onwumechili

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9.

Latitudinal profile of the altitude structure of ionospheric currents (from /50/).

~ Location x

1~O

TABLE 3 Sunnnary Latitudinal ExtentofLIndications, Peak Current Ratio ~ 1, of the Westward Return Currents Evidences of Westward Current From Satellite /9,10,11,15, 16,17,19,53/ From Ground /2,3,5,7,8,20, 21,22,23,26, 27,29,30,31,33/ From Model /28,34,37,39, 40,42,46,48,49/ From Rocket /38,47,52/

Ratio of Peak Westward Current J I.! % ~ o From Satellite —30 in /9/ —23 in /19/ From Ground —24 in /8,29/ —26 in /31/ From Model —29 in /40/ From Rocket —30 in /52/

of the HE.!.

Location of Westward Peak x m

469

km in /9/ 570 km in /19/ 680 km in /8,29/ 662 km in /9,20/ 5.5° in /21,22,23/ 6.0: in /34/ 6.0 in /37/ 3.5° in /46/ 5.0 in /40/ 4.5°in /38/ 537 ion in /47/ 5.00 in /52/

,

and

m

Latitudinal Extent L 1 From Satellite 784 km in /14/ 1290 km in /19/ From Ground 1440 km in /8,29/ 850 km in /31/ 1200 las in /20/

In Brazil, Musmann and Seiler /52/ recorded magnetic variations on a ground profile, and simultaneously measured with rockets in the ionosphere, three components of magnetic field and electron density. About 5 discrepancies arose among the data and their comgarison with /35/. They stated, “These discrepancies can be explained by currents flowing 5 off the magnetic equator, on both sides, with intensities of about 0.3 of the EEJ intensity at the same height but in reversed direction..., the measured values of ~F and ~D (of meridional currents) can be explained only by taking into account the reversed current~”. The reversed current also enabled them to explain the findings of /54/. Thus the reverse westward currents constitute such an important component of the EEJ that it is needed to fit all observed magnetic fields of EEJ and also to interprete the meridional currents of the ER.!. Cases in which views of EEJ from different sources agree include: (i) The values of the parameters in Table 3. (ii) Contours of /17/ from satellite and rocket data agree so well with are close, (iii) Subsolar of EE.! from satellite data /16/ like is those ~rom physical2 model /40/ that altitude and elevation latitude extents of individual contours similar from global observatories data /33/. (iv) Various landmark values of ER.! 1 A/km to and that —2 A/km derived from satellite data /9/ are consistent with those from ground data /20/, model /40/, and rocket data /38,47,52/. (v) The current distribution with reverse westward currents on the flanks originally derived from ground data /2,6/, has been amply confirmed from satellite /16,17/, ground /31,33/, model /39,40,42,46/, and rocket /52/ data. CONCLUDING DISCUSSIONS The review having established the existence of westward currents close by the flanks of dip equator and their important role in the features of HE.! manifestations, it remains their interpretation in relation to the return current of EL!. Considering some views in /20,55/ we firstly discuss whether this is westward current or depletion of current on the flanks due to current channelling to the equator. Hesse /20/ considered them unresolvable. Fukushima /56/ considered inserting an elliptic area of conductivity k times the conductivity of its surrounding. He found that in the dynamo environment, “In general when k becomes larger than unity, an additional current flows circulating in the elliptic area and the surrounding area”. The returning flow within and outside the area is driven by polarization field at the boundaries, agreeing with Suzuki /33/. Thus the higher conductivity at the equator indeed leads to return currents, In any case the westward currents have actually been observed /38,47/. Is the reverse (return) current permanent or occasional?

There are three sources of the

(6)41

Equatorial Electrojet Return Current

5.6.1971

26.10.1971

12 60000

12

40000

40000

20000

14.35 . ‘ 10 15./~~930

20000

I

1530 —41110

-

,/

-20000

13,’

17 I

9

0

-20011 ~

I

20000

I

40000 1EE)

60000 in A

6.39__-.’

~.oo‘8830

~

I

I

21101

I

‘CEO IT

I

40000 A

—

—20000

Fig. 10. Plot of total eastward HE.! current L,,,.~, against the total return current hour by hour on 5 Jun 1971 and 26 Oct 1971 (fr~dW/2O/).

—

SIR,

westward currents. (1) Vertical polarization field reverses or minimizes around 60 dip latitude /34,37/. This is inherent in the geometry of field lines and must be permanent. (ii) The enhanced conductivity at the equator causes polarization field at the boundary that drives return currents /33,56/. This is intrinsic feature of equatorial ionosphere and is therefore permanent. (iii) Local zonal winds cause westward currents /39,40,42/. Fambitakoye, Mayaud and Richmond /39/ concluded, “Nevertheless, there seem to be average winds present through the year which make their presence known through their characteristic effects on the H and Z profiles averaged for the year”. We therefore conclude with Hesse /20/ that the westward currents are directly and permanently connected with the EE.!. Their effects are absent when ER.! is absent, they increase and decrease with ER.!, and they reverse when EEJ reverses into counter electrojet. It becomes compelling to regard the westward currents as the return currents of the equatorial electrojet (ER.!). Are the reverse currents partial or full return currents of ER.!? The source from curvature of field lines appears weak from /23,34,37/ but /40/ considered it significantly large. The source from polarization at the boundaries of high conductivity at the equator is relatively large /33,45/, and so is the source from zonal neutral wind shear /37,39,42,57/. All the three maximize around 5 dip latitude and therefore combine constructively. From the total return current of Hesse /20/, Figure 10, it is estimated that 95+11% of ER.! returned on 5 Jun 1971 but 85±5%of EEJ and 54+4% of counter electrojet in the morning returned on 26 Oct 1971. Thus 90+9% of HE.! returned before his L 1~PPT been The ratio two quanti1 —has 1200 km. found. He stated, “For between all the these investigated days ties is not exactly equal —to~unity, a linear relation between and es~ially not for the events of the counter electrojet. This is likely due to difficulties of the numerical calculations”. Div J — 0 was satisfied in /33,40/ implying 100% return, p overwhelming percentage returned before 300 in /33/ and probably all returned before 20 in /40/. In all the 919 satellite and ground profiles analyzed with continuous distribution 0of current density model, o~~(_l,impl~ing100% return; for about 909 profiles before L1~20 and only about 10 within 20 ~ L, ~ 30 . We therefore conclude that the three sources of return currents combine constructively, taking advantage of the peaks of conductivit~esaround 50 dip latitude, to provide for 100% return of EEJ currents, between about 3.5 dip latitude and the Sq focus. Is ionospheric current systen) in one monolithic, coupled or separate layers? Whether the ionospheric current system is in one monolithic layer or in two or more coupled or separate layers, all its component parts contribute to Sq, contrary to what seems to be implied in /58/ which misrepresented /8,27/. In /49,59/ the HE.! and the worldwide part of Sq are considered coupled. Tke lower layer and the upper layer partially overlap in many places from the equator to 20 latitude, but beyond, only the upper layer has been detected /50/. It is suggested that the lower layer, including the return currents, is the ER.! amd the upper layer is the worldwide part of Sq currents. CONCLUSIONS A review of indications of return currents of HE.! and associated phenomena has been presented. The main conclusions include the following: (1) Reverse westward currents flow on both flanks of the dip equator. No dip equatorial magnetic variation profile has been fitted well without taking the westward currents into account, Investigators using models which do not normally include westward currents had been forced to introduce ad hoc westward currents to improve the fittings. (2) The ratio of peak westward current intensity J , to the eastward peak at the equator J from various sources agree on the interval —l8%~JJJ0~~ —33%, with mea~ —27+3%. ° (3) Various sources are in accord in locating the wesTward peak at 5.3+0.7 dip latitude. (4) ~onsidering the variability of L1, the latitudinal extent of HE.!, we estimate that 10 ~ L1~ 30°,more likely 12°to 200, dip latitude.

(6)42

C. A. Onwumechili

(5)

The negative correlation between the width and intensity of ER.! established from ground and satellite data has been very well confirmed with data derived from physical model. It is becoming clear that the negative correlation of HE.! intensity and its landmark distances arises from the return currents driven by neutral local winds and other sources. (6) Evidences indicate that the reverse currents on the flanks of dip equator are directly and permanently connected with the EEJ. Their effects are absent when EEJ is absent, they increase and decrease with ER.!, and they reverse when ER.! reverses into counter electrojet. It is compelling to regard the reverse currents as the return currents of the ER.!. (7) The three sources of return currents: the geometry of field lines controlling vertical polarization field, the polarization field at the boundaries of enhanced conductivity at the dip equator, the local zonal wind shear effects combine constructively, taking advantage of the peaks of conductivities around 5 dip latitude, to provide fSr the rgturn currents of EEJ. Practically all the eastward EEJ returns between about 3.5 and 20 dip latitude, in any case before the Sq focus. (8) The ionospheric current system so far detected by rockets is basically in two laye~s. It is suggested that the intense lower layer, including the return currents around 5 dip latitude, be associated with the HE.!; and that the weak upper layer which maintains fairly steady altitude characteristics everywhere be associated with worldwide part of Sq current. (9) The review underlines the great0need for more rocket measurements of ionospheric currents in the region of 5.3±0.7 dip latitude, and for measurements of neutral winds at ionospheric and thermospheric altitudes in the equatorial zone. REFERENCES 1.

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