- Email: [email protected]
North-south cross-sections of the equatorial electrojet in the Pacific and the effect of a solar eclipse
Journalof Atmospheric andTerrestrial Phyaiea, 1969,Vol. 31,pp. 781-792.Pergamon Press.Printedin Northern Ireland
North-south cross-sections of the equatorial electrojet in the Pacik and the effect of a solar eclipse G. J. Air Force
GASSMANN
Cambridge
(Received 25 Januay
Research
and R. A. WAGNER* Laboratories,
Bedford,
Massachusetts
1968; in revised form 7 November 1968)
Al&&---The aim was to experimentally observe dynamic features of the electrojet during individual days and hours in order to aid theoretical studies on electrical fields in the ionosphere. During June 1965 six magnetic and ionospheric horizontal cross-sections near local noon across the dip equator in the Central Pacific were obtained by an airborne equipment. The removal of the magnetic effects of the ocean bottom from the magnetic data was sufficiently suocessful to observe variations in width and shape of the electrojet. The observed north--south cross-se&ions The latter stay fixed for two hours or longer on a magshow irregular shape and striations. netically quiet day; on a moderately disturbed day drastic changes in width and form occurred. Ionospheric Esq and Ess echoes appeared consistently in the center of each cross-section. During the 30 May 1965 eclipse the path of totality crossed the electrojet. A gradual deorease of field intensity of up to 507 was observed prior to totality and a quick return to normal thereafter. The path of totality appeared to have caused the electrojet to temporarily deviate from its normal position. 1. INTR~DT~~TI~N
IN THE theory
of the #,-current system the so-called polarization field plays an important role. This static electric field system, together with the electromotive forces, induced by tidal motions, determines the shape of the current flow. It is of particular importance at the equator (ZIKUDA, 1960; SIN~H and MISRA, 1967). The static field is produced in the E-layer and maintained as in any current circuit by the current itself through separation of ionospheric charges under the given boundary conditions. According to theory, the static electric field at the magnetic dip equator must have a large vertical component E, to allow the electrojet current to flow across the magnetic field lines. This requires that a surplus of positive charges exist at the bottom of the E-layer while a surplus of negative charges is maintained above the E-layer. Such a configuration occurs only in regions close to the dip equator, as is obvious from the existence and limited width of the electrojet. The electrojet current, according to theory, (HIRONO 1952; BAKER and MARTYN 1953) is electric field strength component equal to Eeast (a1 + us2/a,), where E eLstis the horizontal toward magnetic east, err and us are Pederson and Hall conductivities respectively (which are functions of electron and ion density, collision frequencies and of the geomagnetic field vector 8). The above is valid only if a strong electric field component El of the strength EL = Eew ua/ul exists which must be perpendioular to Eesst and to Jj. At the dip equator EL, therefore, is vertical and perpendicular to the conducting layer. This does not apply at locations away from the dip equator, and the accumulated electric charges (which produce El and which are located near the upper and lower boundaries of the horizontally stratified E-layer) are subject to forces. In addition, the equalizing shunt currents along field lines increase with magnetic dip. These matters are treated in more detail in existing theoretical models of the equatorial electrojet * Formerly
with Lowell
Tech. Inst. Research
781
Foundation,
Lowell,
Mass.
782
G. J. GASSMANNand R. A. WAGNER
(ONWU~CHILLI, 1966; SUGIURAand CAIN, 1966; UNTIEDT, 1967). According to Untiedt the northern and southern edges of the electrojet must be associated with meridional currents of appreciable strength.
It is of interest to investigate experimentally how flexible this peculiar configuration of accumulated electric charges is and to what extent it might be unstable or subject to fluctuations. One indirect means of investigating the behaviour of the electric fields is the observation of the electrojet current and of its dynamic behaviour. This requires measurements with a time resolution good enough not to smooth out and obscure the dynamic changes. The results reported here were obtained from measurements made in a jet aircraft and represent horizontal north-south magnetic cross-sections across the dip equator of the overhead current. By comparing consecutive cross-sections, the temporal variations occurring during a single crossing turned out to be small. The presented cross-sections therefore, can be regarded as quasiinstantaneous (except during disturbed magnetic conditions of K, w 4). Although cross-sections across the width of the electrojet have been reported before, most were averaged over time and space and are not useful for the purpose stated here. This is also true for the cross-sectional profile derived from rocket measurements near the coast of Peru by DAVIS et al. (1967), since the data were not taken simultaneously but are in fact from several days with attempted normalization. Valuable insight can be gained from the hourly horizontal cross-sections of the overhead current for 3 days in August 1958 in South America published by HUTTON (1965). Those data show that the shape of the cross-section varies during one day and also from one day to the next even during a quiet period. The spatial resolution of the data, however, is too coarse to show whether or not the electrojet has a uniform or streaky structure. In addition, some of the data taken near the coast line may be distorted by electric ocean currents. GASSMANN and WAGNER (1966) found that the horizontal intensity H measured in an aircraft at 12 km altitude was reducd by 50~ while overflying the Peruvian coastline during local morning hours in July 1964. At that time airborne cross-sections were obtained. However, the data were so heavily distorted by ground effects that only coarse dynamic variations of the electrojet were recognizable. These are similar to those in Hutton’s curves, In the present experiments in June 1965, four flights from Honolulu were made with airborne magnetic and ionospheric sounder equipment over the Pacific, along Six cross-sections were obtained from three day approximately 159”W longitude. flights and two from one night flight. General background information as to the average position and width of the electrojet in the Pacific area, and as to its variations from day to day, and with season has been reported by MASON (1963) and HALL (1968). Two more cross-sections were obtained from flights in the path of totality of the 30 May 1965 eclipse that crossed the dip equator. One flight was made during the eclipse, while another along an identical flight track on another day was used as reference. The map, Fig. 1, shows the flight tracks of all flights and Table 1 gives U.T. and local time for flights and the K, indices during flight times.
2. MAGNETIC DATA The instrument used was a fluxgate magnetometer, type AN/ASQ-3A, designed to measure the total magnetic field intensity. The sensor of the instrument was installed in the tail section
00~10-02~20
02.47-04.30
21.33-23.25
00~1@-02*01
21.24-23.19
00~01-01~55
10~50--13*00
13.27-15.10
8 June
9 June
10 June
11 June
6 June (reference) 6 June (reference)
10.49-13.02
20.13-21.42
30 May (eclipse)
4
8
8
0
30
10
w 145’E w 160°W w 75’W w 39OE
0
long. long. long. long. (IAGA)
6
10
16
16
4 4
0
0
0
0
36
14
6 6
0
Hu
0
10
Ho
and Aeronomy
+ 14O dip, +39O dip, + 2O dip, - lo dip,
2
0
10
8
0
0
20
18
10
10
Gu
Bulletin
0
4
0
8
0
0
# 12.
110
80
121
126
13 13
132
-
14 0
HU
AA
Max. minus min. Value of H during 1 Greenwich day, in y
AA
Approx. short period deviation, in y, of H from smooth diurnal curve
of Geomagnetism
Guam Honolulu Huancayo Addis Ababa
Association
= = = =
l-
2_
2_
00
4_
20
Highest index
K,
Gu Ho Hu AA
i
1
* According to scale given in International
11.06-13.25
20.30-22.05
27 May (reference eclipse)
13.21-15.15
10.44-12.39
13.30-15.21
10.53-12.45
13.2&15.06
00~00-01~46
2 June
10.35-12.18
Approximate local time
21.15-22.58
Time U.T.
1 June
1965
Corresponding range in y for Guam*
Table 1
784
G. J. GASSMANN
and R. A. WAGNER
30 MAY1965
Fig. 1. Flight tracks over ocean along w159’W longitude crossing the electrojet region, l-10 June 1965 and during solar eclipse on 30 May 1965 (reference flight on 27 May 1965 was identical). The path of totality is plotted for 80 km altitude, The aircraft’s altitude was approximately 10 km. of a four-engine jet aircraft. The output was proportional to the field intensity and was recorded on a paper chart in units of gamma (ly = lops Gauss) with an arbitrary zero value. The zero level was sufficiently stable over intervals of several hours so that changes in the magnetio field intensity could be recorded with an accuracy of &lOy. The aircraft’s own permanent magnetic field appearing at the magnetometer’s sensor was partly compensated by the field of a small magnet placed and orientated appropriately at a distance of about 1 m. The remaining portions of the aircraft’s own field including the statically induced field were graphically eliminated from the records according to a method described earlier (GASSY and WAONER, 1966). The procedure used was to fly at a constant magnetic heading during the data taking and, in addition, to fly one set of circles at each of various dip locations. For all flights, except for those along the eclipse track, magnetic headings near 0’ and 180’ were chosen and were kept constant within &lo during a flight and within f2.5’ from day to day. However, the flight tracks on different days differed slightly from each other due to the particular wind conditions. The maximum distance between any flight track and the corresponding referenoe flight track, explained below, was 140 km. This necessitated an individual adjustment of the data prior to comparison with the reference flight track data, taking account of the horizontal gradient of the total field intensity in the direction perpendicular to the flight tracks. The required information was taken from global magnetic maps. The flight altitude was constant for each flight track and was approximately 10 km for the north-south tracks and 11.5 km for south-north tracks. The difference of the indicated field intensity for the two flight tracks at different altitudes due to this altitude difference (1.5 km) is nearly the same along the entire length of the flight tracks. No corrections due to altitude were, therefore, applied to the data. The data, so treated, resulted in curves showing relative total geomagnetic field intensity in y vs. dip angle. Dip angle location was taken from the map for 1955 issued by the U.S. Naval Oceanographio Office. The electrojet was then identified by matching and comparing the curves from day flights with the corresponding reference curves. Those were produced during one night flight. An example is shown in Fig. 2. In the case of the eclipse flight, the reference curve was produced during a flight on a non-eclipse day. The matching of the curves was a tedious process. All curves display large and small anomalies of the total field intensity with amplitudes of up to f130y and periods of 4-8 min, which
North-south cross-sections of the equatorial electrojet
785
Fig. 2. Total geomagnetic field intensity in y vs. dip angle as measured in N to S flight along w159’W longitude 1 June 1965 and along nearly identical flight track during night flight of 516 June 65. (y-scale is approximate, for se8 level.) corresponds to horizontal distances of approximately 55-110 km at the aircraft’s speed. These anomalies are caused by magnetic line&ions of the ocean bottom and are characteristic for this Pacific region. They were already shown by BRACEY (1963) in magnetic intensity profiles along 159’W longitude obtained from the magnetic survey by ship made by the U.S. Naval Oceanographic Office 1961 and 1962. A comparison of all eight field intensity vs. dip angle curves showed a high degree of correlation of all those large and small anomalies if allowances (of up to f0.4’ dip angle or f23 km distance along the flight path) were made for loose matching of the curves along the dip axis. This procedure allowed for the fact that the magnetic line&ions of the ground did not run exactly perpendicular to the aircraft tracks. Each of the six y vs. dip curves (obtained during the day flights) was matched and compared with one of the two reference curves (obtained in the night flight of 6 June 1965). The choice of the latter was determined by the closest proximity of the corresponding day flight tracks. In the matching of the dayllight rmd reference curves the aim was to produce the best fit of the before mentioned y anomaly patterns and, in addition, near zero y differences in the regions having dip angles > *loo where no large current is present. Six pairs of y vs. dip curves were obtained, each pair consisting of one dayflight curve and one reference curve. The y difference between the two curves-m the following always called Ay-at each point along the flight path was then plotted. Only small Ay ( ~10) were derived from day minus night curves for locations having dip angles from +35” to +21°, as can be expected outside the electrojet region. To smooth out sudden jumps in the A7 curves caused by incomplete matching a running mean was computed, each averaging a period stretching over 2’ dip or approximately 9 min flying time or 120 km distance. In Fig. 3 Ay (shaded bands) and top frequencies of Esq echoes (solid curves) are plotted vs. dip angle. The upper and lower limits of the shaded bands indicate the standard deviation from the running mean. While those limits merely represent the uncertainty due to incomplete matching (caused by ocean bottom magnetic lineations) all other sources of uncertainty inherent in the data and in their processing (uncertainties of drift of equipment reference voltage, of aircraft heading and location, of reference curves) were found smaller and relatively negligible. 3. IONOSPHERIC DATA During all day flights the Granger sounder installed in the airplane w&s operated. Verticd incidence ionograms with swept frequencies from 2 to 10 MHz were taken continuously; the period for each sweep was 36 sec. During one flight this program was interrupted every 10 min
G. J. GASSMANN and R. A.
786
LOCALNOIJN
I JUNE 1965 2115UT I ” \ START ‘.py_ I,j,~ ‘_ -’
WAGNER
-,..
. . -I IO 8 fi 4I IO 8 6 41 IO 8 6 4I I
\ .
2309UT
IO 8 6 4
tl6" tl4" tl2” tip” tfP ‘7 I
I
I
t4O tf I 100 km
Fig. 3. North-south cross-sections through eleotro jet. Shaded bends give Ay (in units of y) vs. dip. Ay is the difference of field intensities measured: day minus reference flights. Width of shaded bands indicate the uncertainty. Solid curves: top frequency of Esq. Local time is approximately U.T. minus 10.6 hr. Aircraft altitude: 10 km. for 1 min by a sequence of rapid sweeps from 2 to 10 MHz, taking I.8 set each. During this period pulse by pulse echo amplitudes were recorded on rapidly moving film, together with calibration ctmplitudes. 4.
ELECTROJETNORTH-SOUTH CROSS-SECTIONSNEAR 159”W LONGITUDE
The six bands of Ay vs. dip displayed in Fig. 3 represent meridional cross-sections of differences between total magnetic field intensities during day and night (reference). The actual curve representing the electrojet may be assumed to run in an unknown way within each band, possibly as smooth as those limits of uncertainty allow. The meaning of the baseline Ay = 0 is discussed below. For estimating the electrojet current from Ay it must be pointed out that Ay = 0 does not mean zero current. In the matching procedure between a day curve and a reference curve the Ay by definition was made zero for dip angles >lO”. In detlning Ay = 0 we follow a conventional, not necessarily proper, practice (for example see CASAVERDE 1961; OSBORNE 1964; YACOB 1966) which identifies the electrojet by the sudden increase of the diurnal magnetic variation at stations near the dip equator. However, the noon over midnight increase of the magnetic field intensity at locations with about 10’ dip angle is considerable; for example, the monthly quiet-day mean of the daily variation of the horizontal component of the magnetic
North-south
cross-sections
of the equatorial
electrojet
787
field intensity at Fanning Island (f 94” dip), 1ocated in our flight route, in June 1958 (sunspot maximum) was in the order of 607. In June 1965 (at sunspot minimum), for which no data were available for this station, the corresponding value must be expected to be about 8 of the above. This estimated value of 40~ (at 10’ dip) is then the combined result of the noon current The ratio of the two contributions is not precisely known and (negative) midnight current. and for this reason magnetic diurnal variations have been presented against arbitrary reference levels. MATSUSHITA and MAEDA (1965) chose the daily mean, PRICE and STONE (1964), and PRICE and WILKINS (1963), and SUGIURA and HAGAN (1967) chose the midnight value. It is estimated that the current density near midnight is probably of the order of one-tenth of the day-time value. Assuming this for 10’ dip (where we matched the day and night curves) the overhead current during the day near noon caused an estimated 90 per cent of 40~ = 36~ positive deviation from the horizontal component, as it were without external currents. The baselines by = 0 in Fig. 3 do not account for this positive current. The above mentioned position of the baseline, being a matter of detiition, has an estimated uncertainty of &lOy due to limited knowledge of sunspot cycle dependence of the diurnal variation and of the ratio of night to day current. This estimate assumes very quiet magnetic conditions. We add to this uncertainty another f10y due to the fact that the reference night flight took place under K, = 2-. The Table shows that, for instance, at Guam 8-10~ was the deviation from a smooth diurnal curve.
From the foregoing follows that the baseline in Fig. 3, from which the electrojet current may be computed, lies 36 &- 20~ below the level Ay = 0. In order to estimate how much of the electrojet meridional cross-sections, displayed in Fig. 3 as spatial variations, are actually manifestations of short temporal variations a number of magnetograms of fixed stations were analyzed. As shown in the Table no such temporal variations took place on lo/l1 June. On l/2 June and S/9 June, the observed variations at the ground stations suggest that about 10 and 12 per cent respectively of the Ay values, plotted in Fig. 3, might be temporal variations. The latter might have been caused by disturbance ring current. The highest Ay value, indicating the strongest electrojet current, was found during the flights of lo/l1 June, the maximum of 19Oy occurring at local noon time and at’ -2’ dip. The decline of Ay is gradual towards the N of the cross-section, while the electrojet is strong through -8” dip, then decreases rapidly to the S- end. On the return flight the same profile was found with the maximum Ay still at -2” dip. The nearly identical shape of the cross-section curves derived from the N to S and S to N flight suggest that during the 44 hr between beginning and end of the two flights there occurred no change in current strength and no shift in the location of the current maximum. This local and temporal stability coincides with the fact that the flight was made during absolutely quiet geomagnetic conditions, the K, indices being 0, all the time. The width of the electrojet on this day appears to be approximately 1000 km, symmetrical about the dip equator. The cross-section obtained from the flights of lo/l1 June show the most irregular Ay versus dip curves of all flights and the widest error bands. The last feature is mainly due to the fact that the flight tracks of the lo/l1 June have the greatest distance of all flight routes from those of the reference flights. The sharp irregularity near +lO” in both S to N and N to S curves is an effect occurring while overflying Fanning Island. All other flights passed the island at a distance. A striation appears to be indicated near -5” dip, more pronounced in the N to S cross-section. During rather quiet magnetic conditions (with highest K, index of 2, at the beginning of the N to S flight) on l/S June the maximum electrojet with Ay = 140 was
788
G. J. GASSMANN and R. A. WAamn
recorded at -lo dip, shortly before local noon. Not much change was found during the return flight, high electrojet current prevailing between -4 and +2” dip. The irregular shape of the Ay vs. dip curve and the wide error band near + 1” dip show the ground effect of Jarvis Island which was passed in 9 km distance on the S to N section of the flight. The width of the electrojet was about 800 km on l/2 June. During the flight of 8/9 June both flight tracks were close to that of the respective reference flight; the error bands consequently are narrow. Geomagnetically moderately disturbed conditions existed during this flight with K, of 4_ in the beginning and 3, later. The electrojet appears to extend more to the N than on the other 2 days, while its southern limit is the same as found on l/2 and lo/l1 June. The width appears to be about 1100 km. In contrast to the S-N cross-section the N to S cross-section shows a rather uniform electrojet strength between +9 and -8” dip with a maximum Ay of 100 near -6’ dip. The ourrent densities which can be derived from the highest peaks of the cross-sections in Fig. 3 are much higher than those experienced at both Huancayo and Addis Ababa. This can be seen by referring to the last two columns of the Table where the maximum ohange of H at these stations during one Greenwich day is listed. This suggests, although the longitudinal and/or time differences might play a role, that during the days listed the peak current never came close to the stations.
As another indication of the electrojet current, and independent of the magnetic measurements, the ionograms were utilized. According to COHEN,BOWLESand CALVERT(1962) and FARLEY (1963) two-stream instabilities within the electrojet current cause ionospheric irregularities which, in turn, cause radio scatter, which gives rise to typical Esq and Ess echoes in ionograms. The gain setting of the receiver was constant for each flight. The top frequency of the Esq echo, reflected vertically from 115 km, is plotted for each flight. Intercomparison between different days should be exercised with caution, since slight uncontrolled changes in receiver gain setting from day to day might have occurred. The top frequency shows good correlation with the magnetic data in the center of the electrojet. The poor correlation at the northern and sourthern edges is not unexpected, since the ionospheric irregularities are known to occur only above a certain threshold value of the current. In order to relate top frequency to received power a few spot checks were made. The frequency dependence of vertical Esq echo power waz found to be flat from 4.5 MHz (the lowest frequency measured) to 6.5 MHz and then falling toward higher frequencies. Earlier measurements in Peru (GASSMANN and WAamn, 19f3e) yielded a dependence on frequency f in the formf-c with c = 1.4 approximately. For Ibadan, SKINNERand WnrauT (1982) found c = 9.4. As a consequence of the fiat scatter spectrum the recorded top frequency was very sensitive changes in echo power; a reduction from 8 to 4 MHz corresponded to a power reduction of only 4. The top frequency of 8 MHz corresponded to O-2 of the power which would have been received from a metallic reflector. The polarization of the transmitting and receiving antenna was linear and was oriented in the direction of the aircraft axis. 5. EFFECT OF 30 i&y 1965 ECLIPSE ON ELECTROJET A recent short review of both theoretical predictions and observations of eclipse effects on the equatorial geomagnetic field can be found in MATSUSHITA (19ef3). The observed effect at an equatorial magnetic station has been a gradual decrease and increase of the horizontal component following along with the progress of the eclipse. In terms of y0 of the daily variation the maximum eclipse effect has varied from case to case and was between very small or unoertain
North-south cross-se&ions of the equatorial electrojet
789
and 50 per cent. Recently BOMKE et al. (1967) recorded the horizontal field intensity at I&&a, and Humoayo, Peru, during the 12 November 1906 eclipse oocurring at that location in the early morning. A gradual depression and recovery was recorded reaching 30~ maximum depression 2 min after totality in the E-layer. A value for the effective recombination ooefficient was derived and was based on the adoption of a disturbance flow pattern of the current, derived by VOLLAND (1967).
The present measurements were made during two flights in the path of the total solar eclipse of 30 May 1965, which crossed the dip equator. The flight during the eclipse and the corresponding control flight were performed under nearly identical conditions as to location, altitude, time and aircraft’s own field. The data from these two flights were, therefore, compared without applying corrections. The map of Fig. 1 shows both the path of totality and the aircraft track with corresponding time. The shadow of the moon overtook the aircraft at 21.20 U.T. or 12.28 local time. At that particular location the dip angle was near $2”. The y vs. dip curves obtained from the measurements made during the eclipse flight on 30 May and during the reference flight on 27 May, 1965 are shown in Fig. 4.
Fig. 4. Total geomagnetio field intensity vs. dip angle as measured during the flight in the path of totality of the eclipse of 30 May 1966 (y-scale is approximate, for sea level). First and fourth contacts occurred approximately 70 min before and after totality respectively and were not covered by measurements. It was fortunate that the matching of the y vs. dip curves for the eolipse day and reference day was perfeot on both ends of the data, i.e., near -7 and $4“ dip. Since these locations are not outside the eleotrojet width, the perfeot matching indicated that the shape of the electrojet was similar on both days. This fact furnishes con6dence that the observed difference in total field intensity near totality is not merely an incidental dissimilarity of the electrojet cross-se&ions on two different days. An additional check was made as to whether during the periods of the flights on either one of the two days sporadic fluotuations in the order of one and one half hour duration occurred at other stations. The highest K, indicies were 2_ and l_ respectively during the times of flight on 27 and 30 May 1966; the magnetograms of Guam, Honolulu, Huanceyo and Addis Ababa show sporadic fluctuations smaller than 10~ within the identical time interval (see Table 1).
790
G. J. GASSMANN and R. A. WAGNER
The effect of the eclipse may be best described after realizing that the data from 20.30 to 21.40 U.T. displayed in Fig. 4 are taken at magnetic latitudes from -2 to +2”, locations which are well within the width of &-6” dip which is normally occupied by the electrojet. The measurements therefore were practically made at the dip equator (see Fig. 1). Considering these circumstances, the significant feature of the effect (represented in Fig. 4 by the difference between eclipse- and reference-data) appears to be the assymmetry with respect to totality. The effect of the eclipse is noticeable and steady during the whole hour prior to totality. Shortly after totality, 21.27 U.T. the effect ceased rapidly and fluctuated about zero. Although only 20 min after totality were covered by measurements as a consequence of range limitations (the flight required 13 hr) the data allow the drawing of some important conclusions. The explanation of the asymmetry of the data cannot be obtained from existing theoretical treatments of the problem. The theoretical predictions as reviewed by MATSUSHITA (1966) and also as advanced by VOLLAND (1957) do assume the undisturbed current system S, to be infinitely large compared to the area of the moon’s shadow (penumbra). The effect of the eclipse appeared then as a superposition of the original current stream and local disturbance current systems in such a way that the resulting current flow is symmetrical around the area of the moon’s core shadow (umbra). Such flow pattern would produce magnetic eclipse effects symmetrical at totality. It is obvious that this simple concept is not readily applicable to the region of the equatorial electrojet whose width is smaller than the penumbra. The limited width of the channel in which the electrojet flows is controlled by the geometry of the field lines and the position of the space charges (“Polarization Field”). When the penumbra moves across the electrojet the latter cannot simply circumflow the area of lowest conductivity since the position of the channel is predetermined. The channel is flexible only to the degree by which the electrical space charges can be rearranged within the restrictions imposed by the magnetic field. And, that process is not a simple problem of flow theory. It is known (see Introduction) that the optimal effective conductivity along the dip equator depends critically on the electrical field E, arising from accumulated space charges and that the latter are established in their proper positions within the ionospheric E-layer by Hall currents. If the process of rearranging space charges takes time which is comparable to the period of the most severe disturbance, namely of the umbra’s passing across the electrojet (20 min), then the flow pattern around the umbra can hardly be symmetrical. Without attempting a theoretical treatment of this case, it is reasonable to expect that the electrojet indeed rearranged the space charges along its west-east flow in such a way as to utilize regions of higher electron density. It follows from the f,,E, measured during this and other eclipses, that during the whole period while the penumbra was approaching from the south (from 19.45 to 21.20 U.T.) the electron density in 105-125 km altitude above the dip equator increased toward the north with an average slope of +65 per cent per 1000 km. This likely caused a northward shift of the electrojet and this shift amounted to an evasive move away from the approaching umbra prior to totality. It is now of interest to ask how far the electrojet might have deviated from its mormal position along the dip equator. Such displacement away from the dip equator must amount to a deviation from a stable
North-south cross-sections of the equatorial electrojet
791
configuration. Judging from the very existence of the normal electrojet clinging close to zero dip, this position is obviously stable for reasons of magnetic symmetry. The data in Fig. 4 can be best understood if one assumes that during this eclipse the electrojet after an evasive move toward north must have returned to its normal position as soon as the conductivity along the dip equator allowed. This move, however, amounted to a flip-over from the northern side of the umbra to the southern side. It is suggested that this happened at about 21.27 U.T. It is suggested that shortly before 21.27 U.T. the center of the electrojet was located north of +6” dip and that shortly after 21.30 U.T. the center of the electrojet was located south of +2” dip. The above statement refers to the longitude of approximately 132”W where the observation took place. At this eclipse and location the electrojet flow pattern in horizontal space appeared to be non-symmetrical with respect to the umbra and produced at a fixed location a non-symmetric magnetic record with respect to totality. 6. SUMMARY The results describe some observed dynamic variations of the electrojet which might aid theoretical treatments of electrostatic fields in the ionosphere. Airborne magnetic and ionospheric measurements across the dip equator in the Central Pacific during three days in June 1965 showed that the variations in width and individual shape of the electrojet are considerable from day to day. On days of low magnetic activity, however, the width and shape of the electrojet stayed nearly constant as evident from the cross-sections taken before and after local noon on the same day. Irregular features or striations appear superimposed upon a smooth cross-section and modify it by approximately 130 per cent. These striations, however, stay fixed, within 150 km horizontal distance, during a period of two hours and possibly longer. In contrast, on one day with K, near 4 the shape and individual striations of the meridional cross-section underwent drastic changes within two hours. On the ionograms Esq and Ess echoes appeared continuously in the center of the electrojet but seldom were present over its whole width. At the edges of the electrojet occasional strong Esq echoes were observed during short periods. The vertical echo strength of Esq versus frequency turned out to be almost constant for frequencies up to 6.5 MHz and falling off beyond. The eclipse of 30 May 1965 appeared to have caused a reduction of at least 50~ of the total field intensity at the aircraft which was below the center of the electrojet at totality. The reduction of field intensity developed slowly prior to totality and ceased quickly thereafter. The circumstances suggest that the electrojet was forced by the umbra to temporarily shift its normal position possibly by as much as 6’ dip and returned shortly after the umbra had passed the dip equator. Acknowledgements-R. GOWELL, AFCRL, was responsible for the excellent quality of the ionospheric data and R. HOADLEY, Lowell Technological Institute Research Foundation, for the magnetometer instrumentation. The aircraft was under the command of Major K. CROOKS, and the chief navigator was Major R. VINING, both of the USAF. We appreciate the critical reviews of the paper by E. MAPLE, E. J. CKERNOSKY, and K. TONAN, all of AF’CRL, as well as the subsequent comments of two Journal of Atmospheric and Terrestrial Physics reviewers.
G. J. GABS-
792
and R. A. WAONER
REFERENCES BAKER W. G. and MARTYN D. F. Born H. A., BH. A., H.BRIS A. K. HUXSE W. H., SHEPPARD D. J., GIESECEE A. A. and PANTOJA A. CASAVERDE M. COFIENR., BOWLES K. L. and CALVERT W. DAVIS T. N., BURROWS K. and STOLARIK J. D. FARLEY D. T. Jr. GASSMANN G. J. and WAGNER R. A. IXALL S. H. HIRONO M. HUTTON R. MATSUS~TA S. MATSUSHITA S. and MAEDA H. ONWLTMECHILLIC. A. OSBORNE D. G. PRICE A. T. and STONE D. J. PRICE A. T. and WILKINS G. A. SINCR R. N. and MISRA K. D. SKINNER N. J. and WRIGHT R. W.
SUUIURA M. and CAIN G. C. UNTIEDT G. VOLLAND H. YACOB A. ZadDDA A. J.
Rqference
is aLso made
1953 1967
Trans. R. Sot. (A), 246,281. J. Geophys. Res. 72, 5913.
1961 1962
Am. J. Phys. 29, 744. J. Geophys. Res. 67, 965.
1967
J. Geophys. Res. 72, 1845.
1963 1966 1968 1952 1967 1966 1965 1966 1964 1964 1963 1967 1962
J. Geophys. Res. 68, 6083. J. Geophys. Res., 71, 1879. J. Atmosph. Tern. Phys. 30, 1293. J. Geom. Geoel. Kyoto 4, 7. J. Atmosph. Terr. Phys. 29, 1429. Ann. Gkophys. 22, 471. J. Geophys. Res. 70, 2535. Nigerian J. Sci. 1, 11. J. Atmosph. Tern. Phys. 26, 1097. Annals IGY, 35, 65. Phil. Trans. R. Soo. (A), 253, 31. J. Geomug. GeoeZ. 19(2), 143. Ionosphem’c Sporadio E (Edited by E. K.
1966 1967 1957 1966 1960
SETH and S. MATSUSHITA), p. Pergamon Press, Oxford. J. Geophya. Res. 71,1869.
J. J. J. J.
Geophys. Atmosph. Atmosph. Geophys.
37.
Rea. 72, 5799. Terr. Phys. 11,1. Tew. Phya. 28, 581. Res. 65, 2247.
to the follouing unpublished materid:
BRACEY D. R.
1963
MASON R. G.
1966
SIJOIURA, M. and HA~AN M. P.
1967
Marine Science Dept., U.S. Naval Oceanographic Office, Washington, D.C. Unpublished manuscript. Scripps Inst. of Oceanography, San Diego, Calif., Reference 63-13. Dept. of Atm. Sciences, University of Washington, Scienca Rep., Nat. SC. NSF GA-478.
North-south cross-sections of the equatorial electrojet in the Pacik and the effect of a solar eclipse G. J. Air Force
GASSMANN
Cambridge
(Received 25 Januay
Research
and R. A. WAGNER* Laboratories,
Bedford,
Massachusetts
1968; in revised form 7 November 1968)
Al&&---The aim was to experimentally observe dynamic features of the electrojet during individual days and hours in order to aid theoretical studies on electrical fields in the ionosphere. During June 1965 six magnetic and ionospheric horizontal cross-sections near local noon across the dip equator in the Central Pacific were obtained by an airborne equipment. The removal of the magnetic effects of the ocean bottom from the magnetic data was sufficiently suocessful to observe variations in width and shape of the electrojet. The observed north--south cross-se&ions The latter stay fixed for two hours or longer on a magshow irregular shape and striations. netically quiet day; on a moderately disturbed day drastic changes in width and form occurred. Ionospheric Esq and Ess echoes appeared consistently in the center of each cross-section. During the 30 May 1965 eclipse the path of totality crossed the electrojet. A gradual deorease of field intensity of up to 507 was observed prior to totality and a quick return to normal thereafter. The path of totality appeared to have caused the electrojet to temporarily deviate from its normal position. 1. INTR~DT~~TI~N
IN THE theory
of the #,-current system the so-called polarization field plays an important role. This static electric field system, together with the electromotive forces, induced by tidal motions, determines the shape of the current flow. It is of particular importance at the equator (ZIKUDA, 1960; SIN~H and MISRA, 1967). The static field is produced in the E-layer and maintained as in any current circuit by the current itself through separation of ionospheric charges under the given boundary conditions. According to theory, the static electric field at the magnetic dip equator must have a large vertical component E, to allow the electrojet current to flow across the magnetic field lines. This requires that a surplus of positive charges exist at the bottom of the E-layer while a surplus of negative charges is maintained above the E-layer. Such a configuration occurs only in regions close to the dip equator, as is obvious from the existence and limited width of the electrojet. The electrojet current, according to theory, (HIRONO 1952; BAKER and MARTYN 1953) is electric field strength component equal to Eeast (a1 + us2/a,), where E eLstis the horizontal toward magnetic east, err and us are Pederson and Hall conductivities respectively (which are functions of electron and ion density, collision frequencies and of the geomagnetic field vector 8). The above is valid only if a strong electric field component El of the strength EL = Eew ua/ul exists which must be perpendioular to Eesst and to Jj. At the dip equator EL, therefore, is vertical and perpendicular to the conducting layer. This does not apply at locations away from the dip equator, and the accumulated electric charges (which produce El and which are located near the upper and lower boundaries of the horizontally stratified E-layer) are subject to forces. In addition, the equalizing shunt currents along field lines increase with magnetic dip. These matters are treated in more detail in existing theoretical models of the equatorial electrojet * Formerly
with Lowell
Tech. Inst. Research
781
Foundation,
Lowell,
Mass.
782
G. J. GASSMANNand R. A. WAGNER
(ONWU~CHILLI, 1966; SUGIURAand CAIN, 1966; UNTIEDT, 1967). According to Untiedt the northern and southern edges of the electrojet must be associated with meridional currents of appreciable strength.
It is of interest to investigate experimentally how flexible this peculiar configuration of accumulated electric charges is and to what extent it might be unstable or subject to fluctuations. One indirect means of investigating the behaviour of the electric fields is the observation of the electrojet current and of its dynamic behaviour. This requires measurements with a time resolution good enough not to smooth out and obscure the dynamic changes. The results reported here were obtained from measurements made in a jet aircraft and represent horizontal north-south magnetic cross-sections across the dip equator of the overhead current. By comparing consecutive cross-sections, the temporal variations occurring during a single crossing turned out to be small. The presented cross-sections therefore, can be regarded as quasiinstantaneous (except during disturbed magnetic conditions of K, w 4). Although cross-sections across the width of the electrojet have been reported before, most were averaged over time and space and are not useful for the purpose stated here. This is also true for the cross-sectional profile derived from rocket measurements near the coast of Peru by DAVIS et al. (1967), since the data were not taken simultaneously but are in fact from several days with attempted normalization. Valuable insight can be gained from the hourly horizontal cross-sections of the overhead current for 3 days in August 1958 in South America published by HUTTON (1965). Those data show that the shape of the cross-section varies during one day and also from one day to the next even during a quiet period. The spatial resolution of the data, however, is too coarse to show whether or not the electrojet has a uniform or streaky structure. In addition, some of the data taken near the coast line may be distorted by electric ocean currents. GASSMANN and WAGNER (1966) found that the horizontal intensity H measured in an aircraft at 12 km altitude was reducd by 50~ while overflying the Peruvian coastline during local morning hours in July 1964. At that time airborne cross-sections were obtained. However, the data were so heavily distorted by ground effects that only coarse dynamic variations of the electrojet were recognizable. These are similar to those in Hutton’s curves, In the present experiments in June 1965, four flights from Honolulu were made with airborne magnetic and ionospheric sounder equipment over the Pacific, along Six cross-sections were obtained from three day approximately 159”W longitude. flights and two from one night flight. General background information as to the average position and width of the electrojet in the Pacific area, and as to its variations from day to day, and with season has been reported by MASON (1963) and HALL (1968). Two more cross-sections were obtained from flights in the path of totality of the 30 May 1965 eclipse that crossed the dip equator. One flight was made during the eclipse, while another along an identical flight track on another day was used as reference. The map, Fig. 1, shows the flight tracks of all flights and Table 1 gives U.T. and local time for flights and the K, indices during flight times.
2. MAGNETIC DATA The instrument used was a fluxgate magnetometer, type AN/ASQ-3A, designed to measure the total magnetic field intensity. The sensor of the instrument was installed in the tail section
00~10-02~20
02.47-04.30
21.33-23.25
00~1@-02*01
21.24-23.19
00~01-01~55
10~50--13*00
13.27-15.10
8 June
9 June
10 June
11 June
6 June (reference) 6 June (reference)
10.49-13.02
20.13-21.42
30 May (eclipse)
4
8
8
0
30
10
w 145’E w 160°W w 75’W w 39OE
0
long. long. long. long. (IAGA)
6
10
16
16
4 4
0
0
0
0
36
14
6 6
0
Hu
0
10
Ho
and Aeronomy
+ 14O dip, +39O dip, + 2O dip, - lo dip,
2
0
10
8
0
0
20
18
10
10
Gu
Bulletin
0
4
0
8
0
0
# 12.
110
80
121
126
13 13
132
-
14 0
HU
AA
Max. minus min. Value of H during 1 Greenwich day, in y
AA
Approx. short period deviation, in y, of H from smooth diurnal curve
of Geomagnetism
Guam Honolulu Huancayo Addis Ababa
Association
= = = =
l-
2_
2_
00
4_
20
Highest index
K,
Gu Ho Hu AA
i
1
* According to scale given in International
11.06-13.25
20.30-22.05
27 May (reference eclipse)
13.21-15.15
10.44-12.39
13.30-15.21
10.53-12.45
13.2&15.06
00~00-01~46
2 June
10.35-12.18
Approximate local time
21.15-22.58
Time U.T.
1 June
1965
Corresponding range in y for Guam*
Table 1
784
G. J. GASSMANN
and R. A. WAGNER
30 MAY1965
Fig. 1. Flight tracks over ocean along w159’W longitude crossing the electrojet region, l-10 June 1965 and during solar eclipse on 30 May 1965 (reference flight on 27 May 1965 was identical). The path of totality is plotted for 80 km altitude, The aircraft’s altitude was approximately 10 km. of a four-engine jet aircraft. The output was proportional to the field intensity and was recorded on a paper chart in units of gamma (ly = lops Gauss) with an arbitrary zero value. The zero level was sufficiently stable over intervals of several hours so that changes in the magnetio field intensity could be recorded with an accuracy of &lOy. The aircraft’s own permanent magnetic field appearing at the magnetometer’s sensor was partly compensated by the field of a small magnet placed and orientated appropriately at a distance of about 1 m. The remaining portions of the aircraft’s own field including the statically induced field were graphically eliminated from the records according to a method described earlier (GASSY and WAONER, 1966). The procedure used was to fly at a constant magnetic heading during the data taking and, in addition, to fly one set of circles at each of various dip locations. For all flights, except for those along the eclipse track, magnetic headings near 0’ and 180’ were chosen and were kept constant within &lo during a flight and within f2.5’ from day to day. However, the flight tracks on different days differed slightly from each other due to the particular wind conditions. The maximum distance between any flight track and the corresponding referenoe flight track, explained below, was 140 km. This necessitated an individual adjustment of the data prior to comparison with the reference flight track data, taking account of the horizontal gradient of the total field intensity in the direction perpendicular to the flight tracks. The required information was taken from global magnetic maps. The flight altitude was constant for each flight track and was approximately 10 km for the north-south tracks and 11.5 km for south-north tracks. The difference of the indicated field intensity for the two flight tracks at different altitudes due to this altitude difference (1.5 km) is nearly the same along the entire length of the flight tracks. No corrections due to altitude were, therefore, applied to the data. The data, so treated, resulted in curves showing relative total geomagnetic field intensity in y vs. dip angle. Dip angle location was taken from the map for 1955 issued by the U.S. Naval Oceanographio Office. The electrojet was then identified by matching and comparing the curves from day flights with the corresponding reference curves. Those were produced during one night flight. An example is shown in Fig. 2. In the case of the eclipse flight, the reference curve was produced during a flight on a non-eclipse day. The matching of the curves was a tedious process. All curves display large and small anomalies of the total field intensity with amplitudes of up to f130y and periods of 4-8 min, which
North-south cross-sections of the equatorial electrojet
785
Fig. 2. Total geomagnetic field intensity in y vs. dip angle as measured in N to S flight along w159’W longitude 1 June 1965 and along nearly identical flight track during night flight of 516 June 65. (y-scale is approximate, for se8 level.) corresponds to horizontal distances of approximately 55-110 km at the aircraft’s speed. These anomalies are caused by magnetic line&ions of the ocean bottom and are characteristic for this Pacific region. They were already shown by BRACEY (1963) in magnetic intensity profiles along 159’W longitude obtained from the magnetic survey by ship made by the U.S. Naval Oceanographic Office 1961 and 1962. A comparison of all eight field intensity vs. dip angle curves showed a high degree of correlation of all those large and small anomalies if allowances (of up to f0.4’ dip angle or f23 km distance along the flight path) were made for loose matching of the curves along the dip axis. This procedure allowed for the fact that the magnetic line&ions of the ground did not run exactly perpendicular to the aircraft tracks. Each of the six y vs. dip curves (obtained during the day flights) was matched and compared with one of the two reference curves (obtained in the night flight of 6 June 1965). The choice of the latter was determined by the closest proximity of the corresponding day flight tracks. In the matching of the dayllight rmd reference curves the aim was to produce the best fit of the before mentioned y anomaly patterns and, in addition, near zero y differences in the regions having dip angles > *loo where no large current is present. Six pairs of y vs. dip curves were obtained, each pair consisting of one dayflight curve and one reference curve. The y difference between the two curves-m the following always called Ay-at each point along the flight path was then plotted. Only small Ay ( ~10) were derived from day minus night curves for locations having dip angles from +35” to +21°, as can be expected outside the electrojet region. To smooth out sudden jumps in the A7 curves caused by incomplete matching a running mean was computed, each averaging a period stretching over 2’ dip or approximately 9 min flying time or 120 km distance. In Fig. 3 Ay (shaded bands) and top frequencies of Esq echoes (solid curves) are plotted vs. dip angle. The upper and lower limits of the shaded bands indicate the standard deviation from the running mean. While those limits merely represent the uncertainty due to incomplete matching (caused by ocean bottom magnetic lineations) all other sources of uncertainty inherent in the data and in their processing (uncertainties of drift of equipment reference voltage, of aircraft heading and location, of reference curves) were found smaller and relatively negligible. 3. IONOSPHERIC DATA During all day flights the Granger sounder installed in the airplane w&s operated. Verticd incidence ionograms with swept frequencies from 2 to 10 MHz were taken continuously; the period for each sweep was 36 sec. During one flight this program was interrupted every 10 min
G. J. GASSMANN and R. A.
786
LOCALNOIJN
I JUNE 1965 2115UT I ” \ START ‘.py_ I,j,~ ‘_ -’
WAGNER
-,..
. . -I IO 8 fi 4I IO 8 6 41 IO 8 6 4I I
\ .
2309UT
IO 8 6 4
tl6" tl4" tl2” tip” tfP ‘7 I
I
I
t4O tf I 100 km
Fig. 3. North-south cross-sections through eleotro jet. Shaded bends give Ay (in units of y) vs. dip. Ay is the difference of field intensities measured: day minus reference flights. Width of shaded bands indicate the uncertainty. Solid curves: top frequency of Esq. Local time is approximately U.T. minus 10.6 hr. Aircraft altitude: 10 km. for 1 min by a sequence of rapid sweeps from 2 to 10 MHz, taking I.8 set each. During this period pulse by pulse echo amplitudes were recorded on rapidly moving film, together with calibration ctmplitudes. 4.
ELECTROJETNORTH-SOUTH CROSS-SECTIONSNEAR 159”W LONGITUDE
The six bands of Ay vs. dip displayed in Fig. 3 represent meridional cross-sections of differences between total magnetic field intensities during day and night (reference). The actual curve representing the electrojet may be assumed to run in an unknown way within each band, possibly as smooth as those limits of uncertainty allow. The meaning of the baseline Ay = 0 is discussed below. For estimating the electrojet current from Ay it must be pointed out that Ay = 0 does not mean zero current. In the matching procedure between a day curve and a reference curve the Ay by definition was made zero for dip angles >lO”. In detlning Ay = 0 we follow a conventional, not necessarily proper, practice (for example see CASAVERDE 1961; OSBORNE 1964; YACOB 1966) which identifies the electrojet by the sudden increase of the diurnal magnetic variation at stations near the dip equator. However, the noon over midnight increase of the magnetic field intensity at locations with about 10’ dip angle is considerable; for example, the monthly quiet-day mean of the daily variation of the horizontal component of the magnetic
North-south
cross-sections
of the equatorial
electrojet
787
field intensity at Fanning Island (f 94” dip), 1ocated in our flight route, in June 1958 (sunspot maximum) was in the order of 607. In June 1965 (at sunspot minimum), for which no data were available for this station, the corresponding value must be expected to be about 8 of the above. This estimated value of 40~ (at 10’ dip) is then the combined result of the noon current The ratio of the two contributions is not precisely known and (negative) midnight current. and for this reason magnetic diurnal variations have been presented against arbitrary reference levels. MATSUSHITA and MAEDA (1965) chose the daily mean, PRICE and STONE (1964), and PRICE and WILKINS (1963), and SUGIURA and HAGAN (1967) chose the midnight value. It is estimated that the current density near midnight is probably of the order of one-tenth of the day-time value. Assuming this for 10’ dip (where we matched the day and night curves) the overhead current during the day near noon caused an estimated 90 per cent of 40~ = 36~ positive deviation from the horizontal component, as it were without external currents. The baselines by = 0 in Fig. 3 do not account for this positive current. The above mentioned position of the baseline, being a matter of detiition, has an estimated uncertainty of &lOy due to limited knowledge of sunspot cycle dependence of the diurnal variation and of the ratio of night to day current. This estimate assumes very quiet magnetic conditions. We add to this uncertainty another f10y due to the fact that the reference night flight took place under K, = 2-. The Table shows that, for instance, at Guam 8-10~ was the deviation from a smooth diurnal curve.
From the foregoing follows that the baseline in Fig. 3, from which the electrojet current may be computed, lies 36 &- 20~ below the level Ay = 0. In order to estimate how much of the electrojet meridional cross-sections, displayed in Fig. 3 as spatial variations, are actually manifestations of short temporal variations a number of magnetograms of fixed stations were analyzed. As shown in the Table no such temporal variations took place on lo/l1 June. On l/2 June and S/9 June, the observed variations at the ground stations suggest that about 10 and 12 per cent respectively of the Ay values, plotted in Fig. 3, might be temporal variations. The latter might have been caused by disturbance ring current. The highest Ay value, indicating the strongest electrojet current, was found during the flights of lo/l1 June, the maximum of 19Oy occurring at local noon time and at’ -2’ dip. The decline of Ay is gradual towards the N of the cross-section, while the electrojet is strong through -8” dip, then decreases rapidly to the S- end. On the return flight the same profile was found with the maximum Ay still at -2” dip. The nearly identical shape of the cross-section curves derived from the N to S and S to N flight suggest that during the 44 hr between beginning and end of the two flights there occurred no change in current strength and no shift in the location of the current maximum. This local and temporal stability coincides with the fact that the flight was made during absolutely quiet geomagnetic conditions, the K, indices being 0, all the time. The width of the electrojet on this day appears to be approximately 1000 km, symmetrical about the dip equator. The cross-section obtained from the flights of lo/l1 June show the most irregular Ay versus dip curves of all flights and the widest error bands. The last feature is mainly due to the fact that the flight tracks of the lo/l1 June have the greatest distance of all flight routes from those of the reference flights. The sharp irregularity near +lO” in both S to N and N to S curves is an effect occurring while overflying Fanning Island. All other flights passed the island at a distance. A striation appears to be indicated near -5” dip, more pronounced in the N to S cross-section. During rather quiet magnetic conditions (with highest K, index of 2, at the beginning of the N to S flight) on l/S June the maximum electrojet with Ay = 140 was
788
G. J. GASSMANN and R. A. WAamn
recorded at -lo dip, shortly before local noon. Not much change was found during the return flight, high electrojet current prevailing between -4 and +2” dip. The irregular shape of the Ay vs. dip curve and the wide error band near + 1” dip show the ground effect of Jarvis Island which was passed in 9 km distance on the S to N section of the flight. The width of the electrojet was about 800 km on l/2 June. During the flight of 8/9 June both flight tracks were close to that of the respective reference flight; the error bands consequently are narrow. Geomagnetically moderately disturbed conditions existed during this flight with K, of 4_ in the beginning and 3, later. The electrojet appears to extend more to the N than on the other 2 days, while its southern limit is the same as found on l/2 and lo/l1 June. The width appears to be about 1100 km. In contrast to the S-N cross-section the N to S cross-section shows a rather uniform electrojet strength between +9 and -8” dip with a maximum Ay of 100 near -6’ dip. The ourrent densities which can be derived from the highest peaks of the cross-sections in Fig. 3 are much higher than those experienced at both Huancayo and Addis Ababa. This can be seen by referring to the last two columns of the Table where the maximum ohange of H at these stations during one Greenwich day is listed. This suggests, although the longitudinal and/or time differences might play a role, that during the days listed the peak current never came close to the stations.
As another indication of the electrojet current, and independent of the magnetic measurements, the ionograms were utilized. According to COHEN,BOWLESand CALVERT(1962) and FARLEY (1963) two-stream instabilities within the electrojet current cause ionospheric irregularities which, in turn, cause radio scatter, which gives rise to typical Esq and Ess echoes in ionograms. The gain setting of the receiver was constant for each flight. The top frequency of the Esq echo, reflected vertically from 115 km, is plotted for each flight. Intercomparison between different days should be exercised with caution, since slight uncontrolled changes in receiver gain setting from day to day might have occurred. The top frequency shows good correlation with the magnetic data in the center of the electrojet. The poor correlation at the northern and sourthern edges is not unexpected, since the ionospheric irregularities are known to occur only above a certain threshold value of the current. In order to relate top frequency to received power a few spot checks were made. The frequency dependence of vertical Esq echo power waz found to be flat from 4.5 MHz (the lowest frequency measured) to 6.5 MHz and then falling toward higher frequencies. Earlier measurements in Peru (GASSMANN and WAamn, 19f3e) yielded a dependence on frequency f in the formf-c with c = 1.4 approximately. For Ibadan, SKINNERand WnrauT (1982) found c = 9.4. As a consequence of the fiat scatter spectrum the recorded top frequency was very sensitive changes in echo power; a reduction from 8 to 4 MHz corresponded to a power reduction of only 4. The top frequency of 8 MHz corresponded to O-2 of the power which would have been received from a metallic reflector. The polarization of the transmitting and receiving antenna was linear and was oriented in the direction of the aircraft axis. 5. EFFECT OF 30 i&y 1965 ECLIPSE ON ELECTROJET A recent short review of both theoretical predictions and observations of eclipse effects on the equatorial geomagnetic field can be found in MATSUSHITA (19ef3). The observed effect at an equatorial magnetic station has been a gradual decrease and increase of the horizontal component following along with the progress of the eclipse. In terms of y0 of the daily variation the maximum eclipse effect has varied from case to case and was between very small or unoertain
North-south cross-se&ions of the equatorial electrojet
789
and 50 per cent. Recently BOMKE et al. (1967) recorded the horizontal field intensity at I&&a, and Humoayo, Peru, during the 12 November 1906 eclipse oocurring at that location in the early morning. A gradual depression and recovery was recorded reaching 30~ maximum depression 2 min after totality in the E-layer. A value for the effective recombination ooefficient was derived and was based on the adoption of a disturbance flow pattern of the current, derived by VOLLAND (1967).
The present measurements were made during two flights in the path of the total solar eclipse of 30 May 1965, which crossed the dip equator. The flight during the eclipse and the corresponding control flight were performed under nearly identical conditions as to location, altitude, time and aircraft’s own field. The data from these two flights were, therefore, compared without applying corrections. The map of Fig. 1 shows both the path of totality and the aircraft track with corresponding time. The shadow of the moon overtook the aircraft at 21.20 U.T. or 12.28 local time. At that particular location the dip angle was near $2”. The y vs. dip curves obtained from the measurements made during the eclipse flight on 30 May and during the reference flight on 27 May, 1965 are shown in Fig. 4.
Fig. 4. Total geomagnetio field intensity vs. dip angle as measured during the flight in the path of totality of the eclipse of 30 May 1966 (y-scale is approximate, for sea level). First and fourth contacts occurred approximately 70 min before and after totality respectively and were not covered by measurements. It was fortunate that the matching of the y vs. dip curves for the eolipse day and reference day was perfeot on both ends of the data, i.e., near -7 and $4“ dip. Since these locations are not outside the eleotrojet width, the perfeot matching indicated that the shape of the electrojet was similar on both days. This fact furnishes con6dence that the observed difference in total field intensity near totality is not merely an incidental dissimilarity of the electrojet cross-se&ions on two different days. An additional check was made as to whether during the periods of the flights on either one of the two days sporadic fluotuations in the order of one and one half hour duration occurred at other stations. The highest K, indicies were 2_ and l_ respectively during the times of flight on 27 and 30 May 1966; the magnetograms of Guam, Honolulu, Huanceyo and Addis Ababa show sporadic fluctuations smaller than 10~ within the identical time interval (see Table 1).
790
G. J. GASSMANN and R. A. WAGNER
The effect of the eclipse may be best described after realizing that the data from 20.30 to 21.40 U.T. displayed in Fig. 4 are taken at magnetic latitudes from -2 to +2”, locations which are well within the width of &-6” dip which is normally occupied by the electrojet. The measurements therefore were practically made at the dip equator (see Fig. 1). Considering these circumstances, the significant feature of the effect (represented in Fig. 4 by the difference between eclipse- and reference-data) appears to be the assymmetry with respect to totality. The effect of the eclipse is noticeable and steady during the whole hour prior to totality. Shortly after totality, 21.27 U.T. the effect ceased rapidly and fluctuated about zero. Although only 20 min after totality were covered by measurements as a consequence of range limitations (the flight required 13 hr) the data allow the drawing of some important conclusions. The explanation of the asymmetry of the data cannot be obtained from existing theoretical treatments of the problem. The theoretical predictions as reviewed by MATSUSHITA (1966) and also as advanced by VOLLAND (1957) do assume the undisturbed current system S, to be infinitely large compared to the area of the moon’s shadow (penumbra). The effect of the eclipse appeared then as a superposition of the original current stream and local disturbance current systems in such a way that the resulting current flow is symmetrical around the area of the moon’s core shadow (umbra). Such flow pattern would produce magnetic eclipse effects symmetrical at totality. It is obvious that this simple concept is not readily applicable to the region of the equatorial electrojet whose width is smaller than the penumbra. The limited width of the channel in which the electrojet flows is controlled by the geometry of the field lines and the position of the space charges (“Polarization Field”). When the penumbra moves across the electrojet the latter cannot simply circumflow the area of lowest conductivity since the position of the channel is predetermined. The channel is flexible only to the degree by which the electrical space charges can be rearranged within the restrictions imposed by the magnetic field. And, that process is not a simple problem of flow theory. It is known (see Introduction) that the optimal effective conductivity along the dip equator depends critically on the electrical field E, arising from accumulated space charges and that the latter are established in their proper positions within the ionospheric E-layer by Hall currents. If the process of rearranging space charges takes time which is comparable to the period of the most severe disturbance, namely of the umbra’s passing across the electrojet (20 min), then the flow pattern around the umbra can hardly be symmetrical. Without attempting a theoretical treatment of this case, it is reasonable to expect that the electrojet indeed rearranged the space charges along its west-east flow in such a way as to utilize regions of higher electron density. It follows from the f,,E, measured during this and other eclipses, that during the whole period while the penumbra was approaching from the south (from 19.45 to 21.20 U.T.) the electron density in 105-125 km altitude above the dip equator increased toward the north with an average slope of +65 per cent per 1000 km. This likely caused a northward shift of the electrojet and this shift amounted to an evasive move away from the approaching umbra prior to totality. It is now of interest to ask how far the electrojet might have deviated from its mormal position along the dip equator. Such displacement away from the dip equator must amount to a deviation from a stable
North-south cross-sections of the equatorial electrojet
791
configuration. Judging from the very existence of the normal electrojet clinging close to zero dip, this position is obviously stable for reasons of magnetic symmetry. The data in Fig. 4 can be best understood if one assumes that during this eclipse the electrojet after an evasive move toward north must have returned to its normal position as soon as the conductivity along the dip equator allowed. This move, however, amounted to a flip-over from the northern side of the umbra to the southern side. It is suggested that this happened at about 21.27 U.T. It is suggested that shortly before 21.27 U.T. the center of the electrojet was located north of +6” dip and that shortly after 21.30 U.T. the center of the electrojet was located south of +2” dip. The above statement refers to the longitude of approximately 132”W where the observation took place. At this eclipse and location the electrojet flow pattern in horizontal space appeared to be non-symmetrical with respect to the umbra and produced at a fixed location a non-symmetric magnetic record with respect to totality. 6. SUMMARY The results describe some observed dynamic variations of the electrojet which might aid theoretical treatments of electrostatic fields in the ionosphere. Airborne magnetic and ionospheric measurements across the dip equator in the Central Pacific during three days in June 1965 showed that the variations in width and individual shape of the electrojet are considerable from day to day. On days of low magnetic activity, however, the width and shape of the electrojet stayed nearly constant as evident from the cross-sections taken before and after local noon on the same day. Irregular features or striations appear superimposed upon a smooth cross-section and modify it by approximately 130 per cent. These striations, however, stay fixed, within 150 km horizontal distance, during a period of two hours and possibly longer. In contrast, on one day with K, near 4 the shape and individual striations of the meridional cross-section underwent drastic changes within two hours. On the ionograms Esq and Ess echoes appeared continuously in the center of the electrojet but seldom were present over its whole width. At the edges of the electrojet occasional strong Esq echoes were observed during short periods. The vertical echo strength of Esq versus frequency turned out to be almost constant for frequencies up to 6.5 MHz and falling off beyond. The eclipse of 30 May 1965 appeared to have caused a reduction of at least 50~ of the total field intensity at the aircraft which was below the center of the electrojet at totality. The reduction of field intensity developed slowly prior to totality and ceased quickly thereafter. The circumstances suggest that the electrojet was forced by the umbra to temporarily shift its normal position possibly by as much as 6’ dip and returned shortly after the umbra had passed the dip equator. Acknowledgements-R. GOWELL, AFCRL, was responsible for the excellent quality of the ionospheric data and R. HOADLEY, Lowell Technological Institute Research Foundation, for the magnetometer instrumentation. The aircraft was under the command of Major K. CROOKS, and the chief navigator was Major R. VINING, both of the USAF. We appreciate the critical reviews of the paper by E. MAPLE, E. J. CKERNOSKY, and K. TONAN, all of AF’CRL, as well as the subsequent comments of two Journal of Atmospheric and Terrestrial Physics reviewers.
G. J. GABS-
792
and R. A. WAONER
REFERENCES BAKER W. G. and MARTYN D. F. Born H. A., BH. A., H.BRIS A. K. HUXSE W. H., SHEPPARD D. J., GIESECEE A. A. and PANTOJA A. CASAVERDE M. COFIENR., BOWLES K. L. and CALVERT W. DAVIS T. N., BURROWS K. and STOLARIK J. D. FARLEY D. T. Jr. GASSMANN G. J. and WAGNER R. A. IXALL S. H. HIRONO M. HUTTON R. MATSUS~TA S. MATSUSHITA S. and MAEDA H. ONWLTMECHILLIC. A. OSBORNE D. G. PRICE A. T. and STONE D. J. PRICE A. T. and WILKINS G. A. SINCR R. N. and MISRA K. D. SKINNER N. J. and WRIGHT R. W.
SUUIURA M. and CAIN G. C. UNTIEDT G. VOLLAND H. YACOB A. ZadDDA A. J.
Rqference
is aLso made
1953 1967
Trans. R. Sot. (A), 246,281. J. Geophys. Res. 72, 5913.
1961 1962
Am. J. Phys. 29, 744. J. Geophys. Res. 67, 965.
1967
J. Geophys. Res. 72, 1845.
1963 1966 1968 1952 1967 1966 1965 1966 1964 1964 1963 1967 1962
J. Geophys. Res. 68, 6083. J. Geophys. Res., 71, 1879. J. Atmosph. Tern. Phys. 30, 1293. J. Geom. Geoel. Kyoto 4, 7. J. Atmosph. Terr. Phys. 29, 1429. Ann. Gkophys. 22, 471. J. Geophys. Res. 70, 2535. Nigerian J. Sci. 1, 11. J. Atmosph. Tern. Phys. 26, 1097. Annals IGY, 35, 65. Phil. Trans. R. Soo. (A), 253, 31. J. Geomug. GeoeZ. 19(2), 143. Ionosphem’c Sporadio E (Edited by E. K.
1966 1967 1957 1966 1960
SETH and S. MATSUSHITA), p. Pergamon Press, Oxford. J. Geophya. Res. 71,1869.
J. J. J. J.
Geophys. Atmosph. Atmosph. Geophys.
37.
Rea. 72, 5799. Terr. Phys. 11,1. Tew. Phya. 28, 581. Res. 65, 2247.
to the follouing unpublished materid:
BRACEY D. R.
1963
MASON R. G.
1966
SIJOIURA, M. and HA~AN M. P.
1967
Marine Science Dept., U.S. Naval Oceanographic Office, Washington, D.C. Unpublished manuscript. Scripps Inst. of Oceanography, San Diego, Calif., Reference 63-13. Dept. of Atm. Sciences, University of Washington, Scienca Rep., Nat. SC. NSF GA-478.