Equatorial Electrojet Strengths in the Indian and American Sectors Part I. During Low Solar Activity

J. Geomag. Geoelectr., 42, 801-811,1990 Equatorial Electrojet Strengths in the Indian and American Sectors Part I. During Low Solar Activity A. R. PATIL, D. R. K. RAO, and R. G. RASTOGI Indian Institute of Geomagnetism, Colaba, Bombay 400 005, India (Received June 1, 1989; Revised March 5, 1990) The equatorial electrojet strengths are computed by a simple technique at Indian and American sectors utilising the hourly mean values of the horizontal component (H) of the geomagnetic field at pairs of appropriately selected observatories. After testing the validity of the index to its faithful representation of the strengths, detailed comparative study of the monthly mean and seasonal behaviours of the diurnal variations at the two sectors for low solar activities are made. The diurnal variations would permit themselves in identifying the intervals of "Counter electrojet" events separately during the "morning" and "afternoon" hours. Besides the morphological features of this special phenomenon at the two sectors, their dependence on the degree of magnetic activity and lunar control are studied and the results are reported. In the seasonal progression of the occurrence of counter electrojet, during low solar activity period, the afternoon/ early evening events are confined to J-season at Indian sector whereas they are observed more frequently in the morning hours in the D-season at American sector. During high solar activity years, morning counter electrojet events are larger in number in both the sectors. Lunar influence on the occurrence frequency of the counter electrojet appears to be confined to the morning events only. 1. Introduction The strength of the equatorial electrojet current depends on many variables of ionospheric parameters like the eastward electric field strength, neutral winds, the intensity of the geomagnetic field, the electron density, ambient temperature, collision frequencies of the charged particles, longitudinal gradients of the electric fields and conductivities and the degree of ionospheric distortion due to polarization fields, etc. Numerous investigators have attempted to define a reasonable model responsible for the enhanced eastward current distribution in the vicinity of the dip equator from the geomagnetic field variations within and just outside this region. With high interest in this phenomena and impetus from the I.G.Y., more observing stations were established on the equator including temporary nets of several recorders across the equator. Apart from the equatorial electrojet, another special feature, an occasional depression in the daily variation of the horizontal component (H) of the geomagnetic field below its night time level at the observatories in the electrojet region, had led to the concept of a current similar to the electrojet but in the reverse direction. This feature was called by GOUIN and MAYAUD (1967) as the equatorial counter electrojet. Several researchers studied both qualitatively and quantitatively the equatorial electrojet (OSBORNE, 1973; KANE, 1974; BHARGAVA et al., 1983) and the counter 801

802 A. R. PATIL et al. electrojet phenomena either at a particular longitudinal sector or globally. (RASTOGI, 1973, 1974; MAYAUD,1977; SASTRI and BHARGAVA, 1980; ARORA et al., 1982). In this communication, a simple technique is adopted to derive the strength of the equatorial electrojet at each hour of the day from the ground-based geomagnetic field at the Indian and American sectors during an epoch of low solar activity. Incidentally, an index so derived for the strength gives the intervals of counter electrojet events. First the index is tested to prove for its faithful representation of the phenomenon. It is then used for the comparison of the complexities like relative strengths of the electrojets, differences in the occurrences of counter electrojet, etc. 2. Data and Method of Deriving Electrojet Strength For the low solar activity years, 1964 (Rz=10.2) and 1965 (Rz=15.1) mean hourly electrojet are selected for deriving the electrojet strength in the Indian sector. Alibag is situated at about 1000 km from the equatorial station Trivandrum in the Indian sector. Apart from the distance, the annual and semiannual variations at these two stations are very different, Alibag predominantly showing annual progression whereas at Trivandrum this is mainly semiannual. These features are similar to those that are expected of stations under and far away from the influence of equatorial electrojet RASTOGI and IYER (1976). Thus, selecting Alibag as a non-electrojet station in the Indian sector appears to be each of the stations from the local time retabulated data of H by subtracting the first (close to the midnight) local hourly value from each of the 24 hours. The strength of the of the electrojet station from that of the non-electrojet station at each of the sectors. The assumption underlying the above procedure to designate the residual field values as the strengths of the electrojet are as follows: (i) The field of the extra ionospheric currents can be considered as the same at the equatorial station as at the non-equatorial station in a particular sector because of the proximity of the stations compared to the distance of the source currents. During disturbed days, small period fluctuations (periods <60 min) superimposed on the longer periodic variations may not have phase synchronization at the two stations in a longitudinal sector and also the effects of the fluctuating electric field of ionospheric origin may not be the same at the non-equatorial and equatorial stations in a sector. These fluctuations would, however, be smoothed out to a maximum extent in the hourly mean values used in the analysis and the magnitudes of the residual field are considered to be of negligible amplitudes. (ii) The field associated with the worldwide Sq currents at non-equatorial station represents the Sq field at the equatorial station at both the sectors and the excess field at the equatorial station is attributed to the electrojet currents. These assumptions and the method of derivation closely follow that the method of evolving indices adopted by BHARGAVA et al. (1980, 1983) excepting that no correction for the

Equatorial Electrojet Strengths in the Indian and American Sectors: Part I 803 non-cycle variation is applied here and the base chosen for local midnight value is different. It is assumed that a periodic variation between the pairs of stations is almost derived by subtraction. The assumption of the inequalities at a station far from the influence of the equatorial electrojet to represent the worldwide Sq at a station under the influence of the electrojet is also the basis of calculation of the SDI index derived by KANE (1973). At the Indian sector, YACOB (1977) has earlier shown that a suitable estimate of the electrojet field at Trivandrum for any hour can be made by subtraction of inequalities of H at Trivandrum from those at Alibag or at the most 1.1 times of the inequalities at Alibag. Using high time resolution observations of VHF backscatter echoes at Jicamarca, RASTOGI et al. (1977) showed that the E region irregularities disappear or reappear precisely at the time when ƒ Hej is close to zero. RASTOGI and PATIL (1986) demonstrated would indicate the occurrence of counter electrojet events. 3. Results Average diurnal variations for both the years, 1964-65, over the selected midnight time base, in respect of Huancayo (HUA), Fuquene (FUQ) and the strength of the electrojet field in the American sector (HUA-FUQ) and similar variations at Trivandrum (TRD), Alibag (ABG) and the Indian jet strength (TRD-ABG) are shown in Fig. 1. A Fig. 1. Diurnal variations for 1964-65 years. American sector at stations (i) at Huancayo (HUA), (ii) at similar variations at Indian sector stations (i) at Trivandrum (TRD), (ii) at Alibag (ABG) and (iii) the electrojet strength (TRD-ABG) corresponding to 75 E meridian time.

804 A. R. PATIL et al. marginal increase in the magnitude of the electrojet strength in the American over the Indian sector at local noon can be inferred from the figure. The ratio of the American to Indian strengths is 6:5. The increase in jet strength at American sector may not be due to the decrease in the diurnal variations at FUQ over ABG in Indian sector, as HUA variations are apparently larger than those at TRD. There is a suggestion to change in the times of maximum between the equatorial and non-equatorial stations between the two sectors indicating that at Indian sector the jet leads over Sq and the opposite at the American sector. This fact is brought out very clearly in the dialogram plot of annual (1964-65) amplitudes and their times of maximum shown in Fig. 2(a). The first two occurrence of maximum differs between American and Indian sectors between the two solstitial seasons. Earlier reaching the time of maximum of the 1st harmonic amplitude in D-months at American over the Indian sector and later in J-months is apparent from the dialograms. However, the amplitudes and their times of maxima of the 2nd harmonic are American and Indian sectors for 1964-65 years. The symbols are defined with abbreviated station names in the diagram.

Equatorial Electrojet Strengths in the Indian and American Sectors: Part I 805 nearly the same at all the seasons in respect of the 2nd harmonic. calendar months at the two sectors is shown in Fig. 3. Both at Indian and at American sectors, the electrojet is strongest (with maximum amplitudes) in Equinoctial months and they are the least in December solstitial months. Generally the maximum of the electrojet field around the noon is higher at American sector over the Indian sector at all the months, thereby suggesting that the jet strength is larger even on monthly basis in the occurrence of counter electrojet events. Even in the monthly mean values during the low solar activity years, the presence of afternoon-early evening counter electrojet events can be seen in months April to September in the Indian sector. On the other hand, an indication of the occurrence of counter electrojet during morning/ early forenoon hours only is noticed in the E-seasonal months of March, April and October in American sector. However, there is no suggestion of afternoon/ early evening counter electrojet phenomenon in the American sector and morning/early forenoon counter electrojet in the Indian sector in any of the monthly mean diurnal variations. Fig. 2(b). Dialogram points of 1st and 2nd harmonic amplitudes and their times of maxima for each of the seasons at the two sectors. Solid circle and hollow circle dial points correspond to American and Indian sectors respectively.

806 A. R. PATIL et al. Fig. 3. Diurnal variation of electrojet strengths at each of the calender months for the years 1964-65 at American and Indian sectors. With a view to understand the occurrence of the counter electrojet events in both the decreases below -5 nt or more are counted for each of the months during the period 1964-65. To further elucidate the strongness of the occurrence of counter electrojet depressed below this value are determined for each of the hours. The total number of occasions against the local time for each of the months at both the sectors are shown in Fig. 4. It is seen that in Indian sector about 30% of the days are associated with the able in the Indian sector. However, in American sector, there are relatively few occasions when the counter electrojet phenomenon occurred. For the years 1964-65, this sector

Equatorial Electrojet Strengths in the Indian and American Sectors: Part I 807Fig.4. Number of occasions of mod

808 A. R. PATIL et al. months from March to September i.e. in months of northern solar declination. Also, the occurrence of morning/ forenoon counter electrojet does not show any distinct seasonal variation. In the American sector the occurrence of afternoon/early evening counter electrojet is confined to the D-seasonal months only, excepting November month. In case of D-season, the tendency to have opposite times of occurrence of counter electrojet between these two sectors is a noteworthy result. In general, the occurrence of counter electrojet phenomenon in the morning/ early afternoon hours in the American sector and similar occurrence in the afternoon/early evening hours in Indian sector is apparent from the results from Fig. 4. October month in the American sector has the largest number of morning counter electrojet events whereas April, May, June and July have about the same number of higher days of afternoon counter electrojet in the Indian sector during low activity years. With a view to understand the influence of the geomagnetic activity on the counter electrojet) or 14 or 15 hrs LT (afternoon counter electrojet). The number of occasions are counted separately for both morning and afternoon counter electrojet according to the age of moon on that day. The age of the moon is reckoned for the zero Greenwich hour of the day in the ascending order of the lunar phase which varies from 0 and 23. For both the sectors the number of occasions against the age of the moon separately for morning and afternoon events are shown as histograms in Fig. 6. A semidiurnal pattern of the distribution in case of American sector data both for morning and afternoon events is apparent, though it is subdued and shifted in age in case of afternoon events. The larger number of events observed in the Indian sector are, however, not following any regular pattern with respect to lunar age. However, there is a suggestion at both the sectors that the events are to some extent influenced by the motion of the moon.

Equatorial Electrojet Strengths in the Indian and American Sectors: Part I 809 Fig. 5. Percentage occurrence of counter electrojet days, number of counter electrojet days and total number sectors. 4. Discussion OZOEMENA and ONWUMECHILI (1987) showed that the peak eastward intensity J0 and the total eastward current I+ during December and June solstitial months of the years years shown here is in confirmity with the above results of all seasons. As stated earlier in the results, the increase in the strength of the electrojet in the American sector may not be due to the latitudinal difference in the selected non-equatorial stations at the two sectors but appears to be due to increase in the effective conductivity of the equatorial ionosphere in the American sector. Also, there is no correlation of day to day changes in Sq between the two sectors. The linear correlation coefficient computed for all days in

810 A. R. PATIL et al. Fig. 6. Histograms of number of occasions of equatorial counter electrojet events separately for morning and evening occurrence against lunar age for 1964-65 at American and Indian sectors. 11 hr LT at Alibag when correlated with following days values at Fuquene, gives a value of -0.078, indicating the changes of Sq are quite independent at the two sectors. Apart from the occurrence of counter electrojet events at each sector, the simultaneous occurrence of the phenomenon at both the sectors on the same day are examined. The morning/ forenoon events registered simultaneously only on 21 occasions between American and Indian sectors out of the total number of 119 days during 1964-65. Similarly 37 simultaneous observations of the afternoon/ early evening counter electrojet events are noticed out of 289 days of the events in Indian sector. Thus, there are less than 20% of simultaneous occurrence of counter electrojet events at these two sectors. The irregularities of the events at different longitudes have been extensively studied by various authors (GOUIN,1962; RAO and RAJA RAO,1963; KANE, 1973, 1974; RASTOGI, 1974). Summarising the results, MAYAUD (1977) stated that any reliable statement concerning the longitude extension of a given event would need a careful examination of records from observatories widely spread in longitude. He observed that a given event is never restricted to a very narrow longitude band (Say, 2 hr wide) but is commonly spread over 5-8 hr in longitude and sometimes occurs within a much wider band. The results reported here viz; simultaneous presence of the phenomenon though not always for the two regions nearly 180 in longitudinal separation supports this contention. It is known that changes in the horizontal component of the earth's magnetic field is proportional to the product of conductivity and the east-west electric field. The electric field to a large extent is governed by the tidal winds. Also, it is unlikely that during the time of counter electrojet events occurrence, there would be abnormal change in the conductivity. It, therefore, appears that the electric field changes associated with tidal winds are responsible for the counter electrojet. If this were the case, it is surprising to observe the occurrence of counter electrojet in both the sectors only on few occasions. Perhaps the sources responsible for the non association of day to day changes in the field

Equatorial Electrojet Strengths in the Indian and American Sectors: Part I 811 different longitudes may also be responsible for the non-simultaneous occurrence of counter electrojet phenomenon. Lunar modulation in the occurrence frequency of the counter electrojet is noticed to be apparent in the American sector. The results of the lunar influence shown here is consistent with that shown by SASTRI and JAYAKAR (1972), RASTOGI (1973) and BHARGAVA et al. (1983). However, it appears that the moon's effect is not to cause the counter electrojet events but only to modulate the occurrence of the events. This conclusion has also been arrived by MAYAUD (1977). REFERENCES ARORA, B. R., N. S. SASTRI, and B. N. BHARGAVA, Spectral characteristics of the geomagnetic field associated with the equatorial electrojet and counter electrojet in the Indian region, Indian J. Radio Space Phys.,11, 129-132, 1982. BHARGAVA, B. N., N. S. SASTRI, B. R. ARORA, and R. RAJARAM, The afternoon counter-electrojet phenomenon, Ann. Geophys., 36, 231-240,1980. BHARGAVA, B. N., B. R. ARORA, and N. S. SASTRI, Indices of equatorial electrojet and counter electrojet in the Indian region: evolution of the indices and their authenticity, Proc. Indian Acad. Sci. (Earth Planet. Sci.), 92, 45-55, 1983. GOUIN, P., Reversal of the magnetic daily variation at Addis Ababa, Nature, 193, 1145-1146, 1962. GOUIN, P. and P. N. MAYAUD, A propo dell existence possible dun "Counter Electrojet" aux latitudes magnetiques equatorials, Ann. Geophys., 23, 41-47, 1967. KANE, R. P., An estimate of the equatorial strength, J. Atmos. Terr. Phys., 35,1965-1967, 1973. KANE, R. P., Evolution of disturbance daily variation (DS) and interplanetary plasma parameters, Proc. Indian Acad. Sci., 80, 124-139, 1974. MAYAUD, P. N., The equatorial counter electrojet-a review of its geomagnetic aspects, J. Atmos. Terr. Phys., 39,1055-1070,1977. ONWUMECHILI, C. A. and C. E. AGU, Longitudinal variation of equatorial electrojet parameters derived from POGO satellite observations, Planet. Space Sci., 29, 627-634, 1981. OSBORNE, D. G., Electrojet measurement from satellite and ground, J. Atmos. Terr. Phys., 35,1273-1279,1973. OZOEMENA, P. C. and C. A. ONWUMECHILI, Global variations of the POGO electrojet parameters during the solistices, J. Geomag. Geoelectr., 39, 625-636,1987. RAO, K. N. and K. S. RAJA RAO, Quiet day magnetic variations near the magnetic equator, Nature, 200, 460-461, 1963. RASTOGI, R. G., Counter equatorial electrojet currents in the Indian zone, Planet. Space Sci., 21, 1355-1365, 1973. RASTOGI, R. G., Westward equatorial electrojet during day time hours, J. Geophys. Res., 79,1503-1512,1914. RASTOGI, R. G. and K. N. IYER, Equatorial day variation of geomagnetic H field at low latitudes, J. Geomag. Geoelectr., 28, 461-479,1976. RASTOGI, R. G. and A. PATIL, Complex structure of equatorial electrojet current, Curr. Sci., 55, 433-436,1986. RASTOCI, R. G., B. G. FEJER, and R. F. WOODMAN, Sudden disappearance of VHF radar echoes from equatorial E region irregularities, Indian J. Radio Space Phys., 6, 39-43,1977. SASTRI, N. S. and B. N. BHARGAVA, Spectral characteristics of the geomagnetic field associated with the equatorial electrojet in the Indian region, Indian J. Radio Space Phys., 11, 129-132, 1980. SASTRI, N. S, and R. W. JAYAKAR, Afternoon depression in the horizontal component of the geomagnetic field at Trivandrum, Ann. Geophys., 28, 589-591,1972. YACOB, A., Internal induction by the equatorial electrojet in India examined with surface and satellite geomagnetic observations, J. Atmos. Terr. Phys., 39, 601-606, 1977.