Altitude and latitude dependence of the equatorial electrojet

The effects of day-to-day or seasonal variation of altitude and latitude profiles of the Elayer plasma density in the equatorial ionosphere on equatorial electrojet (EEJ) structure are examined numerically using a self-consistent and high resolution dynamo model. It is found that variations in the E-layer peak altitude and amplitude and its gradient below significantly affect EEJ structure. For any realistic shape, the EEJ peak appears at or below the E-layer peak altitude. Distinct double peaks appear in the EEJ structure, such as revealed by rocket measurements, if the E-layer peak is above 105 km or the gradient is large, as when sporadic-E is present. The influence of the latitudinal variation of ionospheric field line integrated conductivities upon the amplitude and altitude of the EEJ peak is demonstrated.     

Field aligned airglow observations of transequatorial bubbles in the tropical F-region

It is possible to form images of the tropical F-region ionization structures, variously labelled as ‘bubbles’, ‘plumes’, or ‘depletions’, in a plane perpendicular to the magnetic field by observing the airglow emissions associated with them in a field aligned direction. Structures which are present at altitudes from 250 km to more than 700 km above the dip equator map down to the 250–350 km region, where recombination and associated airglow emissions occur, ranging from the equator to dip latitudes of 15° or more. The structures can be viewed in a field aligned direction from sites in the range 17°–23° dip latitude. Measurements with high angular resolution (as small as 0.1° in the meridian) could show structures as small as 2 km. It is possible to make simultaneous measurements in both 6300 and 7774 Å recombination emissions, from which the height h_max of the peak plasma concentration n(e)_max on the field line can be estimated from a ratio of the emission rates. It is possible to make maps of n(e)_max and h_max either by raster scanning the sky in the two emissions or by imaging them onto an imaging detector. Useful data can be obtained from one site over a range of 20° in dip latitude and 10° in dip longitude. Observations in the same magnetic meridian as a backscatter radar system are desirable, as also are observations from near magnetic conjugate points. Imaging characteristics for the observation sites in the range of dip latitude 17°–23° have been calculated.     

Effect of the electric field on the equatorial ionospheric plasma distribution

It is known that on a counter electrojet day the noontime electron density at the equator shows enhanced values with no bite-out. The consequences of the absence of the normal equatorial electrojet on the electron density distribution at the equatorial station Kodaikanal (dip latitude 1.4°N, long. 77.5°E) and at an anomaly crest location Ahmedabad (dip latitude 18°N, long. 73°E) are discussed for a strong electrojet (SEJ) day and a counter electrojet (CEJ) day. The electron density distribution with height for a pair of SEJ and CEJ days at the two equatorial stations Kodaikanal and Huancayo (dip latitude 1°N, long. 75°W) are studied. The F-region peak height, hm and the semi-thickness parameter ym on the SEJ day followed a similar variation pattern. On the CEJ days ym exhibited a substantially low and mostly flattened daytime variation compared to the peaked values on the SEJ day. An attempt is made to interpret these differences in terms of the changes in the vertical drift pattern resulting from the E × B drift of plasma at the equator and the varying recombination rate β, which is also a height dependent and a local time dependent parameter.     

Are the equatorial electrojet and counterelectrojet centered invariably on the dip equator?

The H, D and Z variations at Huancayo (dip angle 2°N) in the South American dip equator region were compared during a sequence of days (11–16 January 1964) having extraordinarily large afternoon counterelectrojets. It was noticed that both the electrojet and counterelectrojet showed large latitudinal excursions (exceeding 2° dip angle or 1° latitude) on different days as also at different hours of the same day.     

Temporal variations of POGO equatorial electrojet parameters

An analysis of the POGO satellites observations of the magnetic field of the equatorial electroject for the September equinoctial months of the years 1967, 1968 and 1969, provided about 500 values each of the electrojet half-width w, its peak current intensity J₀, its total eastward current I₊ at 11, 12, 13, 14 and 15 h LT. The all-sector daytime average values of the parameters for the three years are 232 ± 47 A km⁻¹ for J₀, 234 ± 6 km for w and (55± 8)×10³ A for I₊.This first coverage of all sectors of the globe gives the first study of the diurnal variation of the total current and shows that all the three parameters vary substantially with local time; that w has minimal values around local noon; that J₀ has a pronounced peak around local noon as may be expected from the diurnal variation of H; and that I₊ has a broad maximum around 11 h and 12 h LT.     

Internal structure of the equatorial ionospheric dynamo

Using the Sq current profiles measured with rocket born magnetometer, off the coast of Peru, by Daviset al. (J. geophys. Res. 72, 1845) comparison is made between measured and calculated profiles near noon on geomagnetic dip equator, and a mismatch is pointed out in the height pattern of Sq current. Theory is worked out to determine the eastward electric field (E_y) with which computed j_y (eastward current density) coincides with the observed one. It is found that the neutral wind plays very important rôle in keeping E_y height-independent. E_y is found to be about 0.6 mVm⁻¹, ranging from 0.5 to 0.7 mVm⁻¹ in some cases. Flow of meridional current is obtained and its effect on jet current construction is discussed.     

Geomagnetically quiet day ionospheric currents over the Indian sector—III. Counter equatorial electrojet currents

Counter equatorial electrojet (CEJ) occurring at all hours from 0700 to 1700 h LT in the Indian sector have been studied. The percentage occurrence of morning and afternoon CEJ in each hour and each season are given and discussed. The first quantitative determinations of eight landmark parameters that depict the structures of hourly latitudinal profiles of CEJ current have been presented and discussed.These are the peak height integrated forward current density (or intensity) at the centre of the current, the peak return current intensity, the ratio of the peak return to the peak forward current intensity, the total forward current, the dip latitude of the current centre, the distance of the focus from the current centre, the distance of the peak return current intensity from the centre, and the latitudinal extent of the current. They reveal new and interesting features of diurnal and seasonal variations, and marked contrasts between morning and afternoon CEJ. Other evidences support our focal distance w of CEJ. Comparison with equatorial electrojet (EEJ) shows that five of the parameters are similar for EEJ and morning CEJ but substantially different for afternoon CEJ. The counter worldwide part of Sq currents have also been compared with CEJ.     

Diurnal variations of equatorial electrojet parameters derived from POGO solstitial data

The POGO electrojet data have been analysed for the winter and summer solstitial seasons of the two years, 1968 and 1969, respectively. Our analysis yielded a very large number of values (about 432), each of the electrojet half width, w, its peak current intensity, J₀, and its total eastward current, I₊, at 0900–1400 LT in December, and at 1000–1500 LT in June solstitial seasons, respectively. The all-longitude daytime values of the parameters are 246 ± 48 km for w,216 ± 60 A km⁻¹ for J₀, and (58 ± 8) × 10³A for I₊, in December of 1968 and 218 ± 19 km for w, 187 ± 20 A km⁻¹ for J₀, and (45 ± 3) × 10³ A for I₊, in June of 1969, respectively. We therefore present a diurnal study covering the entire Earth, in which for the first time, morning data earlier than 1000 LT are incorporated in the analysis. This has enabled us to chart a clearer picture of the temporal variations of electrojet parameters at two different solstices. This shows that all of the three parameters vary substantially with local time, in such a manner that J₀ and I₊ attain maximal values around local noon, while w is a minimum then, and therefore confirms the finding of Agu and Onwumechili.     

On the IRI model predictions for the low-latitude ionosphere

Measurements of ionospheric electron density vertical profiles, carried out at a magnetic equatorial station located at Fortaleza (4°S, 38°W; dip latitude 2°S) in Brazil, are analyzed and compared with low-latitude electron density profiles predicted by the International Reference Ionosphere (IRI) model. The analysis performed here covers periods of high (1979/1980) and low (1986) solar activities, considering data obtained under magnetically quiet conditions representative of the summer, winter and equinox seasons. Some discrepancies are found to exist between the observed and the IRI model-predicted ionospheric electron density profiles. For high solar activity conditions the most remarkable one is the observed fast upward motion of the F-layer just after sunset, not considered in the IRI model and which precedes the occurrence of nighttime ionospheric plasma irregularities. These discrepancies are attributed mainly to dynamical effects associated with the low latitude E × B electromagnetic plasma drifts and the thermospheric neutral winds, which are not satisfactorily reproduced either in the CCIR numerical maps or in the IRI profile shapes. In particular, the pre-reversal enhancement in the vertical E × B plasma drifts around sunset hours has a great influence on the nighttime spatial distribution of the low-latitude ionospheric plasma. Also, the dynamical control exerted by the electromagnetic plasma drifts and by the thermospheric neutral winds on the low-latitude ionospheric plasma is strongly dependent on the magnetic declination angle at a given longitude. These important longitudinal and latitudinal dependences must be considered for improvement of IRI model predictions at low latitudes.     

Computer simulations of the seasonal variations of the ionospheric equatorial anomaly in East Asia under solar minimum conditions

The previous dynamical, computer simulation model of the ionosphere at low latitudes of Chan H. F. and Walker G. O. (1984a, J. atmos. terr. Phys. 46, 1103; 1984b, J. atmos. terr. Phys. 46, 1113) has been modified to (1) include photoionization of molecular species NO⁺, N₂⁺ and O₂⁺ below 300km, (2) decouple the ionization and wind calculations below 180 km and (3) expand the geographical coverage to 46°N-30°S latitude. The first two modifications improved the model stability and the latter reduced the effect of the lateral boundaries on the equatorial anomaly. Results are presented for the representative seasonal months of January, April and July for East Asia, during solar minimum, comprising latitudinal-local standard time (120°E) contour plots of (1) the atmospheric pressure, (2) the computed meridional wind at 300 km, (3) the foF2 and (4) hmF2, together with latitudinal profiles of foF2 and N_T (electron content) showing the daytime development and nighttime decay of the equatorial anomaly.Comparisons have been made between the computer simulations and various experimental measurements of foF2, M(3000) F2 and N_T obtained in East Asia during periods of low solar activity. Most of the gross features of the development and decay of the equatorial anomaly at the various seasons were reproducible by the model simulations, the best agreement occurring for the equinoctial month of April.     

Two-dimensional high-resolution imaging of the equatorial plasma fountain

The first visual representation of the reverse equatorial plasma fountain during night-time has been made using images obtained by an All Sky Imaging Fabry-Perot Spectrometer observing the OI 630.0 nm airglow emission line from the thermosphere; the intensity of this line emission is directly related to the F-region plasma densities during night-time. From the identifiable features when the enhanced airglow emitting region moves overhead and when it completely leaves the field-of-view, the equatorwards velocity of the EIA has been estimated to be ~150 km/h at the latitude of the measurements (Mt Abu, 20°20′ dip latitude, India). Lower limits for the latitudinal and longitudinal extents of the EIA crest have each been inferred to be 6°.     

New results on the dip equator and the equatorial electrojet in India

The geophysical implications are examined of the continuing southward migration of the magnetic dip equator in India since 1965, its precise ground location in 1971, and thereafter its drift at 1–6 km/yr accelerating to 7 km/yr in the mid-1980s near its mean central position in the 80-yr secular oscillation, estimated to be about 10 km south of Trivandrum. Simultaneously its drift northwards near the antipodal point at Huancayo Observatory, in Peru (South America), is also observed.The ground projection of the mean axis of the equatorial electrojet for 1980 is clearly delineated about 55 km to the north of the dip equator in India, with positive Sq(Z) values of 25 nT recorded right on the dip equator, based on the ground geomagnetic survey 1971 and the magnetometer array experiment of 1980. The half-width and midday peak total current intensity of the Indian electrojet are determined from the H data recorded at Trivandrum, Annamalainagar and Hyderabad for the solar minimum year 1976 (146 ± 46 km, 137 ± 25 Amp/km) and the maximum year 1980 (169 ± 39 km, 203 ±49 Amp/km), assuming a uniform west-east current band model at a height of 107 km centred on its newly discovered axis. These new results are quite different from those of earlier determinations. Severe induction anomalies observed in the region due to subsurface geological bodies are also appropriately incorporated.     

Comparison of equatorial electrojet characteristics at Huancayo and Eusébio (Fortaleza) in the South American region

The H, D and Z average daily variations for five international quiet days are compared for Huancayo (12°S, 75°W, dip +1.9°) and Eusébio near Fortaleza (4°S, 39°W, dip −3.5°) in the South American region, for the 12 successive months, October 1978-September 1979. The H range shows that the electrojet is weaker in the Fortaleza region. Also, the electrojet center has latitudinal excursions from month to month at both the locations, but not in a similar way. The D variations indicate excursions of northern hemisphereS_q current systems into the southern hemisphere (or vice versa) but in a dissimilar way at Eusébio and Huancayo. Also, significant ΔD magnitudes are noticed even at midday, indicating possibility of meridional currents. It seems that the overhead S_q current pattern changes in form, while moving over from the Fortaleza region to the Huancayo region.     

Ion-neutral collisions and the equatorial electrojet

There are various theoretical models of ion-neutral collision and the resultant collision frequencies (v_in) differ significantly in their values. Also there is a range of uncertainty associated with each of the theoretical values of v_in. The effects of the differing theoretical values and the uncertainty in the v_in on the estimates of the equatorial electrojet (EEJ) current strength are examined numerically. It is found that the differences in various v_in-models affect the amplitude of the EEJ, but leave the peak altitude of the EEJ nearly unchanged. However, modification to v_in by the order of its uncertainty does change the peak altitude of the EEJ by at least 2km.     

Tidal winds from the MLT global radar network during the first LTCS campaign—September 1987

Winds and tides were measured by a number of MLT (Mesosphere, Lower Thermosphere) radars with locations varying from 43–70°N, 35–68°S, during the first LTCS (Lower Thermosphere Coupling Study) Campaign, 21–25 September 1987. The mean winds were globally westerly, consistent with early winter-like (NH) and late winter (SH) circulations.The semi-diurnal tide had vertical wavelengths near or less than 100 km at most locations, with some latitudinal variation (longer/shorter at lower latitudes in the NH/SH)—amplitudes decreased at high latitudes. The global tide was closer to anti-symmetric, with northward components being in phase at 90 km. Numerical model calculations [Forbes and Vial (1989), J. atmos. lerr. Phys. 51, 649] for September have rather similar wavelengths and amplitudes; however, the global tide was closer to symmetric, and detailed latitudinal trends differed from observed.The diurnal tide had similar wavelengths in each hemisphere, with short values (~30 km) at 35°, long (evanescence) at 68–70°, and irregular phase structures at mid-latitudes. The tide was neither symmetric nor anti-symmetric. Model calculations for the equinox [Forbes. S and Hagan (1988), Planet. Space Sci. 36, 579] were by nature symmetric, and showed the short wavelengths extending to mid-latitudes (43–52°). Southern hemisphere phases were significantly (6–8 h) different from observations. Amplitudes decreased at high latitudes in model and observation profiles.     

Effect of the large-scale convection electric field structure on the formation of thin ionization layers at high latitudes

A three-dimensional simulation of the high-latitude ionosphere was applied to investigate the geographical distribution of E-region thin ionization layers which may be formed by the action of the convection electric field. The simulation model computes the ion densities (O⁺, O⁺₂, N⁺, N⁺₂, NO⁺, Fe⁺), and temperatures as a function of altitude, latitude, and longitude. The stationary state momentum and continuity equations are solved for each ion species, then the energy equation is solved for electrons, neutrals, and a generic ion having the mean ion mass and velocity. The various electric field patterns of the Heppner and Maynard [(1987) J. geophys. Res.92, 4467–4489] convection electric field model were applied and the ionization density pattern was examined after a time sufficient for the formation of thin layers (≈2000 s). It was found that large areas of thin ionization layers were formed for each of the electric field patterns examined. Southward IMF B_z conditions resulted in thin layers forming in the pre-midnight sector in the latitude range north of about 70° to about 80°, and after midnight between 60 and 70°. For northward B_z conditions, the layers were mainly in the pre-midnight sector and covered a latitude range from about 60 to 80°.     

Geomagnetically quiet day ionospheric currents over the Indian sector—II. Equatorial electrojet currents

With 1986 quiet days data of Indian observatories, the equatorial electrojet (EEJ) has been studied in terms of 8 landmark parameters that reveal the structures of hourly latitudinal profiles of EEJ current from 0700 to 1700 h local time. The landmark distances suggest that near dawn, the EEJ is widest and its centre and focus are most northerly before it contracts towards local noon with its centre and focus moving southwards. The seasonal means of peak current intensity and the total forward current seem to peak earlier when the intensity of EEJ is higher than when it is lower. The seasonal order of EEJ intensity is found not to be the same at all hours of the day. This implies that seasonal variation of EEJ is not semiannual at certain daytime hours. The landmark distances of EEJ current have semi-annual variations with minima in the vernal and autumnal equinoxes and maxima at June and December solstices, but annual variations of the measures of EEJ current are just the reverse. Certain properties of the worldwide part of Sq are found to be markedly different from those of EEJ including some key features of diurnal and seasonal variations.     

Annual variations in the electron content and height of the F layer in the northern and southern hemispheres,related to neutral composition

Total electron content (TEC) data is presented for similar sites at ±35° latitude, and conjugate sites at ±20°, for several years near solar maximum. Comparison with the MSIS atmospheric model shows that the large seasonal anomaly at 35°N (an increase of 80% in TEC from October to April) is fully explained by changes in neutral composition. The small seasonal anomaly at 35°S also agrees with the MSIS model. Composition changes fail to account for the generally higher TEC in the northern hemisphere; this suggests the presence of an overall south-to-north atmospheric wind. Eastern declinations also contribute to enhanced TEC in the northern hemisphere, in the Pacific zone. The MSIS model predicts a semiannual variation of about ±25% in TEC at all sites, while observed changes are only about ±8%; thus we require some enhanced loss process near the equinoxes, particularly in September and October.Peak height calculations assuming a constant pressure level give a large semiannual variation in the F2 region: this is replaced by an annual variation when h_m F2 is calculated from diffusion theory. Heights calculated from the MSIS model are similar to observed values at ±35° latitude on summer days. A decrease of about 20km in observed heights on winter days is attributed to a poleward neutral wind; this wind also reduces the observed TEC. At night the height changes correspond to an equatorial wind, which is largest in summer and equinox. Observed day time TEC is greater at 20°N than at 20°S at all times of year, suggesting a northward transequatorial wind which is strongest near January and gives increased TEC and decreased peak height at 20°N.     

Spread-F and ionization anomaly belt

The present investigation attempts to bring out the dynamics of the F-region at magnetic equatorial and low latitudes in the American zone. Data are examined for two sets of nights, one with strong range-type spread at Huancayo another with complete absence of spread-F. A prominent bulge of the F-region was observed within and below a latitude 10°N in the evening hours of the spread-F nights. Contours of electron distribution during post-sunset hours at the equatorial latitude, Huancayo (Dip 2°N); low latitude, Talara (dip 13°N); and a location near the anomaly crest location, Panama (dip 38°N), indicated a much steeper gradient in electron density at fixed heights on spread-F nights compared to a rather low gradient on the nonspread-F nights. Enhanced concentration of electrons at the anomaly crest location Panama, and a lower density at the equatorial location Huancayo, were observed on spread-F present nights. This is attributed to the phenomena of an evening plasma fountain in operation at equatorial latitudes on spread-F nights.     

Winds in the ionosphere—A review

Winds in the upper atmosphere, and their effect on the ionosphere, are reviewed with an emphasis on information useful to ionospheric studies. The winds are driven by pressure gradients from solar and auroral heating, with some forcing by tidal energy from below. Simple calculations which balance the pressure gradient by ion drag and Coriolis forces are generally unreliable, so large-scale numerical models of the coupled atmosphere and ionosphere are required. The accuracy of these global models is limited by uncertainties in the energy inputs at high latitudes and at the lower boundary (about 90 km). The best current wind data come from incoherent scatter radar or airglow installations, at a few sites and for only a few nights per month. Satellite data are also available for several years, and results to 1989 are incorporated in the global HWM90 model. This seems acceptable for determining mean winds at night, less good during the day, and least good in the southern hemisphere where few data were available. Plots are given to show the mean winds at different latitudes and longitudes, for use in ionospheric calculations.Meridional winds alter the height of the mid-latitude F layer, causing large changes in the effective loss rate. This is the major cause of observed seasonal changes, of differences between the hemispheres, and of changes at different longitudes. An increased knowledge of the winds is essential for further progress in F region studies. Ionospheric data provide the most promising route, using routinely scaled parameters. The simplest calculations compare observed peak heights, obtained from M (3000)F2, with the value h_o predicted by simplified “servo” equations. Errors occurring for some hours after sunrise can be overcome using model results to define h_o this allows rapid and accurate wind calculations at dip latitudes of 23–62°. Winds can also be obtained from full model calculations, designed to match observed values of peak height or density.