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Sharp decreases in ΦoF2 are found to occur frequently in the nighttime low-latitude ionosphere after southward turning of the IMF Bz component, especially under isolated Bz turnings, i.e. when the IMF has been northward for at least 6 h before its turning. These decreases occur simultaneously (within a 1-h time interval) with the Bz turning. The effect is observed both when a substorm or a magnetic storm begins after Bz has turned southward, and when a noticeable substorm does not occur. The effect is more pronounced after midnight and a maximum at 03 LT. Short-term (with scale times of about 1 h) variations of ΦoF2 and hmF2 for Bz southward turning are analysed using a large amount of ground-based and topside sounding data. The decreases in ΦoF2 are shown to occur at first over the magnetic equator and then, during the second hour after the turning, at the crests of the equatorial anomaly. The ionosphere returns to its undisturbed state, on average, in 4–5 h (if other disturbing agents do not arise). These decreases are suggested to be caused by modifications in the electric field in the low-latitude ionosphere associated with Bz southward turning.

The neutral dynamic and electrodynamic coupling between high and low latitudes, and the mutual interactions between these two processes, are investigated. For 22 March 1979, when a sudden increase in magnetic activity occurred, we have analyzed the following experimental data: (a) neutral densities and cross-track neutral winds as a function of latitude (0°–80°) near 200 km from a satellite-borne accelerometer; (b) hourly mean H-component magnetic data from the Huancayo Observatory (0.72°S, 4.78°E; dipole geomagnetic coordinates) magnetometer; and (c) hourly mean foF2 measurements from the ionosonde at Huancayo. Comparisons are also made with a self-consistent thermosphere-ionosphere general circulation model and with observationally-based empirical models of winds and density. In concert with the increase in magnetic activity to K p levels of 5–7, a nighttime (2230 LT) westward intensification of the neutral wind approaching 400 ± 100 ms −1 occurred near the magnetic equator on 22 March 1979, accompanied by a 35% increase in neutral mass density. About 2 h after each of two substorm commencements associated with periods of southward IMF, ∼100γ and ∼200γ reductions in the daytime Huancayo H-component (corrected for ring current effects) are interpreted in terms of ∼0.5 and ∼1.0 mVm −1 westward perturbation electric fields, respectively. An intervening 2-hour period of northward IMF preceded a positive equatorial magnetic perturbation of about 200γ. Time scales for field variations are a few hours, suggesting that processes other than Alfven shielding are involved. Variations in f 0 F 2 (∼ ± 1.0 MHz) over Huancayo are consistent with the inferred electric fields and magnetic variations. Similar equatorial perturbations are found through examination of other magnetic disturbances during 1979.

Meridional winds can be determined from measurements of the peak height of the ionospheric F2 layer (hm), the peak density (Nm), or the total electron content (Nt). At night, ionospheric changes follow the wind with a time constant of 30–40 min. During the day, this increases to 50–100 min for hm, and 3–6 h for Nm and Nt. Thus, peak-height data are most suitable for the direct calculation of atmospheric winds, and daytime results should be advanced by 1 h. The wind calculations require an estimate of the peak height h0 at zero wind. h0 is close to the servo result (hs) at night, if the servo constant c is increased by 25% to agree with current theory. During the day, however, the long time constants prevent the F layer from reaching equilibrium before sunset. h0 is well below hs from sunrise until afternoon, giving serious errors in any results based on servo theory. Results from a full ionospheric modelling program are used to obtain analytic expressions which reproduce the true, zero-wind peak height with an accuracy of a few kilometres, for all times of the day, all seasons and solar flux, and all latitudes in the useful range of 20–60° geomagnetic. The wind W required to produce a given change in hm varies closely as sin1.4 I, where I is the magnetic dip angle, and the variation of hm with W is accurately reproduced by a modified servo equation. Use of these results with accurate peak height data should give horizontal winds with an accuracy of about ±25 m/s. Peak heights derived from scaled ionospheric data (M3000F2 and foF2) have an accuracy of typically 10–20 km, giving overall errors of about 40 m/s in calculated winds.

Visual auroras at low latitudes are rare, documented for only five occasions since 1859. They are important from a theoretical point of view in terms of the lowest latitude at which particle precipitation can been observed. One of the great magnetic storms of the past 150 yr occurred on 25 September 1909, during the prolonged solar activity minimum around the beginning of the present century. A report of an auroral sighting at Singapore, not far from the magnetic equator, the lowest latitude ever reported for a visual aurora. has focussed attention on this event. This paper compiles and discusses the solar, magnetic, auroral, and telegraphic disturbance data related to the event. The data show that the lowest credible magnetic latitude for this event was between 30° and 36°. I conclude that the report of a visual auroral observation at Singapore, based on a single Australian newspaper item, was almost certainly not auroral at all, but rather resulted from cable disturbances, known to be associated with auroras, and confused with the aurora itself. Furthermore, consideration of the height at which particle precipitation would be observed leads to an estimate of a threshold magnetic latitude for a visual auroral event of the order of 15°, greater than that of 'Singapore. This result removes the necessity of theories of major storms to account for such a low latitude sighting.

Equatorial ionospheric irregularities in the F-layer have been the subject of intensive experimental and theoretical investigations during recent years. The class or irregularities which continues to receive much attention is characterized by large scale plasma depletions, generally referred to as ionospheric plumes and bubbles. The OI 630.0 nm F-region night-glow emissions arising from recombination processes can be used to observe the dynamics of transequatorial ionospheric plasma bubbles and smaller scale plasma irregularities. In a collaborative project between the Center for Space Physics of Boston University and Brazil's National Institute for Space Research (INPE), an all-sky imaging system was operated at Cachoeira Paulista (22.7° S, 45.0° W, dip latitude 15.8° S), between March 1987 and October 1991. In addition to the imager, photometer and VHP polarimeter observations were conducted at Cachoeira Paulista, with ionospheric soundings carried out at both C. Paulista and Fortaleza, the latter at 3.9° S, 38.4° W, 3.7° S dip latitude. For this longitude, the observed seasonal variation of the airglow depletions shows a maximum from October through March and a very low occurrence of airglow depletions from April through September. This long series of OI 630.0 nm imaging observations has permitted us to determine that when there are extended plumes, the altitudes affected over the magnetic equator often exceed 1500 km and probably exceed 2500 km at times, the maximum projection that can be seen from Cachoeira Paulista. This holds true even during years of low solar flux.

On oblique ionograms, the maximum frequencies of the ordinary and extraordinary modes are referred to as nose frequencies. The difference in the nose frequencies depends on parameters such as the length of the propagation circuit and the direction of propagation. In this paper, the difference in nose frequencies is studied using the frequency scaling technique of Bennett et at . [(1991) Appl. Comput. Electromagn. Soc. Jl 6 , 192]. For long paths, an explicit formula is obtained which gives the difference approximately as a function of the local magnetic dip and azimuth of propagation at the ray mid point. For shorter paths, it is shown how analytic ray tracing can be used to determine the difference.

The vertical drift velocity of the F-region in the post-sunset period at the magnetic equatorial station Trivandrum has been studied using a HF phase path sounder. The study revealed the presence of quasi-periodic fluctuations with periods in the range 4 30 min superposed on a steady vertical motion as a regular feature of the equatorial F-region in the post-sunset period. The fluctuations in the vertical velocity arc attributed to the east west electric field fluctuations generated by internal atmospheric gravity waves. The vertical velocity fluctuations can provide the necessary seed perturbations for the growth of equatorial spread-F irregularities.

Simultaneous daytime observations of E region horizontal irregularity drift velocities in the equatorial electrojet and F region vertical plasma drifts were made on a few magnetically quiet days at the magnetic equatorial station of Trivandrum (dip 0.5°N). Measurements of the electrojet irregularity velocities by VHF backscatter radar and the F region vertical plasma drifts by HF Doppier radar are used to deduce the daytime East-West electric fields in the E and F regions, respectively. The fluctuating components of the electric fields are separated and subjected to power spectral analysis. The E and F region electric field fluctuations are found to be well correlated; the estimated correlation coefficient is in the range of 0.52–0.8. The fluctuation amplitudes are of the order of 15% over the background for the E region and 25% for the F region. The spectral analysis reveals dominant components in the range of 30–90 min with F region components stronger than those of the E region by a factor of about 1.5 on the average. The F region electric fields during daytime being coupled from the low latitude E region, the good correlation observed between the E and F region perturbations suggests that the electric fields in the E region at low and equatorial latitudes are coherent for the temporal scales of the order of few tens of minutes. The spectral characteristics are such that the commonly occurring medium scale gravity waves could possibly be the source for the observed fluctuations in the E and F region electric fields.

A new set of corrected geomagnetic coordinates (CGM) has been calculated from the magnetic field model DGRF for Epoch 1985 and the IGRF model for Epoch 1990. A new approach to determine the ‘dip’ magnetic equator has been developed, which is based on the vertical (along Re ) projection on the Earth's surface of the B -minimum value point (apex) on each geomagnetic field line. A strip along the ‘dip’ magnetic equator line has been defined where the corrected geomagnetic coordinates could not be found by the definition of CGM. Linear interpolation between the locations of the two last definable CGM latitudes in both hemispheres has been used to calculate the CGM longitudes in the equatorial region. Interpolation between locations of the last definable CGM latitude and ‘dip’ equator in both hemispheres has been used to calculate the CGM latitudes in this region. The constant B -min geomagnetic coordinate system (CBM) is proposed and analysed to replace CGM in the equatorial region.

Using HF Doppler radar in a spaced-receiver configuration, observations conducted on the vector plasma drifts in the equatorial F-region at Trivandrum during the afternoon-midnight period of March–April 1988, are presented. The three components of the F-region plasma drift at the magnetic equator are such that the North-South component corresponds to the meridional neutral air wind while the other two components represent the electrodynamic drift. The vertical component of the plasma drift is characterized by a prereversal upward enhancement which fits the interpretation based on the F-region dynamo operating at sunset. Equatorial spread-F, which is linked to the prereversal enhancement, appears to be inhibited during the main phase of magnetic storms. The East-West component of the plasma drift is westward during the afternoon and eastward during the night with mean peak velocities of 30 and 110 ms−1, respectively. The large night-time drift is believed to be due to a large reduction in the degree of electromagnetic coupling between the E- and F-regions during the night. The North-South component, representing the meridional neutral air wind, is found to be northward during the afternoon and southward during the night with respective mean peak velocities of about 50 and 70 m s −1. The neutral air wind shows a tendency to reverse from southward to northward at around midnight and is thought to be caused by the midnight bulge in the neutral air temperature. The observations which correspond to the lower F-region are found to be consistent with those reported earlier using other experimental techniques.

To study equatorial plasma bubble dynamics, telemetry signals (4 GHz) were recorded simultaneously from two geostationary satellites. INSAT-1B (74°E) and INSAT-1C (94°E) at Sikandarabad satellite Earth station (dip 42.0°) from January to December 1989 and at the Chenglepet satellite Earth station (dip 10.5°) during September–October 1989 along the same geomagnetic meridian. The characteristics and occurrence pattern of the scintillations suggest that these are equatorial plasma bubble induced events. Observations from the two satellites recorded simultaneously at each of these locations were utilized to estimate the east-west plasma bubble irregularity motion. Plasma bubble rise velocities over the magnetic equator were calculated from the systematic onset time differences observed between an equatorial and a low latitude station. The east-west plasma bubble velocity estimated at Sikandarabad, corresponding to 1200 km altitude in the equatorial plane, shows a night time variation pattern with a peak at around 2100 LT. The mean values over Chenglepet, which correspond to 400 km altitude, start decreasing right from 1900 LT and seem to be influenced by the plasma bubble rise velocities. The differences in magnitude observed between the present results and those reported elsewhere by other techniques are interpreted in terms of vertical shears in the plasma zonal flow over the equator. The near alignment of the two observing stations along a common geomagnetic meridian and the simultaneous use of two satellites located twenty degrees apart in longitude provided an excellent data base to study plasma bubble dynamics.

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.

A longitudinal comparison has been undertaken of the vertical ion velocities in terms of ion mobilities and the eastward electrojet currents in terms of the electric conductivities overhead the magnetic equator at African and West Asian regions to search for longitudinal differences in the developments of the equatorial anamoly. It is shown that the vertical ion velocities and electrojet currents in both regions are different. It is proposed that longitudinal differences in (k21i+k22i)/k2i and K1i/K2i ratios give rise to a different vertical ion velocities in these regions. This is likely to cause different equatorial F-layer plasma fountain magnitudes resulting in different developments of the equatorial anomaly.

A model using photochemistry and transport due to electric fields and gravity wave winds has been used to explain the formation of ionisation layers observed over an equatorial station Thumba (dip 0°47′S) with rocket-borne Langmuir probes during two daytime counter-electrojet periods. These layers were seen as blanketing E s -layers with an ionosonde at Thumba. Convergence of the metallic ions due to three-dimensional gravity wave winds and a westward electric field appears to be mainly responsible for the observed ionisation layer over the equator.

Measurements of zonal irregularity drifts were made by the spaced receiver scintillation and radar interferometer techniques from Huancayo and Jicamarca, respectively. The Fabry-Perot Interferometer operated at Arequipa provided the zonal neutral winds. These simultaneous measurements were performed during evening hours in the presence of equatorial spread- F on three nights in October 1988. The zonal drift of 3-m irregularities obtained with the 50-MHz radar showed considerable variation as a function of altitude. The drift of hundreds of m-scale irregularities obtained by the scintillation technique agreed with the drift of 3-m irregularities when the latter were measured near the F -peak. The neutral winds, on the other hand, sometimes exceeded the irregularity drifts by a factor of two. This is a possible result of the partial reduction of the vertical polarization electric field in the F -region caused by the effects of integrated Pedersen conductivity of the off-equatorial night-time E -region coupled to the F -region at high altitudes above the magnetic equator.

Effects due to the transit of a solar eclipse through SE Asia on 18 March 1988 have been investigated using ionograms, electron content measurements, magnetograms and microbarograph recordings, provided by a chain of stations situated roughly parallel to the path of totality. In corresponding time sequence depletions of foE and foF l were observed, while in the F 2-region the only affected parameter of hpF 2, at equatorial anomaly crest locations, showed well-defined maxima. Electron density-height profiles revealed a ‘deep hollowed-out valley’ of electron density within the height range 200–300 km. Estimations of ionization and production loss rates demonstrated that the equatorial anomaly diffusion process was considerably reduced. Even though the eastward electric field E overhead the magnetic equator, providing the ( E × B ) upward diffusion for the equatorial anomaly mechanism, was enhanced on this day it is clear that the local depletion in electron density caused by the solar eclipse succeeded in greatly reducing the transport of ionization to higher latitudes. As a consequence of the depletion of electron density in the E -region at the magnetic equator the northward movement of electrons forming the S q , current was halted, this showing up as a drop in magnetic declination at S q current-focus latitude locations. Afterwards, the resurgence of ionization diffusion down magnetic field lines, associated with the equatorial anomaly, caused a secondary peak in the magnetic declination. Direct evidence for the production of acoustic gravity waves by the moving bow wave front of the solar eclipse is afforded by the observation of waves with periods of 30–33 min on ionosonde sounding virtual heights and a microbarograph, lower atmosphere, pressure recording.

Intensity scintillations of 40 MHz, 140 MHz and 360 MHz radio beacons on board the geostationary satellite ATS-6, (at 34°E longitude), recorded at Ootacamund (magnetic dip latitude 3°N) during the ATS-6 phase II (1975–1976) are described. The scintillation index S4, which is the normalised RMS value of the intensity fluctuations, is found to be very large at 2100-0200 h associated with equatorial spread-F, and in the forenoon and afternoon hours associated with a blanketing type of sporadic E-layer. At 40 MHz there seems to be a weak scintillation (S4 The frequency exponent, n, (with S 4 ∝ ƒ −1 , whereƒ is the signal frequency), was found to decrease monotonically with increasing intensity of scintillations, approaching a value of zero for saturated scintillations. The relationship between the frequency exponent and the scintillation index was found to be independent of the time of the day or the season, in spite of different kinds of irregularities being involved in the scintillation processes.

The nocturnal and seasonal variations of equatorial ionospheric scintillations are presented. Scintillations are classified into two classes, namely, class I and class II depending on their fading rates and association with bottomside spread-F. Power spectra and frequency indices for class I and class II scintillations are presented and their theoretical implications are discussed.

VLF (13.6 kHz) radio waves propagating westward from the Hawaiin Omega transmitter to receivers in Australia exhibit anomalous propagation effects associated with their passage across the magnetic equator. No such effects were seen for transequatorial propagation to the east.

On the night of Mar. 9, 1983 (UT) at Punta Lobos Launch Site, Peru (12.5 deg S, 76.8 deg W, magnetic dip -0.7 deg), a sequence of sounding rockets was flown to study the electrical structure of the equatorial middle atmosphere and to evaluate perturbations on this environment induced by the X-ray star Sco X-1. The rocket series was anchored by two Nike Orion payloads (31.032 and 31.033) which were launched at 0327 and 0857 UT, near Sco X-1 star-rise and after it had attained an elevation angle of 70 deg E. An enhanced flux of X-rays was observed on the second Nike Orion flight (31.033). This increase is directly attributed to Sco X-1, both from the spectral properties of the measured X-ray distribution and by spatial information acquired from a spinning X-ray detector during the upleg portion of the 31.033 flight. Simultaneously, a growth in ion conductivity and density was seen to occur in the lower mesosphere between 60 and 80 km on the second flight, specifically in the region of maximum energy deposition by the Sco X-1 X-rays. The results imply the presence of a significant number of ionized heavy constituents within the lower mesosphere, with masses possibly in the submacroscopoic range.

Ionospheric E -region drift measurements have been made at Ibadan between 1958 and 1974 and the velocity data have been analysed for possible solar-cycle effect. It has been found that both the true velocity V and the apparent velocity V of the full correlation analysis are not dependent on the solar cycle. This result is shown to be consistent with the hypothesis that the solar-cycle variation in the equatorial E -region current intensity is due principally to corresponding variation in the E -region electron density.

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.

During periods of partial counter electrojet, i.e. when ΔH at an equatorial station is positive but lees than ΔH at a non-equatorial low latitude station, the scatter echoes of VHP radio wares from the E-region of the ionosphere are shown to disappear. These events are suggested due to simultaneous flow of eastward current near the peak ionication level of the E-region and the westward current at the base of the E-region.

The examination of H magnetograms of Huancayo for the period, 1948–1971, has shown that the negative effects on the daily variation of H occur primarily in the morning hours around 07 hr and in the evening between 15 and 17 hr. The occurrence of the evening event is maximum around lunar ages 1.4 and 13.4 hr or around lunar times 2.2 and 14.2 hr. The occurrence of the morning events is maximum around lunar ages 4.8 and 16.8 hr or around lunar times 2.3 and 14.3 hr. These lunar times correspond to that of maximum negative effect in H due to lunar tidal wave. These counter electrojet effects are suggested to be due to the superposition of lunar semi-diurnal wave over the solar daily wave of H . The late appearance of sporadic- E in the morning or the earlier disappearance of Fs in the evening is due to these counter electrojet events, when the ionospheric currents over the equatorial regions are reversed to westward direction during the daylight hours.

Periodic amplitude fluctuations of VHF signals from a geostationary satellite monitored from near the magnetic equator have been observed in the evening hours as precursors of strong Rayleigh fading associated with plumes of irregularities. These periodic fluctuations called amplitude waves exhibit amplitude changes of only 1 to 2 dB and have been observed for up to 30 minutes before the onset of strong scintillations. Individual fades are correlated over distances of at least 120 km in the magnetic east-west direction. The velocity of these wavelike disturbances has been found to be approximately 140 m/s eastward with a corresponding wavelength of 25 km.

Observations of the local time dependence of the [OI] 6300 A zenith intensity made with a filter photometer at Natal, Brazil, near the magnetic equator, are presented, together with the local time behavior of theoretical 6300 A intensities calculated using a realistic dynamic computer model of the tropical ionosphere. It is shown that the observed local time variation of the 6300 A emission is controlled by the local time behavior of the vertical E × B plasma drift velocity at the magnetic equator. The typical 6300 A enhancement which occurs near midnight gives an indication of the magnitude and time of reversal of the vertical plasma drift velocity.

Power spectra of day-time amplitude scintillations on VHF and UHF signals from the geostationary satellite ATS-6 received at an equatorial station Ootacamund (magnetic dip 6°N) are studied in order to obtain information about long wavelength (≳ 100 m) irregularities generated in the equatorial electrojet plasma. Some of these spectra show Fresnel oscillations, which are usually absent for night-time amplitude power spectra, indicating thereby the presence of a thin layer of irregularities rather than a thick one, as is the case at night. We find that the drift speeds of irregularities estimated from the respective power spectra show expected variations with changes in the electrojet current strength. The high frequency asymptotes of power spectra yield a two-dimensional irregularity power spectrum index of 3.0 ± 0.4 over a wavelength range of approximately 60–800 m, which is close to the value 8/3 predicted by the unified theory of type I and type II irregularities in the equatorial electrojet for the so-called inertial range of wavelengths.

The ducted and nonducted whistlers observed at ground and aboard satellites respectively are briefly reviewed. Another type (one and two hop whistlers) named partly ducted whistlers is observed aboard satellites in a large area over Europe. Their spectrographic traces are always diffuse and their morphology is consistent with a theoretical study of partly ducted propagation through the magnetosphere of the VLF energy radiated by lightning. This mode of propagation seems to be a magnetospheric permanent feature during magnetic quiet conditions. It implies that: 1. (i) the wave is ducted by electron irregularities aligned along the Earth magnetic field lines in the vicinity of the equatorial plane between L = 2 and 2, 6; and 2. (ii) after leakage out of ducts, it propagates through an homogeneous medium in the nonducted mode and can reach the conjugate ionosphere over a very large area extending sometimes from L = 1, 2 to 3, 6 and over more than 50° in longitude.

Variations in the Faraday fading period of the 40 MHz signal from satellite BE-B have been used to study large-scale irregularities in the equatorial ionosphere. It is found that the irregularities have sizes ranging from 50–400 km, with an average value of 160 km. The observed amplitudes lie between 0.15 and 3.3 per cent of the background total electron content and the latitude variation of amplitude shows a maximum at the magnetic equator and a minimum at a dip of about 22°S. It is suggested that the irregularities are caused by atmospheric gravity waves with a period of around 40 min. The irregularities occur mainly between 0600–1100 LT.

Analyses have been made of the behaviour of cold plasma electron density at 6.6 RE over the geomagnetic equator during very intense magnetic storms, by using the relaxation sounder data measured on board the geostationary satellite GEOS-2 of ESA. It is found that during very intense storms a high density region appears in the afternoon sector, the width of which is generally larger than the plasmaspheric bulge. The electron density on the nightside on storm days is lower than that on quiet and moderately disturbed days, representing a negative phase storm. There are severe perturbations in electron density during intense storms, which occur just after steep changes in Dst index and last one or two days. They appear more severe on the nightside, especially around midnight. It is pointed out that the enhancement of the magnetospheric convection and the invasion of the plasmasheet plasma into lower L regions are responsible for the foregoing phenomena.

The irregularities associated with plasma instabilities in the equatorial electrojet are drifting with the electrons under electric field and neutral winds actions and are used as radar targets to monitor ionospheric parameters in the equatorial E -layer. These irregularities were first detected at Jicamarca and associated with the two stream and the cross field instabilities. Radar technique is shown to be well adapted for E-W investigations of the electrojet and a number of echo parameters deduced from spectral analysis will change with range according to geographical variations of different ionospheric parameters. The influence of temperature, collision frequencies, density gradients, neutral winds and electrostatic field on the phase velocity of both types of irregularities are given by plasma instability theories and dynamics of the E -layer. Radar limitations have also to be taken into account before investigating the profiles. cohen and Bowles (1967), first observed in South America an asymmetry not predicted by the theory and pointed out the possible relation of this asymmetry with the variations with height of the physical properties of the E -region. Balsley (1970) investigated this asymmetry and suggested an origin related to a local magnetic anomaly or to a variation of induction near the coast line of Peru or the continental shelf. The origin of the phenomenon being unclear and possibly associated with instability processes, vertical drifts, E-W gradients of electrostatic field, neutral winds or induced effects, other experiments were performed along the magnetic equator. No noticeable asymmetry was detected by radar in Central Africa (Chad) far away from the coasts and a number of hypothesis could then be discarded. Confirmation of the existence of the asymmetry was obtained in East Africa near the Red Sea in a tectonically very active zone. The different hypotheses are discussed in the light of the radar data and new investigations are proposed.

Electron content-latitude profiles from 20°S to 30°N have been obtained for 1975 (a period of low solar activity) from an analysis of Faraday fading recordings of INTASAT taken at Hong Kong (long. 114.1°E, lat 22.2°N, dip 30°N) and Bali (long. 115.2°E, lat. 8.9°S, dip 35.5°S) during the periods 0800–1000 and 2000–2200 LST (120°E). Hourly median profiles have been computed for both the morning and evening periods for the northern summer and winter, the March/April equinox and also corresponding to a period during which the sun was overhead the magnetic equator (9°N). Profiles were most symmetric with respect to the magnetic equator at the equinox. In the morning at the solstices the formation of a single crest of the equatorial anomaly was observed on the winter side of the magnetic equator, though at higher latitudes the electron content values were greatest in the summer hemisphere compared with those of winter hemisphere for the same dip location. The former result is explainable by the action of a wind blowing from the summer hemisphere across the magnetic equator while the latter is probably due to the proximity of the sun giving rise to a faster ionization production rate in the summer hemisphere. In the evening at the solstices the electron content was greatest in the summer hemisphere and for the northern winter a single crest of the equatorial anomaly remained at about 35°S dip as late as 2200 LST. These results are explainable as being a consequence of the movement of ionization to high levels in the summer hemisphere and low levels in the winter hemisphere by the daytime wind, re-combination after sunset being the most rapid for that ionization in the winter hemisphere. At the equinox the profiles showed the greatest content values and the two equatorial anomaly crests were symmetric with respect to the magnetic equator for 2000–2100 LST. However from 2100–2200 LST the northern crest collapsed near to the magnetic equator and it is thought that this was due to a southward wind blowing at that time. Representative individual profiles corresponding to the varying overhead position of the Sun throughout the year have been given. The evening profiles, in particular, show marked changes of form reflecting the effects of prevailing equatorward winds. From the December solstice to the March equinox no such a wind was apparent but between the September equinox and the December solstice this wind did occur in the summer hemisphere, having a maximum effect on the evening profiles around the end of October. With the movement of the Sun (and the thermal pressure bulge) towards the most southward position in the summer hemisphere during the day the effect of the evening equatorward wind on the electron content profiles decreased.

It is shown that the quiet day afternoon counter-electrojet events at equatorial stations are the result of a strong semi-diurnal tidal force, with a maximum around 0900 LT and a minimum around 1500 LT, superimposed over the normal Sq (H) variation. This additional eastward electric field in the forenoon hours over the magnetic equator is shown to cause an enhancement of the equatorial F-layer plasma fountain, and results in a lower ƒoF2 at the equator. The daily variation of the differential values of ƒoF2, being almost counter-electrojet minus normal electrojet days, and in antiphase with the corresponding curve of ΔH. At the anomaly crest station, larger values of ƒoF2 are observed in the afternoon on counter-electrojet with a delay of about 3 h with respect to the decrease of ƒoF2 at the equator.

Longitudinal gradients in the equatorial electrojet are observed with scales ranging from several thousands of kilometers on down. Since previous theoretical models of the electrojet electric fields and currents have assumed that no longitudinal gradients exist, we examine theoretically the consequences of allowing for them. The vertical structures of the electric fields and currents are altered in predictable ways, the alterations being increasingly important as the electric field and current gradients increase. In contrast to the infinite-electro jet model, reversals of the electric field and current directions with height become possible when longitudinal gradients are present. We show how electric fields produced by hypothetical generating sources located at various latitudes are attenuated between the source and the Equator. For electric fields with small longitudinal scale lengths, the source must lie close to the magnetic equator in order to cause noticeable effects in the electrojet.

The daily change in illumination over a VLF propagation path produces a regular variation in the phase and amplitude of a received signal. Significant differences have been reported for such variations when measured at the same frequency over propagation paths of similar length but different orientation. Past attempts have been made to explain these observations in terms of stellar X-ray sources and a major variation in VLF propagation parameters at the magnetic equator. This paper shows that a minor azimuthal variation in the relative phase velocities of interfering waveguide modes propagating at night can account for the observations in some detail.

Equatorial 6300 A arcs observed by the ISIS—II satellite close to the magnetic equator over the African and Asian zones are studied for night-time conditions from 21:00 h to 02:00 h local time in the summer and spring of 1972–1974 and 1976, respectively. Case studies of the arcs have been made for quiet geomagnetic conditions and for minor storms. Sometimes very intense arcs with intensities of 1–2 kR are observed. Arcs of moderate intensities (300–400 R) are observed during geomagnetically disturbed periods. It is confirmed that these intensities can be fully accounted for theoretically by the dissociative recombination of molecular oxygen ions. Since the emission intensities are found to be sensitive to the geomagnetic activity, the influence of the latter has been taken into account and discussed. Equatorial spread-F (ESF)/bubble conditions are usually present at these local times. The data presented here show a correlation between the 6300 A emission rate at one of the anomaly crests, the gradient in h (the lowest scaled real height from topside ionosonde trace) and the existence of ESF and gravity waves. This correlation is consistent with the scenario put forward by Maruyama and Matuura that the occurrence of ESF requires a symmetrical electron density distribution around the magnetic equator, so that a transequatorial wind causes an asymmetry and inhibits the formation of ESF. For the ISIS data we conclude that where strong transequatorial winds exist the 6300 A emission rate at one of the anomaly crests is very large and there is a steep gradient in h. When these winds are weak, the 6300 A emission is low and the gradient in h is also small. In the latter case, gravity waves of wavelength 200–400 km were present as well, which suggests that ESF is promoted by the existence of gravity waves. However, the magnetic disturbance level was higher during these orbits, which offers another source of gravity waves.

An effective plasma temperature model appropriate to sunspot maximum conditions has been constructed for low latitudes taking into consideration previously reported incoherent backscatter and satellite probe measurements of electron and ion temperatures. Using this temperature model and electron density latitude profiles, obtained from an analysis of ISIS A topside ionograms (see Walker and Chan , 1976), the variations of mean ion mass at heights of 800, 900 and 1000 km over a range of dip latitude 30°S to 20°N have been estimated during the daytime (1100–1600 LST, long. 105°E) for the period November 1969–January 1970. The results show 1. (1) that there has been a general increase in mean ion mass with increase in solar activity, 2. (2) a latitude crest of ion mass peaking near the magnetic equator to a value ~16 a.m.u., indicating a predominance of O+ ions at a height of 800 km and 3. (3) that the mean ion mass estimates (and consequently O+ ion concentrations) were consistently higher in the summer hemisphere compared with those of similar dip locations in the winter hemisphere. The predominance of O+ ions over the magnetic equator is clearly related to the (E × B) upthrust mechanism at this location giving rise to the ‘equatorial anomaly’ in the ionosphere while the predominance of O+ ions in the summer hemisphere can be explained by a meridional wind blowing from the summer to the winter hemisphere.

Values of vertical incidence HF absorption calculated using the International Reference Ionosphere (IRI) are compared with published values obtained using the A1 technique. It is found that the IRI overestimates the absorption under most conditions, especially near the geomagnetic equator. The causes of the discrepancies are discussed, and required changes to the IRI E -region N ( h ) profile indicated.

At dip angles of 26°–30°, the group retardation of the extraordinary ray is a constant (depending on frequency) times the retardation of the ordinary ray reflected at the same true height. The true height can therefore be found directly from the virtual heights of the two rays, with no assumptions about the shape of other parts of the profile. Results for three days show that any valley between the E- and F-layers was less than about 15 km wide, from sunrise to sunset. At night the addition of a simple extraordinary ray correction reduces the errors in standard methods of analysis from about 50 km to about 5 km, at frequencies near 2 MHz.

This paper describes some recent theoretical modeling efforts which primarily study the effects of E×B drift, both vertical and horizontal, on the equatorial ionosphere. Solving the time-dependent ion continuity equation for plasma densities as a function of altitude, latitude and local time it is shown that: (1) by incorporating F-region, vertical E×B drifts which are typical of solar cycle maximum and minimum equinoctial conditions, good agreement is achieved between observed and calculated ƒoF2 values at the magnetic equator (American sector) for these two periods. (2) Modeling the seasonal anomaly in ƒof2 at Kodaikanal for October 1975 and July 1976 requires different vertical E×B drift patterns for these two months and both of these patterns are significantly different from the drifts measured at Jicamarca. (3) By incorporating a realistic E×B drift model which includes the post-sunset enhancement in upward drift, bottomside F-region electron density scale heights are found to be in good agreement with measured values in the 1900–2400 LT period. (4) When a realistic east-west E×B drift pattern is included, calculated electron density profiles at the magnetic equator between 1800 and 2400 LT do not differ significantly from calculated profiles obtained when this component of E×B drift is omitted.

A global representation of the large scale variations of ionospheric electron density with the annual, diurnal and solar activity cycles is described. The phenomenological model, requiring minimal computer storage space, is constructed from monthly-averaged hourly ionospheric sounding data from some 50 stations during the epoch 1957–1970, as provided by World Data Center (A). The model, describing noontime F2-layer critical frequencies to better than 2 MHz average maximum error and to better than 0.5 MHz root mean squared error, is particularly suitable for applications in space communication and ionosphere-atmosphere coupling studies. Magnetic dip angle and north-south asymmetry effects are incorporated in the model. We show that a ‘secular’ variation of the ionospheric density exists in the epoch 1960–1971. The basis of comparison with data is broadened to include the 1958–1968 chronologies of 18 stations in addition to the original samplings of 50 stations. In addition, a modelling of the polar ionospheres is included.

The ordinary ray trace of topside ionograms is consistently observed down to the extraordinary ray cut-off frequency fxS. It is suggested that the virtual height of the ordinary ray should be routinely scaled at this frequency. The result is called hox'. Scaling is rapid and accurate, since fxS is clearly marked and is already being scaled. hox' gives a measure of the height scale of the recorded traces. For records taken over the magnetic equator it is related to the scale height Hs just below the satellite by the relation hox' = 2Hs tanh−1 (fB/ fxS) solsu1 2 , where fB is the electron gyrofrequency. At other dip angles the value of Hs is up to 20% less than given by this relation. Expressions are given for computing Hs from hox', at any dip angle. If the scale height varies with height, the calculated value is correct at a distance of about hox'/3 below the satellite.

Magnetic storm effects on the F2-Layer at Ibadan (lat. 7.4°N, long. 3.9°E, magnetic dip 6°S) were observed over a solar cycle period (1956–1966). During the period of high solar activity day-time decreases in NmF2 occur during the initial phase of storms. The main and early part of the recovery phases of storms cause either increases or no significant changes in the build-up interval (0500–0900 h L.T.) of the F2 layer usually, increases being most frequent during the March equinox season. This same part of the storm causes increases in NmF2 in the day-time (0900–1800 h L.T.) and night-time (1800-0800 h L.T.) intervals usually, at all seasons of the year. Similar effects occur during the period of low solar activity, but on a reduced scale.

The undisturbed ambient total electron content of the ionosphere in the equatorial region exhibits two characteristic features: 1. (i) a longitudinal behavior of the post-sunset variation of the ionization near the crests of the equatorial anomaly 2. (ii) an enhancement at lower latitudes following the post-sunset decay. During high solar activity periods the southern crest of the equatorial anomaly in the African longitude sector is characterized by a post-sunset maximum often exceeding the afternoon maximum. In the Indian and other longitude zones, the post-sunset peak is not so prominent. Instead, a ledge is obtained in the corresponding local time period. At lower magnetic latitudes, the ionization decays very rapidly around sunset, but an enhancement lasting 2–4 h is observed afterwards. Numerical solution of the plasma continuity equation, including the effects of ionization production by solar ultraviolet radiation, loss through charge exchange and transport by diffusion, electrodynamic drift and neutral wind, has been used to investigate the above two features. It is found that the pre-reversal peak of the E × B drift at the magnetic equator around sunset is the dominant mechanism responsible for the post-sunset behavior near the crests of the equatorial anomaly. The zonal wind causes an asymmetry of the total content in the northern and southern hemispheres. In African longitudes, where the magnetic declination is about 20°W, the southern crest is more developed at the expense of the northern counterpart. The north-south asymmetry is practically absent in the Asian sector, with its low (

A relatively simple procedure for the calculation of ionization production rate profiles due to precipitated electrons is described. It is used to show that the exospheric temperature can have an important effect on the production rates in the F-region and that the magnetic dip angle must be taken into account when calculating the profiles for the E-region at lower geomagnetic latitudes. A very quick method for the rough estimation of such profiles is also given.

Conjugate observations of ELF-VLF emissions, magnetic pulsations and aurorae were carried out at Syowa Station in Antartica and Husafell in Iceland from 29 July, 1977 to 18 September, 1977. Husafell is located at a distance of about 50 km from the point of Syowa. The conjugacy of ELF-VLF emissions depends on their type, whether polar chorus, quasi-periodic emissions, bursts of discrete emissions or auroral hiss. Polar chorus and quasi-periodic emissions are observed simultaneously at the conjugate-pair stations. This suggests that the emissions are generated near the equatorial region in the outer magnetosphere and propagate to both hemispheres along the field lines. The conjugacy of bursts of discrete emissions and auroral hiss is generally low. The interrsity of auroral hiss is much stronger at Syowa (winter hemisphere) than at Husafell (summer hemisphere); just the opposite terrdency is observed for bursts of discrete emissions. The low conjugacy of these emissions may be due to differences in the geophysical parameters, such as the magnetic field, magnetic dip angle and the plasma density in the two conjugate ionospheres. Preferential precipitation in the southern hemisphere explains the strong discrete emissions in the northern hemisphere. The growth rate of auroral hiss is large in the low plasma density (winter hemisphere) ionosphere.

Using ƒoF2 and M(3000)F2 data, taken at stations spread over narrow longitude ranges and giving a good coverage of dip location near the magnetic equator, a comparison has been undertaken of the diurnal variations of the ionospheric equatorial anomaly at different seasons in East Asia and America. The very significant differences observed have been explained in terms of (1) the relative inferred locations of the thermal pressure bulges with respect to the magnetic equator, (2) magnetic declination effects (only significant at high latitudes in South America) and (3) the greater magnetic field in Asia. For both regions maximum symmetry of the equatorial anomaly crests has been observed in the particular month for which the midday overhead sun occurred near the magnetic equator. The equatorial anomaly was found to persist later at the solstices in America but at equinox it persisted until 0300–0400 LT in both regions. Some re-generation of the anomaly seems to take place after sunset but in addition it appears that poleward winds from the so-called ‘midnight pressure bulge’ converge with the winds from the poles thereby delaying the decay of the anomaly crests.

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 Eusebio 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 hemisphere S q current systems into the southern hemisphere (or vice versa) but in a dissimilar way at Eusebio 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.

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.

Data on ƒ 0 F2 from the satellites Alouette and ISS-b are used to study the diurnal, seasonal and longitudinal variations of the location of the equatorial trough. The trough is located at about 40–100 km to the north of the magnetic equator, which can be explained by the systematic deviation of the equatorial electrojet towards the northern hemisphere. The trough is situated to the north of the mean trough during day-time and at the June solstice, while it is to the south by night and at the December solstice. These features are consistent with the neutral wind system inherent to different local hours and different seasons. There is also a marked declination effect of the trough location, which is related to the subtropical crests of ƒ 0 F2 and may also be related to the zonal wind as a secondary effect.