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.     

Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set

The magnetic field expressions from the current ribbon and thick current versions of the continuous distribution of current density model and their merits have been presented. For the first time both the latitudinal and vertical parameters of the equatorial electrojet (EEJ) have been derived from the same set of data. The local noon and daytime means of certain key parameters of the EEJ are shown to be in good agreement with those from other sources. Selected local noon means include: peak current density j_o, 10.58 ± 0.34 A/km²; peak current intensity j_o, 224 ± 9 A/km; total eastward current I₊, 74 ± 5 kA ; EEJ current focal distance w, 300 ± 5 km ; half thickness at half of peak current density p, 7.0 ± 0.1 km; peak westward current location x_m, 5.13 ± 0.08° dip latitude; and EEJ latitudinal extent L₁, 12 ± 1° dip latitude. The problem of model calculated landmark distances of EEJ being consistently shorter than observations, encountered by Onwumechiliet al. [J. geomagn. Geoelecl. 41, 443 (1989)] has been solved.     

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.     

Retrieval of east-west wind in the equatorial electrojet from the local wind-generated electric field

The paper presents a method to retrieve the height-varying east-west wind U(z) in the equatorial electrojet from the local wind generated electric field E_W(z), or from the radar-measured phase velocity V_p^II(z) of the type II plasma waves. The method is found to be satisfactory when E_w ⪢ E_p, where E_p is the vertical polarization electric field generated by the global scale east-west electric field, EY, and E_y < 0.2 mV m⁻¹. Measurements of V_p^II by a VHF backscatter radar can be inverted to obtain the causative wind profile by this method. The method is tested using a simulation study in which E_w(z) and V_p^II(z) as generated by two different wind models are used. The retrieved winds are compared with the original wind profiles and it is found that the error in the retrieved winds is mostly under 5%, for the case of no errors in the model Pedersen conductivity (σ₁) profile and the E_w(z) or V_p^II(z)(z) profiles used in the inversion. Even with a ±20% error in the above profiles, the errors in the retrieved winds are found to be less than 20% over 75% of the altitude range and 20–30% for the remaining 25% of the altitude range, on the average.     

Effects of the ionospheric conductivity on the longitudinal structure of the F-region equatorial anomaly

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 (k²_1i+k²_2i)/k_2i and K_1i/K_2i 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.     

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°.     

Positive and negative ion composition measurements in the D- and E-regions during the 26 February 1979 solar eclipse

Two rockets bearing quadrupole mass spectrometers capable of measuring both positive and negative ion composition were launched from Red Lake, Canada, during the solar eclipse. Both instruments had liquid helium cryopumps and shock-attaching conical samplers. The payloads also contained two Gerdien condensers to measure total positive and negative ion concentrations and ion mobilities. Attitude control systems aligned the payloads with the velocity vector throughout ascent and descent. The first rocket was launched so that the D-region was in darkness 35 ± 8 s on the upleg and about 150 ± 15 s on the downleg for the study of ionospheric decay processes. The second rocket was fired after totality into 75% solar illumination for the study of ionospheric recovery. The positive ion composition above 105 km exhibited a strongly increasing NO⁺/O₂⁺ ratio with time after second contact due to O₂⁺ charge transfer with NO and a sharply diminished ionization rate. However, in both nights, the ionization below 105 km was created mainly by energetic particle deposition as exemplified by the increased ion concentrations and the composition signatures of a particle event: asignificant enhancement of O₂⁺ below 105 km and large amounts of H₅O₂⁺ ions in the D-region which result from the O₂⁺ clustering scheme. H₅O₂ was the major ion in the upper D-region while H₇O⁺₃, H₉O₄⁺ and H₅O₂⁺ were dominant ions at lower altitudes. Numerous minor species were also detected. The negative ion distributions in both flights exhibited a distinct shelf at 83 ± 2 km, decreasing by more than an order of magnitude by 90 km and with minima near 75 km. In the 75–90 km range, a significant percentage of the negative ions had masses exceeding 160 a.m.u. Comparisons are made with prior negative ion measurements during similar daytime auroral zone absorption (AZA) events. Two striking characteristics of the precipitating particles were apparent from these and past observations in daytime AZA events: there is a near absence of low energy electrons capable of ionizing above about 105 km and there is'a significant spatial and/or temporal variability in the electron flux. This paper is devoted principally to a presentation of the ion composition measurements and associated uncertainties.     

Anomalous diurnal phase shifts of Omega VLF waves (10–14 kHz) on the east–west low latitude and transequatorial paths

The phase of the Omega HAIKU (Hawaii, U.S.A.) and REUNION (La Reunion) signals were measured at Inubo, Japan and onboard ship at Fremantle. Australia. Strong east–west non-reciprocities of the diurnal phase shift are obtained both on the low latitude and transequatorial paths, and it is found that the non-reciprocity on one path is in an opposite sense to the other. The diurnal phase shift, ϕ_DN for the west-to-east (WE) propagation is 7.8–8.7 µs Mm^–1 at 13.6 kHz on the transequatorial and mid-latitude paths, indicating no significant latitude dependence of the phase velocity in WE propagation. On the other hand, ϕ_DN for the east-to-west (EW) propagation greatly depends on the geomagnetic latitude; at 13.6 kHz ϕ_DN = 11.3µs Mm^–1 on the low latitude path and ϕ_DN = 50 µs Mm^–1 on the transequatorial path, which are 40% greater and 35% less than ϕ_DN in WE propagation, respectively. The east-west non-reciprocities of ϕ_DN on the low latitude and transequatorial paths are interpreted in terms of a single mode propagation in the conventional anisotropic waveguide model with β_D = 0.3 km^–1, β_N = 0.5 km^–1 and h_N–h_D = 12.5 km. In particular, the anomalously small ϕ_DN on the EW transequatorial path is explained as due to the high phase velocity of the night-time first-order mode in the equatorial region within ±12° geomagnetic latitude.     

Ionospheric sources of Pi(c) pulsations

Data collected at Macquarie Island (invariant latitude 64.5°S) shows average delays of 0.6 s and 0.2 s between D and H component Pi(c) pulsations, respectively, and the N₂⁺1NG (0,1) optical band pulsations. The dominant phase of the cross-correlations between the H component pulsations and the optical pulsations is consistent with the H component fluctuations being generated by increases in a westward electrojet. The D component fluctuations are consistent with either an increase in an equatorward Pedersen current or an increase in a field aligned current. The polarization of the Pi(c) pulsations may be explained in terms of the altitude of the respective currents.     

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.     

Twilight optical studies of the El Chichón volcanic dust over Ahmedabad,India

Following the spectacular twilights associated with the El Chichón volcanic eruption, noticed in Ahmedabad, India (23°03′N, 72°30′E) since late May 1982, twilight sky brightness measurements were carried out from the same place for about a year between June 1982 and May 1983. The measurements made mainly in the near infrared I band (λ_eff = 0.87 μm) show a novel feature representative of the volcanic aerosol layer in the plot of sky brightness against solar depression. The height of the lower end of the aerosol layer (H_L) derived from the measurements has remained at ∼ 18 ± 2 km over a period of about one year. The strength of the aerosol feature, determined by an index Q, shows a gradual decrease, suggesting a slow dispersal of the layer with a detectable existence in the I band till the middle of 1984. Multispectral observations indicate that the near infrared transmission window (λ_eff = 2.2 μm) may be a more sensitive indicator of aerosol layer than visible wavelengths.     

Ionospheric ELF radio signal generation due to LF and/or MF radio transmissions—I. Experimental results

A new non-linear ionospheric effect has been discovered from the analysis of ground based FLF/VLF radio data obtained in Scandinavia during 1979. Apart from signals of natural origin, timing signals (six pips which occurred on the hour) were received. The pips of frequency 1000±0.5 Hz, duration 105 ± 8 ms, and field strength ~ 0.1 pT at Sodankyla, Finland, exhibit a favoured source location ~ 150 km south-south-east of Sodankyla. A close association between the reception of these ELF pips and the auroral electrojet is demonstrated by the positive temporal correlation between ELF pip generation and periods of enhanced local magnetic activity, and also by the spatial correlation between source location and the latitude over F'inland at which riometer absorption is a maximum. In the evening and midnight sectors the latter is interpreted as indicating the electrojet position. The originating signals are shown to emanate from one or more Soviet LF/MF broadcast transmitters, all of which are several hundred kilometres or more from the favoured generation region.     

Equatorial penetration of magnetic disturbance effects in the thermosphere and ionosphere

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 f₀F2 (∼ ± 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.     

Electrodynamic structural features of the auroral zone as deduced using data from a meridional chain of ionospheric and magnetic stations

This study has used ionospheric and magnetic observational data obtained at a meridional chain of stations during the high latitude geophysical experiment ‘Taimir-82’ in the winter of 1982–1983. Mean statistical latitude-time distributions of the occurrence probability of various types of E_s, their blanketing frequency and of the amplitude of geomagnetic field H-variations have been constructed. Based on these distributions and taking the E_s properties into account, an analysis is made of the mutual correspondence of large-scale structures of the auroral ionosphere and ionospheric currents.Ionospheric currents flow mainly in the region of high E-layer ionization. With increasing magnetic activity, the zone of currents and the zone of ionization expand simultaneously toward lower latitudes. The evening eastward electrojet and the morning westward electrojet are localized inside the zone of diffuse auroral precipitation which is responsible for the formation of E_s type r. The equatorial part of the midnight westward electrojet is also located in the zone of diffuse precipitation which coincides also with the region of maximum ionization of the E-layer. The polar part of this electrojet, which extends far into the dusk sector, is located in the zone of discrete auroral precipitation (a type E_s). Whereas there exists in the meridional cross-section quite a definite relationship between the Harang discontinuity and ionospheric parameters, such a relationship is not manifested in the zonal cross-section of the Harang discontinuity.     

The solar eclipse of 26 February 1979: analysis and modelling of primary pair production,electron density and ionic composition

The solar eclipse of 26 February 1979 was observed from Red Lake, Canada, (52 °N, 91 °W) where totality occurred at about 1053 local time. Several research groups and government agencies participated in an extensive ground- and rocket-based observational program directed at the middle atmosphere. At the time of the eclipse, an extensive geomagnetic storm was in progress and the middle atmosphere was undergoing temperature and circulation changes associated with a stratospheric warming. Concurrent observations of atmospheric constituents, solar radiation, electron flux and other middle atmosphere parameters were obtained as inputs for a D-region predictive chemical computer code, DAIRCHEM, tailored to eclipse conditions. Ion pair production rates were computed by an E-region infrared radiance model and were used as necessary source function input values for DAIRCHEM computations. The computations yielded predictions of electron and total positive ion densities about totality. The positive ion measurements of a supersonic Gerdien condenser and a subsonic blunt probe during the eclipse were in agreement with the model computations and provided normalizing summations of total positive ions for the interpretation of mass spectrometer measurements. The chemical computer code identified principal routes for increase and removal of key species such as O₂⁺, NO⁺, hydrated clusters and negative ions. The dominant precursor ion for pair production hydrates was O₂⁺ and the chemistry was characteristic of the disturbed D-region.     

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.     

Subsidiary maxima in late evening NmF2 at a low latitude station at solar maximum

At solar maximum during the late evening hours (2100–2400 LT), N_mF2 at Tahiti frequently does not decrease monotonically but exhibits temporary subsidiary maxima. Thus, in 1980, of 66 evening periods for which good data were available, 20 showed monotonie decreases but the remainder exhibited such subsidiary maxima. In summer the subsidiary maxima correspond to hmF2 significantly lower than the monotonie decreases. This lower hmF2 during subsidiary maxima corresponds to a weakening or reversal of the equatorward neutral wind, accompanied by an increase in the flux from the equatorial fountain. In winter the subsidiary maxima are fully accounted for by increases in the flux from the fountain effect, probably due to increases in the strength of the equatorial electrojet.