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.     

Diurnal variations in F2 peak density and ionization flux above Tahiti

The effects of composition and ionization fluxes on the diurnal variation of NmF2 at an equatorial anomaly zone station (Tahiti) are separated. The calculated diurnal variation of the fluxes agrees well with what would be expected from published equatorial E × B drift observations and global neutral wind models. A correlation analysis shows that lower hmF2 is often accompanied by larger NmF2, in spite of a much larger recombination rate. This illustrates the dominance of the fountain effect and neutral wind induced interhemispheric transport at this station.     

The isobaric F2-layer

Cyclic diagrams, obtained by plotting the daily variation of the ionospheric electron density NmF2 against the height hmF2, are drawn for typical conditions at Slough (52°N) and Watheroo (30°S). Using the MSIS86 thermospheric model to relate the heights hmF2 to values of atmospheric pressure, the F2-peak is found to lie at nearly the same pressure-level at any given local time, over a wide range of geophysical conditions (season, solar cycle, magnetic disturbance). As local time varies, the pressure level corresponding to hmF2 varies in a way that is mainly determined by the local time variation of the thermospheric winds. This is verified for noon and midnight, using the MSIS86 model to compute the winds. The noon values of peak electron density (NmF2) are fairly consistent with theory (using values of solar ionizing flux as quoted in the literature), but with some discrepancies—particularly at sunspot maximum—that are probably due to uncertainties in chemical composition, or to the effects of vibrational excitation of molecular nitrogen. Overall, the analysis shows a remarkable consistency between ionospheric theory, the data and the MSIS model.     

Role of vertical winds on the Rayleigh-Taylor mode instabilities of the night-time equatorial ionosphere

In view of the recent observations on the presence of vertical winds in the equatorial ionosphere in the evening and night-time, the role of vertical winds in the Rayleigh-Taylor (R-T) mode instability has been re-examined. The mathematical treatment of Chiu and Straus, earlier developd for a case of horizontal winds, is extended to evaluate the role of vertical winds in causing the R-T mode instability. It is shown that the vertical (downward) winds of small magnitude have a very significant effect on the instability growth rate in the. F-region. A downward wind of l m s⁻¹ can cause the same growth rate as a 200 m s⁻¹ eastward wind at 260 km altitude. Furthermore, a downward wind of 16m s⁻¹ at 300 km can be as effective as that due to the gravitational drift itself. Similarly, an upward wind can inhibit the instability on the bottomside of the F-region. It appears that the polarity of the vertical winds (upward or downward) at the base of the F-layer plays an important role in the growth of the R-T mode plasma instability in the equatorial ionosphere.     

A study of the meridional structure of the equatorial red arcs in the night-time ionosphere from the ISIS-II satellite

Equatorial 6300 Å 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 Å 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 Å 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 Å 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.     

Vertical neutral wind in the equatorial F-region deduced from electric field and ion density measurements

DC electric field and ion density measurements near density depletion regions (that is, equatorial plasma bubbles) are used to estimate the vertical neutral wind speed. The measured zonal electric field in a series of density depletions crossed by the San Marco D satellite at 01.47-01.52 UT on 25 October 1988, can be explained if a downward neutral wind of 15–30 m s⁻¹ exists. Simultaneously, the F-region plasma was moving downward at a speed of 30–50 m s⁻¹ These events appear in the local time sector of 23.002̄23.15 in which strong downward neutral winds may occur. Indeed, airglow measurements suggest that downward neutral velocities of 25–50 m s⁻¹ are possible at times near midnight in the equatorial F-region.     

Modelling studies of the conjugate-hemisphere differences in ionospheric ionization at equatorial anomaly latitudes

The relative importance of the equatorial plasma fountain (caused by vertical E x B drift at the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis on conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospherec electron content (IEC) and peak electron density (Nmax) computed at conjugate points in the magnetic latitude range 10–30° at longitude 158°W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and Nmax, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes.At equinox., the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements. in IEC and Nmax (with delayed peaks) in the southern hemisphere at all latitudes (10–30°). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F-region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and Nmax and stronger daytime Nmax in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.     

The Origin of African Sheep: Archaeological and Genetic Perspectives

Indigenous African sheep genetic resources have been classified into two main groups, fat-tailed and thin-tailed sheep. The fat-tailed sheep are the most widely distributed, being found in a large part of North Africa (from Egypt to Algeria) and in Eastern and Southern Africa (from Eritrea to South Africa). The thin-tailed sheep are present mainly in Morocco, Sudan and in West Africa. African sheep were domesticated outside Africa. They share a common ancestry with European and Asian sheep. Archaeological information supports separate introductions and dispersion histories for the African thin-tailed and fat-tailed sheep. The first sheep entered Africa via the Isthmus of Suez and/or the southern Sinai Peninsula, between 7500 and 7000 BP. They were likely of the thin-tailed type. Fat-tailed sheep entered Africa through its northeastern part and the Horn of Africa. Mitochondrial DNA analysis supports a common maternal ancestral origin for all African sheep, while autosomal and Y chromosome DNA analysis indicates a distinct genetic history for African thin-tailed and sub-Saharan fat-tailed sheep. The main ancestral population of southern African fat-tailed sheep likely originated in East Africa. Further work is needed to assess the possible dispersion of sheep from western Africa to the southern African regions.     

Seasonal variations of equatorial night-time thermospheric meridional winds

Using h'F data at two equatorial stations, night-time equatorial thermospheric meridional winds have been deduced for a period of two years to study their seasonal characteristics. It has been found that the thermospheric wind shows trans-equatorial flow from summer to winter hemisphere. During equinoxes the flow is mainly equatorward with a reversal to poleward direction around midnight hours. The abatement and reversal of equatorward wind which is weaker in summer compared to equinoxes is attributed to Midnight Temperature Maximum (MTM). The results of the present investigation are compared with those at other equatorial stations and also with the empirical model of Hedin et al. (1991).     

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.     

Distortions in the height structure of the equatorial electrojet during counter electrojet events

This paper presents simultaneous observations made near the magnetic equator during counter electrojet events using a coherent VHF backscattcr radar, magnetometer and digital ionosonde to understand the physical processes that generate the counter electrojet conditions. The VHF backscatter radar gives the height structure of the drift velocity or the ionization irregularities, the equatorial electrojet current variations are obtained from the magnetometer and the digital ionosonde provides the presence of blanketing E-layers at the F-region heights which give rise to the backscatter signals. These observations have provided direct experimental evidence for the theoretically predicted distortions in the height structure of the polarization electric field in the equatorial electrojet due to the local effects of shearing zonal neutral winds.     

Formation of ionisation layers responsible for blanketing Es during daytime counter-electrojet over magnetic equator

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.     

Diurnal variations in the flux of ionisation above the F2 peak in the northern and southern hemispheres

The flux of ionisation at 850 km height is calculated using the MSIS atmospheric model, a simplified form for the continuity equation at the peak of the F2-layer, and observed values of NmF2. Results are given for stations at latitudes of 32°N, 21°N, 21°S and 37°S during 1971 and for Tahiti (18°S) in 1980. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at mid latitudes. Increased recombination due to N₂ vibrational excitation produces a large afternoon decrease in NmF2 in summer, near solar maximum, and an increased downward flux. At all stations the day-time flux has a much larger downward component in winter than in summer. Because of the eastward magnetic declination, zonal winds produce opposite effects on the diurnal variations of hmF2, NmF2 and flux in the northern and southern hemispheres. Downward fluxes are largest in the morning in the southern hemisphere and in the late afternoon and evening in the north. At ± 21° latitude, neutral winds have a major effect on the distribution of ionisation from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than in summer. Averaged over both hemispheres, the total flow at 21° latitude is approximately the same for solstice and equinox conditions. At mid latitudes there is a downwards flux of about 1–2 × 10¹² m² s⁻¹ into the night ionosphere.     

Simulations of the behaviour of the polar thermosphere and ionosphere for northward IMF

The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of auroral precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B_z, regions of sunward thermospheric winds (up to 200 m s⁻¹) may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by low-latitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF B_z. The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 2–4 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B_y negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting E-and F-region plasma densities. For example, in northern mid-winter, sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.     

Ionospheric changes in response to IMF variations

This paper attempts to summarize the results of investigations of IMF effects on the ionosphere, published mostly in Russian, and to place them in context in up-to-date knowledge of IMF/magnetosphere/ionosphere relationships. Effects of the IMF sector structure and of the IMF B_z component turnings on the ionospheric F-layer are considered, including variations of position of the main ionospheric trough (MIT). The paper includes results of both theoretical calculations and observational data obtained mostly by the Cosmos-900, Intercosmos-19 and Cosmos-1809 satellites at subauroral, middle and low latitudes. The MIT position dependence on longitude has been derived as a background for further study. It has been shown that the nightside winter trough position at the storm growth phase correlates best with K_P index taken with a time delay τ, which is proportional to a disturbance growth rate ΔK_p/Δt. The MIT position dependence on D_st, B_z and B_y is also shown. Two troughs have been found to be formed usually in the storm recovery phase at postmidnight hours: these are the MIT (main ionospheric trough) and RIT (ring ionospheric trough) associated with the DR-current. In general the MIT position's response to B_z southward turnings corresponds well to changes of the amoral diffuse precipitation equatorial edge. For B_z southward turnings the height of the equatorial night-time F-layer lowers, and at equatorial latitudes f_oF2 decreases sharply, the latter effect being most pronounced at 03 LT. Large-scale internal gravity waves arriving at equatorial latitudes from the auroral oval cause intensification of the equatorial anomaly, both in daytime and night-time. A schematic pattern of a global ionospheric response to a magnetic disturbance is constructed using as an example the strong storm on 3–4 April 1979.     

Improved world-wide maps of monthly median foF2

New global maps of monthly median values of foF2 have been prepared using over 45,000 station months of foF2 observations, semi-empirical model values of foF2 in the mid-latitude ocean areas and empirical model values for the equatorial and high latitude regions. These observations have been carefully screened and mapped, using the Jones-Galley technique, to produce monthly median maps of foF2 for each hour, each month and for high and low levels of solar activity.     

Zonal irregularity drifts and neutral winds measured near the magnetic equator in Peru

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.     

Evidence of an Eleventh-Century AD Cola Nitida Trade into the Middle Niger Region

Kola nut (Cola cf. nitida) and Safou fruit (Dacryodes edulis) remains have been discovered in eleventh- to fourteenth-century archaeological contexts at Togu Missiri near Ségou in Mali. These remains are evidence of early trade in perishable foodstuffs from the West African forest zone into the Middle Niger region. On the basis of these finds, this paper argues that long-distance trade links were well established by the end of the first millennium AD. It thereby supports the hypothesis that dates the inception of trade between the West African forest zone and the savanna regions to the first millennium AD. The circumstances of the find are discussed, as are the implications for our understanding of the wider exchange network based on the Niger River system in the late first and early second millennium CE.

     

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.