The effect of the mid-latitude ionospheric trough on whistler mode ducting during magnetic storms

Whistler-mode signals observed at Faraday, Antarctica (65° S, 64° W, Λ=50.8°) show anomalous changes in group delay and Doppler shift with time during the main phase of intense geomagnetic activity. These changes are interpreted as the effect of refracting signals into and out of ducts near L=2.5 by electron concentration gradients associated with edges of the mid-latitude ionospheric trough. The refraction region is observed to propagate equatorwards at velocities in the range 20–85 ms⁻¹ during periods of high geomagnetic activity (K_p ≥ 5), which is in good agreement with typical trough velocities. Model estimates of the time that the trough edges come into view from Faraday show a good correlation with the observed start times of the anomalous features. Whistler-mode signals observed at Dunedin, New Zealand (46° S, 171° E, Λ=52.5°) that have propagated at an average L-shell of 2.2 (Λ=47.6°) do not show such trough-related changes in group delay. These observations are consistent with a lower occurrence of the trough at lower invariant latitudes.     

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.     

IMF effects in the appearance of slant E conditions (SEC) in the northern and southern polar regions

Vertical sounding data of the polar ionosphere are used to study slant E condition (SEC) related to the development of instabilities in the ionospheric plasma. The measurements from three Arctic and three Antarctic stations located correspondingly in the polar cap, daytime cusp and auroral zone are analyzed. It is shown that the SEC daily variations in these three regions are different. Distinctions between SEC features in the opposite hemispheres are affected by the azimuthal and vertical interplanetary magnetic field (IMF) components.     

The mid-latitude trough in the electron concentration of the ionospheric F-layer: a review of observations and modelling

Experimental observations and theoretical modelling of the terrestrial mid-latitude trough are reviewed. The mid-latitude trough is considered as an F-layer phenomenon, and its relationships to the lightion trough in the topside ionosphere and to the plasmapause are discussed. The observed morphology of the mid-latitude trough is summarised. Recent evidence on plasma temperatures in the trough is examined. The physical processes that may be important in the trough region are listed. Large-scale computational models that include some of those processes are described and the results compared with observations. Deficiencies in the models and possible future developments are mentioned.     

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

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

Diurnal and seasonal variability of the southern-hemisphere main ionospheric trough from differential-phase measurements

A differential-phase technique utilizing the radio transmissions of NNSS satellites was used to make measurements of the latitudinal variations of ionospheric vertical total electron content (TEC) in the southern mid-latitude trough region from the location of Macquarie Island (a cis-auroral site; geographic coordinates 54.5°S, 154.95°E, geomagnetic coordinates 64.5 S, 177.67 E, L = 5.38) for a period of 14 months during 1987–1989. The differential-phase method provided a means of observing a relatively large expanse of ionosphere whilst centered on the cis-auroral region which was well suited for trough studies. By monitoring the two transmitted radio signals at 150 and 400 MHz from the Navy Navigation Satellite System (NNSS) polar orbiting satellites it was possible to deduce the latitudinal variation of ionospheric vertical TEC for the duration of the satellite pass. The absolute TEC was derived from Faraday-rotation and ionosonde data obtained during the same period.The main findings of this work have been the high incidence of daytime troughs for all seasons and the relative low incidence of night-time troughs. Both summer and vernal equinox seasons display a greater occurrence frequency of daytime troughs than the winter and autumnal equinox seasons. Winter-time troughs at any time are less frequent than for any other season. Comparisons with the northern-hemisphere trough display a marked difference in occurrence frequency and distribution of troughs. An attempt to explain some of these features in the light of the high-latitude convection theory is offered. Case studies are given for all seasons to highlight these findings.     

Plasma convection and auroral precipitation processes associated with the main ionospheric trough at high latitudes

Intervals of F-region electron density depletions associated with the main (mid-latitude) ionospheric trough have been studied using latitude scanning experiments with the EISCAT UHF radar. From 450 h of measurements over a one year period at solar minimum (April 1986–April 1987) the local time of appearance of the trough at a given latitude is observed to vary by up to about 8 h. No seasonal dependence of location is apparent, but troughs are absent in the data from summertime experiments. A weak dependence of trough location on K_p is found, and an empirical model predicting the latitude of the trough is proposed. The model is shown to be more appropriate than other available quantitative models for the latitudes covered by EISCAT. Detailed studies of four individual days show no relationship between local magnetic activity and time of observation of the trough. On all four of these days, however, the edge of the auroral oval, evidenced by enhanced electron densities in the E-region, is found to be approximately co-located with, or up to 1° poleward of, the F-region density minimum. Simultaneous ion drift velocity measurements show that the main trough is a region of strong (> several hundred metres per second) westward flow, with its boundary located approximately 1°–2° equatorward of the density minimum. Within the accuracy of the observations this relationship between the convection boundary, the trough minimum and the precipitation boundary is independent of local time and latitude. The relevance of these results is discussed in relation to theoretical models of the F-reregion at high latitudes.     

Types of ionospheric scintillations in southern mid-latitudes during the last sunspot maximum

A 5-yr study (1987–1992) has been undertaken at a southern mid-latitude station, Brisbane (35.6°S invariant latitude) on scintillation occurrences in radio-satellite transmission (at a frequency of 150 MHz) from polar orbit Transit satellites, within a sub-ionospheric invariant latitude range 20–55°S. Over 7000 recorded passes were used to define the spatial and temporal occurrence pattern of different types of scintillation events. Two predominant scintillation types were found: so-called type P (associated with a scintillation patch close to the magnetic zenith) and type S (characteristic of the equatorward edge of auroral scintillation oval). Type S was by far the most frequent during sunspot maximum (1988–1992), with sharp occurrence peaks in the summer-autumn period. Its seasonal occurrence showed a high degree of correlation (correlation coefficient r = 0.8) with the seasonally averaged 10.7 cm solar radio flux. This type occurred mainly at night-time except in austral summer where 40% of scintillations were detected in daytime, coinciding with the well-known summer peak of sporadic-E occurrence. Type P was more predominant during a year (1987) of ascending sunspot activity but decreased to a much lower level during the sunspot maximum.     

The influence of ionization and recombination processes on the dynamics of ionospheric plasma clouds

The dynamics of a one-dimensional ionospheric irregularity interacting with the magnetosphere is studied by numerical simulation. The polarization electric field produced by charge separation within the irregularity propagates along magnetic field lines with the Alfvén velocity V_A and drives polarizational and field-aligned currents in the magnetosphere. Their values and localization are controlled by motion and deformation of the irregularity resulting from its electrostatic coupling to the background ionosphere. The pattern of the field-aligned currents varies with time and depends primarily on gradients of the polarization electric field. The latter is controlled by the ambient electric field, diffusion, recombination process, intensity of the initial perturbation, etc. Feedback effect of the magnetospheric conductance on the development of the irregularity is examined.     

Uganda: Contradictions of the IMF Programme and Perspective

This critique of the Structural Adjustment Programme in Uganda is argued at three levels. A discussion of the immediate consequences of SAP is based on empirical data on economic performance compiled by the Government of Uganda, IMF and World Bank. Second, information for a longer-term historical analysis is culled from the author's own research. Its purpose is to raise more fundamental issues of social transformation. Finally, these perspectives are reinforced through a comparative discussion using South East Asian development experience. The author argues that there are diverse paths to capitalist development, with diverse and contradictory social and political consequences. The real issue in contemporary Uganda is not one of the state or the market, but of the transformation of relations internal to both from the point of view of democratization.     

The dynamics of the poleward edge of the trough deduced using data from a meridional chain of vertical-incidence sounding stations

Data from four ionospheric stations located along the 902E meridian in the range 55–702 of corrected geomagnetic latitude, were used to construct latitude-time electron density distributions in the F2-layer peak for 17 winter nights of 1982–1983. It is concluded that under stationary convection conditions the poleward edge of the trough during the nighttime displaces only 0.5-l°/h, that is, significantly less than obtained from existing analytical models of the ionospheric trough. When the stationarity is upset (due to the development of a substorm or abrupt changes of the north-south component of the IMF), the poleward edge of the trough is observed to displace abruptly equatorward. In the substorm expansion phase these displacements can amount to 4–5° in less than an hour. Such displacements of the poleward edge in the evening hours can characterize the dynamics of the inner edge of the plasma sheet.     

A theoretical study of the distribution of ionization in the high-latitude ionosphere and the plasmasphere: first results on the mid-latitude trough and the light-ion trough

A model of the O⁺ and H⁺ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O⁺ and H⁺ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner plasmasphere, and to an altitude of 1400 km in the outer plasmasphere and high-latitude ionosphere. Account is taken of high-latitude convection, departure from corotation inside the plasmasphere, and neutral air winds. The neutral air winds are consistent with the assumed convection pattern. For equinox and magnetically quiet conditions the calculations show that a mid-latitude trough in F-layer electron concentration is present from 1600 to 0600 LT and the trough may occur either inside or outside the plasmasphere. The movement of the trough in this period is from higher to lower latitudes and is in qualitative agreement with AE-C and ESRO-4 data. A light-ion trough feature is apparent in the H⁺ distribution in the topside ionosphere at all local times. During the day the upward H⁺ flow increases with latitude to produce the light-ion trough. At night the H⁺ trough may be directly produced by the occurrence of the mid-latitude O⁺ trough. The relationships between the position of the plasmapause and the trough are discussed. Also discussed are the influence of particle ionization in the auroral zone and the effect of the neutral air wind.     

The effect of rapid changes in ionospheric flow on velocity vectors deduced from radar beam-swinging experiments

The effects on the horizontal ionospheric velocity vectors deduced from radar beam-swinging experiments, which occur when changes in the flow take place on short time scales compared with the experiment cycle time, are analysed in detail. The further complications which arise in the interpretation of beam-swinging data, due to longitudinal gradients in the flow and to field-aligned flows, are also considered. It is concluded that these effects are unlikely to seriously compromise statistical determinations of the response time of the flow, e.g. to changes in the north-south component of the IMF, such as have been recently reported by Etemadi et al. (1988, Planet. Space Sci. 36, 471), using EISCAT ‘Polar’ data.