The global distribution of ionospheric small-scale irregularities from topside sounding data

This paper examines the global distribution of electron density irregularities with scales of the order of several tens to hundreds of meters in the ionosphere by using topside sounder data from the COSMOS-1809 satellite obtained in May–June and December 1987. The diffuse traces of Z-waves on topside ionograms in a frequency band just below the upper hybrid resonance are used for diagnostics. These traces are attributed to the scattering of sounder-generated ordinary and slow extraordinary mode waves.     

A comparison of foF2 obtained from a time-dependent ionospheric model with Argentine Islands' data for quiet conditions

In this study a comparison is made of the Utah State University Time-Dependent Ionospheric Model (TDIM) and an ionosonde data set from Argentine Islands. This study is unique in that the Argentine Islands data set of foF2 spans complete diurnal, seasonal and solar cycle conditions for low geomagnetic activity. The TDIM reproduces these foF2 variations extremely well. Although the observed winter and summer solstice foF2 diurnal curves have opposite phases, they are readily modelled. At equinox where a sharp transition occurs from winter to summer, or vice versa, the monthly average is complicated by this feature and hence the TDIM does not reproduce the diurnal fine structure.The neutral wind induced vertical plasma drift is the only free parameter in this study. All the other inputs are fixed for the specific solar, seasonal and diurnal conditions. A vertical plasma drift variation is presented; although simplistic, it couples the geographic and geomagnetic frames. With additional information such as hmF2, it would be possible to deduce a unique vertically induced drift pattern.     

Effects of neutral temperature on meridional winds estimated from ionospheric data

Indirect determination of meridional winds using ground-based ionosonde data from low latitude regions, under the assumption that the thermosphere and the F-region of the ionosphere behave as a closely coupled system, has been critically examined. The significance of neutral temperature and its variations in the above estimates has been demonstrated through individual case studies after duly validating the procedure adopted. Since the measured neutral temperatures have shown large deviations from the existing atmospheric models on many occasions and more so during high solar activity periods, it has been shown that the neutral temperature effects on the F-region heights should be properly accounted for before one attempts to estimate meridional winds. However, it has also been shown that during low solar activity periods, use of atmospheric models may still provide a fairly reasonable average picture. Examples of these effects are presented and discussed.     

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.     

Changes of the electron concentration profile during local heating of the ionospheric plasma

Using numerical simulation of a non-stationary problem of thermodiffusion and diffusive spreading of the electron component of the dense cold ionospheric plasma, the processes of formation and relaxation of strong disturbances of the electron temperature and concentration in the E- and F-regions of the middle-latitude ionosphere are examined, taking into account the altitudinal distribution of the electron transport coefficients. The cases of local heating and heating at separated altitudes of the ionospheric plasma by powerful radio waves generated from ground-based HF-facilities are numerically investigated. The numerical simulations of the non-stationary problem are compared with the analytical evaluations carried out for the stationary and quasi-stationary heating models. Results obtained from numerical experiments give good explanations of the experimentally observed deformation of the altitudinal ionospheric plasma density profile and the creation of negative cavities in the upper ionosphere and positive cavities in the lower ionosphere during the process of plasma heating.     

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 accuracy of simple methods for determining the height of the maximum electron concentration of the F2-layer from scaled ionospheric characteristics

The techniques for estimating hmF2 from M(3000)F2 are reviewed with particular stress put upon those in which the effects of underlying ionization are accounted for by a correction (ΔM) to M(3000)F2, formulated in terms of the ratio foF2/foE(=x_E). The simplifying assumptions involved in the three practical implementations (Bradley and Dudeney, 1973; Dudeney, 1974; Bilitza et al., 1979) are emphasised and their consequences investigated quantitatively using a numerical simulation. The factors considered are the dependence upon ymF2, the importance of the underlying layer shape (in particular the significance of the F1-ledge), and the influence of the geomagnetic field.It is demonstrated that the correction technique relies upon ymF2 being a direct polynomial function of hmF2. Analysis of observational data suggests that this relationship holds in practice. Fluctuations in ymF2 about this mean variation are shown to produce only small effects which decrease in magnitude as the amount of underlying ionization increases. The results indicate that underlying layer shape becomes very important when a large amount of underlying ionization is present (x_E<2.5). However, the global morphology of the occurrence of the F1-ledge is such that it is invariably present in such circumstances (ignoring the polar regions). Hence, the ionosphere tends to assume a specific profile form for low x_E cases. The three implementations are shown all to fortuitously incorporate this behaviour. It is demonstrated that exclusion of the geomagnetic field introduces a very small extra uncertainty dependent upon gyrofrequency and geomagnetic latitude, which decreases as the amount of underlying ionization increases.The three implementations are compared and it is concluded that the Dudeney (1974) scheme gives the best overall performance. The more modern and complex Bilitza et al. (1979) scheme appears to have no performance advantages, whilst containing a sunspot number dependent geomagnetic term whose behaviour is irreconcilible with the numerical simulation. The Dudeney (1974) equation is shown to be accurate to between 4 and 5% at magnetic mid-latitudes. The scope for further refinement is considered but rejected as being unlikely to produce an increase in accuracy commensurate with the effort required.     

The height of electron content changes in the ionosphere from ATS6 beacon data

A technique is described which uses relative changes in Faraday rotation and modulation phase of satellite radio signals to determine the median height of the enhancement (or depletion) in the electron density of the ionosphere. During the post sunrise formation of the F mlayer the incremental layers have a median height of around 210 km (±40) and in the afternoon the decremental median is above the peak at 340 km (±40) on a winter day. A winter night-time enhancement just after midnight appears as a thick layer extending upwards from the peak, with a median height at about 730 km. The method applies to large scale irregularities but not to small, dense, scintillation-causing irregularities for which Faraday and modulation phases do not represent the total electron content.     

Analysis of Es traces from a calibrated oblique ionosonde

An analysis is performed on ionosonde data produced during five years of operation of an oblique sounder transmitting on a path from Darwin (12.4°S, 130.9°E) to Alice Springs (23.5°S, 133.7°E). It is found that the occurrence of sporadic-E (E_s) shows a relatively mild diurnal dependence, with a significant amount of E_s occurring in the early evening before midnight. It appears that, on average, nighttime E_s produces weaker reflections than daytime E_s.The power of the E_s reflections as a function of frequency is collated for all ionograms. The resulting power curve exhibits total and partial reflection sections. In trying to reproduce the partial reflection section of the curve it is shown that a layer without horizontal structure is required to be only 100 m thick. A second model involving a layer consisting of horizontally localised clouds of scatterers, with scale sizes ranging from 100 to 1000 metres, reproduces the partial reflection section of the curve quite well. The size, intensity and distribution of the clouds affects the curve shape on individual ionograms, resulting in the suggestion that nighttime layers are more irregular than daytime layers.     

Design and realization of a goniopolarimeter with application to the study of ionospheric tilts

The purpose of the paper is to describe a high frequency goniopolarimeter operating in the range of 3–30 MHz. The system uses four sensors, each of them is composed of two cross polarization active loop antennas oriented east-west and north-south. By “goniopolarimeter” we mean a system which determines both arrival angles (goniometer function), the type of the mode (O or X), the magnitude of this mode and the corresponding Doppler shift. It works with known broadcast transmitters and permits measurements over a large geographic area. The signal process deals with narrowband signals, polarization filtering, nonlinear frequency analysis and interferometry technique. We present some experimental results and discuss the application of this system to the study of ionospheric tilts.     

Nocturnal stratification of the ionospheric F-layer over Wuchang,China

Examples are given in the present paper of nocturnal ionospheric stratifications in lower midlatitudes, with the following characteristics:

• 1.(1) clear night-time stratification or low frequency F retardation, implying stratification
• 2.(2) a marked decrease in foF2 and a marked increase in h'F2 and hpF2
• 3.(3) a marked increase in electron concentration under the peak
• 4.(4) a close correlation with polar substorms, as deduced from the local magnetograms. It is suggested that the stratifications are due to particle precipitation and to modification of the low-latitude electric field by polar substorms.

     

Symbolic engravings on Byzantine sounding leads from the Carmel coast of Israel

Two Byzantine sounding leads recovered from a shipwreck off the Israeli coast were identified and dated by associated coins and artefacts to the 5th–6th centuries AD. The leads are similarly shaped. A cross and a sign like a capital'P' mark both. They may be from a set including an inshore and a deep-sea lead. A depiction of the stellar constellation Cassiopeia is seen on top of the lug of the small lead. Interpretation of the sign is supported by the navigational role of the constellation as well as by a classical tradition associating it with Jaffa and by its symbolic, anti-storm role.     

A model of ionospheric image structure underneath a braking,cross-field plasma jet

A plasma jetting across the geomagnetic field above the ionosphere tends to brake by ohmic dissipation of Pedersen currents. The braking can affect the ionosphere underneath if the associated Pedersen drifts are intense and prolonged enough to cause cumulative image structuring. We examine such image structuring for the parameter regime of forthcoming CRRES releases, involving photo-ionization of kilograms of barium vapor moving at orbital velocity. The resultant structuring in the upper E-region offers possible diagnostic tell-tales of the braking process.     

Properties of ionospheric gyroechoes in the presence of a sporadic E-layer

Generation and properties of the ionospheric gyroechoes in the presence of a thin sporadic E-layer are theoretically investigated. The intensification of the echo and the decrease of its virtual height occurring at the onset of an Es-layer are explained in terms of mode coupling. A mechanism is found capable of producing gyroechoes when an otherwise totally blanketing flat-type Es-layer is present.     

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.     

Measurements of gradients in ionospheric parameters with a new nine-position experiment at Millstone Hill

A new nine-position experiment is now routinely carried out with the Millstone Hill incoherent scatter radars which allows estimation of spatial gradients in the measured ionospheric scalar parameters N_e, T_e, and T_i, and in the components of the ion velocity vector v_i. Use of this technique results in improved estimates of basic and derived parameters from incoherent scatter data at times of significant gradients. We detail the data analysis method and present the first results from this new experiment. The gradients in N_e and in the components of vi are used to compute the motion term in the ionospheric F region continuity equation ▿ · (Nv), which is then combined with ∂N/∂t to estimate the O⁺ recombination rate β at night. Meridional neutral winds U_mer are computed from the field-aligned ion velocity v_∥ and a calculation of the O⁺ diffusion velocity v_d, and it is found that horizontal gradients in the ion velocity field at times significantly affect the calculation of the neutral winds.     

Double structure of ionospheric conductivity in the midnight auroral oval during a substorm

Measurements of precipitating particles on board DMSP F7 spacecraft are used to analyze the distribution of ionospheric conductance in the midnight auroral zone during substorms. The distribution is compared with the meridional profile of ionospheric currents calculated from magnetic data from the Kara meridional chain. Two regions of high Hall conductance are found; one of them is the traditional auroral zone, at latitudes 64–68°, and the other is a narrow band at latitudes 70–73°. The position of high conductance zones is in agreement with the location of the intense westward currents. The accelerated particle population is typical of electrons E_e > 5 keV in the high conductance region.     

The effect of travelling ionospheric disturbances on the group path,phase path,amplitude and direction of arrival of radio waves reflected from the ionosphere

A fixed frequency amplitude modulated transmission was reflected from the ionosphere and changes in the group path, phase path, signal amplitude and directions of arrival of this transmission caused by travelling ionospheric disturbances were measured. These measurements enabled approximate determinations of the horizontal wavelength, period and horizontal phase velocity of the disturbances, which were compared with the theory of disturbances for atmospheric waves. A simple model is proposed to explain the phase relationships between the TIDs observed in the group and phase paths, and the faster decrease in power of the phase path than the other measured parameters indicated by spectral analysis.     

Synthesis of oblique ionograms from vertical ionograms using quasi-parabolic segment models of the ionosphere

This paper describes a method of synthesizing oblique ionograms from vertical ionograms based on representing the ionosphere by multiple quasi-parabolic segments (QPS). The advantage of this approach is that it allows analytical solutions to be obtained for several ray parameters when the Earth's magnetic field is neglected and the ionosphere is spherically stratified. The no-field results are representative of the ordinary mode. In addition, results for the extraordinary mode can be obtained by introducing a perturbation to the effective frequency and modifying the no-field results. The method is illustrated by comparing synthesized oblique ionograms with observed ionograms and with results obtained for the ordinary mode using the classical method.