Winds in the ionosphere—A review

Winds in the upper atmosphere, and their effect on the ionosphere, are reviewed with an emphasis on information useful to ionospheric studies. The winds are driven by pressure gradients from solar and auroral heating, with some forcing by tidal energy from below. Simple calculations which balance the pressure gradient by ion drag and Coriolis forces are generally unreliable, so large-scale numerical models of the coupled atmosphere and ionosphere are required. The accuracy of these global models is limited by uncertainties in the energy inputs at high latitudes and at the lower boundary (about 90 km). The best current wind data come from incoherent scatter radar or airglow installations, at a few sites and for only a few nights per month. Satellite data are also available for several years, and results to 1989 are incorporated in the global HWM90 model. This seems acceptable for determining mean winds at night, less good during the day, and least good in the southern hemisphere where few data were available. Plots are given to show the mean winds at different latitudes and longitudes, for use in ionospheric calculations.Meridional winds alter the height of the mid-latitude F layer, causing large changes in the effective loss rate. This is the major cause of observed seasonal changes, of differences between the hemispheres, and of changes at different longitudes. An increased knowledge of the winds is essential for further progress in F region studies. Ionospheric data provide the most promising route, using routinely scaled parameters. The simplest calculations compare observed peak heights, obtained from M (3000)F2, with the value h_o predicted by simplified “servo” equations. Errors occurring for some hours after sunrise can be overcome using model results to define h_o this allows rapid and accurate wind calculations at dip latitudes of 23–62°. Winds can also be obtained from full model calculations, designed to match observed values of peak height or density.     

Lightning transient fields in the atmosphere—low ionosphere

The complexity of the electric properties of the atmosphere, which influences the excitation and propagation of electromagnetic signals generated by lightning discharges, gives rise to a diversity of the typical signal wave forms and amplitudes in the various space-time domains. This causes difficulties and controversies in the interpretation of the experimental data. In geophysical problems of such a kind, considerations accounting for a broadband spectrum are particularly important.In the present paper, lightning induced ULF-VLF fields in the daytime inhomogeneous atmosphere and D-region of the ionosphere above the thundercloud are analyzed for a standard model of the vertical return stroke. The developed analytical approach makes it possible to unite the canonical model of the electromagnetic radiation into a free-half-space, the discharge (ELF-VLF spectrum) and ‘post-discharge’ (ULF spectrum) low-frequency models and to clarify the temporal scales in the mechanism of lightning coupling with the atmosphere and low ionosphere with respect to the local relaxation time, discharge characteristic time scales and observer location.     

Can the high latitude ionosphere support large field-aligned ion drifts?

Ion velocities perpendicular and parallel to the geomagnetic field have recently been deduced by Smith et al. from bistatic measurements at 71° geomagnetic latitude in the afternoon sector. The results of this experiment include large (>400 m s⁻¹) downward ion velocities parallel to the magnetic field that persist for hours, small (100 m s⁻¹) ion velocities perpendicular to the magnetic field and electron density profiles with extremely narrow full-width at half-maximum. The explanation of these results was that the ionospheric flux tubes observed were near the terminator, and thus, sunlit at the top and in darkness at the bottom. The difference in production between the top and bottom of the flux tube creates an excess of ions at the top, which rapidly diffuse downwards. A three-dimensional, time-dependent model of the ionosphere has been used to test this explanation. Numerical experiments were performed to determine upper limits for the downward ion velocity. Assuming reasonable vertically-induced ion drifts due to either neutral winds or plasma convection, these upper limits were substantially smaller than the measurements. The location of the terminator was found to contribute a maximum of about 60 m s⁻¹ to the vertical ion velocity due to diffusion in a partially illuminated flux tube. In an attempt to explain the narrow density profiles without invoking an additional ionization source, the downward force in the model was arbitrarily increased, as would occur due to parallel electric fields in the ionosphere. Since the interpretation of these measurements as large field-aligned flows seems untenable by a model thought to be consistent with the currently accepted physics of the atmosphere, an alternate hypothesis is presented. If the common volume measurement is made in a region of O⁺ precipitation, then the line profile would not be Doppler shifted when viewed off-zenith. Therefore, the field-aligned velocities would be small, and the narrow width of the profiles would be due to enhanced electron densities in an O⁺ arc.     

Palaeontology of the Chatham Islands,SW Pacific—a review

Despite their small area, the Chatham Islands host diverse and abundant fossils. Fossil assemblages of Permian to Late Cretaceous age preserved in terrestrial to shallow marine deposits represent the only record of plants and animals that once inhabited the eastern extension of Zealandia. Lower Cenozoic sediments have yielded a shallow marine fauna, including a rich molluscan assemblage linked to the oceanic inundation of this landmass. The late Cenozoic biota documents the re-emergence of the Chatham Islands and the establishment of major oceanic currents, which meet along the Chatham Rise. This review summarizes the fossil record of the Chatham Islands and the Chatham Rise, integrating data from published and unpublished sources.     

Magnetosheath,magnetopause and low latitude boundary layer research, 1987–1989

Important developments over the past two years (1987–1989) in studies of the magnetosheath, the magnetopause and the magnetopause boundary layers are reviewed. The pace of research shows no sign of abating, but progress has become focused on certain topics. These include flux transfer event reconnection and the relevance of solar wind dynamic pressure variations for magnetopause and polar cusp dynamics. A promising development in both these areas is the use of ground observations under the polar cusp ionosphere for monitoring solar wind-magnetosphere coupling processes at the magnetopause. In contrast to these progress areas, the magnetosheath and the low latitude boundary layer remain topics which are being unjustifiably neglected. The controversies and the key remaining questions are highlighted and suggestions are made on how to proceed further.     

Atmospheric tides—a review

Studies on atmospheric tides are reviewed with an emphasis on theory and recent important observations. Numerical simulation seems to be promising to elucidate the variability of observed tides. Radar observation will be powerful to obtain the precise vertical structure of tides in the middle atmosphere as well as the thermosphere. Various unsolved problems are pointed out.     

Equatorial scintillations—a review

The scintillation technique, as is well known, provides an integrated measure of phase and amplitude fluctuations imposed on radio signals over a wide range of frequencies during their propagation through the ionosphere. The large amplitude of equatorial irregularities necessitates the use of frequencies in the GHz band to obtain unambiguously the temporal variation of irregularity intensity and the effect of irregularity anisotropy. Recent observations of equatorial scintillations will be reviewed with an emphasis on GHz measurements. The steep spatial gradients observed in in-situ data and their relationship to intense GHz scintillations will be explored. Co-ordinated measurements of equatorial irregularities by such techniques as radar backscatter, in-situ rocket and satellite, total electron content and 6300 Å airglow will be discussed, insofar as they provide a better understanding of the scintillation phenomena. While it is difficult to critically assess results that are so recent and constantly evolving, we have attempted to focus attention on the outstanding problems that still remain in the field.     

Neutral atom precipitation—a review

The production of energetic neutral atoms by charge exchange of ring current ions with neutral hydrogen in the geocorona was predicted many years ago, and there are now a number of measurements of the effect of the impact of these energetic atoms on the thermosphere. Theoretical models of the process have been developed. The latitude variation of the precipitating flux depends very much on the pitch angle distribution of the ions in the ring current, and on the L shell on which they are located.The production of a belt of trapped particles at low altitude near the magnetic equator may occur when neutral atoms re-ionize and become trapped on impacting the thermosphere, and this belt has been found in particle measurements near the equator and is enhanced during periods of magnetic activity.A region of enhanced optical emission due to precipitating neutrals is found in the thermosphere near the magnetic equator in both disturbed and quiet times, implying a low L value and/or pancake pitch angle distribution for the ring current particles that give rise to these neutrals. An isotropic pitch angle distribution is present in parts of the ring current at time during magnetic storms. This gives rise to neutral atom precipitation at all latitudes, and particularly of particles near 90° pitch angle in the region of SAR arc occurrence, about 10° in dip latitude equatorward of the isotropic region.The rate of energy deposition and the rate of production of ionization in the thermosphere depend on the ion species present in the ring current; their energy spectra, and on the distributions of the ions with L value and pitch angle. The rate of energy deposition may at times reach 10⁻² to 10⁻¹ mWm⁻², sufficient for significant heating and wind generation. The rate of production of ionization in the thermosphere at night may be much greater than that of other low latitude night-time ionization sources.     

Vertical structure of AGW associated ionospheric fluctuations in the E- and lower F-region observed with EISCAT—a case study

The vertical structure of AGW (atmospheric gravity wave) associated fluctuations of ionospheric plasma parameters for the 100–240 km altitude range in the daytime of 7 September 1988 has been investigated by making use of the data provided by the Tromsø measurements in the EISCAT CP1 observation mode.The wave power profile vs height has been studied by integrating the power spectral density in each altitude. The essential feature of the power variation can be explained in terms of the energy conservation of AGWs propagating in a dissipative thermosphere. Intrinsic propagation parameters of the dominant AGW have been successfully estimated with a method based on the retrieval of the Doppler effect due to the horizontal prevailing wind. From the fluctuation structure analysis in a time-altitude frame, a downcoming AGW has been clearly identified. This downcoming wave might have been reflected from a wind shear at the altitude around 200 km, which is inferred from the meridional prevailing wind profile.     

The MST technique—a brief review

The purpose of this paper is to briefly review the MST (mesosphere-stratosphere-troposphere) radar technique for remotely observing the atmosphere in the height range 1–100 km. Particular emphasis will be directed here toward aspects of the subject related to the equatorial atmosphere—a region in which the overall circulation and dynamics is poorly understood. Some of the current MST capabilities will be demonstrated via data obtained from existing observatories located in the tropics. A case will be made for the necessity of having additional equatorial-region observatories, and an example of a reasonably economical system will be discussed.     

VLF radiation generated by a loop antenna in P2 layer of the ionosphere—2. Interpretation of the measurements

The results of an experiment on radiation and reception of VLF waves with a frequency of 5 kHz in the F2 layer of the ionosphere are compared with theoretical calculations made in the framework of the linear theory of a loop antenna in a plane-stratified plasma. The measurements made outside the Storey cone can be explained in the framework of the linear theory by the proper modification of the ionosphere model. The wave polarization differs from circular polarization on the boundary of caustic cone, due to non-linear effects.     

Neutral density cells in the high latitude thermosphere—2. Mechanisms

NCAR-TIGCM simulations predict mesoscale cellular structures in the high latitude neutral density at altitudes from 120–350 km. During magnetically active conditions, the density structure at 200 km consists of low-density cells near dawn and dusk and high-density cells near noon and midnight. Mechanisms causing the structured density cells are a result of thermosphere-ionosphere coupling and can be explained in terms of dynamic meteorology. For example, at high latitudes ion drag causes the neutral circulation to flow cyclonically in the dawn sector and anticyclonically in the dusk sector. Low densities are contained within the cyclonic circulation at all altitudes. Below about 170 km, the densities inside the anticyclonic flow are high, while above that altitude densities within the anticyclonic flow are low. While typical dynamic meteorology explains low densities in the centre of cyclonic circulation and high densities inside anticyclonic circulation, the dusk low-density cell in the centre of anticyclonic flow is unexpected. The anticyclonic dusk low-density cell is explained by anomalous antibaric flow due to high-speed winds. 120 km and 200 km altitudes are used to demonstrate the relationship between the high latitude densities and winds as well as the effect of joule heating and auroral particle precipitation on the density structures.     

VLF radiation generated by a loop antenna in F2 layer of the ionosphere—1. Theory

In 1987 an experiment on the transmission and reception of VLF electromagnetic waves was carried out in the Earth's ionosphere. The interpretation of the experiment required an elaboration of the theory of a loop antenna in a plane-stratified plasma with parameters changing along the external magnetic field. Formulae for the wave field at arbitrary distances from the antenna have been obtained. Asymptotic expansions for the wave field permit us to construct an effective numeral algorithm for the calculation of the signal when the far zone approximation does not hold and in the presence of the fine structure.     

Drifting patches of equatorial spread-F irregularities—experimental support for the spatial resonance mechanism in the ionosphere

The spatial resonance mechanism which sets in when the plasma drift velocity matches the phase velocity of atmospheric gravity waves in the ionosphere is applied to explain large-scale structures of the equatorial spread-F. The resonance condition is examined for reasonable parameters of travelling ionospheric disturbances, plasma drift and neutral wind velocities. It is shown that the resonance condition can be fulfilled in equatorial regions during the post-sunset hours. The measured drift velocities of patches of range-type equatorial spread-F are similar to measured plasma drift velocities. Some spread-F structures observed with the RTI-technique at Jicamarca, range-type spread-F observed on ionograms of Huancayo as well as the occurrence of spread-F patches observed with transequatorial propagation experiments do occur after the vertical plasma drift reverses from upward to downward. The periods, wavelengths and velocities of the large-scale structure of the spread-F patches appear to be similar to those parameters for medium-scale TID's. It is argued that a non-linear break-up of resonance-steepened TID's gives rise to the quasi-periodic structures which are typical for the range-type equatorial spread-F.     

In-situ studies of equatorial spread-F over SHAR—steep gradients in the bottomside F-region and transitional wavelength results

RH-560 rockets instrumented with Langmuir probes were launched from SHAR, India (dip 11°N) for in-situ studies of electron density irregularities associated with equatorial spread-F (ESF) when the F-region plasma was drifting down and strong range spread-F was observed with an ionosonde at SHAR. A high variability was observed in the steepness of the base of the F-region. The bases were found to be steeper during the periods when the F-region plasma was drifting down. On one of the flights irregularities were observed in the region around 280 km where the gradients in electron density were downwards, indicating that the gradient drift instability is the main mechanism for their generation. Assuming a power law of the type P_k∝ kⁿ for irregularities of transitional scale (20–200 m), it was found that the spectral index n ranges between −1.5 and −4.6, when the mean integrated spectral power P_T of the irregularities in the above scale size range varied from −45 to −12 db. A relationship between n and P_T was observed and can be represented by a Gaussian function using the above expression; the altitude variation of n normalized for a P_T value of −10 db showed that the nature of spectral index remains the same between 230 km and the apogee of the rocket. This is at variance with the observations of Kelley et al. [(1982), J. geophys. Res. 87, 1575] that 280 km is the threshold altitude for the steep drift wave type of spectra to a shallower spectra.     

Total electron content of the ionosphere at 31°S, 1967–1974

The total electron content (TEC) of the ionosphere at 31°S (geographic) has been calculated on the basis of Faraday rotation measurements made between September 1967 and January 1975 using geostationary satellites. The day-to-day, diurnal, seasonal and solar cycle variations of TEC are illustrated and discussed in relation to the maximum electron density of the F-layer, NMAX. A regression analysis is used to derive curves corresponding to fixed high and low levels of activity. The variations of slab thickness S = TEC/NMAX are also illustrated and discussed. The results overlap the observation periods of other published results and general agreement is found with these other results.     

Scattered power from non-thermal,F-region plasma observed by EISCAT—evidence for coherent echoes?

Three rapid, poleward bursts of plasma flow, observed by the U.K.-POLAR EISCAT experiment, are studied in detail. In all three cases the large ion velocities (> 1 kms⁻¹) are shown to drive the ion velocity distribution into a non-Maxwellian form, identified by the characteristic shape of the observed spectra and the fact that analysis of the spectra with the assumption of a Maxwellian distribution leads to excessive rises in apparent ion temperature, and an anticorrelation of apparent electron and ion temperatures. For all three periods the total scattered power is shown to rise with apparent ion temperature by up to 6 dB more than is expected for an isotropic Maxwellian plasma of constant density and by an even larger factor than that expected for non-thermal plasma. The anomalous increases in power are only observed at the lower altitudes (< 300 km). At greater altitudes the rise in power is roughly consistent with that simulated numerically for homogeneous, anisotropic, non-Maxwellian plasma of constant density, viewed using the U.K.-POLAR aspect angle. The spectra at times of anomalously high power are found to be asymmetric, showing an enhancement near the downward Doppler-shifted ion-acoustic frequency. Although it is not possible to eliminate completely rapid plasma density fluctuations as a cause of these power increases, such effects cannot explain the observed spectra and the correlation of power and apparent ion temperature without an unlikely set of coincidences. The observations are made along a beam direction which is as much as 16.5° from orthogonality with the geomagnetic field. Nevertheless, some form of coherent-like echo contamination of the incoherent scatter spectrum is the most satisfactory explanation of these data.     

Palaeobotany is blooming: 1970–1979, a review

During the ten years covered by this review, the foci of studies in palaeobotany of plant megafossils and certain non-palynological plant microfossils included Precambrian palaeobotany, early vascular plants, Palaeozoic pteridosperms, conifers, and angiosperms. Botanically orientated investigations displayed an increasing use of the electron microscope. Interpretations of palaeoecology and reproductive biology of fossil plants gained increasing momentum. Biostratigraphic studies underwent some new activation but generally took second place to the amount of morphological work. The number of palaeobotanists who completed or received their training during the decade augurs well for the future; there is still much to be done.