High-latitude D- and E-region investigations using imaging riometer observations

The imaging riometer technique has proved a valuable tool for investigations of a variety of ionspheric and magnetospheric disturbances. To illustrate the potential of the new technique, this presentation will discuss the observations at cusp latitudes (approx. 75° inv. lat.) of PCA events, substorms and poleward progressing absorption features for which imaging riometer observations have provided important new information. For the PCA studies an exceptionally nice data set for the sunrise/sunset asymmetry is presented. It is argued that the asymmetry is so modest that temperature effects offer a simple explanation. For the substorm generation an augmented current wedge model is suggested on the basis of imaging riometer observations. Finally, imaging riometer observations of IMF-dependent poleward progressing absorption events are presented. This type of disturbance is considered the convecting ionospheric footprint of the B_Y component of the interplanetary magnetic field. A typical example is examined.     

Day-to-day variability of riometer absorption

Statistical variations of absorption data measured with a riometer on different days at a given hour are examined. Parameters of the cumulative amplitude-probability distributions are selected and the approximate relationships between these that have been found in earlier investigations are reviewed. A new mean relationship is derived from a study of an exterrded data set and its applicability to morphological mapping is considered.     

The day to night absorption ratio in auroral and subauroral zone riometer measurements during auroral absorption

The day to night ratio of auroral absorption has been studied using data from auroral and subauroral latitudes and by application of different kinds of statistical analyses. Ratios between 0.5 and 3.0 are obtained, depending on the criteria applied to the selection of data. Previous studies obtained similar ratios, but reached different conclusions about the effective solar control of auroral absorption. It is concluded here that evidence of solar control of the day to night ratio of auroral absorption, or the lack thereof, cannot be extracted by these statistical analyses.     

Investigations of ionospheric radio wave absorption processes using imaging riometer techniques

The recent development of imaging riometer techniques has enabled a range of new, interesting observations of the complex dynamics of auroral and polar radio wave absorption events. These events mostly relate to the precipitation of energetic particles, creating enhanced ionization in the D-region. However, E-region heating by large electric fields and F-region electron density enhancements may also—at times—be responsible for observable absorption effects. Observations of ionospheric radio wave absorption processes using imaging riometer techniques may provide detailed characteristics of the spatial and temporal structures of small-scale disturbance events, velocity vectors for drifting features and frequency spectra for modulated events. This presentation will give a brief summary of imaging riometer techniques and a survey of existing and planned imaging riometer installations. Furthermore, the characteristics of frequently occurring absorption event types are summarized. In a companion paper imaging riometer observations are presented for some selected absorption events.     

Case study of modelled and measured D-rmregion plasma densities

Two of the many sounding rockets launched as part of the MAC/EPSILON campaign are particularly suited to test the validity or quality of D-region models under night-time conditions. The measurements covered atomic oxygen, neutral temperatures and fluxes of charged particles. With these parameters as inputs, one is in the position to assess their relevance for the plasma parameters ion composition, transition heights of clusters and negative ions, and total plasma density. The two IOMAS payloads are believed to be the only night-time flights with associated measurements of atomic oxygen extending below the ledge at about 80 km. The relevance of atomic oxygen for the ion chemistry of the lower D-region could be experimentally demonstrated.     

Comparisons of EISCAT and dynasonde ionospheric measurements: simple to moderately structured plasma densities

Plasma densities obtained from EISCAT's UHF incoherent scatter system are compared with profiles inverted from the digital ionograms of a co-located dynasonde. Excellent agreement is found for the bottomside ionosphere when conditions of horizontal stratification and classical photochemical equilibrium prevail. However, departures from such conditions are frequent and intense at Tromsø. Compensating errors of EISCAT calibration and long pulse convolution are resolved by analysis of power profile data. Good agreement is recovered for tilted and more complex ionospheric structure, provided that accurate echo location data are used to confirm a common volume. Monotonic inversion of the ionograms is inadequate. Dynasonde recordings are analysed to show characteristic structure in vertical and horizontal planes as a context for EISCAT measurements along a fixed (magnetic field) direction. Incoherent scatter and modern total reflection sounding, used together and coordinated in one consistent data reduction system, could produce a far more powerful ionospheric diagnostic program than either technique seems capable of providing alone.     

High-latitude tidal behavior in the mesosphere and lower thermosphere

The high-latitude structure of the mean winds and tides is described in this paper using climatologies prepared from radar data during the Atmospheric Tides Middle Atmosphere Program. The monthly evolution of the amplitude and phase of the tides is discussed. Comparison between the southern and northern hemispheres indicate that the diurnal tide is stronger in the southern hemisphere and that the antisymmetric diurnal tidal modes are dominant. The semidiurnal tide is larger than the diurnal tide. The vertical wavelength structure is significantly different between the southern and northern hemisphere. Comparisons with recent tidal models show several discrepancies.     

Planetary waves in the strato- and mesosphere during the Western European Winter Anomaly Campaign 1975/76 and their relation to ionospheric absorption

The planetary-scale circulation features have been analysed for the period 28 December 1975 to 11 February 1976 by means of radiosonde, rocketsonde, and satellite data (infrared radiances). Geostrophic horizontal winds, vertical motions, and amplitudes and phases of the planetary waves are provided from the stratosphere to the mesosphere (from 20 to 80 km). The development in time and space of the planetary-scale waves is discussed and it is shown how the ionospheric absorption over Spain and Germany during the Winter Anomaly Campaign is connected with these waves which can be traced from the stratosphere through the whole mesosphere.     

Validity of IRI electron density profiles in relation to vertical incidence absorption measurements

The observed discrepancies between A1 absorption meaurements and numerical estimation of the same using IRI electron density profiles are attributed to the assumption made in the Sen-Wyleer generalized magneto-ionic theory that the momentum transfer collision frequency of electrons with neutrals is proportional to the square of the electron thermal speed. Based on Budden's (1985) suggestion that, in the lower thermosphere and mesosphere, the momentum transfer collision frequency is proportional to the electron thermal speed, a generalized magneto-ionic theory has been outlined. The new theory brings experimental measurements of A1 absorption closer to the theoretical deductions based on IRI-90 electron density profiles.     

Morphology of low-frequency waves in the solar wind and their relation to ground pulsations

Three classes of low frequency waves (period range 20–80 s) were identified using data from the UCLA fluxgate magnetometer experiment on board the ISEE 2 spacecraft. These are continuous pulsations similar in type to Pc 3, band-limited oscillations distinguished by mixed period fluctuations, and relatively isolated wave bundles. The waves were preferentially observed when the interplanetary magnetic field (IMF) direction was sunward and were most common when the cone angle, i.e. the angle between IMF and the Sun-Earth line (θ_xb) was often between 15° and 45°. Their frequency is proportional to the IMF magnitude.Comparison between the waves observed on board the ISEE 2 spacecraft and the Pc 3–4 recorded simultaneously at a mid-latitude ground station, Oulu (L = 4.5), showed that similarity of spectra of the waves in the spacecraft and on the ground was very rare and that correspondence between the events in space and on the ground was extremely low.     

Experimental studies of daytime LF radio propagation and their implications for D-region electron densities

Measurements are presented of interference phenomena in amplitude and phase of VLF and LF signals along propagation paths from central England to the Norwegian Sea. The data are interpreted by means of the ‘wave-hop’ propagation theory, incorporating full wave evaluation of ionospheric reflection coefficients with realistic D-region models. No published electron density profiles are found which completely satisfy the experimental data, but modified profiles are presented which provide a better fit to the observations.     

Pi(c) magnetic pulsations: their relation to ionospheric currents

The association of the phase of the H and D components of the Pi(c) pulsations with the phase of the broadscale H component magnetic bays confirms that Pi(c) pulsations result from auroral electron precipitation induced conductivity enhancements of existing current systems. Statistically determined relationships between the time delays and phases of the H and D components of magnetic Pi(c) pulsations with respect to the optical pulsations are used to infer a delay between the E-region Hall and Pedersen current fluctuations associated with the pulsating electron fluxes. Theoretical modelling of an auroral pulsation patch, as per Oguti and Hayashi, is used to show that the polarization of the Pi(c) pulsations is controlled principally by the delay between the Hall and Pedersen currents and the direction of the background E-region electric field.     

Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes

Standard riometer data from a southern auroral station were compared with ionograms obtained at five stations positioned from sub-auroral to equatorial latitudes. The rapid onset in riometer absorption, during intense substorm activities in an equinoctial period, was associated with a sequential propagation of ionospheric disturbances deduced from the F-region parameters h′F and range spread-F. The time shift between absorption maxima and extrapolated commencement times of the disturbances was consistent with the presence of large-scale travelling ionospheric disturbances (TIDs), propagating equatorwards with velocities lying typically in the range 600–900 m s⁻¹, and with a median velocity of 720 m s⁻¹. It is suggested that the onset of TIDs is associated with high-energy particle precipitation, manifested by the occurrence of auroral absorption events. Similarity of absorption increases at the southern and northern conjugate points, found from a previous riometer study, would indicate that large-scale TIDs are simultaneously generated in both hemispheres.     

An American controversy about the localization of cutaneous sensory regions and their relation to motor regions

Three American neurologists (C. K. Mills, C. L. Dana, and M. A. Starr) explored the anatomical limits of the motor and tactile systems in the brain from 1884 to 1895. Their papers and critiques of one another show contemporary knowledge, limits of their thinking, and difficulties deciding between alternatives. The issue for them was whether there were separate sensory and motor regions or whether there was a combined sensory-motor region. They based their localization arguments on clinical and laboratory findings and on the conclusions of H. Munk and D. Ferrier. There is a discussion about why differences were unresolved.     

Physicochemical characterization and provenance of colouring materials from Akrotiri-Thera in relation to their archaeological context and application

An assemblage of approximately 500 colouring materials and objects related to their use, found in different contexts and forms at Akrotiri, Thera, was recently investigated. The items date to the Early, Middle and Late Cycladic Bronze Age (c. 3000?C1600 bc) and include materials or pigments found in their natural state, in specifically made forms or inside vessels. Some of the contexts do not provide any indication of the pigments?? use, storage, trade or application. Among the investigated materials, which include red and yellow ochres and blue riebeckite, are those that can be associated with pigments that were widely used in the past in wall paintings or to decorate pottery. There is also evidence for the selective use of purple pigments, which were derived from shellfish, in wall paintings. However, there are other raw colouring materials, such as lead pigments that have not been previously known to have been used in wall paintings or in any other applications during the prehistoric period in the Aegean. A thorough macro- and microscopic visual examination of the quality and morphology of these items enabled the identification of physical features (colour, homogeneity, grain size and shape) that indicate their nature or degree of processing. To identify the mineral composition of the pigments and to investigate their provenance from a geological perspective, quantitative X-ray diffraction (XRD) analysis was conducted. For those samples requiring additional information on their composition and for the refining or confirming of the XRD data, scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX), micro-Raman and Fourier transform infrared (FT-IR) spectroscopies were performed. This work is part of a larger project, supported by Institute for Aegean Prehistory, that aims to clarify major archaeological queries that are not restricted to the identification of the nature, composition and provenance of the colouring materials but extend to aspects of their selection, preparation and application techniques used during the Early to the Late Bronze Age.     

Winds,wind-shears and plasma densities during the initial phase of a magnetic storm from equatorial latitudes

Results of a sodium vapour release experiment carried out from SHAR (India), an equatorial rocket launching station, immediately after (⩽ 2 h) a storm sudden commencement (SSC) during the initial phase of a magnetic storm, followed by electron density measurements are presented. Many of the relevant atmospheric parameters, namely, neutral winds and their altitude variation, the magnitude of the shears in them, the neutral temperature with altitude by spectroscopic methods, diffusion measurements on the released trail, clues on the turbopause level and the electron densities including the structures (irregularities) in them were obtained. The results of the temperature measurements carried out independently on the sodium trail by means of a ground-based Fabry-Perot spectrometer, operating on the sodium D 1 line, resonantly scattered by the trail have already been reported by us (Ranjan Guptaet al., 1986). In this paper the effects of the excess temperature reported earlier and the rest of the measured parameters on the electron density profiles are evaluated using models and compared with the measurements.The formation of sharp layers of ionization have been explained on the basis of the electro-dynamical processes associated with the wind shears at a location, close to the edge of the equatorial electrojet belt. The significance of the changes in the neutral composition due to the enhanced neutral temperature and the low turbopause level, in increasing the base-level plasma densities by a factor of 3–5 are demonstrated and the possible role of plasma dynamics discussed.     

Instabilities in pitch angle diffusion and their possible relation to a 3 Hz modulation in pulsating aurora

The Coroniti-Kennel theory of electron precipitation pulsations has been reviewed, and an additional restriction of differential relaxation time has been introduced, making the model unstable under most conditions. The solution to the model was found to be nonlinear, and a very crude numerical solution has been attempted. Although suffering from several nonrealistic approximations, this model has confirmed the idea that a 3 Hz modulation in pulsating auroral forms is related to short bursts of Coroniti-Kennel type strong pitch angle diffusion. Strong pitch angle diffusion will fill the loss cone and is therefore not consistent with a self-excited steady state diffusion, i.e. it must be of a transient nature. The repetition frequency of such bursts depends on the size of the diffusion region along the magnetic field line combined with the parallel velocity of precipitating particles. Estimates based on reasonable diffusion region and velocity give a modulation frequency comparable to observations. The suggested mechanism is consistent with self-excited pitch angle diffusion only; it excludes the possibility that the diffusion is parasitic.The numerical model also indicated that the abnormal pitch angle distribution generated as a result of a magnetic disturbance might survive for hundreds of seconds and thus acts as a ‘memory’ on the fieldline, making it more susceptible for further disturbances.     

A comparison of plasma densities by EISCAT and the Dynasonde from auroral altitudes: evidence of intense structure

Under conditions of moderately-energetic particle precipitation typical of the equatorward side of the auroral oval, plasma densities obtained from routine analysis of EISCAT Common Program data are often a factor 2 to 5 smaller than those suggested by co-located digital ionograms. We consider the reasons for this disagreement, and in particular we reject the implications of diffractive and multiplyrefractive scatter as alternatives to the usual plasma-frequency interpretation of ionogram echoes. We examine the effects of the (5 min and shorter) temporal averaging applied to the EISCAT data and conclude that together with the evidently small size (perhaps as little as 20 km) and high velocity of these structures, this accounts for much, if not all, of the disagreement. We point out the significance of the higher plasma densities in the 100–150 km height range for estimates of Joule and particle heating.