Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.     

Auroral infrasonic waves and the auroral electrojet

Using models of a line current and a band current with a parabolic intensity distribution across the width, techniques to deduce the speed, the direction of motion and the zenith crossing time of the electrojet from observations of surface magnetic perturbations are studied.From the current motions deduced by these techniques and the observed traces of the Auroral Infrasonic Waves (AIW), the following four facts are established.

• 1.(1) The time between the zenith crossing of the current and the arrival of AIW at the ground is reasonable.
• 2.(2) The direction of travel of AIW is considered to be parallel to the current drift or parallel to the current flow.
• 3.(3) The trace velocity of AIW is equal to the calculated drift velocity of the current.
• 4.(4) AIW can be produced only by a line (or a narrow band) current.

Several minutes before the AIW arrive at the ground, the existence of certain motions of westward current which satisfy the AIW emission conditions proposed by Wilson (1972) have been confirmed. However there are several cases in which a succession of two equatorward supersonic zenith crossings of westward current have induced only one AIW.     

Auroral rays: filamentation in the auroral accelerator

In situ measurements of particles, fields and optical emissions from a rocket that encountered auroral rays are reported. The measurements give insight into the production of rays, as well as the production of large fluctuations in electric fields perpendicular to the magnetic field. The fine structure and rapid variations of the electron energy flux associated with the rays are apparently produced by modulation in the degree of electron acceleration. Rays are produced when the energy flux increases in localized regions to values even higher than those normally encountered in bright auroral forms. Close and consistent similarities in the variations of the electron energy flux, the light and the electric fields suggest that the field variations were produced as a direct result of the changes in the stream of accelerated electrons. In examining possible causes of the velocity changes that produce the rays, two acceleration processes are considered; acceleration as a consequence of a potential difference between the magnetosphere and the atmosphere and acceleration by waves.     

On the prediction of auroral radio absorption on the equatorial side of the absorption zone

Observations of auroral radio absorption made in Finland over the years 1972–1983 have been compiled for use in absorption predictions. The data are presented as values of Q(1), the probability of at least 1 dB absorption being recorded by a 30 MHz riometer, and the results are compared with the predictions of an existing model regarding the latitudinal variation, the influence of solar and magnetic activity indices, and the pattern of daily variation. New formulae for basing absorption predictions on the A_p index are proposed.     

Loss cone fluxes and pitch angle diffusion at the equatorial plane during auroral radio absorption events

The flux and pitch angle distribution of energetic electrons near the loss cone have been investigated over the energy range 15–300 keV, using measurements on the geosynchronous satellite GEOS-2 at the times of auroral radio absorption events detected by riometers in Scandinavia. It is shown that conditions of strong pitch angle diffusion apply only during the most intense absorption events ( 6 dB at 30 MHz) which are relatively infrequent. During most events the loss cone is partially depleted, with the degree of depletion increasing as the absorption becomes weaker. The variation of the pitch angle diffusion coefficient with the observed radio absorption is estimated. A consequence of loss cone depletion is a tendency to overestimate the smaller events when computing the radio absorption from flux measurements in the 0°–5° range of detector pointing angles. An empirical law is derived which enables the computation of radio absorption consistent with measurements. D-region recombination laws are discussed and limits are set on the height profile of the effective recombination coefficient.     

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.     

Oscillations in D-region absorption at periods of one to two months

One to two month oscillations in D-region absorption are found in seven years of daily ƒ-min data from low latitude stations at Singapore (1°N, 104°E) and Rarotonga (21°S, 160°W). Coherency (cross-spectral) analyses reveal that solar flux variations account for much of the ƒ-min variance at these periods. Over the range of periods from 10 to 200 days, statistically significant linear correlation is found between the ƒ-min time series and contemporaneous 10.7cm solar flux measurements at periods of 16–19 days, the 26–29 day solar rotation band, and a broad band covering 43–80 day periods.     

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.     

Bromine in teeth and bone as an indicator of marine diet

Nitrogen isotope ratios in tissues are often used to aid in the reconstruction of the marine component of diets in past populations. Elevated δ¹⁵N values normally found with high trophic level marine resource consumption can, however, also be mimicked by physiological conditions (breastfeeding, pathological states affecting nitrogen balance), climate (aridity), and anthropogenic environments (manuring). This paper presents a pilot study testing whether bromine concentrations can provide a tool for teasing apart possible causes of variation in nitrogen isotope ratios. Blind analyses of bromine in archaeological samples correctly identified individuals derived from coastal vs. inland regions and high vs. low marine consumption time periods. Thus, bromine can provide an important supplement to other isotopic analyses of diet.     

Equatorial HF radio wave absorption measurements and the IRI

A modification to the International Reference Ionosphere (IRI-79) electron density profile between altitudes of about 80 km and the peak of the E-region is proposed for compatibility with equatorial HF absorption measurements for Thumba, India. This is tested against independent absorption data for Colombo, Sri Lanka, and Ibadan, Nigeria.     

Predictability of ionospheric variations for quiet and disturbed conditions

Empirical models can predict the average monthly behaviour of the ionosphere and allow for day-to-day variability. Significant departures from these predicted conditions can be forecast using a range of solar, magnetic and ionospheric observations. The main improvements needed in the prediction models is that they should be made more precise, by introducing variable prediction limits, and should be improved by harnessing the impressive power of the current physical models so that ionospheric data can be assimilated in near real time. This strategic objective will lead to ionospheric weather forecasting.     

General formulae for absorption of radio waves in the ionosphere

The relationship between group path, phase path and absorption of radio waves is discussed and new approximations relating these quantities are presented. The new relationships include dependence on the angle between the wave normal and magnetic field directions and so, in contrast to other approximations they are not restricted to quasi-longitudinal or quasi-transverse situations. For deviative absorption it is found that the ordinary mode quasi-transverse approximation introduces errors of less than 5% except in the case of purely longitudinal propagation. For non-deviative absorption, use of the quasi-longitudinal approximation can introduce significant errors which particularly affect the determination of latitudinal variations in absorption.     

The relationship between electron temperature and electron density under quiet conditions in the auroral zone

A theoretical model is described which predicts electron temperature in the day-time F-region above EISCAT on geomagnetically quiet days, given the observed values of electron concentration, ion temperature and heat conduction, the daily average of solar radiation at 10.7cm and the MSIS-86 model of the neutral atmosphere. The values predicted by the model agree very closely with the observed temperatures, both for average conditions and for individual days. On two occasions the onset of a geomagnetic disturbance after a period of quiet conditions was accompanied by a growing divergence between the predicted and observed values, which corresponds to an additional source of electron heating such as would be provided by low energy particle precipitation.     

The seasonal variation of radio wave absorption in Europe

The seasonal variation of radio wave absorption of three mid-latitudes is calculated using an atmospheric model and compared to measurements of two winters and one summer. From the correlations of the absorption patterns of three A3 paths, velocities of these patterns are derived and the relevance of such velocities to enhanced absorption is examined.     

Recent advances in the study of the D-region winter anomaly

D-region work as concerns the winter anomaly of electron density is screened for the period 1974–78. The following topics are dealt with: electron density distribution; ion production processes; neutral atmosphere effects by trace constituents, temperature and transports; ion composition and chemistry; coupling between atmospheric/ionospheric layers. A summary is given which might be read first, and which leads to some aspects of future work.     

Rock art as an archaeological and social indicator: The neolithisation of the Iberian Peninsula

Rock art has been regarded as a second class archaeological data source. In this paper we use Levantine rock art as a case study, to show how this situation can be reversed. This rock art, found along the Mediterranean region of the Iberian Peninsula, has been considered to be typically Mesolithic due to its distinctive hunting scenes. A review of certain archaeological indicators provides new arguments about its chronology, its socio-cultural attribution, and its significance. We first deal with the chronological issue, presenting the evidence that led to the recent consideration of the Levantine style as Early Neolithic, in synchrony with two other rock art styles (Schematic and Macroschematic). We will further propose that rock art itself is a central and independent source of information to explore the historical context of the Neolithisation of Mediterranean Iberia. Its wide geographical distribution allows us to study different and complementary territories as a single entity. This contrasts with the limitations posed by the study of settlements, whose differences, at a regional scale, are usually interpreted in terms of cultural variability. If we consider rock art as a key aspect in the constitution of social landscapes, it must not be treated just as an ideological by-product of the Neolithisation, but as a key factor in understanding this historical process.     

Auroral Research and the Character of Astronomy in Enlightenment Sweden

Abstract

The observation of the magnetic effects of the aurora borealis by Olof Hiorter in the 1740s was hailed by Swedish scientists as one of the major discoveries in contemporary research. This article investigates the political and academic context of the discovery, focusing on the astronomical ideals promoted by Hiorter. He used the discovery in order to buttress the importance of his own scientific character – technically competent, hard-working and research-oriented. He contrasted this ideal with the character of an ordinary university professor who was described as more of a bureaucrat, interested in science only as long as it could boost his reputation, and not averse from stealing results of technically more competent underlings. Hiorter's opponents at the university decried his lack of theory and devalued the importance of technical skill. This conflict is discussed in the context of ideals concerning cultural, political and economic values of science and scientists.     

Modelling of the electron density profile in the lowest part of ionosphere D-region on the basis of radiowave absorption data: 2. Seasonal variations

A new model of the lowest part of the D-region is obtained by a trial-and-error inversion method. Its basic feature is a step-like transition between 55 and 70 km which is not depicted in most ionospheric models. The seasonal differences of this are considered to be quite important: the bottom of the ionsphere is found to be lower in summer and spring, the gradient of the profile below the CR-layer is stronger in winter, and a well defined ‘valley’ exists around 70 km in spring. By simulating the ionospheric response to a, solar flare (SID-effect) in summer and in winter, an attempt was made to verify the obtained seasonal peculiarities of the quiet ionosphere.     

Caesium-137 as an indicator of geomorphic processes in a drainage basin system

Caesium-137 from fallout from nuclear weapons tests is adsorbed on fine sediments and becomes an effective tracer. It is hypothesised that within a drainage basin, sites undergoing little or no erosion accumulate ¹³⁷ Cs in their upper layers; cultivated soils will have ¹³⁷ Cs uniformly distributed within the cultivated layer; eroded soils, cultivated or not, will have relatively less ^l37 Cs, depending on the severity of erosion. Accumulated sediments will have characteristic ¹³⁷ Cs profiles reflecting temporal fallout variations and sedimentation history. This hypothetical model is largely confirmed by results from Maluna Creek basin, where erosion and accumulation of sediments has taken place. Soils under viticulture have about one third the ¹³⁷ Cs content of soils with grass cover, indicating more severe erosion under cultivation. Caesium-137 profiles in alluvial fan and flood plain deposits correlate with sediment layers and known cultivation history.