The investigation of long-distance HF propagation on the basis of a chirp sounder

Experimental data on round-the-world HF radio signals near the terminator are given. The critical frequency of the ionospheric waveguide is found to be F_c ∼ 16–17 MHz. At frequencies F < F_c the group delay has a negative dispersion $\text{τ}\text{dot}{}_{\mathrm{<mtext>g</mtext>}}\text{= ∂τ/∂F ⋍ −100}$ μs/MHz and $\text{τ}\text{dot}{}_{\mathrm{<mtext>g</mtext>}}\text{⋍ 80}$ μs/MHz for frequencies f > f_c. Ray-tracing calculations are carried out. It is found that the low frequency branch of round-the-world signals (F < F_c) is formed mainly by waveguide modes and the high frequency branch (F > F_c) by 0 ricochet and hop modes.Experiments on waveguide modes escaping from the ionospheric channel due to field-aligned scattering by artificial ionospheric turbulence are carried out. The conditions for trapping of radio waves in the ionospheric waveguide are investigated. It is shown that if the gradient of the critical frequency F₀F2 is less than minus 2 × 10⁻² MHz/100 km radio wave trapping takes place in the ionospheric waveguide at frequencies exceeding by 1–2 MHz the maximum observed frequency of the hop mode. The frequency time characteristics of the mode and the geophysical conditions for the effective control of radio waves escaping from the waveguide are defined.     

The height distribution of radio meteors: observations at 6 MHz

As part of a program aimed at deriving the true influx to the Earth of small meteoroids we have measured the height distribution of radio meteors to limiting magnitude + 6 (mass ~ 1.0 mg) at a frequency of 6 MHz for the combined Daytime Arietid and Zeta Perseid showers, and also for the Eta Aquarids; these showers have widely different velocities but no substantial dependence upon velocity is apparent. The distributions peak at ~ 105 km, 10 km above the peak found using conventional VHP meteor radars and in line with observations previously made by us at 2 MHz. Instrumental limitations confine the span of heights to 84 <h <116 km, but it is notable that within this range the 6 MHz height distributions appear to be symmetric, with a swift drop-off above 105 km; this contrasts with the asymmetric 2 MHz distribution which showed a slow drop-off with many meteors to 140 km. The origin of this difference is probably due to diffusion and the finite-velocity effect, which will be considered in a subsequent paper.     

The use of dental crown heights for constructing age profiles of red deer and similar species in archaeological samples

The analysis of dental crown heights in a sample of known-age American Red deer (known in America as “elk”) has permitted the derivation of formulae for the prediction of age from crown height. The ages predicted from crown heights closely approximate true ages, when both predicted and true ages are grouped into broad, but still useful age classes. At least for the Red deer or “elk” dP₄, M₂ and M₃, age prediction formulae very similar to the empirically derived ones may be constructed by making certain hypothetical assumptions about the nature and rate of crown attrition. This suggests that these assumptions may be used to construct age prediction formulae in situations where an empirical relationship between age and crown height has not been or cannot be established. With certain provisos noted in the text, the age prediction formulae that result will probably provide reasonably reliable age profiles for the numerous ungulate species whose teeth are similar in structure to those of Red deer or “elk” and at least as hypsodont.     

An inversion procedure for obtaining collision frequency profiles from swept-frequency absorption measurements

A method is presented which inverts swept-frequency Al absorption data to obtain collision frequency profiles in the E- and F-regions of the ionosphere. The method gives consistent results from successive sets of measurements and the profiles obtained are consistent with other measurements of collision frequency. Accounting for D-region absorption is a difficulty affecting the accuracy of the collision frequencies obtained at the lowest heights, but model simulations show that values at higher heights are not affected seriously. The technique can be used to obtain results for the F1-region for which there are very few previous measurements.Comparison with theoretical calculations of collision frequency show agreement in the form of the altitude variation. That is, there is a rapid decrease with altitude through the E-region which becomes much less in the F-region so that the collision frequency becomes almost constant with height. This change is caused by electron-ion collisions becoming more dominant than electron—neutral collisions. However, consistent with other observers, we find a major discrepancy between the magnitude of the experimental and theoretical values. If the electron and ion temperatures are assumed equal, the experimental values are approximately five times greater. The discrepancy increases if Te >Ti in the theoretical calculations.     

Large simultaneous disturbances (LSDs) in the Antarctic ionosphere

A high frequency radio Doppler experiment was deployed in the Antarctic Peninsula region, centred on Argentine Islands (65°15′S, 64°16′W; L = 2.3), to investigate the morphology and sources of ionospheric disturbances. The experiment consisted of a three-transmitter dual frequency network which permits horizontal and vertical propagation velocities to be estimated over a north-south baseline of 200 km and an east-west baseline of 100 km.A new class of ionospheric disturbance has been observed, in the period range 10 min−1 h. These disturbances are characterised by unusually good correlation between perturbations on all available Doppler signals, but are apparently non—propagating and occur simultaneously at each reflection point. Several of these events display large (2 Hz at about 5 MHz transmitted frequency) Doppler shifts, thus we have labelled them Large Simultaneous Disturbances (LSD_s).Criteria for identification of LSD_s are established and the analysis of one event is described in detail. The occurrence statistics of the LSD_s are presented, including their seasonal and diurnal distributions.There is no clear general relationship between LSD_s and local geomagnetic field perturbations. However, examination of the magnetic indices AE and I_RC indicates that there is a loose association between the occurrence and amplitude of LSD_s and magnetic activity.Several possible mechanisms for the generation of LSD_s at middle latitudes are reviewed. The most likely explanation is that high latitude electric fields penetrate to magnetic middle latitudes and drive the ionospheric plasma via the E × B drift.     

Modelling of transequatorial propagation of HF radio waves

In the geometrical optics approximation, a synthesis oblique ionogram of ionospheric and magnetospheric HF radio wave signals propagating between magnetic conjugate points has been carried out. The magnetospheric HF propagation is considered for a model of the waveguide formed by field-aligned irregularities with depleted electron density. The characteristic peculiarities of the magnetospheric mode have been determined: (i) strong disperion of the group delay with a frequency at 14–18 MHz, from − 1.4 to 0.6 ms/MHz for magnetically conjugate points at geomagnetic latitudes φ = 30°, 40° and 50°, respectively, (ii) spreading ∼ 1–2 ms, and (iii) a possibility of propagation between magnetic conjugates points at moderately low geomagnetic latitudes φ₀ ∼ 30–40° at frequencies exceeding 1.5 times the maximum usable frequency (MUF) of multi-hop ionospheric propagation.     

Power spectra of VHF intensity scintillations from F2- and E-region ionospheric irregularities

An experiment is described for the routine study of scintillations and ionospheric irregularities at high-latitudes using NNSS satellites with additional coordinated observations by means of the EISCAT ionospheric radar facility. Early results, obtained during the development phase of the experiment, are presented of the power spectra of intensity fluctuations at 150 MHz observed at the equatorwards edge of the high-latitude irregularity zone. The spectra of 165 samples of night-time scintillation recorded during October 1982 to May 1983 show a spectral index with a mean value of −3.58 and a steepening of the spectral slope with increasing S₄. Some examples of scintillation arising from irregularities at E-layer height show spectral indices of magnitude generally smaller than for F-region cases. A few spectra have been found with a clear break in spectral slope at around 10 Hz, suggesting two regimes for irregularities of different scale sizes.     

Multi-instrument observations of mesospheric motions over Arecibo: comparisons and interpretations

The MF/HF partial-reflection technique of observing the mesosphere and lower thermosphere has been employed for more than two decades to measure motions, but there has never been complete agreement as to what motions were being detected. This paper reports on observations made during a major international campaign—AIDA '89—that was initiated with the objective of resolving this question.The partial-reflection system employed was an Imaging Doppler Interferometer operating at 3.175 MHz, but it stands here as a prototype for all MF/HF partial-reflection radar systems: its raw data were analyzed both in its own basic mode, derived on the assumption that it sees wind-borne multiple scattering centers and in modes adopted by other interferometric and ‘spaced antenna’ systems. The motions thus revealed are compared here with those found by what we consider to be more certain measurers of winds: an incoherent-scatteer radar at heights of 65–95 km, a meteor-wind radar at heights of 80–100 km and a Fabry-Perot interferometer measuring 0(¹S) emissions near a height of 97 km.Comparisons of the different sets of observations oblige us to conclude that

• 1.(1) MF/HF partial-reflection systems may be expected to give a good representation of ambient winds up to a height of about 80 km;
• 2.(2) they fail to give a consistently reliable measurement of the ambient winds above a height of about 80 km
• 3.(3) they yield, at the greater heights, what appears in our data to be some convolution of the horizontal phase velocities of atmospheric gravity waves, with the wave spectrum having been modified by passage through the underlying wind system and containing, on occasion, locally generated Kelvin-Helmholtz waves; and
• 4.(4) when the underlying winds change, the local wave spectrum will change in response and, in MF/HF partial-reflection measurements, will give the appearance of a changing local wind: if the underlying winds undergo tidal changes, the wave spectrum will undergo tide-like changes that will masquerade as true tidal winds.

These results are, of course, limited to a single site over a limited period of observation. Nevertheless, taken at face value they suggest that current methods of data reduction are inappropriate for partial-reflection velocities at heights above 80 km and that new methods of data reduction, perhaps extending certain older methods that have been applied successfully in the past to total-reflection measurements, should be employed in their place if the full potential of the MF/HF partial-reflecton technique is to be realized.     

The height distribution of radio meteors: observations at 2 MHz

Conventional meteor radars, operating at wavelengths of around 5–15 m, are unable to detect high-altitude meteors due to the wavelength-dependent echo ceiling. It is suggested that the ‘missing mass’ in the 10⁻⁶–10⁻² g range of interplanetary material is in fact a high-velocity component which is normally undetected since it ablates at high altitude. This contention is supported by previous work. In this paper we describe measurements of the heights of radio meteors (limiting magnitude about +7) at a wavelength of 150 m (frequency 2 MHz), for which the echo ceiling is above 140 km. The resultant true height distribution is found to peak at ~ 104 km, about 10 km above the peak found by conventional meteor radars. The majority of meteors are detected at or above this peak, and substantial numbers are seen right up to 140 km. It is therefore concluded that the ‘missing mass’, comprising the vast majority of the meteoric input to the atmosphere, ablates well above 100 km.     

A simplified approach to internal gravity wave damping by viscous dissipation

Internal gravity wave perturbation velocity subject to constant dynamic viscosity and constant kinematic viscosity are approximately derived based on an energy conservation principle. When the dynamic viscosity is assumed to be a constant, the velocity at any height relative to the velocity at the saturation height, Z_sat, is found to be solely dependent on the number of scale heights measured from Z_sat. Gravity wave energy dissipation due to constant dynamic viscosity primarily occurs from one scale height below to one scale height above the saturation height. When the kinematic viscosity is assumed to be a constant, the perturbation amplitude either increases or decreases monotonically with height depending on whether the energy dissipation rate due, to viscosity, can offset the effect of the decreasing atmospheric density with increasing height. The derivations are made simple by assuming that the non-dissipative dispersion relation is applicable to the dissipative situation. The condition for the assumption to be approximately valid is also given.     

Non-homogeneous structure of neutral winds in the lower thermosphere during the MAC/EPSILON campaign

The paper presents the results of an investigation of the height variations of dynamic processes in the 80–110 km height region, carried out in Kazan, U.S.S.R. (56°N, 49°E) by the radiometeor method during the MAC/EPSILON campaign. Experimental results show that the largest values of vertical wind gradients, as well as zonal and meridional temperature gradients can be found at heights of ~ 83 km. At heights of 80 ⩽ h ⩽ 100 km, we can observe energy absorption of IGW and tides which are the major sources of turbulent energy in the above-mentioned height interval. Using the effects of IGW energy absorption, values of the turbulent eddy diffusion coefficient K_l ranging from 1600 to 4400 m²/s were calculated for October 1987. The energy dissipation rate ϵ was estimated to be from 0.1 to 0.4 W/kg.     

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.     

Phase velocity studies of 34-cm E-region irregularities observed at Millstone Hill

Phase velocity observations at E-region heights made with the Millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0° magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For a 34cm volume backscatter cross-section, σ_v, less than ∼5 × 10⁻¹³ m⁻¹, the phase speed is at or below the usual ion acoustic speed in the E-region (350m/s), and increases only slowly with the observed backscattered power amplitude (∼50 m/s per 10dB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750 m/s at times, and increases more rapidly with backscattered power (∼200 m/s per 10dB). Phase velocity/time maps observed over a 3° span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed magnetospheric pulsations.     

The isobaric F2-layer

Cyclic diagrams, obtained by plotting the daily variation of the ionospheric electron density NmF2 against the height hmF2, are drawn for typical conditions at Slough (52°N) and Watheroo (30°S). Using the MSIS86 thermospheric model to relate the heights hmF2 to values of atmospheric pressure, the F2-peak is found to lie at nearly the same pressure-level at any given local time, over a wide range of geophysical conditions (season, solar cycle, magnetic disturbance). As local time varies, the pressure level corresponding to hmF2 varies in a way that is mainly determined by the local time variation of the thermospheric winds. This is verified for noon and midnight, using the MSIS86 model to compute the winds. The noon values of peak electron density (NmF2) are fairly consistent with theory (using values of solar ionizing flux as quoted in the literature), but with some discrepancies—particularly at sunspot maximum—that are probably due to uncertainties in chemical composition, or to the effects of vibrational excitation of molecular nitrogen. Overall, the analysis shows a remarkable consistency between ionospheric theory, the data and the MSIS model.     

Distinguishing between calcites formed by different mechanisms using infrared spectrometry: archaeological applications

Infrared spectrometry is a well-established method for the identification of minerals. Due to its simplicity and the short time required to obtain a result, it can be practiced on-site during excavation using portable infrared spectrometers. However, the identification of a mineral may not be sufficient. For example, a lime plaster floor and a crushed chalk surface have a similar appearance and are composed of the same mineral – calcite. Here we exploit differences in the infrared spectra of geogenic, biogenic and pyrogenic calcites for the identification of each calcite type. The infrared calcite spectrum has three characteristic peaks in the region of 400–4000 cm⁻¹, designated ν₂, ν₃, and ν₄. When a calcite sample is ground, as part of the measurement preparation procedure, some grinding dependent changes will be revealed in the infrared spectrum. With additional grinding, the ν₃ peak narrows and the heights of the ν₂ and ν₄ peaks decrease, when both are normalized to the ν₃ height. By plotting the normalized heights of the ν₂versus the ν₄ of several grindings of the same sample, a characteristic trend line is formed for each calcite type. The trend lines of geogenic calcites have the shallowest slopes and highest ν₄ values when compared to pyrogenic calcites, which can be further divided to ash and plaster/mortar samples. This method can assist in distinguishing between the various calcites, and provide insights into homogeneity and preservation state of the calcitic materials in question.     

EISCAT observations of tidal modes in the lower thermosphere

EISCAT has made regular measurements of plasma velocity at heights between 101 and 133 km in the E-region and at 279 km in the F-region as part of the Common Programme CP1. Correcting for the effect of the electric field as determined in the E-region, it is possible to estimate the neutral wind velocity in the E-region for a number of days in the period 1985–1987 when magnetic conditions were relatively quiet. These velocities display diurnal and semi-diurnal tidal oscillations. The diurnal tide varies considerably from day to day in both amplitude and phase. The semi-diurnal tide also varies in amplitude but displays a fairly consistent phase at each height and the variation of phase with height below 110 km indicates a dominant (2,4) mode. Above 120 km the variation of phase with height is slower which suggests that at these heights the (2, 4) mode is attenuated and the (2, 2) mode is more important. The results agree well with previous measurements at high latitude.     

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.     

Asymmetric incoherent scatter spectra from a relaxation collision model

A relaxation collision model for ion flow through a stationary neutral gas has been used to obtain ion velocity distributions and line-of-sight incoherent scatter spectra for a range of values of collision frequency and electric field. The mean velocity of the line-of-sight ion velocity distribution has been compared with the Doppler shift of the corresponding spectra. The latter is not always a good estimate of the former, because the ion velocity distribution in the plane perpendicular to the magnetic field direction is highly distorted. For ion-neutral collision frequency to ion gyrofrequency ratios 0.1 ≤V_i,/Ω_i≤0.5, the greatest inaccuracies in mean velocity estimation take place along the electric field direction, while for 0.5 ≤ V_i/Ω_i ≤ 1.0 the greatest inaccuracies occur across the electric field direction. These inaccuracies would be reduced but not eliminated in a more realistic model. At F-region altitudes, other processes must be invoked to explain observed asymmetrical spectra, but the comparison of mean line-of-sight ion velocity and spectrum Doppler shift may still have relevance.     

A model for the electron temperature variation along geomagnetic field lines and its effect on electron density profiles and VLF paths

A realistic model for the temperature variation along geomagnetic field lines is described. For high altitudes (>1500 km) the temperature is taken to increase as the nth power of radial distance (n−2), giving temperatures consistent with those measured in situ by high altitude satellites. For realistic temperatures at low altitude an extra term is included. The temperature gradient along the field line is then 0.9–1.6° km⁻¹ during the day and 0.5–0.7° km⁻¹ during the night at 1000 km, reducing to about half these values at 2000 km, for the latitude range 35–50°. This is consistent with calculations made from nearly simultaneous satellite measurements at 1000 and 2500 km. It is shown that assuming diffusive equilibrium, including the new temperature model, more realistic equatorial electron density profiles result than for isothermal field lines.The temperature gradient model is also purposely formulated to be of a form that enables the temperature modified geopotential height to be obtained without numerical integration. This renders the model particularly suitable for ray-tracing calculations. A ray-tracing model is developed and it is shown that unducted ray paths are significantly altered from the corresponding paths in an equivalent isothermal model; there is greater refraction and magnetospheric reflection takes place at lower altitudes. For summer day conditions, an inter-hemispheric unducted ray path becomes possible from 26° latitude that can reach the ground at the conjugate.