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.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.     

Electron density profiles over the Southern Ocean from oblique incidence ionograms

This paper presents a first attempt to use oblique incidence ionograms over the 4500 km path from Sanae, Antarctica, to Grahamstown, South Africa, to deduce information about the ionosphere in the intervening regions. It is shown that existing methods for the reduction of oblique incidence ionograms to N(h) profiles give reasonable results even over the two-hop path involved. By comparison with vertical incidence ionograms made from a research ship below the reflection regions it is shown that the maximum observed frequency is normally limited by conditions at the southernmost reflection point, though this may be modified by ionospheric tilts, sunrise and sunset.     

Inter-relations between current and electron density profiles in the equatorial electrojet and effects of neutral density changes

A simple model of the equatorial electrojet is used to try to reproduce observed current density profiles and it is found that an increase in neutral density is required. The effects of neutral density changes of various kinds are investigated. Changes in the electron density profile due to the j × B force are found to be fairly small and, in the cases studied here, are decreases at all heights.     

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.     

Indirect phase height measurements of the lower ionosphere compared with rocket measurements of D-region electron density

A comparison between rocket-measured electron density profiles of the lower ionosphere and the results of ground-based indirect phase height measurements in the LF range, carried out near the Soviet rocket sounding station Volgograd over several years, confirms—to a first-order approximation—the height of the level of electron density which according to magnetoionic ray theory is necessary for reflection of the waves. In a second-order approximation, however, an additional phase path change has to be taken into account, which is caused by the ionization below the reflection level. This makes the observed phase height always slightly smaller than the real geometric height, on average by −1 km, but in extreme cases by up to −4 km, depending on the actual height gradient of electron density below the reflection level. Due to systematic diurnal and seasonal variations of this gradient, the amplitude of the diurnal variation of the observed phase height is found to be slightly larger than that of the real geometric height, whereas the reverse is true for the seasonal variation at constant solar zenith angle.     

Comparison of total electron content calculated using the IRI with observations in China

Values of total electron content (TEC) calculated using the International Reference Ionosphere (IRI-86 and IRI-90) are compared with the observations at Xinxiang based on the Faraday rotation measurement. It is found that the IRI gives acceptable values with respect to the observations during low solar activity. Generally the IRI-90 is better than the IRI-86 and the URSI coefficients are better than the CCIR coefficients in the calculation of TEC. Making use of the foF2 and M(3000)F₂ calculated using the Asia Oceania Region F₂-layer mapping (AOR) instead of using the CCIR or the URSI coefficients, the IRI gives more accurate TEC values. In October-April during high solar activity, however, the IRI obviously underestimates TEC in the daytime, which could be due to an improper topside electron density profile.     

Incoherent scatter radar measurements of electron density and ion velocity during an isolated substorm

A study of the average pattern of F-region plasma densities and velocities measured by the Chatanika incoherent scatter radar has previously suggested that the main ionospheric F-region trough is formed in the evening sector by the westward transport of plasma under the influence of convective electric fields. This paper examines the role of convective electric fields on the electron density profile and the formation of the F-region density trough for a particular night. Incoherent scatter radar data from Chatanika are presented.On 25 May 1972 an isolated substorm occurred near 0900 UT after a long period of magnetic quiet. The substorm was manifested at Chatanika, in the evening sector, by a small positive bay and a concurrent onset of westward motion of plasma associated with a rapid decrease in the F-layer electron density in the region of the moving plasma. Analysis of plasma densities and velocities during this event indicates that

• 1.(1) temporal changes of plasma motion are associated with changes in the convective electric field pattern in response to substorm activity
• 2.(2) the electric field pattern created a north-south gradient in the F-layer electron density which is interpreted as the formation of the ionospheric trough near its equatorward edge, and
• 3.(3) large scale electron density fluctuations were observed in the evening sector resulting from westward travelling density variations originating in the midnight sector.

The study emphasizes the complexity, and difficulty in interpretation, of single station auroral zone measurements of the F-region ionosphere.     

Average electron density profiles in the plasmasphere between L = 1.4 and 3.2 deduced from whistlers

985 whistlers recorded at Tihany, Hungary (L = 1.9) between December 1970 and May 1975 have been processed for equatorial crossing radius L, equatorial electron density n_eq and tube electron content N_T. The obtained L values lie in the range L = 1.4-3.2. The median values of n_eq and N_T as a function of L fit well to the density profiles published by Park et al. The results also indicate that at lower L values the whistlers are ducted by stronger density enhancements.     

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.     

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.     

High-latitude plasma densities and their relation to riometer absorption

A large number of D- and E-region electron density profiles from high latitudes have been analysed. These were derived from rocket-borne wave propagation experiments and—after careful screening—arranged according to riometer absorption. Statistical profiles for various degrees of absorption, including 0 dB, were established both for day and night. Furthermore, the height region predominantly contributing to the absorption has been identified. Finally a mean variation of the density of negative ions has been derived.     

Ledge effect in A1 absorption measurements

The short-term variability in A1 absorption measurements for Camden (34°S, 151°E) is caused by changes in the deviative component of absorption. These changes, in turn, are caused by enhanced electrondensity gradients (referred to here as ‘ledges’) near the reflection level. The ledges perturb the normal E-region electron density profile and introduce a bias (termed the ‘ledge effect’) into the absorption measurements. In the present experiment, the ledge effect was isolated by using the virtual reflection height information to interpret the A1 absorption measurements. During the period of this study (February 1980–January 1981), the ledge effect was observed to have both diurnal and seasonal variations. The effect was most noticeable in the afternoon around the equinoxes, but was essentially absent during the summer months.     

Passage of a powerful HF radio wave through the lower ionosphere as a function of initial electron density profiles

The results of numerical modelling of powerful HF radio wave propagation through the ionosphere plasma are presented. Comparison of the heating wave parameters with those of a low powerwave gives the possibility to study the self-action of the powerful HF wave. At low altitudes the ‘translucence’ of the ionosphere plasma takes place. At high altitudes the wave absorption sharply increases. Both self-action effects lead to the reduction of the altitude range of the heated region. The dependence of self-action effects and the resulting electron temperature profiles on the initial electron density profiles are studied.     

Riometer measurements of ionospheric radio wave absorption

Absorption was calculated at two height levels in the ionosphere, from different electron density profiles. The correlation between absorption and foF2 was studied. This study bears out theoretical results that riometer absorption occurs mainly in the D-layer and less in the upper parts of the ionosphere.     

An assessment of the value of bone density measurements to archaeoichthyological studies

This paper reviews the meaning of the term ‘density’ and the problems associated with the methods of density determination for animal bones in archaeology. It has often been assumed that density is the intrinsic property of most influence in controlling the rate of a bone's decay. Values for whole bone density have been published only for large mammal bone, however. Fish bone appears to be particularly vulnerable to decay, and usually a restricted range of skeletal elements are recovered from archaeological sites. The object of this study was to examine the relationship between fish bone density and the ability of the bone to survive on occupation sites and in archaeological deposits. A set of ‘density’ measurements was established for the bones of cod (Gadus morhua). The usefulness of these measurements as a predictive tool in archaeoichthyological studies is assessed. It was found that ‘density’ as measured did not explain adequately the relative survival of skeletal elements after mechanical abrasion and weathering, or within archaeological deposits.     

Beam-generated electrostatic electron waves: relevance to EISCAT VHF radar measurements

The generation by an electron beam of (slowly) growing electrostatic waves, detectable with the EISCAT VHF radar, is investigated. A set of graphs is derived from which it is possible to estimate, by interpolation, frequency, growth rate and resonant beam velocity for most situations of interest. The dependence of the mentioned variables on propagation direction, beam temperature and density and on ionospheric density over the part of parameter space that is of practical interest in the auroral and subauroral ionosphere is presented and discussed.     

Simultaneous measurements of large vertical winds in the upper and lower thermosphere

High resolution vertical wind measurements of the upper and lower thermosphere were made at Poker Flat, Alaska, using a scanning Fabry-Perot spectrometer (FPS). Observations of the λ558 and λ630 nm emissions of atomic oxygen were made on 21 nights and allowed for the simultaneous determination of wind and temperature at altitudes of about 130 and 240 km, respectively. On two occasions, significant upwelling events were measured which lasted between 15 and 25 min. Peak velocities were up to 42 m/s at 130 km and 138 m/s at 240 km. Auroral activity was monitored using a meridian scanning photometer (MSP). On both occasions, the region of upwelling was located on the poleward side of the auroral oval during geomagnetically active conditions. A schematic model is used to describe an event from which the horizontal scale of the upwelling region is estimated to be less than 320 km in the lower thermosphere and less than 800 km in the upper thermosphere.     

Effective electron collision frequency measurements in the E- and F-regions

The collision frequency v in the ionosphere has often been determined by measuring differences in the amplitude and group path of two closely spaced signals reflected in the region of high group retardation. In this paper we describe a method of measuring v using a CW double-side-band modulated signal reflected obliquely in the ionosphere. This allows v to be determined on a continuous basis and it is found that the value of v obtained is 1–5 × 10⁴ s⁻¹ for the E-region and ~ 10³ s⁻¹ for the F-region. It is shown that measurements made just after sunset, when the E-region is still present, are more representative of E-region values than F-region.     

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.