The electron content of the ionosphere and the Southern boundary of diffuse Aurora

Measurements of latitudinal dependence of ionospheric electron content over Europe exhibit strong enhancements around 60° geomagnetic latitude, occuring during some winter nights. One of these events — on December 4th, 1977 — could be related to the precipitation of soft electrons (<400 eV) at the equatorward boundary of a bright diffuse Aurora.     

An improved model of the variation of electron concentration with height in the ionosphere

An empirical model of the variation of electron concentration with height is described which overcomes some limitations found in practice with a previous widely used model (Bradley and Dudeney, 1973). In particular, the new model will generate more realistic variations of electron concentration with real height and virtual height, both including and excluding an F1-ledge. The model has no gradient discontinuities and will reproduce cases in which the F1-ledge does not have a true turning point. The model should prove very valuable for a wide range of propagation problems and for certain aeronomical applications.     

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.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.     

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.     

Lunar and planetary influences upon the peak electron density of the ionosphere

Observable effects of semidiurnal tidal character caused by the Sun (radiation and gravity) and Moon (gravity only) seem to be influenced by the planet Jupiter and, during approach, by Venus. These effects can be identified during summer for suitably selected planetary configurations. The importance of the lunar effect is found to depend on the solar hour, season and planetary constellation.     

Annual variations in the electron content and height of the F layer in the northern and southern hemispheres,related to neutral composition

Total electron content (TEC) data is presented for similar sites at ±35° latitude, and conjugate sites at ±20°, for several years near solar maximum. Comparison with the MSIS atmospheric model shows that the large seasonal anomaly at 35°N (an increase of 80% in TEC from October to April) is fully explained by changes in neutral composition. The small seasonal anomaly at 35°S also agrees with the MSIS model. Composition changes fail to account for the generally higher TEC in the northern hemisphere; this suggests the presence of an overall south-to-north atmospheric wind. Eastern declinations also contribute to enhanced TEC in the northern hemisphere, in the Pacific zone. The MSIS model predicts a semiannual variation of about ±25% in TEC at all sites, while observed changes are only about ±8%; thus we require some enhanced loss process near the equinoxes, particularly in September and October.Peak height calculations assuming a constant pressure level give a large semiannual variation in the F2 region: this is replaced by an annual variation when h_m F2 is calculated from diffusion theory. Heights calculated from the MSIS model are similar to observed values at ±35° latitude on summer days. A decrease of about 20km in observed heights on winter days is attributed to a poleward neutral wind; this wind also reduces the observed TEC. At night the height changes correspond to an equatorial wind, which is largest in summer and equinox. Observed day time TEC is greater at 20°N than at 20°S at all times of year, suggesting a northward transequatorial wind which is strongest near January and gives increased TEC and decreased peak height at 20°N.     

Modelling of the electron density profile in the lowest part of ionosphere D-region on the basis of radiowave absorption data: 1. Theoretical model

A simplified full-wave method adapted to the propagation of very obliquely incident LF radio waves is developed. For a selected ionosphere model the wave-field structure is calculated inside a horizontally stratified ionosphere and the peculiarities of the reflected field are clearly described. The penetration of the investigated radio waves in the lower ionosphere at noon-time is found to be restricted to a layer several wavelengths thick. The reflected wave is created entirely by the mechanism of partial reflections and the region responsible for its formation is usually below 70 km. The influence of some typical parameters of the electron density profile, as well as the atmospheric pressure and temperature, on the attenuation of the investigated radio waves is demonstrated. It is also found that the reflection at very oblique incidence depends mainly on the height of the bottom of the ionosphere.     

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.     

Acceleration of electrons in the ionosphere under the action of intense radio-waves near electron cyclotron harmonics

The effect of electron acceleration by intense ionospheric plasma turbulence induced by a high-power radio wave is studied theoretically, under conditions when the turbulence frequency is nearly equal to harmonics of the electron-cyclotron frequency. The turbulence located in a thin layer at the region of the pump-wave upper-hybrid resonance gives rise to the formation of an intense electron distribution function tail at high energies. Given the resonantly absorbed fraction of the pump-wave energy and the typical plasma turbulence scale, one can estimate the turbulence energy density and calculate the modified electron distribution function.     

The maintenance of the night-time ionosphere at mid-latitudes. I. The ionosphere above Malvern

The total rate of recombination in the night-time ionosphere above Malvern (at L = 2.6) was estimated using a model atmosphere, and the results were compared with the observed rate of change of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was an important factor on a time-scale of an hour or less but when averaged throughout the night made little contribution. The main influx of plasma was a downward diffusion from the protonosphere, especially before midnight. The average downward flux increased steadily as the protonosphere filled after a magnetic storm, with a saturation time of at least 8 days.     

The angular distribution of radio waves partially reflected from the lower ionosphere

Measurements of radio waves partially reflected from the D-region made using two antennae of very different beamwidth are reported. The arrays are composed of 40 and 4 dipoles respectively. It is shown that the gain of the larger array over the smaller is often variable—both in height and time. These results can be used to estimate the off-vertical angles from which significant energy is returned. For altitudes less than 80 km angles less than 10° seem to be usual but at higher altitudes the angles increase to values of the order of 15°–20°. Other important properties of the echoes, such as the probability distribution of the amplitude were also measured. The results are discussed with particular reference to the differential absorption method of measuring electron densities and also to the nature of the irregularities responsible for the partial reflections.     

Amplitude statistics of signals reflected from the ionosphere

Statistical analysis methods used to define the amplitude distributions of signals returned from the ionosphere are discussed in this paper. Emphasis is placed on determining accurately the parameter B, which is the ratio of steady to random components present in a signal. Thus B > 1 if the signal is dominated by the steady component, and B < 1 when the random components dominate. This study investigates the characteristics of B for F-region and E-region ionospheric echoes, as well as some types of spread-F, observed at the southern mid-latitude station Beveridge (37.3 S and 144.6 E). The results indicate that amplitude measurements obtained in approximately 100 s are adequate for determining B. The results also illustrate some effects that the E-region can have on F-region echoes.It is found that frequency spreading, the most common type of spreading observed at Beveridge, displays strong specular reflections and some signal variation due to interference at the leading edge of the F-region echo (i.e. B > 2). Within the spread echo B fluctuates between 0 and about 1.5 but is typically less than 1. The autocorrelation function of signal amplitude has a relatively large coherence interval, suggesting that this type of spread-F is due to interference of specular reflections from coherent irregularity structures with horizontal scale sizes of tens of kilometres rather than scattering from small scale irregularities. A second form of spread-F which would generally be classified as frequency spreading on standard ionoerams is actually due to off-vertical reflections from patches ol irregularities which originate south (poleward) of Beveridge. Echoes within this oblique spread-F (OS-F) do not exhibit coherence indicating that the irregularities responsible are of a smaller scale than those producing normal frequency spread. Finally, the phenomenon of spreading occurring on the second hop, but not the first hop trace is studied. It is shown that the form of the second hop echoes can be reproduced using a simple geometric model of ground scatter. The interpretation is supported by the fact that B for spread second hop echoes is less than 1 whereas it is much greater than 1 for the corresponding first hop echoes.     

Electron temperature and electron density in the F-region of the ionosphere. I. Observed relationship

Ionospheric data from three incoherent scatter stations over the height range 225–450 km were studied for all daylight hours over a wide range of solar conditions. The relationship between electron temperature T_e, electron density Nand solar flux at 10.7 cm wavelength S_10.7 was expressed as T_e = A−B·(N−5 × 10¹¹) + C·(S_10.7−750), where N is in units of m⁻³ and S_10.7 in kJy.This provided a very satisfactory expression for all data taken at Malvern and St. Santin between 0800 and 1600 LT. For data taken at Arecibo, however, the linearity broke down at low electron densities. The data from all three stations were therefore divided into two sets according to electron density and reexamined.ForN < 5 × 10¹¹ m⁻³ B increased steadily with height and decreased steadily with latitude.For N > 5 × 10¹¹ m⁻³ B did not appear to vary with height, with season or with latitude. C was approximately constant for all sets of data.The different mechanisms involved in the heat balance of the electron population are discussed and a qualitative explanation for the relationship is proposed.     

Role of chemical effects in the formation of electron concentration depletions during HF heating of the ionosphere

A chemical mechanism which reduces the electron concentration in the upper ionosphere during HF heating is presented. It is based on the excitation of nitrogen vibrational levels by fast electrons which have appeared as a result of absorption of a radio wave. The vibrational excitation of nitrogen leads to an increase in ion-molecular exchange, followed by a depletion of the electron concentration because the positive molecular ions are very effective in electron -ion recombination. Two different models arc discussed. In the equilibrium model, the vibrational temperature has been established in the region disturbed by the radio wave. In the nonequilibrium model, the fast electrons are moving inside a thin duct where the time of vibrational-vibrational relaxation is greater than the time required by the excited molecules to leave the channel due to diffusion, so that the vibrational temperature cannot be established.     

The accuracy of simple methods for determining the height of the maximum electron concentration of the F2-layer from scaled ionospheric characteristics

The techniques for estimating hmF2 from M(3000)F2 are reviewed with particular stress put upon those in which the effects of underlying ionization are accounted for by a correction (ΔM) to M(3000)F2, formulated in terms of the ratio foF2/foE(=x_E). The simplifying assumptions involved in the three practical implementations (Bradley and Dudeney, 1973; Dudeney, 1974; Bilitza et al., 1979) are emphasised and their consequences investigated quantitatively using a numerical simulation. The factors considered are the dependence upon ymF2, the importance of the underlying layer shape (in particular the significance of the F1-ledge), and the influence of the geomagnetic field.It is demonstrated that the correction technique relies upon ymF2 being a direct polynomial function of hmF2. Analysis of observational data suggests that this relationship holds in practice. Fluctuations in ymF2 about this mean variation are shown to produce only small effects which decrease in magnitude as the amount of underlying ionization increases. The results indicate that underlying layer shape becomes very important when a large amount of underlying ionization is present (x_E<2.5). However, the global morphology of the occurrence of the F1-ledge is such that it is invariably present in such circumstances (ignoring the polar regions). Hence, the ionosphere tends to assume a specific profile form for low x_E cases. The three implementations are shown all to fortuitously incorporate this behaviour. It is demonstrated that exclusion of the geomagnetic field introduces a very small extra uncertainty dependent upon gyrofrequency and geomagnetic latitude, which decreases as the amount of underlying ionization increases.The three implementations are compared and it is concluded that the Dudeney (1974) scheme gives the best overall performance. The more modern and complex Bilitza et al. (1979) scheme appears to have no performance advantages, whilst containing a sunspot number dependent geomagnetic term whose behaviour is irreconcilible with the numerical simulation. The Dudeney (1974) equation is shown to be accurate to between 4 and 5% at magnetic mid-latitudes. The scope for further refinement is considered but rejected as being unlikely to produce an increase in accuracy commensurate with the effort required.     

The maintenance of the night-time ionosphere at mid-latitudes. II. The ionosphere above St. Santin

The total rate of recombination in the night-time ionosphere above St. Santin (at L = 1.8) was estimated using a model atmosphere and the results were compared with the observed rate of change of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was measured but at the latitude of St. Santin this was always small and averaged over the night as a whole the contribution was negligible. Downward diffusion provided the main source of plasma and the flux predicted was compared with the flux measured at 450 km. The comparison was good provided the model atmosphere was modified to use exospheric temperatures based on actual measurements by the incoherent scatter radar. A comparison with the results obtained at Malvern (Paper I) confirmed that the saturation time for the protonosphere at L = 1.8 is far less than at L = 2.6 and that the downward flux from the saturated protonosphere was also less.     

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.     

Lunar tides in the stratosphere and mesosphere from NIMBUS 6 data

Infra-red radiance data from the NIMBUS 6 satellite have been analysed for lunar tidal variations in temperature at heights from 45 to 80 km. The tide at 45 km agrees well with that previously found from NIMBUS 5 data. The tide at 80 km has an amplitude of about 0.4K, and shows a phase change in the expected sense relative to that at 45 km.     

The theory of radio windows in the ionosphere and magnetosphere

Radio waves in a stratified plasma can sometimes penetrate through a region where, according to a simple ray theory, they would be evanescent. They emerge on the far side in a different magnetoionic mode. This occurs when the incident wave normal is within a small cone of angles, called a radio window. The best known example is the Ellis window, used to explain the Z-trace in ionosonde records. Other phenomena where windows may be important have recently been studied. Simple approximate formulae are given for the transmission coefficient of a window and for its angular widths. These show the dependence on frequency, electron concentration gradient and direction of the ambient magnetic field. Comparison with more accurate calculations shows that these formulae are likely to be reliable in practical applications. The tracing of rays near a window is discussed, and the properties of a second kind of window are described.