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.     

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.     

Pulse propagation through a plane stratified ionosphere

The paper concerns the propagation of pulses or wave packets through an ionospheric plasma. The rise time, which appears in the ionospheric weighting function, is shown to be a realistic measure of the magnitude of the pulse shape distortion and the shortest usable pulse length in single-path, single-mode communication. Although rather general, smooth models of the medium can be dealt with, we consider in particular the case of a horizontally stratified, unmagnetized and collisionless ionosphere. Two such simple models are studied, which describe the lower part of the layer and the region of the maximum of electron density, respectively. These models allow a completely analytic treatment, and hence one may obtain explicit expressions, in terms of the parameters of the profiles, for the rise time. The illustrations show that the rise time may be less than 1 μs for a steep profile typical for sporadic E-layers, but in the range of one to a few tens of μs for propagation through a normal layer. Results of preliminary estimates indicate, however, that pulse mixing due to the differences in group path between the O- and X-mode rays dominates over pulse shape distortion in many cases.     

Ionospheric modelling in support of single station location of long range transmitters

Position estimates derived from a large data base of bearing and elevation angles of signals from distant HF transmitters have been analysed, with a view to comparing the validity of available ionospheric models and to examining ionospheric limitations to the accuracy of single station location of such transmitters. In general, the accuracy of the position estimates is almost entirely controlled by a limited ability to model in sufficiently accurate detail the ionospheric effects on the signal propagation. Median miss distances for those cases with a reliable identification of the propagation mode were about 7% for both E and F2 propagation for all models considered. Difficulties were encountered with the International Reference Ionosphere, which failed to support the observed propagation in half the F2 propagation cases. Standard deviations of the bearing errors were about 0.5° for E modes and 0.7° for F2 modes and were largely attributable to the effects of the ionosphere and not to instrumental errors     

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.     

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.     

Diurnal,semi-diurnal and terdiurnal Hough components of surface pressure

Diurnal and semi-diurnal Hough components of surface pressure are evaluated by classical tidal theory for previously presented profiles of water vapour and ozone heating. Values are compared with the observational results of Haurwitz and Cowley (1973) and show significant discrepancies which are considered to indicate the need for additional heating to be identified. The present calculations are in satisfactory agreement with those for semi-diurnal modes evaluated by Walterscheid et al. (1980).Profiles of terdiurnal heating due respectively to ozone and water vapour absorption of solar radiation are calculated for the (3,3,3) to (3,3,6) Hough modes and corresponding surface pressure oscillations are evaluated by classical tidal theory. Comparisons are made with earlier evaluations and with observationally derived results. The calculated solstitial (3, 3, 4) mode, which is the mode of the largest surface pressure amplitude, shows good agreement in phase with observation but underpredicts observed amplitudes by about 36% in contrast to earlier evaluations which were based on a now unacceptable basic temperature profile.     

Comparison of Pc3 and Pc4 pulsation characteristics on an East-West mid-latitude chain of magnetometers

We compare the results of analysing Pc3 frequencies on an East-West chain of 4 magnetometers at mid-latitudes with the results of an earlier analysis of the same data at Pc4 frequencies. The Pc3 signals show some remarkable similarities to those at the lower frequencies. Near local midnight, when the higher frequencies are a component of Pi2 pulsations, they share the characteristic of very high coherence across the chain. At other times, Pc3 signals resemble the Pc4 band studied earlier in that the longitudinal wave number is small, and no clear diurnal propagation pattern is systematically observed but at times there is evidence of preferentially sunward phase motion in all daylight hours. By night westward propagation dominates. We conclude that our results are consistent with field line resonance theory, but not with the Kelvin-Helmholtz instability model.     

Reconstruction of horizontally-inhomogeneous ionospheric structure from oblique-incidence backscatter experiments

Fridman and Fridman [(1994) J. atmos. terr. Phys. 56, 115] suggested a method of reconstructing the horizontally-inhomogeneous ionospheric structure using vertical- and oblique-incidence backscatter sounding (OBS) ionograms measured at a single location. In the present paper this technique has been used to analyze experimental data and tested against independent vertical sounding (VS) measurements. By using the OBS and VS ionograms measured at Irkutsk as source data for the method we reconstructed ionization profiles over Tomsk (1050 km to the west of Irkutsk). We found that the reconstructed profiles are in reasonable agreement with the profiles obtained from VS measurements at Tomsk.     

Analytic calculation of the ordinary (O) and extraordinary (X) mode nose frequencies on oblique ionograms

On oblique ionograms, the maximum frequencies of the ordinary and extraordinary modes are referred to as nose frequencies. The difference in the nose frequencies depends on parameters such as the length of the propagation circuit and the direction of propagation. In this paper, the difference in nose frequencies is studied using the frequency scaling technique of Bennettet at. [(1991) Appl. Comput. Electromagn. Soc. Jl6, 192]. For long paths, an explicit formula is obtained which gives the difference approximately as a function of the local magnetic dip and azimuth of propagation at the ray mid point. For shorter paths, it is shown how analytic ray tracing can be used to determine the difference.     

HF Doppler observations of medium-scale travelling ionospheric disturbances at mid-latitudes

From 1972 to 1975 F-region medium-scale travelling ionospheric disturbances (MSTIDs) were observed at Leicester, U.K. (52°32′N 1°8′W) by means of the HF Doppler technique. Most of the features of the disturbances previously reported in the literature are confirmed, with the exception of the apparent seasonal variation in the propagation direction. The measured wave azimuth rotates clockwise through 360° in 24 h, supporting theoretical predictions concerning the filtering effect of the neutral wind in the northern hemisphere. The most commonly observed direction of wave propagation, however, is displaced from the antiwind direction and is located at an azimuth of 130–140° relative to the wind. A periodic variation of the direction of wave propagation with respect to the anti-wind direction is evident, which may indicate that lower atmospheric winds can have a greater influence on waves at thermospheric heights than previously supposed.A synoptic survey of the data set reveals little correlation between wave occurrence and auroral processes, and it is unlikely that high-latitude sources are responsible for many of the MSTIDs observed at mid-latitudes.     

Numerical rise time calculations for obliquely propagating BY pulses

For planning spread spectrum communication systems over ionospheric HF channels it is important to determine pulse rise times or dispersive bandwidths which are characteristic for wideband propagation. 1(n this paper a numerical technique for the calculation of pulse rise times is proposed. This technique has been developed on the basis of known theoretical results concerning the pulse propagation through a plane stratified ionosphere. The calculations were carried out for several cases of propagation through the lower E-region, using the Jones-Stephenson three-dimensional ray-tracing program. The obtained rise times are in the range from a few μs (for the wave-packet reflected from sporadic-E) to several tens of μs (for the wave-packet propagating nearly along the Pedersen path in the lower E-region). The results are shown to be in good agreement with those previously obtained by other authors, either by measurements or theoretical approaches.     

Transmission of large-scale TIDs in the ionospheric F2-region

Large-scale travelling ionospheric disturbances (1.s. TIDs) have been investigated in order to derive the horizontal velocity dispersion by using f₀F2 data from four ionospheric observatories in Japan. It was found that the horizontal phase trace velocity lies between 300 and 1000ms⁻¹ with periods in the range 50 to 150 min. There is evidence that the derived velocity generally increases with increase of wave period. This is consistent with the dispersion predicted by the theory of the internal gravity waves. The azimuthal angles are distributed in ±35° sectors centered around 197° (measured clockwise from north), indicating that 1.s. TIDs may be obtainable when they are excited along the auroral zone of the same sector in longitude as that of the observatories. The average propagation direction shifts by 17° from south towards west. This clockwise shift is consistent with the rotation caused by the Coriolis effect. This means that the Coriolis effect cannot be ignored for the wave propagation of 1.s. TIDs. In addition to the positive correlation between TID speed and geomagnetic activity, the direction of wave propagation is found to be correlated with polar magnetic activity. The propagation direction is mostly southward during the period of large polar magnetic disturbances, while during the period of low magnetic activity the direction scatters considerably.     

The winter anomaly as seen in the propagation of low frequency 40 kHz radio waves

An analysis of propagation data for LF 40 kHz radio waves shows that the field strength of the sky wave is enhanced during day-time on winter anomaly days (WAD), in striking contrast to the severe attenuation of HF radio waves. This peculiar enhancement of the field strength may be ascribed to an increase in the reflection coefficient. The analysis also demonstrates that the reflection height is lower on WAD, which seems to be associated with enhancements of ionization in the D-region. Moreover, it was found that WAD are characterized by an earlier occurrence in the morning and a delayed occurrence in the evening of pronounced interference maxima and minima, respectively.     

Anomalous diurnal phase shifts of Omega VLF waves (10–14 kHz) on the east–west low latitude and transequatorial paths

The phase of the Omega HAIKU (Hawaii, U.S.A.) and REUNION (La Reunion) signals were measured at Inubo, Japan and onboard ship at Fremantle. Australia. Strong east–west non-reciprocities of the diurnal phase shift are obtained both on the low latitude and transequatorial paths, and it is found that the non-reciprocity on one path is in an opposite sense to the other. The diurnal phase shift, ϕ_DN for the west-to-east (WE) propagation is 7.8–8.7 µs Mm^–1 at 13.6 kHz on the transequatorial and mid-latitude paths, indicating no significant latitude dependence of the phase velocity in WE propagation. On the other hand, ϕ_DN for the east-to-west (EW) propagation greatly depends on the geomagnetic latitude; at 13.6 kHz ϕ_DN = 11.3µs Mm^–1 on the low latitude path and ϕ_DN = 50 µs Mm^–1 on the transequatorial path, which are 40% greater and 35% less than ϕ_DN in WE propagation, respectively. The east-west non-reciprocities of ϕ_DN on the low latitude and transequatorial paths are interpreted in terms of a single mode propagation in the conventional anisotropic waveguide model with β_D = 0.3 km^–1, β_N = 0.5 km^–1 and h_N–h_D = 12.5 km. In particular, the anomalously small ϕ_DN on the EW transequatorial path is explained as due to the high phase velocity of the night-time first-order mode in the equatorial region within ±12° geomagnetic latitude.     

The quiet midlatitude D-region: a comparison between modeling efforts and experimental measurements

Two long-standing problems in the atmospheric sciences have been the correct modeling of the ion chemistry in the earth's atmosphere and the proper determination of the ion species and densities through in situ measurements. Comparison between experimental data and simulations of the data by computer modeling of atmospheric chemistry is a means of validating the model as well as indicating which processes are in need of further study. The DAIRCHEM computer code is used here to simulate data taken in the midlatitude D-region during quiet conditions. On the one hand, comparison between the total positive ion density profile derived from rocket measurements and the one computed by the code shows very good agreement in the 30–90 km range, with the exception that the simulated ion profile is somewhat smaller than the experimental one in the 60–75 km region. Such discrepancy is only partially explained by the inherent uncertainties in the NO density profile or the total ionization rate profile. On the other hand, comparison between the measured and the computed electron density profiles shows that the measured profile is consistently smaller than the computed profile in the 65–85 km range. We interpret this discrepancy as a deficiency in the modeling of the negative ion chemistry. Also, this deficiency is probably the main cause of the disparity between the total positive ion density profiles in the corresponding altitude range. It is felt that the positive ion chemistry of the D-region is reasonably well understood. However, the negative ion chemistry is in need of further study. Specifically, alternate electron attachment/detachment processes should be considered, as well as an as yet undetermined, possibly very massive, negative species which may affect the ion recombination rates.     

Simultaneous observations of the quasi 2-day wave at Mawson,Antarctica, and Adelaide,South Australia

Observations of the Austral quasi 2-day wave at Mawson, Antarctica (67°S, 63°E) are presented and compared with those from Adelaide (35°S, 138°E). The data were obtained from partial-reflection radars which have been measuring winds continuously since mid-1984, and the results presented here are the first to record the 2-day wave in middle atmosphere winds from Mawson. They show that 2-day period oscillations of 10–15 m s⁻¹ are a regular feature of the high latitude southern hemisphere summer. The wide longitude and latitude separation of the radar stations permits estimates of propagation velocity and latitude phase structure, and results are consistent with the passage of a westward travelling Rossby-gravity (3, 3) wave.