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 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.     

The dynamic F2-layer over Arecibo

The idealized ‘servo’ model of the ionospheric F2-layer, developed by Rishbeth, ganguly and Walker (1978), is used to simulate the observed behaviour of the daytime F2-peak at Arecibo for sunspot minimum. Taking the east-west electric field to be given by the observed plasma drift velocity perpendicular to the magnetic field, the theoretical equations are integrated using a trial-and-error approach to match the observed values of field-parallel plasma velocity, and the height and electron density of the F2-peak. From the calculation is determined empirically the meridional pressure-gradient force associated with the meridional neutral-air wind. The local time variation during the day is found to be consistent with the semidiurnal variation given by the MSIS atmospheric model of Hedinet al. (1977a, b), though with a phase shift that varies with season; on some days in the fall the pressure-gradient force displays a strong equatorward ‘surge’ in the evening. The values of F2-layer loss and diffusion coefficients needed to match the data are broadly consistent with the MSIS model. The analysis thus validates the MSIS model by way of ionospheric parameters quite independent of the data from which MSIS was originally derived.     

Field-aligned and field-perpendicular velocities in the ionospheric F2-layer

Incoherent scatter observations have shown that there is sometimes a detailed anticorrelation or ‘mirroring’ between V_∥ and V_⊥, the components of F2-layer plasma velocity parallel to and perpendicular to the geomagnetic field. In this paper we develop a simple theoretical model of the F2-layer and compute its response to applied perturbations of V_⊥, both steplike and oscillatory; in particular we investigate the phase and amplitude relationships between V_∥ and V_⊥ resulting from ion-drag and plasma diffusion. For periods of a few hours, the oscillations of V_∥ lag behind exact anticorrelation with V_⊥ by 0.1–0.2 cycle, but the time lags corresponding to these phase differences are only a fraction of 1 h and seem broadly compatible with observations previously reported from Arecibo and Malvern. We do not study the question of what causes the velocities to fluctuate in the first place.     

Incoherent scatter results for co-ordinated special intervals at St. Santin (France)

The development of the French Quadristatic Incoherent Scatter Facility is briefly discussed, and measurements of winds and temperatures over Nançay (47°N, 2°E) in August 1974, and Mende (44°N, 3°E) during October 1975 and January 1976 are presented.     

Seasonal mean structure of the night-time F2 region over Arecibo

Seasonal mean night-time variations of ion and electron temperatures, electron density, ion drift velocity, and light ion composition of the F2 region are derived from incoherent scatter observations at Arecibo based on 19 nights of observation over the latest sunspot minimum years 1974–1976. It is shown that the downward flux of ionization is sufficient to maintain the nocturnal F2 region against recombination at low latitudes. The difference in the electron density decay rate from summer to winter is consistent with the seasonal variation in magnitude of the ionization flux. The mean eastward electric field, which is responsible for any vertical component perpendicular to B, is very small throughout the night. However, the southward electric field, i.e. east-west ion drifts, shows a substantial systematic variation during the night, being southward (eastward ion drifts) before midnight and northward after midnight, with a mean amplitude of 1–2 mVm⁻¹. The H⁺ ion concentration shows a marked seasonal variation. The mean relative concentration of H⁺ ion to electron density at 500 km sometimes exceeds 50% before sunrise in winter. A strong anti-correlation of H⁺ ion concentration with magnetic activity is observed. The observed ion temperatures average about 20–30 K higher than the prediction of the Jacchia (1971) neutral model for the observed range of the 10.7 cm solar flux.     

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.     

Ionospheric F2-kegion storms at midlatitudes starting in the daytime and the night-time

Disturbances in the F2-region during geomagnetic storms have been studied statistically, using foF2 data for two midlatitude stations in Japan. It is found

• 1.(i) that during the initial stage of the storm foF2 increases on the average for storms starting in the daytime and decreases for storms starting in the night-time, in all seasons,
• 2.(ii) foF2 decreases in summer and at the equinoxes and increases in winter irrespective of the local time of the storm onset.
• 3.(iii) disturbances of foF2 in winter for storms starting in the night-time are larger than those for storms starting in the daytime. Greater positive disturbances occur on the average for strong geomagnetic storms.

     

A comparison of the height of the maximum electron density of the F2-layer from real height analysis and estimates based on M(3000)F2

Estimates of the height of the F-layer peak based on formulations using the ionospheric transmission factor M(3000)F2 are compared with hmF2 derived from the real height analysis of digital ionograms acquired at a mid-latitude station. Based on the analysis of 27 hours of quiet data, our result shows that the M(3000)F2 methods are highly accurate and that the formulation developed by Bradley P. A. and Dudeney J. R., (1973, J. atmos. terr. Phys. 35, 2131) is most accurate.     

F1-layer model application to the analysis of yearly variations of thermospheric composition

A method of using experimental data on the F1-layer to study variations in the mean thermospheric gas composition is described and a comparison made with modern empirical thermosphere models. Good agreement is obtained for relative variations in atomic oxygen density at 150 km.     

Diurnal variations in F2 peak density and ionization flux above Tahiti

The effects of composition and ionization fluxes on the diurnal variation of NmF2 at an equatorial anomaly zone station (Tahiti) are separated. The calculated diurnal variation of the fluxes agrees well with what would be expected from published equatorial E × B drift observations and global neutral wind models. A correlation analysis shows that lower hmF2 is often accompanied by larger NmF2, in spite of a much larger recombination rate. This illustrates the dominance of the fountain effect and neutral wind induced interhemispheric transport at this station.     

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.     

Diurnal variations in the flux of ionisation above the F2 peak in the northern and southern hemispheres

The flux of ionisation at 850 km height is calculated using the MSIS atmospheric model, a simplified form for the continuity equation at the peak of the F2-layer, and observed values of NmF2. Results are given for stations at latitudes of 32°N, 21°N, 21°S and 37°S during 1971 and for Tahiti (18°S) in 1980. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at mid latitudes. Increased recombination due to N₂ vibrational excitation produces a large afternoon decrease in NmF2 in summer, near solar maximum, and an increased downward flux. At all stations the day-time flux has a much larger downward component in winter than in summer. Because of the eastward magnetic declination, zonal winds produce opposite effects on the diurnal variations of hmF2, NmF2 and flux in the northern and southern hemispheres. Downward fluxes are largest in the morning in the southern hemisphere and in the late afternoon and evening in the north. At ± 21° latitude, neutral winds have a major effect on the distribution of ionisation from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than in summer. Averaged over both hemispheres, the total flow at 21° latitude is approximately the same for solstice and equinox conditions. At mid latitudes there is a downwards flux of about 1–2 × 10¹² m² s⁻¹ into the night ionosphere.     

Influence of fluctuations in neutral gas density height distribution on F1 -layer dynamics

Model ionograms are analyzed, corresponding to conditions of disturbances to the diffusive-equilibrium gas density distribution. The results obtained show that the F1-layer clearly demonstrates a response to the dynamics of the vertical thermospheric structure. Features of the model virtual height-frequency curves correspond to the character of experimental ionograms recorded in the case of internal gravity waves.