Neutral winds in the F-region

The Navier-Stokes equations are solved by introducing a statistical model of density and a distinct temperature model. The non-linear terms of inertia and the viscous term are not included. The computations have been made for equinoctial conditions at a height of 300 km and at different latitudes.The results are presented in the form of a global wind pattern. The most important features are: the existence of transequatorial winds by night, the asymmetrical structure of the wind pattern and the agreement of the computed velocities with observations.     

Gravity waves in low latitude F-region

The paper presents a study of large scale travelling ionospheric disturbances detected by riometers operating at 30 MHz, over São José dos Campos (23°S, 45°W) and a nearby location in Sao Paulo, Brasil. The TID's are observed mainly at night and have wavelengths greater than 500 Km. In a few cases it is possible to determine the E-W component of the velocity of propagation, which is of the order of 450 Km per hour. Most of the events are characterized by disperions; the period is found to increase from half an hour to nearly two hours. These and other features are identified with the propagation of atmospheric gravity waves in the F-region, whose source might be located far away from the observing site. The results also suggest that a suitably designed riometer system could profitably be used for future investigation of gravity waves in the F-region in low latitude.     

A numerical computer investigation of the polar F-region ionosphere

Results of a numerical computer investigation of the geomagnetically quiet, high latitude F-region ionosphere are presented. A mathematical model of the steady state polar convective electric field pattern is used in conjunction with production and loss processes to solve the continuity equation for the ionization density in a unit volume as it moves across the polar cap and through the auroral zones.Contours of electron density (～ 300 km altitude) over the polar region are computed for various geophysical conditions. Results show changes in the F-region morphology within the polar cap in response to varying the asymmetry of the global convective electric fields but no corresponding change in the morphology of the mid-latitude ionospheric trough. The U.T. response of the ionosphere produces large diurnal changes in both the polar cap densities and trough morphology. In agreement with observations, the model shows diurnal variations of the polar cap density by a factor of about 10 at midwinter and a negligible diurnal variation at midsummer. The phase of the polar cap diurnal variation is such that the maximum polar cap densities occur approximately when the geomagnetic pole is nearest to the Sun (i.e. when the polar cap photo-ionization is a maximum).Within the accuracy of this model, the results suggest that transport of ionization from the dayside of the auroral zone can numerically account for the maintenance of the polar cap ionosphere during winter when no other sources of ionization are present. In addition, east-west transport of ionization, in conjunction with chemical recombination is responsible for the major features of the main trough morphology.There is little seasonal variation in the depth or latitude of the ionization trough, the predominant seasonal change being the longitudinal extent of the trough.The polar wind loss of ionization is of secondary importance compared to chemical recombination.     

Electrical coupling of the E- and F-regions and its effect on F-region drifts and winds

Electric currents, generated by thermospheric winds, flow along the geomagnetic field lines linking the E-and F-regions. Their effects on the electric field distribution are investigated by solving the electrical and dynamical equations. The input data include appropriate models of the F-region tidal winds, the thermospheric pressure distribution and the E-and F-layer concentrations. At the magnetic equator, the calculated neutral air wind at 240 km height has a prevailling eastward component of 55 m sec^-1 and the west-east and vertical ion drifts agree in their general form with incoherent scatter data from Jicamarca     

Polarization fields produced by winds in the equatorial F-region

Neutral air winds blowing across the magnetic field cause a slow transverse drift of the positive ions, perpendicular to both the winds and the magnetic field. This drift sets up an electric polarization field which can only be neutralized by currents flowing along magnetic field lines and through the E-layer. But at night the E-layer conductivity may be too small to close this circuit, so that polarization fields build up in the F-layer, causing the plasma to drift with the wind. This polarization effect may influence the behaviour of the nighttime equatorial F-layer and contribute to ‘superrotation’ of the atmosphere.     

Role of Coulomb collisions in the equatorial F-region plasma instabilities

The effect of collisions on electrostatic instabilities driven by gravity and density gradients perpendicular to the ambient magnetic field is studied. Electron collisions tend to stabilize the short wavelength (k_y?_i ? 1, where k_y is the perpendicular wavenumber of the instability and ?_i is the ion Larmor radius) kinetic interchange mode. In the presence of weak ion-ion collisions, this mode gets converted into an unmagnetized ion interchange mode which has maximum growth rate one order smaller than that of the collisionless mode. On the other hand, electron collisions can excite a long wavelength resistive interchange mode in a wide wavenumber regime ($\text{10}{}^{\mathrm{?}}\text{3 ? k}{}_{\mathrm{y}}\text{?}{}_{\mathrm{i}}\text{? 0.3}$) with growth rates comparable to that of the collisional Rayleigh-Taylor mode. The results may be relevant to some of the spread F irregularities.     

Numerical solution of the time-dependent heat energy equations for the magnetosphere

A new solution of the magnetospheric heat equations capable of covering the whole region from 300 km along a field line to the equatorial plane has been achieved by adapting the searching procedure of Murphy (1974). It has been found that the protonospheric heat reservoir is sufficient to maintain T_e >T_n down to the height of the F2-peak electron density all through the night at mid-latitudes. Full solution of the equations has also shown that T_i >T_e in the protonosphere at night and the ions constitute a significant source of heat for the electrons.     

Photoionization rates in the night-time E- and F-region ionosphere1

Quantitative estimates of ionization sources that maintain the night-time E- and F-region ionosphere are given. Starlight (stellar continuum radiation in the spectral inverval 911–1026 Å) and resonance scattering of solar Ly-β into the night sector are the most important sources in the E-region and are capable of maintaining observable electron densities of order (1–4) × 10³ cm^?3. Starlight ionization rates have substantial variations (factors of 2–4) with latitude and time of year since the brightest stars in the night sky occur in the southern Milky Way and Orion regions. In the lower F-region the major O⁺ source in the equatorial ionosphere is 910 Å radiation from the O⁺ recombination in the F2-region, whereas in the extratropical ionosphere interplanetary 584 Å radiation only exceeds resonance scattering of solar 584 and 304 Å radiation as the dominant O⁺ source during the month of December.     

The source of midlatitude metallic ions at F-region altitudes

A survey of metallic ions detected by the Bennett Ion Mass Spectrometers flown on the Atmosphere Explorer satellites, including both circular and eccentric orbital configurations, shows that patches of these ions of meteoric origin are frequently present during magnetically active periods on the bottomside of the F-layer at middle and high latitudes. In particular the F-region metals statistically tend to appear at night in the vicinity of the main ionospheric trough (in a band of invariant latitudes approx. 10 degrees wide) and on the day side of the polar cap. These distributions were previously associated with the expected dynamics of ions in the F-region above 140 km where meridional neutral wind drag and convection electric fields are the dominant ion transport mechanisms. However, the main meteor deposition layer—the presumed source region of the metals—is located below 100 km where these transport mechanisms do not prevail. It is demonstrated that the Pedersen ion drifts driven by intense electric fields such as those associated with sub-auroral ion drifts (SAID) are sufficient to transport the long-lived metallic ions upward from the main meteor layer to altitudes where the drag of equatorial directed neutral winds and electric field convection can support them against the downward pull of gravity and transport them to other locations. The spatial and temporal distribution of the middle and high latitude F-region metals are consistent with the known characteristics of the electric fields and with the expected F-region ion dynamics.     

Lower F-region barium release experiments at a sub-auroral location

A series of barium release experiments have been conducted at altitudes near 160 km from the R.A. Hebrides Range (L = 3.5). A predominantly westward neutral drift has been observed for all the releases conducted during evening twilight. The development of the ionized clouds are characterized, in this situation, by a lack of separation from the neutral release material and by a rapid onset of the formation of structure. The implications of these features are discussed and numerical modelling used to describe the steepening and striation onset in the ion clouds. It is concluded that the most likely striation sizes are those which are of sufficiently small scale to be electrostatically isolated from other regions of the ionosphere but not so small as to be diffusively dispersed.     

Interpretation of single-site scintillation measurements to estimate F-region drift velocities

A method of estimating ionospheric drift velocities using single-site scintillation measurements is applied to determine a correlation coefficient of 0.55 between magnetic activity and F-region drift velocity near the auroral ionosphere. This method is based on the relationship between the drift velocity and the scintillation spectral breakpoint.     

Modeling the midlatitude F-region ionospheric storm using east-west drift and a meridional wind

Under magnetically quiet conditions, ionospheric plasma in the midlatitude F-region corotates with the Earth and relative east-west drifts are small compared to the corotation velocity. During magnetic storms, however, the enhanced dawn-to-dusk magnetospheric convection electric field often penetrates into the midlatitude region, where it maps into the ionosphere as a poleward electric field in the 18:00 LT sector, producing a strong westward plasma drift. To evaluate the ionospheric response to this east-west drift, the time-dependent O⁺ continuity equation is solved numerically, including the effects of production by photoionization, loss by charge exchange and transport by diffusion, neutral wind and $\text{E}\text{×}\text{B}$ drift. In this investigation only the neutral wind's meridional component and east-west $\text{E}\text{×}\text{B}$ drift are included. It is found that an enhanced equatorward wind coupled with westward drift produces an enhancement in the peak electron density (NMAX(F2)) and in the electron content (up to 1000 km) in the afternoon sector and a subsequent greater-than-normal decay in ionization after 18:00 LT. These results agree in general with midlatitude F-region ionospheric storm observations of NMAX(F2) and electron content which show an afternoon enhancement over quiet-time values followed by an abrupt transition to lower-than-normal values. Westward drift appears to be a sufficient mechanism in bringing about this sharp transition.     

Diurnal transport effects on the F-region plasma at chatanika under quiet and disturbed conditions

The predictions of a time-dependent, three-dimensional model of the high-latitude ionosphere have been compared with the diurnal variations of plasma convection velocities and electron densities observed at Chatanika on geomagnetically quiet and disturbed days near equinox. The model predictions for the quiet day are in good agreement, both qualitatively and quantitatively, with the measurements. The model predictions for the disturbed day are in qualitative agreement with the measurements, but at certain local times there are significant quantitative differences. Also, the model cannot produce the detailed fine structure in the electron density that was observed on the disturbed day owing to the lack of fine structure in the model convection pattern and auroral precipitation fluxes. For the quiet day, the gross features of plasma convection are consistent with a symmetric two-cell pattern with a cross-polar cap potential of 52 kV. For the disturbed day, on the other hand, the observed plasma convection is consistent with an asymmetric two-cell pattern with enhanced plasma flow in the dusk sector and a cross-polar cap potential of 90 kV. For both the quiet and disturbed days, the lower latitude region of the high-latitude ionosphere was found to be sensitive to the competition between the vertical components of the electrodynamic and wind-induced ion drifts. For both days, horizontal plasma transport was found to be very important. One consequence of transport is that on the dayside the peak density at a given altitude occurs at a later local time as altitude increases. Another consequence of transport is that high electron densities are maintained in regions devoid of ionization sources, particularly on the disturbed day.     

On a mechanism for the formation of VLF electrostatic emissions in the high latitude F-region

The presence of highly anisotropic ion velocity distributions in the weakly-ionized plasma of strongly convecting areas of the high latitude F-region leads to the excitation of electrostatic microinstabilities (λ ～ 50 cm) at frequencies of the order of the lower hybrid frequency and smaller. We have estimated the threshold conditions for the excitation of the unstable waves under various physical circumstances. For some representative conditions we have also calculated the frequencies, growth rates, and wavelengths for the fastest growing modes using the linear approximation. We stress that the present theory breaks down in regions where the plasma cannot be treated as locally homogeneous. The altitude range over which the theory is applicable also varies with conditions. For highly disturbed conditions the upper altitude limit may be as high as 400 km.     

Latitudinal gradients in the electron temperature of the lower F-region at mid- to low latitudes

In this paper we present the results of a study of the thermal balance of the lower F-region at mid- to low latitudes. By using a mathematical model with input data based on in situ measurements along AE-C orbits 457, 666 and 677 (26 January, 1974, 14 February, 1974 and 15 February, 1974, respectively) we demonstrate that electron heat conduction along the magnetic field lines has to be included in the model if good agreement between the calculated and observed electron temperatures is to be achieved. This gives support to the suggestion made by Hoegy and Brace (1978), that the discrepancy in the shape of the electron heating and electron cooling rate distributions reported by Brace et al. (1976) resulted mainly from neglecting heat conduction in the electron gas. In addition, our results indicate that the currently used plasma heating and plasma cooling rates and the photoelectron heating rates calculated by Brace et al. (1976) for the orbits used in this study are consistent with the AE-C in situ measurements.     

Medium scale gravity waves in the ionospheric F-region and their possible origin in weather disturbances

This study results from a coordinated experiment involving ionospheric observations of Faraday rotation between a geostationary satellite and three ground based receivers at Aberystwyth and Bournemouth in the U.K. and Lannion, France, together with incoherent scatter observations at St. Santin-Nancay, France.Quasi-periodic variations of electron content observed simultaneously at the three stations are interpreted in terms of medium scale gravity waves travelling in the ionospheric F-region. Characteristics of these waves are derived by means of a cross-correlation technique.A reverse ray tracing computation, using data on the neutral atmosphere and neutral wind stratification from the incoherent scatter observations, has been used in an attempt to locate the sources of these waves.The results show that some of the waves are almost certainly generated above 100 km altitude, probably by auroral phenomena, while the others could be produced near ground level by meteorological sources. The reverse ray tracing indicates that the latter sources are in general located in a geographic area in the vicinity of a weather disturbance. A production mechanism for these waves is proposed involving ageostrophic perturbations of the neutral wind in a jet stream.     

The behaviour of the midlatitude F-region at the time of four magnetospheric substorms during the night of 29/30 October 1973

The data from observations of the geomagnetic field, ionospheric parameters and atmospheric emissions, carried out at four midlatitude station in Bulgaria are analysed. The observations refer to the geomagnetic disturbance on 28/30 October 1973 (K_p^max = 7) and also to a very quiet period before it. It is shown that all four geomagnetic substorms during the night of 29/30 October influenced the midlatitude F-region. This is indicated by a lowering of the height of the F-region by ca. 50–70 km. Owing to this downward drift of ionisation the dissociative recombination and the intensity of the red line is accordingly increased. As an explanation of this phenomenon we suggest the action of the electric fields, which can at the same time be transported from the magnetosphere to the ionosphere.     

Auroral E-region: Ion composition and nitric oxide

Data from eight auroral ion composition measurements, seven of which have been reported in the literature, are analyzed and compared in terms of a single model format. We find, contrary to conclusions published previously for two of the experiments, that there is no discrepancy concerning O⁺ ions. In general, the mean CIRA 1972 neutral model is found to be quite suitable as a representative of the major gas composition required for auroral E-region calculations which agree with the data. Nitric oxide profiles inferred from analysis of the data range from about normal non-auroral E-region nitrix oxide distributions with peak concentrations near 10⁸ cm^?3 to profiles with peak populations near 10⁹ cm^?3. Although the higher concentrations are generally correlated with intense aurora, we acknowledge that the length and strength of auroral activity prior to the individual rocket flights can have an even greater bearing, at times, on the NO “snapshot” profile deduced from the auroral ion composition data.