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     

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.     

Electron density profiles above the F-layer

A method for the determination of electron densities up to 1000 km by using data from vertical sounding and Faraday rotation techniques is suggested. Electron density profiles and contours plotted following this method are compared with results obtained by Evans using the incoherent scatter technique.     

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.     

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 day-sector polar F-layer during a magnetospheric substorm

Ionogram and all-sky camera data have been recorded on the Air Force Cambridge Research Laboratories' Flying Ionospheric Laboratory in the day sector of the auroral oval under conditions of darkness. The airborne measurements show that the polar F-layer irregularity zone, which is characterized on ionograms by a generally non-retarded and spread F type echo, exhibits meridional motions similar to the day-sector auroras. The polar F-layer irregularity zone and the day-sector auroras move equatorward and then move poleward in harmony with the development and decay of a magnetospheric substorm. We suggest that the polar cusp also moves in essentially the same fashion.     

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.     

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.     

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.     

Thermal balance of the mid-latitude F2-region at night

The thermal balance of the plasma in the night-time mid-latitude F2-region is examined using solutions of the steady-state O⁺ and electron heat balance equations. The required concentrations and field-aligned velocities are obtained from a simultaneous solution of the time-dependent O⁺ continuity and momentum equations.The results demonstrate the systematic trend for the O⁺ temperature to be 10–20 K greater than the electron temperature during the night at around 300 km, as observed at St. Santin by Bauer and Mazaudier. It is shown that frictional heating between the O⁺ and neutral gases is the cause of the O⁺ temperature being greater than the electron temperature; the greater the importance of frictional heating in the thermal balance the greater is the difference in the O⁺ and electron temperatures. A study is made of the roles played in the thermal balance of the plasma by the thermal conductivity of the O⁺ and electron gases; collisional heat transfer between O⁺ electrons and neutrals; frictional heating between the O⁺ and neutral gases; and advection and convection due to field-aligned O⁺ and electron motions. The results of the study show that, at around 300 km, electron cooling by excitation of the fine structure of the ground state of atomic oxygen plays a major role in the thermal balance of the electrons and, since the temperature of the ions is little affected by this electron cooling process, in determining the difference between the ion and electron temperatures.     

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.     

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.     

Ambipolar diffusion in the F1-region of the ionosphere

Ambipolar diffusion is discussed for the case of a weakly-ionized, multicomponent plasma. Shortcomings of some of the previous formulations of this problem are pointed out. In particular, it is shown that the polarization electrostatic field acts to couple the motion of the various ions with the result that the diffusion velocity of each ion depends on the density gradients of all the ions.     

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.     

Ducted propagation of hydromagnetic waves in the F2 layer

Calculations of the properties of the ionospheric duct centered at the F₂ layer are carried out with a view to investigating the ducted propagation of Pc1 micropulsations in directions out of the geomagnetic meridian plane. For a horizontally uniform ionosphere, duct properties are found to be essentially the same in all horizontal directions. Propagation characteristics of ducted waves, however, vary according to ionospheric and sunspot conditions. In practice, therefore, it is expected that horizontal propagation over a large recording network is not isotropic because of the diurnal changes in the ionosphere.     

Observations of ELF electromagnetic waves associated with equatorial spread F

Extreme low frequency electromagnetic waves have been observed below the F peak in the equatorial ionosphere by instruments onboard OGO-6. Electrostatic wave observations indicate that the steep gradient was unstable to the process which causes equatorial spread F above the region where the electromagnetic waves were observed. The data are very similar to observations near the polar cusp and give further evidence that ELF waves are excluded from regions of rapid and irregular density increases. Low level electromagnetic waves with similar properties were occasionally observed on the nightside by the OVI-17 electric field sensor and may be plasmaspheric hiss which has propagated to low altitude.     

A model of ionospheric F-2 region storms in middle latitudes

The proposed ionospheric storm model is based on a heat source located at magnetic noon on Feldstein's auroral oval. The rotation of the Earth produces an apparent motion of the source which is greater than the speed of the disturbance. This gives rise to a wake or front which sweeps over the globe and determines the onset time of the negative phase which results from a change in chemical composition. At the front, focussing will occur which accounts for the sudden drop in electron density (or contents) sometimes observed. The calculated onset times of the negative phase are compared with observations for a number of storms. The local onset times vary from 12 at the latitude of the source to around 24 at 10° geomagnetic latitude. This model predicts that the onset of the negative phase at a given location, for storm which commence between about 2000 LT to about 1000 LT, is independent of the time of storm commencement.     

Coupling between the F-region and protonosphere: Numerical solution of the time-dependent equations

This paper describes a new method of solution of the time-dependent continuity and momentum equations for H⁺ and O⁺ in mid-latitude magnetic field tubes from the F-region to the equator. For each ion the equations are expressed as an integro-differential equation. This equation is treated as an ordinary differential equation and solved by a searching method. By means of this method, the distribution of H⁺ in the O⁺?H⁺ transition region and the protonosphere can be investigated and the influence of H⁺ fluxes on the F layer examined.As an example of application of the method a suggestion by Park (1971) about observed night-time enhancements of N_mF2 is examined. He suggested that lowering of the F layer some hours after a magnetic substorm may cause N_mF2 to increase because of increased ion influx from the protonosphere. In the present calculations the Flayer is maintained around a constant height for some time and then abruptly lowered. Under the conditions adopted the resulting increase in downward H⁺ flux is sufficient to maintain N_mF2 against the increased recombination but not to increase N_mF2 significantly. It is emphasised that these results are not conclusive.