Comparative studies of E-region ionospheric drifts and meteor winds

Ionospheric drifts using total reflections from the E-region have been compared with neutral winds measured by meteor radar. Close agreement was found when both measurements were made in a common volume of atmosphere. Even with a separation of 700 km between the measuring regions the results were very similar. It is concluded that the drift technique does measure the movement of the neutral atmosphere in the altitude range 95–120 km. The agreement between measurements from widely separated regions indicates the horizontal scale of the wind structure is at least 700 km.     

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.     

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.     

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.     

Determination of electric fields and neutral wind velocities in the ionospheric E- and F-regions from observations of artificial ion clouds

A new method of interpreting the behaviour of artificial ion clouds released in the Earth's ionosphere is presented. It is shown that values for the ionospheric electric field, neutral wind velocities and, in some circumstances, ion collision frequency, can be deduced from a study of the motion and deformation of the ion clouds, including those released in the E-region.     

Observation of vertical E-region neutral winds in two intense auroral arcs

Vertical winds have been observed by optical Doppler measurements of the 557.7 nm emission in the aurora, using a Fabry-Perot spectrometer. Both upward and downward winds were observed, of 15 m s^?1 magnitude. The upward winds were associated with westward overhead currents, and with low altitude aurora (～ 110 km) as determined by the auroral temperature, while a high altitude aurora (～ 135 km) and eastward currents were associated with the downward wind. The Lorentz force of these currents has the wrong direction to act as a direct forcing mechanism. It is concluded that Joule heating is directly responsible for the upward winds, while the divergence of horizontal winds is responsible for the downward winds.     

Ion composition in the E- and lower F- region above kiruna during sunset and sunrise

A magnetic type mass spectrometer has been flown on two ESRO sounding rockets from ESRANGE (Kiruna 68°N) on February 25 and 26, 1970. The first launch was at sunset (16:33 UT) and the second the next morning, during sunrise (04:47 UT). For both flights the solar zenith angle was approximately 98°. The instrument was measuring simultaneously the neutral gas and positive ion composition and the total ion density. In this paper the results of the ion composition measurements are presented. For both flights the main ion constituents measured between approximately 110–220 km were O⁺, NO⁺ and O₂⁺. Only at sunset were N⁺ and N₂⁺ detected above 200 km. In spite of the identical solar UV-radiation, pronounced sunset/sunrise variations in the positive ion composition were found. The total ion densities at sunrise were between 5×10³ and 5 × 10⁴ ions cm^?3 and therefore too high to be explained without a night-time ionization by precipitated particles. At sunrise the NO⁺ and O₂⁺ profiles show a correlated wavelike structure with three pronounced almost equally spaced layers in the E-region. Only the highest layer is present in the O⁺ profile. Locally enhanced field aligned ionization originated by particle precipitation and an E × B instability are the most likely source for this structure. In the E- and lower F-regions the $\text{NO}{}^{\mathrm{+}}\text{O}{}_2{}^{\mathrm{+}}$ ration increased overnight from values around 7 at sunset to 15 at sunrise, correlated with an increase of the local magnetic activity index K from 0⁺ to 2°. This could be explained if the NO density and magnetic activity are correlated.     

Antarctic substorm events observed by sounding rockets,ionization of the D- and E-regions by auroral electrons

The ionization structure of the auroral arc was measured on a sounding rocket which penetrated into a bright auroral arc. The E-region electron density becomes large (2 ～ 5 × 10⁵ el/cm³ only in the moving auroral arc, whose N₂⁺ 4278 Å brightness is 1 ～ 2·5 kR. The electron density in the D-region beneath the lower boundary of the arc (75 ～ 98 km in altitude) is also considerably enhanced to 2 ～ 5 × 10⁴ el/cm³.The observed E-region electron density can be interpreted theoretically as due to the direct ionization by precipitating electrons, whose energy spectrum is approximately represented by an exponential type having the characteristic energy of 2 keV. The correlation between the electron density and the N₂⁺ 4278 Å brightness can be reasonably explained by considering the simultaneous effects on the ionization and the optical excitation caused by the primary electrons having a flux of 9 × 10⁹ el/cm²/sec per 1 kR of the 4278 Å emission.Further analyses using the electron density data from four other sounding rockets have shown that the D-region ionization has good correlations to the cosmic noise absorption (CNA) and the magnetic substorm activities observed simultaneously at the ground station, whereas it has poor correlation to the same quantity of the E-region measured in the same experiment. It is found that the observed D-region ionization is much larger than that predicted by the theory which takes into account the Bremsstrahlung X-ray ionization along with the direct impact ionization when it is applied to the precipitating electron flux spectrum consistent to the E-region ionization and optical excitation.After all the present experimental results suggest a dual nature of the electron precipitation spectrum in the substorm, i.e. the softer part which is localized in the auroral arc and the harder part which is spatially wide-spread over the substorm area.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Empirical model of the E-region electron temperature around noon and at low magnetic activity

Electron temperature measurements made with Langmuir probes at E-region heights together with deviative absorption data show that the electrons are not in thermal equilibrium with the neutrals. Moreover, for very quiet days (A_p ? 7, K_p ? 1⁺ throughout the whole day) and hours close to noon the quotients between the electron and neutral gas temperature profiles have a similar behaviour. In this paper T_e profiles measured in situ with Langmuir probes and T_n, profiles given by neutral atmosphere models, both in the specified ionospheric conditions, are used to compute $\text{T}{}_{\mathrm{e}}\text{T}{}_{\mathrm{n}}$ profiles. Each of the profiles thus obtained is fitted by a Lorentzian curve and the variation with F_10.7 of its parameters is also fitted by simple mathematical expressions.