Using a constraint on the parallel velocity when determining electric fields with EISCAT

A method is proposed to determine the perpendicular components of the ion velocity vector (and hence the perpendicular electric field) from EISCAT tristatic measurements, in which one introduces an additional constraint on the parallel velocity, in order to take account of our knowledge that the parallel velocity of ions is small. This procedure removes some artificial features introduced when the tristatic geometry becomes too unfavourable. It is particularly well suited for the southernmost or northernmost positions of the tristatic measurements performed by meridian scan experiments (CP3 mode).     

The generation of electric fields by aerosol particle flow in the middle atmosphere

The generation mechanism of electric fields in the middle atmosphere based on the interaction between charged aerosol particles and an updrafting air flow is considered. Due to the gravity force there occurs a relative motion of air and aerosol particles which excites electric space charge waves. The mechanism is analogous to that of the resistive beam-plasma instability. It is shown that the most favourable conditions for the instability are realized at heights of 80–90 km in regions where the electron density is relatively low and heavy ions are predominant. Estimates are given for the aerosol component parameters which are necessary for the instability to be switched on.     

Equatorial spread-F by electric fields and atmospheric gravity waves generated by thunderstorms

It is assumed that atmospheric gravity waves, resulting in travelling ionospheric disturbances (TIDs), and electric fields, generated by convective thunderstorms, have a reasonable influence on the large-scale structure of premidnight equatorial spread-F irregularities. The responsible mechanisms, viz the superposition of thunderstorm generated electric fields on the ionospheric electric fields being the determining factor for irregularity generation and the steepening of TID structures due to spatial resonance, are briefly outlined. It is recalled that convective activity is most pronounced in the intertropical convergence zone over the African and South American continents. A model based on the typical features of seasonal and geographical variation of tropical convection generating the TIDs is presented which can explain seasonal and geographical variations of premidnight equatorial spread-F occurrence.     

Winds and electric fields in the upper E-region over Malvern

Some preliminary hourly values of total ionic velocity in the upper E-region (122–144 km) are presented and the magnetic meridian component of horizontal neutral wind, averaged from observations on 14 days spread over about two years, is studied. The semi-diurnal variation is found to be predominant at 122 km, almost exactly in phase with the same component at St. Santin (France). Knowledge of the complete ionic velocity allows the rigorous calculation of the meridian perpendicular component of the electric field, some preliminary values of which are presented.     

Accelerated drying of water-wetted materials in electric fields

The electric currents, I, associated with drying rates of discs of paper toweling and cotton cloth wetted with water have been measured as functions of several parameters: E, the strength of the electric field in which the discs were placed; T, the temperature; H, the percent relative humidity. Drying rates increased monotonically with E, and typical drying times decreased by factors of up to about 10 as E rose from 0 to above 7 kV/cm. Measured electric currents over this range of E increased 3–4 orders of magnitude. A solid dielectric object placed in the electric field caused I to be reduced proportionately to the area of the disc that was blocked by the object.     

Modelling the penetration of thundercloud electric fields into the ionosphere

In a previous paper, we considered the penetration of DC thundercloud electric fields E into the ionosphere and also into the region between the ionosphere and the ground (Velinov and Tonev, 1994). In the present paper, we extend the analysis by making a more precise approximation of the electric conductivity profiles by 5–10 piecewise exponential functions of altitude instead of the two functions used up to now. This allows a much more realistic representation of the atmospheric conductivity profile. Besides, Maxwell's equations are solved for more general boundary conditions, taking into account that the electrosphere is not a perfect conductor. This leads to the appearance not only of the transverse E_r (as had been assumed until now), but also of the geomagnetic field-aligned E_z component of the penetrating thundercloud electric fields. The computations show that both E_r and E_z cause significant variations of the electron density profiles N(z) in the ionosphere.     

Enhanced drying rates of wetted materials in electric fields

Laboratory experiments have been performed in which the drying rate of discs of paper towelling moistened by water was measured as a function of the strength of the electric field E in which they were placed. The drying rate increased monotonically with E, the associated drying time (in a typical experiment) decreasing by a factor of about 6 as E rose from 0 to 7 kV/cm.     

Satellite observations of zonal electric fields near sunrise in the equatorial ionosphere

We report here on a number of examples of anomalous enhancements of eastward electric fields near sunrise in the equatorial ionospheric F-region. These examples were selected from the data base of the equatorial satellite, San Marco D (1988), which measured ionospheric electric fields during a period of solar minimum. The eastward electric fields reported correspond to vertical plasma drifts. The examples studied here are similar in signature and polarity to the pre-reversal electric field enhancements seen near sunset from ground-based radar systems. The morphology of these sunrise events, which are observed on about 14% of the morning-side satellite passes, are studied as a function of local zonal velocity, magnetic activity, geographic longitude and altitude. The nine events studied occur at locations where the zonal plasma flow is generally measured to be eastward, but reducing as a function of local time and at satellite longitudes where the magnetic declination has the opposite polarity as the declination of the sunrise terminator.     

Early Trimpi events from lightning-induced electric fields in the ionosphere: An alternative explanation

Two classes of ‘Trimpi’ modulation of VLF signals in the Earth-ionosphere waveguide have been identified in the literature. The more common type occurs l s or more after causative lightning strokes, the second in less than 100 ms. We explore the possibility that these early Trimpi events result from lighting-generated, electric field impulses lowering the mirror altitudes of trapped electrons. To overcome the mirror force on energetic electrons, upward-directed electric fields with strengths of a few tens of mV/m are required. This is well within the range of electric fields observed on sounding rockets above thunderstorms.     

A quasi-DC model of electric fields in the ionosphere-ground region due to electrified clouds

The penetration of the quasi-DC electric fields, E due to electrified clouds, into the middle- and high-latitude ionosphere is theoretically studied during the initial stage of cloud charge separation. The electrification process is characterized by a source function S(t), whose variations are on the time scale of the relaxation process in the cloud. A first-order approximation solution for the time variation of E is obtained as an explicit function of the electric field in the steady-state case. Some features of the time variations of E at different altitudes (in the ionosphere and also in the atmosphere) are investigated, depending on the source function and the relaxation time constants. This result can also be applied for slowly electrifying clouds which do not produce lightning.     

Ion composition changes during F-region density depletions in the presence of electric fields at auroral latitudes

F-region density depletions in the afternoon/evening sector of the auroral zone are studied with the EISCAT UHF radar. Four case studies are presented, in which data from three experiment modes are used. In each case the density depletion can be identified with the main ionospheric trough. For the two cases occurring in sunlit conditions the electron densities recovered significantly after the trough minimum. Tristatic ion velocity measurements show the development of poleward electric fields of typically 50–100 m Vm⁻¹, which maximize exactly in the trough minimum. A special analysis technique for incoherent scatter measurements is introduced, based on the ion energy equation. By assuming that the ion temperature should obey this equation it is possible to fix this parameter in a second analysis and to allow the ion composition to be a free parameter. The results from two experiments with accurate velocity measurements indicate that the proportion of O⁺ near the F-region peak decreased from 100% in the undisturbed ionosphere to only 10% and 30%, respectively, in the density minimum of the trough. The loss of O⁺ is explained by the temperature dependence of recombination with nitrogen molecules. Temperatures derived from radar measurements are very sensitive to the assumed ion composition. For the above case of 10% O⁺ the deduced electron temperature in the trough was transformed from a local minimum of < 2000 K to a local maximum of 4000 K.     

On the role of auroral electric fields in the formation of low altitude sporadic-E and sudden sodium layers

The characteristics of metallic and molecular ion sporadic-E (E_s) layers, formed by the action of strong electric fields at auroral latitudes, are examined using computer simulations. It is found that, for electric fields directed between northward and westward (northern hemisphere), thin metallic ion layers (<2 km thick) can be formed above about 105 km altitude. For electric fields directed from westward, through southward, to south-eastward, slightly thicker (4–6 km thick) metallic ion layers can form between 90 and 105 km altitudes. Thin layers of molecular ions can be formed by electric fields directed between north and west if the ion density is low. Examples of E_s layers observed by the EISCAT radar, together with simultaneous observations of electric fields and ion drifts are presented which show good agreement with the simulations. The relationship between the lower-altitude E_s layers and sudden sodium layers (SSLs) is discussed leading to an explanation of some of the characteristics of SSLs at high latitude. A possible involvement of smoke particles in the formation of both E_s layers and SSLs is proposed.     

Modifications in the nighttime low-latitude ionosphere after southward turning of the IMF

Sharp decreases in Φ_oF2 are found to occur frequently in the nighttime low-latitude ionosphere after southward turning of the IMF B_z component, especially under isolated B_z turnings, i.e. when the IMF has been northward for at least 6 h before its turning. These decreases occur simultaneously (within a 1-h time interval) with the B_z turning. The effect is observed both when a substorm or a magnetic storm begins after B_z has turned southward, and when a noticeable substorm does not occur. The effect is more pronounced after midnight and a maximum at 03 LT. Short-term (with scale times of about 1 h) variations of Φ_oF2 and h_mF2 for B_z southward turning are analysed using a large amount of ground-based and topside sounding data. The decreases in Φ_oF2 are shown to occur at first over the magnetic equator and then, during the second hour after the turning, at the crests of the equatorial anomaly. The ionosphere returns to its undisturbed state, on average, in 4–5 h (if other disturbing agents do not arise). These decreases are suggested to be caused by modifications in the electric field in the low-latitude ionosphere associated with B_z southward turning.     

Small-scale fluctuations of magnetic and electric components of the ELF and VLF wave fields in the sub-auroral topside ionosphere : stochastic characteristics of the wave field

When the Interkosmos-14 and Interkosmos-19 satellites crossed the region of spatially varying electron concentration in the topside ionosphere adjacent to the high-latitude boundary of the main ionospheric trough, it was discovered that there were simultaneous fluctuations of plasma density, temperature and the amplitudes (H_x and E_y) of the ELF and VLF radio/plasma emissions. The probability characteristics of the naturally perpendicular H_x and E_y fluctuations are analysed. The correlation coefficient R(_H, E_y) turned out to be less than 0.6 at frequencies of F ⩽ 4.65 kHz, while at higher frequencies R increases, up to 0.9 at 15 kHz. The following interpretations are proposed:

1. 1.1. While measuring noise emissions, as a rule a mixture of numerous elementary waves is recorded.
2. 2.2. At frequencies exceeding the local lower hybrid resonance frequency (in our case f_LHR ≈ 5 kHz), a mixture of electromagnetic waves experiencing the influence of the inhomogeneous electron concentration N_e is registered.
3. 3.3. At frequencies which are lower than the local value f_LHR the mixture mainly consists of ELF waves. The wave field has a complicated structure, and the dynamical coherence between electric and magnetic field components is not as simple as at VLF frequencies (f ≈ 15 kHz).
4. 4.4. It is shown that the wave components for a mixture of electromagnetic and electrostatic waves (for instance a mixture of VLF and lower hybrid frequency waves) have a lower correlation coefficient because the electrostatic waves are unrelated to the electromagnetic waves.
5. 5.5. The correlation analysis offers an opportunity to detect the presence of waves of various types in the wave mixture.

     

The nonlinear mechanism of gravity wave generation by meteorological motions in the atmosphere

Using asymptotic expansions of the hydrodynamic equations in the Rossby number and the method of multiple time scales, we derive approximate expressions for the inhomogeneous “forcing” terms which describe the continuous generation of inertio-gravity waves by quasi-geostrophic motions. As a result of numerical modelling applied to the evolution of tropospheric meso- and macro-scale wave sources, the values of these forcing terms are estimated. A three-dimensional numerical simulation of wave propagation from a mesometeorological tropospheric eddy into the upper atmosphere was done to estimate the gravity wave response to the sources described. The results of the calculations show that the most part of the wave energy propagates quasi-horizontally carried by two-dimensional inertio-gravity waves. At the same time, a part of the energy is transported into the upper atmosphere by internal-gravity waves which can create regions of wave disturbance in the upper atmosphere at considerable distances from the source site. The amplitudes of these waves increase with increasing intensity and decreasing time scales of the wave sources and can reach the values observed in the upper atmosphere.     

Meridional electric currents in the equatorial F-region

A study of the boundary conditions for the equatorial thermospheric transport equations by the authors has led to the theoretical prediction of the vertical electric field at the base of the F-region. Earlier, this result was applied to the calculation of the zonal wind field in the equatorial F-region. In this work, the aforementioned model is applied to the calculation of the F-region electric current field in the meridional plane as a function of time and the east-west magnetic field generated by these currents. In particular, the field at sunset is compared with the observations made by Magsat.     

Three-dimensional ionospheric currents and fields generated by the atmospheric global circuit current

Three-dimensional ionospheric currents and fields generated by atmospheric global circuit currents, using the distribution of air-Earth currents as a lower boundary conditions of the ionosphere, have been studied. The air-Earth currents are obtained taking geomagnetic and orographic effects into account, under the assumption of an ionosphere with infinite conductivity. Three dimensional ionospheric currents due to thunderstorm sources are calculated, considering the conductivity distribution in the ionosphere and the configuration of the magnetic field. The calculated potential difference in the ionosphere is 55 V and according to our model the horizontal electric field is too weak to affect the ionosphere and magnetosphere significantly. Horizontal currents are not distributed uniformly, but preferably in the day-side hemisphere and especially in the equatorial region, and vertical currents and fields do not simply decrease with altitude near the equator because of anisotropy and nonuniformity in the conductivity.     

Fluxes of electrons generated by proton interaction with the atmosphere

In this paper the interaction of precipitated protons with the atmospheric components is analyzed using a new approach. The method described presents a different way of calculating the proton effects yielding the production rate and fluxes of the generated electrons as a function of height and energy. Also the model gives the possibility of estimating independently the effects of protons and resulting secondary and tertiary electrons in proton events.     

Simultaneous measurements of the ionospheric electric field by probes and radar methods

Ionospheric electric field values are presented, obtained simultaneously by the double probe technique on board a rocket and by two incoherent backscatter radar installations. The measurements were performed during auroral activity over northern Scandinavia. The spatial distribution of the field reveals pronounced local variations.     

Analytical modelling of the electric field distribution in the high-latitude ionosphere

An analytical approach is implemented for self-consistent modelling of the high-latitude convection electric field. Input parameters are determined as distributions of field-aligned currents and height-integrated conductivity. The high-latitude ionosphere is approximated with an arbitrary number (N) of concentric rings. The height-integrated conductivity (∑) is independent of co-latitude within any ring, but depends on the longitude ~ sin λ. The field-aligned currents flow only along the boundaries of each ring and are presented by Fourier series in longitude. The analytical solution for the potential φ as a function of longitude is also presented as a Fourier series. An analytical solution is obtained for the potential dependencies on co-latitude. For the extreme case, when the integrated conductivity does not depend on longitude, this solution coincides with the analytical results, obtained by other authors. Based on this solution, the potential distribution in the high-latitude ionosphere, an example with N = 5 is shown, the values of conductivity and field-aligned currents being similar to those values used by other authors.