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.     

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban (L = 1.69, 29°53′S, 31°00′E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L (L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds (T_i ∼ 5 ms) and large wave amplitudes (B_w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.     

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.     

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.     

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.

     

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.     

Ion-neutral dynamics in the high latitude ionosphere: first results from the INDI experiment

The INDI experiment consisted of a series of joint observations between EISCAT and a Fabry-Perot interferometer (FPI) situated at Kiruna. The FPI measured the 630 nm neutral oxygen emission at eight positions on a 30° elevation circle, giving a spatial average of the neutral wind field with a time resolution of about 15 min. The radar performed a seven-position, near-meridional scan in a region common to the optical measurements. Such simultaneous observations of the ionised and neutral components of the Earth's atmosphere allow a study of the ion energy balance and the coupling between species. The first stage of the analysis was to derive the neutral wind from the EISCAT data using the simplified ambipolar diffusion and ion energy equations. This was then compared with the direct measurements from the FPI. There was good agreement between derived and measured meridional winds, but the zonal wind values, although showing the same trends, differed in magnitude by a factor of 3–5. The reasons for this are discussed. The meridional wind data was used to derive the ion-neutral collision frequency. This was a factor of 2 or 3 less than recent model values. Preliminary comparisons of the measured electron densities with the 630 nm emission intensity gave clues to the chemistry of the emission process.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.     

Modifications introduced in the nonlinear whistler-electron interaction by parallel electric fields

Large scale magnetospheric parallel electric fields expected to occur beyond the plasmapause modify the nonlinear behaviour of the cyclotron resonance between electrons and coherent whistler mode waves. Enhancement of the effects originated in the interaction requires waves of varying frequency which might be generated in natural emissions, or artificially injected into the magnetosphere. Adopting a static electric field model based on a differential pitch-angle anisotropy in the geomagnetic mirror, we contrast the evolution of the resonant particles and the adequate ground transmitter frequency format with the corresponding results for the zero electric field case. The outcome, obtained for a field line with L = 6.2, demonstrates the strong influence of the parallel electric field on the nonlinear cyclotron resonance.     

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.     

Equatorial ionospheric electric fields during magnetospheric disturbances: local time/longitude dependences from recent EITS campaigns

Data sets collected during a few coordinated Equatorial Ionosphere-Thermosphere System (EITS) observational campaign periods, mainly from the Brazilian and Asian longitude sectors are analysed in this paper. Ionosonde magnetometer and Ionospheric Electron Content (IEC) data from the EITS-1 and -2 campaigns (during March and December 1991) are complemented by interplanetary magnetic field and some ground based data sets from other campaigns. The analysis focuses on the response of the equatorial ionospheric heights and ionization anomaly to disturbance electric fields, identified as a direct penetration electric field associated with IMF B_z changes and development of the ring current (especially the asymmetric component), and that produced by a disturbance zonal neutral wind. New evidence on the local time and longitudinal dependences of these electric fields constitute the main results of this paper. Especially, a large eastward electric field (associated with the asymmetric ring current) in the dusk-dawn sector causes significant expansion of the EIA in this sector, and amplification of the evening prereversal uplift of the F-layer over Brazil. Significant inhibition of the evening prereversal electric field enhancement seems to be produced by the disturbance zonal wind associated with the magnetic disturbances prevailing several hours earlier. Some tentative evidence on the Brazilian dusk sector disturbance field being larger than that of the Asian dusk sector support the existence of a longitude asymmetry in the intensity of the disturbance electric field.     

Early Paleoindian Mobility and Watercraft: An Assessment from the Mississippi River Valley

Several researchers have suggested use of watercraft during the Early Paleoindian period 11,500 and 10,800 rcybp (13,400–12,700 cal B.P.), but none have brought empirical data to bear on this possibility. This paper addresses the potential for fluted point-making groups to have made and used boats circa 11,000 rcybp (13,000–12,800 cal B.P.). Fluted point data from a large region of the upper and central Mississippi River valley strongly suggest that the Mississippi River was a barrier to movement and that Early Paleoindians in the midcontinent did not routinely use watercraft.     

An improved model of the variation of electron concentration with height in the ionosphere

An empirical model of the variation of electron concentration with height is described which overcomes some limitations found in practice with a previous widely used model (Bradley and Dudeney, 1973). In particular, the new model will generate more realistic variations of electron concentration with real height and virtual height, both including and excluding an F1-ledge. The model has no gradient discontinuities and will reproduce cases in which the F1-ledge does not have a true turning point. The model should prove very valuable for a wide range of propagation problems and for certain aeronomical applications.     

Magnetopause plasma transients: signatures in the auroral ionosphere

The magnetopause and adjacent boundary layers of the Earth's magnetosphere play important roles in transferring momentum and energy from the solar wind to the magnetosphere-ionosphere system. The details of the different boundary processes, their ionospheric signatures and relative importance are not well known at present. Particle precipitation, field-aligned current, auroral emission, ionospheric ion drift and ground magnetic perturbations are among the low-altitude parameters that show signatures of various plasma processes in the LLBL and the magnetopause current layer. Magnetic merging events, Kelvin-Helmholtz waves, and pressure pulses excited by the variable solar wind/magnetosheath plasma are examples of boundary phenomena that may be coupled to the ionosphere via field-aligned currents. In this paper, attention is focussed on a specific category of auroral activity occurring in the cusp/cleft region predominantly during the southward directed interplanetary magnetic field (IMF). Co-ordinated observations from the ground and satellites in polar orbit have been used to study the temporal/spatial development of the events in relation to the background patterns of particle precipitation and ionospheric convection as well as the field-aligned current and ion drift characteristics of the individual events. The auroral phenomenon is characterized by a sequence of elongated forms moving laterally into the polar cap. Spatial scales of major events repeating every 5–10 min are ∼200 km (N-S) times 300–1000 km (E-W). Smaller scale auroral structures with more irregular occurrence rates are observed at times. The preliminary evidence suggests that the motion pattern is regulated by the IMF orientation, that is, the direction of longitudinal motion along the polar cap boundary is determined by the IMF B_Y polarity. The examples reported here occurred within 1000–1400 MLT, near the zero point potential line separating the morning and post-noon convection cells. During nonzero IMF B_Y the auroral structures are associated with channels of enhanced zonal ionospheric ion flow and Birkeland current sheets of opposite polarity, imbedded within the larger scale IMF B_Y-related cusp-mantle current system. These characteristics are discussed in relation to model predictions of ionospheric signatures of magnetopause plasma transients, with particular emphasis placed on impulsive magnetic merging events.     

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.     

Angular characteristics of radio pulses reflected from the subpolar ionosphere

This paper presents the results derived by measuring angular spectra of HF-radio pulses reflected from the subpolar ionospheric F2-region (62°N) using vertical-incidence soundings and a phase direction finder with Doppler filtering. The results correspond to three main types. One is the classical mirror reflection from the undisturbed ionospheric F2-region, typical of mid-latitudes (deviations from zenith do not exceed 3°; the angular spectrum width is less than 1°). The second type includes oblique diffuse reflections with a deviation from zenith of from 10 to 45°. The azimuth of arrival of these reflections is distributed in the range from 0 to 360°, the angular spectrum width is from 5 to 10°, and the range varies from 400 to 600 km. The third type includes anomalous mirror reflections with small deviations from zenith (not greater than 3°) but with substantially larger detection ranges (for example, 500km) as compared with the main reflections (250–300 km).