Adiabatic and isothermal ion-acoustic speeds of stabilized Farley-Buneman waves in the auroral E-region

The influence of ion and electron energetics on the propagation speeds of stable Parley Buneman waves which are excited by E × B drifts in the auroral E-region is studied theoretically in the fluid limit, with the effects of anomalous collisions on electron thermal conduction included for the first time. In particular, the ratio of the phase speed of waves, stabilized by enhanced diffusion effects, to the isothermal ion-acoustic speed are calculated for realistically modelled E-region ion and electron temperatures, as functions of altitude, flow velocity and wavelength. It is found that the phase speeds of these stabilized waves begin to increase above isothermal ion-acoustic speeds as wave frequencies increase to values where they are comparable with the electron inelastic collision frequency. However, at still higher frequencies their phase speeds tend to fall back towards their isothermal values due to the increasing effects, with increasing wavenumber, of electron thermal conductivity. It is also found that the phase speeds are not always isotropic with respect to flow angle. The relationship between the predictions of the present fluid theory and a previous kinetic theory calculation is also briefly discussed.     

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10⁴ cm⁻³. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.     

Coherent radar (SABRE) studies of dynamical processes in the auroral E-region

In the coherent radar technique, bacsccatter is obtained from plasma irregularities even though the radar frequency can greatly exceed the ionospheric plasma frequency maximum. From the velocity spectrum of the received signals an estimate of the flow velocity can be obtained and hence the electric field determined. Information regarding the irregularity scattering cross section is obtained from the amplitude of the backscatter return. Current radar studies of a range of geophysical phenomena are presented. In addition, attention is drawn to the problems of interpreting the radar observations in terms of the underlying geophysical processes.     

On Joule heating of the equatorial electrojet E-region

Simultaneous data on electron density, electron temperature and current density obtained from a rocket borne Langmuir probe, a glass-sealed Langmuir probe and a proton precession magnetometer flown from Thumba (geomag. lat. 0.99°S, geomag. long. 146.79°E, magnetic dip 0°47'S) have been used to calculate the Joule heating in order to assess whether it contributes significantly to the thermal imbalance in the E-region. It is envisaged that the changes in electron temperature are partially brought about by changes in collision frequency and the energy loss factor. It is found that the Joule heating alone is not sufficient to explain the observed differences in electron and neutral gas temperatures. The inclusion of photoelectron heating and adjustments of profiles of the collision frequency and the energy loss factor bring the computed temperature differences closer to the observed differences.     

Empirical electron recombination coefficients in the D- and E-region

Effective electron recombination coefficients were deduced from measured ion production rates and plasma densities of 29 D-region rocket flights. They were grouped according to day and night conditions and arranged vs. ion production rate. Corresponding results from an ion-chemical model are presented for comparison.     

Auroral infrasonic waves and the auroral electrojet

Using models of a line current and a band current with a parabolic intensity distribution across the width, techniques to deduce the speed, the direction of motion and the zenith crossing time of the electrojet from observations of surface magnetic perturbations are studied.From the current motions deduced by these techniques and the observed traces of the Auroral Infrasonic Waves (AIW), the following four facts are established.

• 1.(1) The time between the zenith crossing of the current and the arrival of AIW at the ground is reasonable.
• 2.(2) The direction of travel of AIW is considered to be parallel to the current drift or parallel to the current flow.
• 3.(3) The trace velocity of AIW is equal to the calculated drift velocity of the current.
• 4.(4) AIW can be produced only by a line (or a narrow band) current.

Several minutes before the AIW arrive at the ground, the existence of certain motions of westward current which satisfy the AIW emission conditions proposed by Wilson (1972) have been confirmed. However there are several cases in which a succession of two equatorward supersonic zenith crossings of westward current have induced only one AIW.     

Direction of arrival of radio waves reflected from the E-region

A simple system for measuring the direction of arrival of continuous high frequency radio waves reflected obliquely from the ionosphere is described. Results obtained over a near vertical path are used to evaluate changes in the electron density gradients (tilts) in the E-region. Wave interference on the transmission due to multiple hop or ground wave propagation is discussed.     

Thermodynamics and electrodynamics of the auroral E-region during the D salvo of the MAP/WINE campaign

With the help of incoherent scatter (EISCAT) data the thermodynamics and electrodynamics of the auroral E-region north east of Andøya Rocket Range has been investigated between 1740 UT and 2040 UT on 31 January 1984. This time period covers the D salvo of MAP/WINE and the EISCAT incoherent scatter data comprise a useful supplement to interpret the rocket data. Good agreement has been found between the EISCAT temperatures and those derived from mass spectrometer data. Neutral wind velocity estimates from EISCAT and from a falling sphere rocket experiment are in satisfactory agreement for the zonal wind component, but disagree for the meridional component.     

The relationship between electron temperature and electron density under quiet conditions in the auroral zone

A theoretical model is described which predicts electron temperature in the day-time F-region above EISCAT on geomagnetically quiet days, given the observed values of electron concentration, ion temperature and heat conduction, the daily average of solar radiation at 10.7cm and the MSIS-86 model of the neutral atmosphere. The values predicted by the model agree very closely with the observed temperatures, both for average conditions and for individual days. On two occasions the onset of a geomagnetic disturbance after a period of quiet conditions was accompanied by a growing divergence between the predicted and observed values, which corresponds to an additional source of electron heating such as would be provided by low energy particle precipitation.     

Relativistic electron precipitation and resonance with ion cyclotron waves

It is shown that only highly relativistic electrons can resonate with ion-cyclotron waves when finite temperatures are considered. The application to dayside relativistic electron precipitation events is discussed.     

Optical observations of gravity waves in the auroral zone

Night-time observations of O(¹D) λ630 nm and O(¹S) λ558 nm thermospheric emissions were made at Mawson, Antarctica (67.6°S, 62.9°E) from 1982 to 1989, using a three-field photometer. Crossspectral analysis of the data was used to extract frequencies and horizontal trace velocities of periodic structures. Structures in the λ630 nm emission were characteristic of large-scale waves, and those in the λ558 nm emission were characteristic of medium-scale waves. The results showed distinct polarisation of the propagation azimuths; waves in the λ630 nm emission propagated approximately northwestward throughout the 8 yr period, whilst propagation azimuths of waves in the λ558 nm emission appeared to be solar-cycle-dependent. It is suggested that waves observed in the λ630 nm emission were of predominantly auroral electrojet origin, whilst those observed in the λ558 nm emission were of both auroral and tropospheric origin.     

Generation of stationary irregularities due to heating of the high-latitude ionospheric E-region

The nonlinear equations describing the generation of artificial irregularities in the E-region of the high-latitude ionosphere due to Joule electron heating under the action of high-power radio waves are derived, including the low-frequency nonlinearities, and investigated in the three-wave approximation. Expressions for the stationary spectra of the short wavelength two-stream and long wavelength gradient-drift and current-convective irregularities are presented. The typical saturation amplitudes of the density fluctuations are obtained.     

Electrodynamic effects of metal ions in the noon equatorial E-region

The observations of metal ions in significant concentrations and in layer formation in the lower E-region are briefly reviewed. It is expected that a metal ion layer may alter the electro-conductivity of the ionosphere and modify the distribution of ionospheric dynamo current. The variation in the electroconductivity is theoretically calculated, and it is shown that the Pedersen conductivity is reduced by less than 14%. In the equatorial ionosphere, field line integration of metal ion effects is likely to be large. A metal ion layer, similar to those observed, is numerically modelled in the equatorial region and its electrodynamic effects on the equatorial electrojet (EEJ) are numerically examined using our self-consistent model of the ionospheric dynamo. The effects are found to be significant on the amplitude of the EEJ, but not effectively large on its peak altitude.     

The generation and propagation of atmospheric gravity waves from activity in the auroral electrojet

EISCAT measurements of the electric field in the auroral electrojet are compared with the signature of TIDs propagating equatorward as observed by an HF-Doppler network. At night-time the onset of auroral activity is usually followed by the arrival of a TID at lower latitude. Cross-correlation of the time variations of the electric field measured by EISCAT and the frequency offset recorded by the HF-Doppler system confirms a relationship between the auroral activity and the gravity wave, indicating both the travel time and the periodicity of the wave. The relationship is especially close under quiet conditions when the cross-correlation coefficient is typically 60%, significant at 0.1%. When the observed electric field is used as input to a thermosphere-ionosphere coupled global model it predicts the time signature of the observed HF-Doppler variation reasonably well but seriously underestimates the amplitude of the disturbance. Examination of this discrepancy may lead to a better understanding of the mechanisms involved in the generation and propagation of atmospheric gravity waves.     

The effects of differential ion flows on EISCAT observations in the auroral ionosphere

During geomagnetic storms different partial pressure gradients in the auroral ionosphere may result in H⁺, He⁺, O⁺ and molecular ions drifting with different velocities along the Earth's magnetic field line. For relative drift velocities ⪡ 400 m s⁻¹ it is shown that differential ion flows may be identified by two signatures in the autocorrelation function (ACF) measured by EISCAT. For larger relative drifts numerical simulations show that these signatures still exist and may result in an asymmetry in the incoherent scatter spectrum for O⁺ and molecular ions. It is demonstrated that UHF data can be reliably analysed for k²λ_D² ≲ 1, but at high altitudes, where O⁺–H⁺ flows are expected, UHF observations will be restricted by large Debye lengths (k²λ_D² > 1). Examples of ACFs based on polar wind theory are presented and discussed for the VHF system and finally it is shown that large ion temperature ratios (T_i(H⁺) >T_i(O⁺)) can significantly affect the velocity determination.     

The E-region electron density diurnal asymmetry at Saint-Santin: observations and role of nitric oxide

Measurements of the E-region electron density were made with the Saint-Santin incoherent scatter radar during consecutive days in June 1978, March 1979 and December 1980. On the basis of a statistical study, the observations show the presence of a diurnal asymmetry of the electron density, with morning values usually exceeding the afternoon densities by 3–20%. Two possible causes of the dissymmetry are examined: the asymmetry in the diurnal variation of the neutral composition and the effect of nitric oxide. The presence of NO partly converts O₂⁺ into NO⁺ ions and increases the effective recombination rate of the electrons in the afternoon. Numerical simulations assessing the relative importance of the two factors are, in general, in good agreement with the measurements.     

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983.The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) (Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian.The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R_E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r_e on the same field lines.     

Non-Maxwellian velocity distribution functions and incoherent scattering of radar waves in the auroral ionosphere

For studies or the high latitude ionosphere it is important to calculate the effect of convection electric fields on the velocity distribution functions. At an altitude where the plasma is weakly ionized, the appropriate Boltzmann equation is solved in the spatial homogeneous case. We discuss the characteristics of the ion non-equilibrium steady state reached for large electric fields, from the macroscopic point of view. From the microscopic point of view, we show that the Grad expansion fails to converge rapidly for large electric fields. Generalized polynomial expansions are developed for different models of ion-neutral interactions. The consequences of these non-Maxwellian ion distribution functions on radar waves are presented and erroneous interpretations of measurements are discussed.