Sondrestrom and EISCAT radar observations of poleward-moving auroral forms

During many magnetospheric substorms, the auroral oval near midnight is observed to expand poleward in association with strong negative perturbations measured by local ground magnetometers. We show Sondrestrom and EISCAT incoherent scatter radar measurements during three such events. In each of the events, enhanced ionization produced by the precipitation moved northward by several degrees of latitude within 10–20 min. The electric fields measured during the three events were significantly different. In one event the electric field was southward everywhere within the precipitation region. In the other two events a reversal in the meridional component of the field was observed. In one case the reversal occurred within the precipitation region, while in the other case the reversal was at the poleward boundary of the precipitation. The westward electrojet that produces the negative H-perturbation in the ground magnetic field has Hall and Pedersen components to varying degrees. In one case the Hall component was eastward and the Pedersen component was westward, but the net magnetic H-deflection on the ground was negative. Simultaneous EISCAT measurements made near the dawn meridian during one of the events show that the polar cap boundary moved northward at the same time as the aurora expanded northward at Sondrestrom. Most of the differences in the electrodynamic configuration in the three events can be accounted for in terms of the location at which the measurements were made relative to the center of the auroral bulge.     

The day to night absorption ratio in auroral and subauroral zone riometer measurements during auroral absorption

The day to night ratio of auroral absorption has been studied using data from auroral and subauroral latitudes and by application of different kinds of statistical analyses. Ratios between 0.5 and 3.0 are obtained, depending on the criteria applied to the selection of data. Previous studies obtained similar ratios, but reached different conclusions about the effective solar control of auroral absorption. It is concluded here that evidence of solar control of the day to night ratio of auroral absorption, or the lack thereof, cannot be extracted by these statistical analyses.     

Auroral rays: filamentation in the auroral accelerator

In situ measurements of particles, fields and optical emissions from a rocket that encountered auroral rays are reported. The measurements give insight into the production of rays, as well as the production of large fluctuations in electric fields perpendicular to the magnetic field. The fine structure and rapid variations of the electron energy flux associated with the rays are apparently produced by modulation in the degree of electron acceleration. Rays are produced when the energy flux increases in localized regions to values even higher than those normally encountered in bright auroral forms. Close and consistent similarities in the variations of the electron energy flux, the light and the electric fields suggest that the field variations were produced as a direct result of the changes in the stream of accelerated electrons. In examining possible causes of the velocity changes that produce the rays, two acceleration processes are considered; acceleration as a consequence of a potential difference between the magnetosphere and the atmosphere and acceleration by waves.     

On the development of folds in auroral arcs

The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F-region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.     

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.     

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.     

Modulation of the auroral proton spectrum in the upper atmosphere

An energetic auroral proton entering the atmosphere will alternate between being a proton and a neutral hydrogen atom by charge-exchange collisions with atmospheric constituents. This study uses a simple procedure to evaluate the energy degradation of the penetrating protons/hydrogen atoms by using semi-empirical range relations in air, and derives the particle energy variation as a function of altitude, starting from proton spectra observed from rockets above the main collision region. The main assumptions are that the geomagnetic field is homogeneous and vertical and that the pitch angle of the proton/hydrogen atom is preserved in collisions with atmospheric constituents before being thermalized. The calculations show that the incoming particle flux first loses the low energy particles at the highest pitch angles, even if the beam itself widens as it penetrates the atmosphere. The largest energy loss for particles with initial energy between 10 and 1000 keV occurs in the height interval between 100 and 125 km.     

Diurnal propagating tides in the low-latitude middle atmosphere

Using an equivalent gravity wave f-plane model it is shown that longitude variations in diurnal insolation absorption by tropospheric H₂O can account for longitudinal variations of at least ± 12–15% about zonal mean values in the diurnal wind amplitude at low latitudes (0–20°) between 80 and 100 km, by virtue of the non-migrating propagating tidal modes which are excited. Phase variations of about ± 0.75 h also occur. These percentage variations are conservative estimates, since the background migrating (1,1,1) mode appears to be slightly (20–25%) overestimated in amplitude. In addition, the assumed eddy dissipation values, which appear necessary to model the breaking (1,1,1) mode, are larger than generally considered ‘reasonable’ by photochemical modellers. For a photochemically more reasonable eddy diffusion profile, estimates of longitude differences in diurnal wind amplitude are quite similar to the above values below 87 km, but increase to ± 17–25% near 100 km, with accompanying phase variations of ± 1–2 h about zonal mean values. In addition, it is shown that radiative damping by CO₂ parameterized by a scale-dependent Newtonian cooling coefficient accounts for no more than a 20% reduction in the amplitudes of diurnal propagating tides above 80 km.     

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.     

Calculation of auroral Bahner volume emission height profiles in the upper atmosphere

Energetic protons entering the atmosphere will either travel as auroral protons or as neutral hydrogen atoms due to charge-exchange and excitation interactions with atmospheric constituents. Our objective is to develop a simple procedure to evaluate the Balmer excitation rates of H_α and H_β, and produce the corresponding volume emission rates vs height, using semi-empirical range relations in air, starting from proton spectra observed from rockets above the main collision region as measured by Reasoneret al. [(1968) J. geophys. Res.73, 4185] and Søbraaset al. [(1974) J. geophys. Res.79, 1851]. The main assumptions are that the geomagnetic field is parallel and vertical, and that the pitch angle of the proton/hydrogen atom is preserved in collisions with atmospheric constituents before being thermalized. Calculations show that the largest energy losses occur in the height interval between 100 and 125 km, and the corresponding volume emission rate vs height profiles have maximum values in this height interval. The calculted volume emission rate height profile of H_β compares favorably with that measured with a rocket-borne photometer.     

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.     

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.     

Double structure of ionospheric conductivity in the midnight auroral oval during a substorm

Measurements of precipitating particles on board DMSP F7 spacecraft are used to analyze the distribution of ionospheric conductance in the midnight auroral zone during substorms. The distribution is compared with the meridional profile of ionospheric currents calculated from magnetic data from the Kara meridional chain. Two regions of high Hall conductance are found; one of them is the traditional auroral zone, at latitudes 64–68°, and the other is a narrow band at latitudes 70–73°. The position of high conductance zones is in agreement with the location of the intense westward currents. The accelerated particle population is typical of electrons E_e > 5 keV in the high conductance region.     

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.     

Temperature anisotropy of drifting ions in the auroral F-region,observed by EISCAT

During relative drifts between the ions and the neutrals perpendicular to the geomagnetic field, the ion temperature in the auroral F-region becomes anisotropic with a higher temperature perpendicular than parallel to the magnetic field (T_⊥ >T_∥). It has been shown that for a gyrotropic ion velocity distribution the ion temperatures T_⊥ and T_∥ can be expressed as a function of the neutral temperature and of the squared normalized relative ion-neutral drift, with parameters β_⊥ and β_∥ describing the anisotropy and the collision process.In this paper, five increases of the F-layer ion temperature and ion drift velocity, found in EISCAT-CP1F data, were analyzed to obtain information about the anisotropy and the collision process. In the CP1F experiment, the angles between the magnetic field line ending in Tromsø and the antenna directions remain small, and the ion drift velocities of the investigated events in general were below 1500 m/s. Thus the ion velocity distributions were approximated by a bi-Maxwellian, and NO⁺ was assumed to remain a minor constituent at the F-layer maximum. For a quantitative analysis, generalized theoretical β-values for a bi-Maxwellian ion velocity distribution drifting through a mixture of different neutral components and for arbitrary observation directions were calculated. With these expressions it was possible to compare the drift dependence of the measured ion temperature for every antenna position directly with the theory. A statistical analysis of the heating events showed a good correlation between the ion temperatures of Tromsø, Kiruna and Sodankylä and the squared normalized ion drift, and values β_T, β_K, β_S could be calculated by linear regression. The fitted curves corresponded well with theoretical curves for a bi-Maxwellian velocity distribution of O⁺ ions drifting through a neutral atmosphere consisting of O and N₂.     

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.     

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 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.     

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.