J.-P. St.-Maurice

Naturally enhanced ion acoustic waves in the auroral ionosphere observed with the EISCAT 933-MHz radar

Observations of strongly enhanced ion acoustic shoulders of the incoherent scatter spectrum at 933 MHz at altitudes from 138 to 587 km have been obtained with the European Incoherent Scatter UHF radar. The enhancements can be up to 1 or 2 orders of magnitude in total backscattered power and can occur at either one or both of the ion acoustic shoulders. They show a variation of frequency with height of about 2 to 1, the same as the normal ion line spectral width and the ion temperature. These unusual spectra appear in two preferred height regions having different characteristics, one below 200 km and one above about 300 km. The enhancements are associated with geomagnetic disturbance, high electron temperatures, auroral arcs, and red aurora in the F region. The observations, which are mainly along the magnetic field direction, indicate that field-aligned thermal electron drifts are destabilizing the ion acoustic waves. They confirm and extend the one other publication reporting on similar echoes. We suggest that field-aligned flows of soft electrons depositing their energy at horizontally poor conducting F region heights are the cause of parallel electric fields in the ionosphere. These fields then produce the thermal electron motions that we argue have to be the cause of the observations.

A morphological study of vertical ionospheric flows in the high‐latitude F region

We have studied the vertical bulk ion drift data recorded by the DE 2 satellite between 200 and 1000km altitudes. For this data set we have found that field-aligned ion flows between 100ms−1 and 3km s−1 are a common occurrence in the high-latitude F region. The flows are predominantly upward near the cusp region and throughout the auroral zone. Strong downward flows of somewhat smaller magnitude are also recorded but mostly over the polar cap. These statements are true for all drift speeds in excess of 50ms−1 and for all altitudes and magnetic activity levels sampled. The morphology of low-altitude upward flowing ions agrees well with the morphology of outflowing ions, ion beams, and ion conics observed at much higher altitudes, but the low-altitude fluxes are often considerably greater. This suggests that a large fraction of the upflowing ions actually returns to the ionosphere, to be observed as large downward ion fluxes. We propose that upflowing ion events are generated by sudden large changes in the ion temperature below the neutral exobase, where ion frictional heating dominates the ion energy balance. The sudden changes in temperature occur when the horizontal velocity of a convecting field tube increases rapidly in regions like the cusp.

Super Dual Auroral Radar Network observations of meteor echoes

Radar echoes from ranges less than 500 km are routinely observed by the Super Dual Auroral Radar Network (SuperDARN) on most days. Many of these echoes have properties which are markedly different from what one would expect from E or F region irregularities. We show that these unusual short-range HF echoes are due to scattering off meteor trails. This explains why, among other things, the Doppler shift from the short-range echoes taken from the SuperDARN Saskatoon antenna are consistent with the mesospheric winds observed by the Saskatoon MF radar. This means that the SuperDARN radars can be used to study neutral winds at meteor heights, a result which is especially interesting since it opens up the capability for a global coverage of mesospheric winds using the worldwide distribution of SuperDARN radars.

Diffusion and heat flow equations for the mid-latitude topside ionosphere

Abstract For application to the mid-latitude topside ionosphere, we have derived diffusion and heat flow equations for a gas mixture composed of two major ions, electrons and a number of minor ions. These equations were derived by expanding the velocity distribution of each constituent about its 13 lower order velocity moments. As a consequence, each constituent was allowed to have its own temperature and drift velocity. The restriction to mid-latitudes results because we have assumed that the species temperature and drift velocity differences were small. In deriving the diffusion and thermal conduction equations, we have discovered some new transport effects. For the major ions, we have found that: (1) a temperature gradient in either gas causes thermal diffusion in both gases; (2) a temperature gradient in either gas causes heat to flow in both gases; and (3) a relative drift between the major ion gases induces a heat flow in both gases. Similar transport effects have also been found for the minor ions.

On the origin of narrow non‐ion‐acoustic coherent radar spectra in the high‐latitude E region

Many types of coherent radar spectra have a width in Doppler velocity units that is less than the ion acoustic speed of the medium. In spectra labeled as type 1 the mean Doppler shift of these narrow spectra matches the ion acoustic speed of the medium. There also exist narrow high-latitude spectra for which the mean Doppler shift is either markedly less or markedly more than the ion acoustic speed. We propose that electron density gradients with scale lengths as small as 100 m are at the origin of a large fraction of these narrow spectra near 50 MHz. The sharp density gradients in that case are created in regions of discrete auroral precipitation associated either with multiple narrow arcs or with sharp edges of broader features. Using the same principle at radar frequencies in the 10- to 20-MHz range, we find that gradient scales from 20 to 30 km in size create a combination of fast and slow phase velocities closely resembling the spectral characteristics expected from NO+ ion cyclotron waves. However, gradients are not always responsible for slow narrow spectra; a detailed analysis of available observations has led us to conclude that the high-latitude E region cannot always be considered as fully turbulent even when appreciable coherent echo returns are registered by the radars. In particular, slow narrow spectra at 50 MHz are at times produced under gradient-free weakly turbulent conditions. In addition, at lower radar frequencies (10 to 20 MHz) the narrow spectral width of slowly moving waves and the morphology of these waves both suggest that the irregularities are generated indirectly via mode coupling of linearly unstable modes and that these “secondary” waves are themselves not coupling efficiently. This implies that processes other than mode coupling are contributing to the overall wave energy budget. In that case we suggest that the convective properties of the slowly growing modes are an important factor in removing wave energy, even for waves as small as a few meters in wavelength. We also propose that there may be two distinct generation mechanisms for secondary waves at 10 MHz, each with its own mean Doppler shift behavior.

The interplanetary electric field, cleft currents and plasma convection in the polar caps

Abstract This report investigates the suggestion that the pattern of plasma convection in the polar cleft region is directly determined by the interplanetary electric field (IEF). Owing to the geometrical properties of the magnetosphere, the East-West component of the IEF will drive field-aligned currents which connect to the ionosphere at points lying on either side of noon, while currents associated with the North-South component of the IEF will connect the two polar caps as sheet currents centered at noon. The effects of the hypothesized IEF driven cleft current systems on polar cap ionospheric plasma convection are investigated through a series of numerical simulations. The simulations demonstrate that this simple electrodynamic model can account for the narrow “throats” of strong dayside antisunward convection observed during periods of southward interplanetary magnetic field (IMF) as well as the sunward convection observed during periods of strongly northward IMF. Thedawn-dusk shift of polar cap convection which is related to the By component of the IMF is also accounted for by the model.

A new nonlinear approach to the theory of E region irregularities

In the study of E region irregularities the standard procedure is to Fourier analyze the irregularities in both time and space, that is, to describe them as a superposition of plane waves. This introduces difficulties when the amplitude of the plane waves becomes large, thereby adding nonlinear terms to the original equations and forcing all the plane waves to become coupled to one another. In the present work we stay away from Fourier analysis and use the standard fluid description of the instabilities in the limit of perturbed electric fields that are strictly perpendicular to the geomagnetic field. We obtain a nonlinear generalization of the standard results whereby the diffusion-like operator found in linear theory is now a function of the density itself. Therefore, as the structures grow, the net electric field seen by the ambient plasma inside the structures changes in time: it rotates and its amplitude decreases. Consequently, one possible saturation mechanism for the instabilities is a reduction in the net electric field inside the structures, which brings them to threshold velocity conditions. This being stated, other nonlinear saturation mechanisms remain possible if they require smaller saturation amplitudes than the present work. Either way, our work is consistent with intermittency and implies that the largest amplitude structures in the medium should be rotated away from zero flow angle conditions by a measurable amount. Finally, we show that when compared to an irregularity-free situation, there should be a measurable reduction in the average Hall current carried by the plasma, while the average Pederseri current should not be affected.

A time-dependent gyro-kinetic model of thermal ion upflows in the high-latitude F region

Ample evidence supports the significance of the high-latitude ionospheric contribution to magnetospheric plasma. Assuming flux conservation along a flux tube, the upward field-aligned ion flows observed in the magnetosphere require high-latitude ionospheric field-aligned ion upflows of the order of 108 to 109 cm−2 s−1. Since radar and satellite observations of high-latitude F region flows at times exceed this flux requirement by an order of magnitude, the thermal ionospheric upflows are not simply the ionospheric response to a magnetospheric flux requirement. Several ionospheric ion upflow mechanisms have been proposed, but simulations based on fluid theory do not reproduce all the observed features of ionospheric ion upflows. Certain asymmetries in the statistical morphology of high-latitude F region ion upflows suggest that the ion upflows may be generated by ion-neutral frictional heating. We developed a single-component (O+), time-dependent gyro-kinetic model of the high-latitude F region response to frictional heating in which the neutral exobase is a discontinuous boundary between fully collisional and collisionless plasmas. The concept of a discontinuous neutral exobase and the assumption of a constant and uniform polarization electric field reduce the ion guiding center motion in the frame of a convecting flux tube to simple one-dimensional ballistic trajectories. We thus are able to analytically calculate a time and height-dependent ion velocity distribution function, from which we can compute the ion density, parallel velocity, parallel and perpendicular temperature, and parallel flux. Using our model, we simulated the response of a convecting flux tube between 500 km and 2500 km to various frictional heating inputs; the results were both qualitatively and quantitatively different from fluid model results, which may indicate an inadequacy of the fluid theory approach. The gyro-kinetic frictional heating model responses to the various simulations were qualitatively similar: (1) initial perturbations of all the modeled parameters propagated rapidly up the flux tube, (2) transient values of the ion parallel velocity, temperature, and flux exceeded 3 km s−1, 2 × 104 K, and 109 cm−2 s−1. respectively, (3) a second transient regime developed wherein the parallel temperature drops to very low values (a few hundred Kelvins), and (4) well after heating ceased, large parallel temperatures and large downward parallel velocities and fluxes developed as the flux tube slowly returned to diffusive equilibrium. The ion velocity distributions during the simulation are often non-Maxwellian and are sometimes composed of two distinct ion populations.

HF detection of slow long-lived E region plasma structures

During the equinox and winter seasons, and in the range 300–1000 km the Saskatoon Super Dual Auroral Radar Network (SuperDARN) radar often detects extended patches of coherent echoes with remarkably uniform properties and low Doppler speeds, in the range 0 to 200 m/s. Typically, these echoes last for ∼3 hours, and are observed between 1300 and 2300 MLT, at times of moderate to high Kp values. The echo Doppler shift changes systematically with azimuthal angle and a vector reconstruction of the implied drift indicates westward velocities in the range 150 to 250 m/s, well below the threshold speed associated with Farley-Buneman waves. When ionosonde observations are available, they invariably show the presence of a thick sporadic E layer. This feature, plus the facts that the IMF By is always negative and that the echoes are equatorward of the regions of discrete precipitation (as indicated by comparison with coincident DMSP satellite observations), indicate that the echoes are associated with the diffuse aurora in regions where the electric field is of the order of 10 mV/m or less. We infer from these echo properties that the irregularities are triggered by a primary gradient-drift mechanism which then cascades to the observed structures through weakly turbulent mode-coupling processes. Several events were observed during special multifrequency experiments using the Saskatoon SuperDARN radar. It was found that the Doppler speed, power, and spectral width all increase systematically with increasing radar frequency. The findings for Doppler speed and power appear to arise, at least in part, from the increase in height of the radar echoes with increasing frequency. The frequency dependence of spectral width may be related to instability lifetimes; it was found to agree well with the results of numerical simulations [Keskinen et al., 1979].

Derivation of the ion temperature partition coefficient β∥ from the study of ion frictional heating events

This paper reports the results of a study evaluating the ion temperature partition coefficient β∥ over a range of F region altitudes. The data have been selected from three EISCAT CP-0 experiments, each of which displayed clear evidence of ion frictional heating. The data were averaged at a time resolution of 30 s. These observations, made in an observing direction parallel to the Earths magnetic field, have the advantage that the line-of-sight ion thermal velocity distribution can be closely approximated to a Maxwellian, while still giving a line-of-sight ion temperature which can be interpreted on the basis of a well-known energy balance equation. The technique for determining β∥ depends upon fitting the variation of the field-parallel ion temperature to simplified forms of this balance equation. The method is an extension of the curve-fitting approach previously used by Glatthor and Hernandez (1990) to deduce both ion temperature partition coefficients at a single F region altitude. The results of this procedure are compared to the theoretical predictions for β∥ obtained from an equation originally due to St.-Maurice and Hanson (1982). By introducing measured and modeled parameters, it is found that values of β∥ close to those expected for resonant charge exchange collisions are predicted around the F region peak. At greater heights, however, the increased influence of Coulomb collisions is predicted to give rise to an increase in β∥ with altitude, corresponding to ion thermal velocity distributions which tend toward isotropy. This height dependence, which has been theoretically predicted in some other recent studies, would be an important factor in calculations of the energy balance in the upper F region. While the experimentally derived values of β∥ are close to the RCE predictions near the F region peak, the expected increase in β∥ with altitude is seen in only one of the three selected events. In the remaining cases, the increases are notably smaller. Possible reasons for this discrepancy are discussed. Changes in ion and neutral composition, together with effects such as heat conduction and thermal diffusion can act to bias the curve-fitting technique. The degree of coupling between the ionized and neutral atmospheres is also an important factor, and there is considerable uncertainty in some of the terms used in the theoretical part of the study. Nonetheless, the results suggest that Coulomb collisions play a potentially important role in determining the energy balance of the upper F region. A clear need exists for further studies in this area to establish more fully the contribution of Coulomb processes.