C. La Hoz

Charged dust in the Earth's mesopause; effects on radar backscatter

We discuss to what extent small scale density inhomogeneities in the dust distribution may influence radar backscatter in the summer mesopause. We show for a reasonable range of parameters that falling dust interacting with a neutral gas vortex cannot penetrate to the centre of the vortex. The size of the hole in the dust space density distribution around the vortex centre depends on the vortex size and rotation speed and on the free fall velocity of the dust. If the dust contains a non-negligible fraction of the space charge (this requires that the dust is charged by the photoelectric effect), a substantial gradient in the positive charge density across the dust hole edge results. The positive charge density profile in the thin boundary layer will depend on the size distribution of the dust grains. A corresponding gradient in the electron density will appear and we discuss the conditions under which it will lead to a radar reflection of the magnitude observed in the summer polar mesosphere (PMSE - Polar Mesospheric Summer Echoes). This mechanism appears to have the potential of explaining the observed characteristics of the radar echoes such as the strong wavelength dependence and the narrow and sometimes complex and composite spectral signal profiles.

Measurements of HF‐enhanced plasma and ion lines at EISCAT with high‐altitude resolution

Measurements of plasma and ion lines induced during HF ionospheric interaction experiments have been made with the European Incoherent Scatter (EISCAT) facility at Tromso with sufficiently high-altitude resolution to compare with theories of Langmuir turbulence. Recent Langmuir turbulence models predict a change from broad structureless spectra to line or cascade spectra within a few hundred meters for VHF (224 MHz) observations assuming typical ionospheric density gradients. In a campaign in May 1994 we found VHF spectra that were grouped into two regions separated in altitude by ∼2 km, with broad, unstructured plasma line spectra in the upper region and cascade type spectra in the lower region. The ion line channels showed detectable spectra mainly in the upper altitude region, which corresponds to that which had the broad plasma lines. The background ionospheric density profile showed an unusually low plasma density gradient near the HF reflection heights, thus allowing the two regions, which arc normally so close together that one only sees a transition from one type of spectra to the other, to be clearly separated in height. Thus, in the high-latitude ionosphere there can, at times, be a simultaneous existence in spatially separate regions of cavitation (often referred to as strong turbulence) and cascading (normally associated with saturated parametric decay) as predicted by some simulations. Another new feature is a height variation in the plasma line cascades with the highest-order cascades strongest at the lowest heights, in accordance with expectations based on the parametric decay instability.

Aspect angle dependence of HF enhanced incoherent backscatter

Abstract An HF ionospheric interaction experiment was performed in November and December of 1997 using the EISCAT HF transmitter and 931 and 224 MHz incoherent scatter radars, all co-located near Tromso, Norway. During this experiment the pointing of the UHF radar was varied in a predetermined and repeating cycle between elevation angles of 90 and 77.2 degrees south, that is, between vertical and geomagnetic field aligned. The HF transmitter duty cycle was intentionally kept to the relatively low value of 2% (200 ms every 10 s) in order to minimize the effects of ionospheric irregularities. Here we report on variations in the intensity of the enhanced incoherent scatter ion and plasma lines observed during the experiment. Bottomside and topside F region enhanced lines were seen with both radars, and while intensity enhancements observed with the UHF radar were clearly correlated with pointing angles between the Spitze angle and field aligned, no correlation between the intensity of the lines observed with the scanning UHF radar and the vertically pointing VHF radar was observed. Consistent with HF propagation theory, the field aligned backscatter observed by the UHF radar originated several kilometers below the HF reflection height.

Optical and radar observations of auroral arcs with emphasis on small-scale structures

Abstract During a campaign in January 1988 we observed auroral arcs with the EISCAT UHF radar system and a low light level TV camera. The aim was to compare apparent motions of the arc and its small scale structures with plasma motions in the adjacent F-region. It was found (1) that there was a relative motion between arc and plasma of the order of 100–200 m/s southward and (2), in confirmation of earlier findings, that folds and curls, i.e. auroral rays seen edge-on, with wavelengths ranging from 2 to 20 km, moved along the are with average velocities of 10 to 20 km/s. At the same time, the tangential plasma velocity at 300 km height varied between −0.3 and 0.8 km/s westward. Most importantly, for all luminous bands exhibiting fast motions of small-scale structures, the direction of this motion was westward on the equatorward edge, and on most occasions, an eastward directed counterflow could be observed on the poleward edge. The existence of such a counterflow appears to be a fundamental property and may even be used for a physical definition of an auroral arc. We support the interpretation of Davis (1978) that auroral ray, fold, or curl motion is a visible expression of a physical motion of small-scale structures in the local plasma. environment of the auroral acceleration region at about 1 R E altitude. The attendant electric fields are shorted out by parallel potential drops. The counterflow pattern is consistent with the existence of a U-shaped potential at that altitude. The observed asymmetry in brightness, definition, and speed of the countertiowing folds or curls is attributed to their location at the leading or trailing edges of the arc propagating relative to the plasma frame.

Steepening of reflectivity structures detected in high-resolution Doppler spectra of polar mesosphere summer echoes (PMSE) observed with the EISCAT 224 MHz radar

Abstract Polar mesosphere summer echoes observed with the EISCAT 224 MHz radar frequently exhibit significant discontinuous offsets or jumps in the Doppler frequency. We can explain these frequency jumps as a result of a lifting of partially reflecting or scattering layers, which are distorted by bumps. These bumps can be caused by steepened refractivity variations, i.e. reflectivity structures. These suggestions are supported by model computations. We also notice that a relation exists between these structure shapes and gravity waves, which are steepened, but which do not necessarily break into enhanced turbulent velocity fluctuations.

Recent EISCAT heating results using chirped ISR

Abstract The chirp technique has recently become fully operational on the EISCAT UHF radar system and has been used for daytime observations of the HF-modified ionosphere over Ramfjordmoen in November 1992 and March 1993. During certain periods the UHF observations show a difference in the frequencies of the photoelectron-enhanced plasma line and the HF-enhanced plasma line (HFPL) similar to the one seen during chirp observations of heating at Arecibo. The frequency difference seen at EISCAT, however, varied dramatically with time and howetimes vanished completely, which was not the case at Arecibo. This frequency difference indicates that the HFPL source region is located several kilometres above the height where the linear Langmuir dispersion equation indicates a resonance should occur at the HF pump frequency. Simultaneous long-pulse measurements of the HFPL spectrum show a cascade-type structure in the HFPL spectra which, according to current theories, indicates that the HFPL must follow the Langmuir dispersion relation. This may be interpreted to mean that the HF-induced plasma waves are excited within plasma density depletions whenever the frequency difference is present. UHF observations also sometimes show a feature in the HF-modified plasma line spectrum which appears to be the same as that observed in an experiment performed at EISCAT in August 1986 by Isham et al. (1990). This new feature has been dubbed the “outshifted” line as it appears downshifted (upshifted) from the heating frequency in the downshifted (upshifted) plasma line spectrum. The magnitude of the shift is in the range of 100–300 kHz.

Magnetometer and incoherent scatter observations of an intense Ps 6 pulsation event

Abstract In the morning sector of 21 April 1985, during the recovery phase of a geomagnetic storm, a Ps 6 pulsation event was recorded by the EISCAT magnetometer cross in northern Scandinavia. Simultaneously, the EISCAT incoherent scatter radar measured E - and F -region plasma parameters with a latitudinal scanning program. Electric fields and height-integrated Hall and Pedersen conductivities are derived. Two-dimensional patterns of these quantities are constructed for one Ps 6 period. The conductance patterns closely resemble the typical auroral forms of eastward drifting Ω bands with low and high conductances at the northern and southern edges of the scanned area, respectively. From the equatorward region a tongue of high ionization extends poleward into the dark area. The location of the maximum southward current is slightly displaced towards the west from the centre of the conductance tongue. The east-west disturbance electric field points towards the tongue; the north-south fields are enhanced outside and reduced inside the high conductance region. As has been previously suggested, the observations can be explained with a model which superposes currents caused by conductance variations and electric fields. Both effects need to be taken into account for this event. The current structures move within a few degrees in the direction of the background E × B drift, but their speed is about 15% lower than the average F -region plasma drift.

HF-pump-induced parametric instabilities in the auroral E-region

Abstract In November 1999 the EISCAT high-power, high-frequency (HF) facility located near Tromso, Norway, was used to create artificial plasma turbulence in the ionosphere. During the experiment the EISCAT 224 MHz radar and sometimes the 931 MHz radar were used to obtain measurements of incoherent scatter ion and plasma lines, and artificially enhanced spectra of E-region plasma waves were measured for the first time at auroral latitudes with both radars. During periods with suitable peak E-region electron density, Z-mode propagation of the HF pump wave to the topside E-region occurred, and topside instability-enhanced plasma waves were observed. In addition to HF-pump-induced effects, an unusual F-region echo was seen in both the ion and plasma line channels, which appears to be due to an auroral arc intersecting the radar beam.

Dust charging and density conditions deduced from observations of PMWE modulated by artificial electron heating

observed relaxation, since this requires winds of around 100 m s � 1 . The most probable cause is photo detachment by which negatively charged dust can lose excess electrons by photon absorption with energies less than the dust material’s work function. By comparing the observed heating with heating model profiles, the electron density at 65 km height must have been of the order of 3 � 10 9 m � 3 . This agrees with PMWE occurring mainly during disturbed conditions with high electron densities. Our results also indicate that in the strongest PMWE layers, electron bite-outs exist consistent with the role of charged dust particles in the mechanism of PMWE and implying larger dust densities.

Evidence of anisotropic temperatures of molecular ions in the auroral ionsphere

Using incoherent scatter measurements obtained from Kiruna and Sodankyla EISCAT remote stations, and corresponding to different aspect angles, the authors show, for the first time, anisotropic temperatures of molecular ions. The electric field obtained for the 5 hours period of the 2nd February 1990 experiment presented here range from 20 to 50 mV/m. They show that the line of sight ion temperature deduced from Kiruna incoherent scatter spectra is larger than the one deduced from Sodankyla spectra which correspond to a smaller aspect angle. A statistical approach has been chosen to evaluate a {beta} parameter for Sodankyla measurements which describes the anisotropy. They found a value of 0.60 {plus minus} 0.02 which compares very well with values inferred from theoretical studies.