Experimental results obtained with the.Jicamarca radar and a new digital processing system during spread F conditions are presented. The data consist of two-dimensional maps showing backscatter power and samples of frequency spectra of the backscatter signals as a function of altitude and time. Almost simultaneous spread F backscatter power and incoherent scatter observations of electron density and vertical drifts are presented for one occasion. It is shown that spread F can occur at the bottomside, at the topside and the steep bottom of the F region, and in the valley between the F and E regions when the electric field is either positive, negative, or null. The existence of plumelike structures extending hundreds of kilometers in altitude and physically connecting the spread F on the topside with the bottomside is one of the highlights of the experimental results. They are interpreted as evidence of a Rayleigh-Taylor instability. A mechanism involving 'bubbles' or low-density plasma is proposed to extend the instability to the stable regions on the top. Other unstable processes are proposed for spread F at other altitude ranges. The frequency spectra show a large variety of shapes. Simple or multiple peak spectra from a few hertz to a few hundred hertz wide are found. An interpretation of the spectral shapes is presented in terms of turbulent motions and the angular extent of k vector angles of the fluctuation waves with respect to perpendicularity. A puzzling phenomenon, referred to as explosive spread F, which consists of the simultaneous onset of short time enhancements in the backscatter power and involves selected heights in an altitude range of the order of 100 kin, is presented. probes, and propagation of satellite beacons (phase and amplitude scintillation). Ionosondes and satellite scintillations have been used mainly to obtain statistical data regarding temporal and spatial (latitude and longitude) spread F behavior and its relationship to other geophysical phenomena (e.g., magnetic and solar activity). Freemouw and Rino [1971] have collected the satellite scintillation observations into an empirical mathematical model. Ionosonde observations have been reviewed by Skinner and Kelleher [1971]. Most of the results reported in the literature have made use of these techniques. That these techniques are limited is shown by the fact that despite the long and intensive effort to find a theoretical explanation for the physical nature of the phenomenon, this effort has been unsuccessful.

.[1] We report from the first campaign, with the EISCAT radars and heating facility, which looked for a proposed new PMSE overshoot effect resulting when PMSE is being affected by a specific cycling of artificial electron heating. The overshoot was predicted to appear after unaffected PMSE dusty plasma had been acted upon, for a comparatively short time, by the EISCAT Heating facility. We show model results and observational examples of the overshoot effect. For the overshoot to be clearly observable, a long relaxation time after the heater is switched off is necessary to get the dust charges back to those of dust unaffected by the heated electrons or, to bring unaffected PMSE dusty plasma into the radar beams by horizontal wind transport. The overshoot characteristic curve (OCC) contain a substantial amount of information on the conditions during PMSE and must lead to new possibilities in the study of PMSE and the conditions close to the summer mesopause.

Abstract. 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 are 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. Using the pulse-to-pulse technique, a continuous transition 7429

We present evidence of filamented structure in the auroral ionosphere, observed through enhanced radar echoes produced by plasma instabilities in the filaments. Enhancements are observed in up‐ or down‐shifted ion‐acoustic peaks, or both, with power well above thermal levels. Detailed theoretical understanding of these enhancements is still lacking, and several different theories are currently used to explain the observations. Using an interferometric technique, we have measured the horizontal scales of the structures, and their evolution in time, with unprecedented resolution. To explain simultaneous up‐ and down‐shifted enhancements using theories that could only enhance one of these shoulders, spatial and/or temporal averaging has previously been suggested. However, the present observations show that enhancements occur simultaneously and in the same volume. The observed scale size in the plane perpendicular to the magnetic field is comparable to the smaller scale size of optical aurora, which, despite extensive attempts, has not been successfully explained.

Abstract. Several explanations have been proposed for Naturally Enhanced ion-acoustic Echoes observed at midand high-latitude Incoherent Scatter observatories. A decisive measure for distinguishing between these explanations is whether or not simultaneously observed up-and downshifted enhancement occur simultaneously, or if they are the result of temporal and/or spatial averaging.The EISCAT Svalbard Radar has two antennas in the same radar system, which can be used as an interferometer when pointed parallel. In observations from 17 January 2002, between 06:46:10 and 06:46:30 UT, we used this possibility, in combination with direct sampling of the received signals, to yield measurements of "naturally enhanced ion-acoustic echoes" with sufficiently high resolution to resolve such averaging, if any. For the first time, radar interferometry has been employed to estimate the sizes of coherent structures. The observations were coordinated with an image intensified video camera with a narrow field of view. Together, this forms the initial study on the causal relationships between enhanced echoes and fine structure in the auroral activity on sub-kilometer, sub-second scales.The results confirm that the enhanced echoes originate from very localised regions (∼300 m perpendicular to the magnetic field at 500 km altitude) with varying range distribution, and with high time variability (≈200 ms). The corresponding increase in scattering cross section, up to 50 dB above incoherent scattering, eliminates theoretical explanations based on marginal stability. The simultaneously observed up-and down-shifted enhanced shoulders, when caused by sufficiently narrow structures to be detected by the interferometer technique, originate predominantly from the same volume. These results have significant impact on theories attempting to explain the enhancements, in particular it is found that the ion-electron two-stream mechanism favoured by many authors is an unlikely candidate to explain the observations. The video data has helped establish a clearCorrespondence to: T. Grydeland (tom.grydeland@phys.uit.no) correlation between the enhanced echoes and auroral activity, on sub-second time scales, showing a threshold connection between the auroral intensity and the triggering of the radar enhancements. It appears that the up-and down-shifted enhanced echoes correlate with fine auroral structures in different ways.