DUAL-BEAM EISCAT RADAR OBSERVATIONS OF THE DYNAMICS OF THE DISTURBED D-REGIONS AND E-REGIONS IN THE EARLY-MORNING SECTOR

We have used the two EISCAT UHF and VHF radar facilities in Scandinavi a simultaneously to measure, in two different directions, the electron density profiles associated with three electron precipitation events in the early morning sector. They may be classified by: (1) diffuse pr ecipitation during the 12 August 1988 observations, associated with we ak precipitation fluxes, low magnetic activity and slowly varying abso rption. (2) A discrete absorption event during the 6-7 June 1990 exper iment, associated with strong fluxes, a moderately high magnetic activ ity and electric field strengths above the threshold for the Farley-Bu neman instability in the electrojet region. (3) A mixed event during t he 8-9 June 1990 experiment, showing similar features to cases (1) and (2) and associated with moderately strong fluxes, stronger absorption and higher magnetic activity. Absorption peaks in this period do not seem to correlate with electric fields above the Farley-Buneman instab ility threshold. During both June 1990 periods we may be observing a p ulsating radio-aurora which modulates (over 10-15 min) the precipitati on of electrons into the ionosphere. This would explain the discrete a bsorption peaks on 8-9 June. On 6-7 June, moreover, the Farley-Buneman instability may be screening the absorption in the D-region. From the cross-correlation of the two simultaneous electron density time-serie s (from 80 to 120 km height at separations of almost-equal-to 20 km) w e obtain time delays of a few tens of seconds for both the June 1990 p eriods and of a few minutes for the August 1988 period and, thus, appa rent N-S sizes of the induced ionization patches ranging between 20 an d 100 km, and 6 and 60 km, respectively. Also, the ionization lifetime inferred from the auto-correlation time (from 30 s to 2 min in June 1 990 and from 2 to 10 min in August 1988), seems to decrease with incre asing precipitation flux. In the June experiments, the implied drift v elocities from both the cross-correlation analysis and the triangulati on of absorption data agree well (within the limits of our comparisons ), and are mainly to the north-east (along the E x B direction). On 6- 7 June 1990 the triangulation velocities also agree with the measured E x B-drift. The gradient-curvature motion of the precipitating electr ons may only be detectable from cross-correlation analysis in the E-W plane. On 12 August 1988 the observed structure may not correspond to the same ionization patch (the estimated lifetime is greater than the recombination time), but to sustained ionization from slowly varying p recipitation, and carried by the neutral wind. On the other hand, the 6-7 June 1990 data show that the in-situ dynamics of the ions may also be radically affected by the onset of the Farley-Buneman instability. In this case, the measured ion-drifts at 105 km height are exceptiona lly strong and comparable with the E x B-drift.