Novel artificial optical annular structures in the high latitude ionosphere over EISCAT

Abstract

The EISCAT low‐gain HF facility has been used repeatedly to produce artificially stimulated optical emissions in the F‐layer ionosphere over northern Scandinavia. On 12 November 2001, the high‐gain HF facility was used for the first time. The pump beam zenith angle was moved in 3° steps along the north‐south meridian from 3°N to 15°S, with one pump cycle per position. Only when pumping in the 9°S position were annular optical structures produced quite unexpectedly. The annuli were approximately centred on the pump beam but outside the −3 dB locus. The optical signature appears to form a cylinder, which was magnetic field‐aligned, rising above the pump wave reflection altitude. The annulus always collapsed into the well‐known optical blobs after ∼60 s, whilst descending many km in altitude. All other pump beam directions produced optical blobs only. The EISCAT UHF radar, which was scanning from 3° to 15°S zenith angle, shows that enhanced ion‐line backscatter persisted throughout the pump on period and followed the morphology of the optical signature. These observations provide the first experimental evidence that Langmuir turbulence can accelerate electrons sufficiently to produce the optical emissions at high latitudes. Why the optical annulus forms, and for only one zenith angle, remains unexplained.