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

• Main Authors: M J Kosch, M T Rietveld, A Senior, I W Mccrea, A J Kavanagh, B Isham, F Honary
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2004
• Subjects: Langmuir; EISCAT
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1082.9013; http://www.treurniet.ca/spirals/EISCAT_AnnularStructure.pdf

[1] The EISCAT low-gain HF facility has been used 8 repeatedly to produce artificially stimulated optical 9 emissions in the F-layer ionosphere over northern 10 Scandinavia. On 12 November 2001, the high-gain HF 11 facility was used for the first time. The pump beam zenith 12 angle was moved in 3°steps along the north-south meridian 13 from 3°N to 15°S, with one pump cycle per position. Only 14 when pumping in the 9°S position were annular optical 15 structures produced quite unexpectedly. The annuli were 16 approximately centred on the pump beam but outside the 17 À3 dB locus. The optical signature appears to form a 18 cylinder, which was magnetic field-aligned, rising above the 19 pump wave reflection altitude. The annulus always 20 collapsed into the well-known optical blobs after $60 s, 21 whilst descending many km in altitude. All other pump 22 beam directions produced optical blobs only. The EISCAT 23 UHF radar, which was scanning from 3°to 15°S zenith 24 angle, shows that enhanced ion-line backscatter persisted 25 throughout the pump on period and followed the 26 morphology of the optical signature. These observations 27 provide the first experimental evidence that Langmuir 28 turbulence can accelerate electrons sufficiently to produce 29 the optical emissions at high latitudes. Why the optical 30 annulus forms, and for only one zenith angle, remains 31 unexplained.