Doppler radio observations of 3‐meter irregularities in the polar cap E region

Abstract

The first VHF Doppler radar system to measure echoes from ionospheric plasma instabilities in the polar cap was set up in 1982 in the Northwest Territories. High‐resolution spectra were achieved through the use of CW transmissions. A scattering volume over Sachs Harbour (geomagnetic latitude ∼77°) was defined by the intersection of the narrow antenna beams from the two widely spaced transmitter arrays (at Inuvik and Cambridge Bay) and the common receiver array (at Fort Franklin). The two bistatic radio links so formed had bisectors which differed in azimuth by 60°, and which were at mean magnetic aspect angles of ∼9° and 13° from perpendicularity with the earth's field. In this report, Doppler spectra are presented for a moderately disturbed period on July 7, when the scattering region was in the polar cap. In spite of this scattering location, the Doppler spectra show all the usual spectral types (I, II, and III) typical of the auroral zone. Although type III (ion cyclotron) echoes might have been anticipated at these large aspect angles (the k∥/k⊥ ratio could be as large as 0.25) in association with strong polar cap field‐aligned currents, the presence of the type I and II echoes is not explicable on the basis of present two‐stream and gradient drift instability theories. Whereas the spectral types in the polar cap and auroral zone show substantial similarities, the temporal morphology of the polar cap echoes is quite different from that of auroral zone echoes.