Abstract
Abstract. The CUTLASS Finland radar, which comprises an integral part of the SuperDARN system of HF coherent radars, provides near continuous observations of high-latitude plasma irregularities within a field-of-view which extends over some four million square kilometres. Within the Finland radar field-of-view lie both the EISCAT mainland and EISCAT Svalbard incoherent scatter radar facilities. Since the CUTLASS Finland radar commenced operation, in February 1995, the mainland EISCAT UHF radar has been run in common programme 1 and 2 modes for a total duration exceeding 1000 h. Simultaneous and spatially coincident returns from these two radars over this period provide the basis for a comparison of irregularity drift velocity and F-region ion velocity. Initial comparison is limited to velocities from four intervals of simultaneous radar returns; intervals are selected such that they exhibit a variety of velocity signatures including that characteristic of the convection reversal and a rapidly fluctuating velocity feature. Subsequent comparison is on a statistical basis. The velocities measured by the two systems demonstrate reasonable correspondence over the velocity regime encountered during the simultaneous occurrence of coherent and incoherent scatter; differences between the EISCAT UHF measurements of F-region ion drift and the irregularity drift velocities from the Finland radar are explained in terms of a number of contributing factors including contamination of the latter by E-region echoes, a factor which is investigated further, and the potentially deleterious effect of discrepant volume and time sampling intervals.Key words. Ionosphere (ionospheric irregularities; plasma convection)
Highlights
Coherent scatter of radio waves o ®eld-aligned irregularities in the ionosphere, as reviewed by for example Fejer and Kelly (1980) and Haldoupis (1989), provides measurements of the drift speed of the plasma irregularities along the radar look direction
For HF coherent radars, such as those which form the SuperDARN chain of auroral radars (e.g. Greenwald et al, 1995), orthogonality can be achieved in both the E- and F-regions due to the refractive nature of radio wave propagation in the 3 to 30 MHz frequency band, this, and the eect of group retardation, results in increased diculty in determining the exact location of the scattering region
It should be reiterated that the location of the EISCAT UHF radar within the CUTLASS Finland radar ®eld-of-view is by no means optimum for such a comparison as the nature of HF propagation severely limits Finland radar returns from the vicinity of EISCAT (Milan et al, 1997); this is even more applicable to the location of EISCAT within the CUTLASS Iceland ®eld-of-view
Summary
Coherent scatter of radio waves o ®eld-aligned irregularities in the ionosphere, as reviewed by for example Fejer and Kelly (1980) and Haldoupis (1989), provides measurements of the drift speed of the plasma irregularities along the radar look direction. VHF and UHF coherent radars, for example STARE (Greenwald et al, 1978), SABRE (Nielsen et al, 1983) and COSCAT (McCrea et al, 1991), whose operating frequencies are signi®cantly greater than the ionospheric plasma frequency can, at high-latitude, only satisfy the orthogonality condition necessary for coherent backscatter in the E-region.
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