Электродинамические процессы в высоких широтах: материалы междунарордного симпозиума «Полярные геомагнитные явления», 25-31 мая 1986 г., Суздаль, СССР / Междунар. геодез. и геофиз. союз, Междунар. ассоц. геомагнетизма и аэрономии ; Акад. наук СССР, Кол. фил. им. С. М. Кирова, Поляр. геофиз. ин-т ; [редкол.: О. М. Распопов (отв. ред.). и др.]. - Апатиты : Кольский филиал АН СССР, 1988.

very sophisticated instruments in recent years (e.g. Timofeev and Miroshnikov 1982; McNamara et al.,1983; Hielsen,1982; H i e l se n et al.,1983; Providakes et al.,1983). S u c h a rad a r measu r e s the radio wa v es backscatter from the u n stable waves created by the instabilities (Figure 5). The Doppler shift of the backscattered signal, is proportional to the phase velocity of these waves. This phase velo c it y co/kj in turn, is a function of the relative electron- i on drift velo c it y v^ (see e q . 1 ). AUR OPAL RADAR (VHF) K , S * 0 t r SOQ-lfOOKM Figure 5. Principle of a n auroral ra d a r experiment. Since the unstable w a ve s observed w i t h such a rad a r propagate perpendicular to the magnetic field, the ra d ar has h a d to be. i nstalled far south of the observed region. Until a f e w years ago a v e r y simplified di s persion relat i o n has b e e n u se d to infer v & fro m those D op p l er v e l oc i t y m ea surements n a m el y (Tsunoda, 1975) Ш = к Ѵ 0( (2) It can be shown that this dispersion r e l a t i o n follows fr o m the general exp r e ssion (1) und e r the f ollowing as s um p t io n (Schlegel and St-Maurice, 1983) k „ = 0 (wave pr o p ag a t io n only p e rp endicular to the magnetic fi e l d lines) (growth phase of the wa v e s small compared to the wave frequency) (collision frequency large compared to the wave frequency) Since these assumptions are not always valid i n the h i g h latitude E - r e gion, p a rticularly during strongly d isturbed conditions, the app l i ca t i on of e q . (2) seems questionable. In deed the first comparisons b et w e en drift v e l o c i ties derived f r o m auroral radars and those derived fro m incoherent scatter data showed quite a d iscrepancy (Uielsen a n d S c h l e g e l ,1983). W h er e a s the d i rection of the v el o cities are in r e l a ti v e ly good agreement, the magnitudes differ in a w a y that the auroral rad a r a lways u nderestimates the velocities f or ѴдХбОО m/s, if eq.(2) is use d (Figure 6 ). This discrepancy 1 6d to a n e w w a y to calculate the drift v e lo c i ty v^ fro m the meas ur e d auroral ra d a r phase velocity, n a m e l y the so-called ion- acoustic approach (Nielsen and S c h l eg e l ,1985). In this a p p r o a c h it is assumed that the irregularity phase v e l o c it y m e a s u re d b y the radar is always close to the i o n acoustic velocity: i This v e l o ci t y increases w i t h in c r easing v^ since the electron temp e ra t ure T 0 increases (see Figure 2). The r e l a ti o n b e t w e e n tha m e a su r e d D o ppler v elocity a n d the drift v e lo c i ty v^ thus becomes more complicated, but 82