where r is the time of day in hours and the subscripts 1 IN, 12N, and 2N refer respectively to the diurnal An mode, diurnal S,_2 mode, and the semidiurnal mode. A0N is the seasonal average northward velocity. The A's and A's are generated as fitted functions of altitude, latitude, time of year, and solar activity level, or a subset of these variables. The seasonal average term A0N is generated with a computer subroutine that was kindly furnished by J. M. Forbes. It is based on fits to results of computations by Robie, Dickinson, and Ridley [11]. Values of the parameters A nN, A 12n,A2N, A,1n, A12N, and A2N were determined as functions of altitude by Harper [12,18] from incoherent scatter measurements at Arecibo for solar minimum non-winter conditions. We have fitted Harper's data with approximate analytic functions of altitude. Of the two diurnal modes SH and S, _2, the former is dominant at low latitudes, while the latter dominates at high latitudes. The relative strengths of the two modes as functions of latitude have been computed by Forbes and Garrett [8]. We use Forbes' and Garrett's results to scale Harper's Arecibo values of A1IN and A,2N to non-Arecibo latitudes. The scaled amplitudes are in rather good agreement with amplitudes computed by Forbes and Garrett for solar minimum conditions at 45° latitude. For solar maximum conditions, A,,N increases at low latitudes (R. M. Harper, private communication [20]). The quantities A2N, AUN, A12N, and A2N are all assumed, for lack of detailed information, to be independent of latitude, season, and solar activity. 4. Ion transport equations The transport of ions is influenced strongly by the geomagnetic field. For this reason we have chosen coordinates which are aligned with the magnetic field. B is in the xz plane and has components BX=B cos I and B^-B sin I.
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