A role of neutral motions in formation of midlatitude E-region field-aligned irregularities

To explain observations of the E-region 3.2-m and 6.1-m backscatter measure d by the MU and FAR radars over Japan we propose that a source for generati on of field-aligned irregularities (FAI) is neutral gas motions operating o ver a wide altitude range. Due to an altitude dependence of ionospheric par ameters these neutral motions may result in different dominating processes at different altitudes. On the basis of recent theories of neutral wind-dri ven gradient drift and thermal instabilities [Kagan and Kelley, 1998; 1999] we show that thermal processes may both support the gradient drift instabi lity and be responsible for sporadic-E independent backscatter. in this sce nario the time of FAI occurrence seems to follow a tidal mode behavior. The nighttime echo occurrence is most probably due to slower recombination.