To explain observations of the E-region 3.2-m and 6.1-m backscatter measured by the MU and FAR radars over Japan we propose that a source for generation of field-aligned irregularities (FAI) is neutral gas motions operating over a wide altitude range. Due to an altitude dependence of ionospheric parameters these neutral motions may result in different dominating processes at different altitudes. On the basis of recent theories of neutral wind-driven gradient drift and thermal instabilities [Kagan and Kelley, 1998; 1999> we show that thermal processes may both support the gradient drift instability and be responsible for sporadic-E independent backscatter. In this scenario the time of FAI occurrence seems to follow a tidal mode behavior. The nighttime echo occurrence is most probably due to slower recombination. ¿ 2000 American Geophysical Union