THE 630 NM MIG AND THE VERTICAL NEUTRAL WIND IN THE LOW-LATITUDE NIGHTTIME THERMOSPHERE

It is shown that large negative divergences (gradients) in the horizon tal neutral wind in the equatorial thermosphere can support downward n eutral winds in excess of 20 m/s. With attention to the meridional and vertical winds only, the pressure tendency equation is used to derive the expression U-z0 approximate to [partial derivative U-y/partial de rivative y]H for the vertical wind U-z0 at the reference altitude for the pressure tendency equation; H is the atmospheric density scale hei ght, and [partial derivative U-y/partial derivative y] is the meridion al wind gradient. The velocity gradient associated with the Meridional Intensity Gradient (MIG) of the O(D-1) emission (630 nn) at low latit udes is used to estimate the vertical neutral wind in the MIG region. Velocity gradients derived from MIG data are about 0.5 (m/s)/km) or mo re, indicating that the MIG region may contain downward neutral winds in excess of 20 m/s. Though direct measurements of the vertical wind a re scarce, Fabry-Perot interferometer data of the equatorial F-region above Natal, Brazil, showed downward winds of 30 m/s occurring during a strong meridional wind convergence in 1982. In-situ measurements wit h the WATS instrument on the DE-2 satellite also show large vertical n eutral winds in the equatorial region.