The mesosphere-stratosphere-troposphere radar technique deduces wind speeds and directions from the Doppler velocities of small-scale irregularities in the refractive index. In the upper mesosphere, where the refractive-index irregularities are due to corresponding irregularities in plasma density, the velocities may depart significantly from the wind vectors, especially at auroral-zone latitudes, where electric fields can be large. The magnitude of the effect is estimated for spherical irregularities in the height region from 60 to 120 km. In the case of horizontal drifts, the departure of the plasma velocity from that of the neutral wind is small in comparison to typical horizontal wind velocities at heights below 90 km except under highly disturbed conditions. The average vertical component of the neutral wind, however, is small, and attempts to measure it in the upper mesosphere can be seriously affected by electric fields mapped down from the auroral-zone magnetosphere, and even by the dynamo action of local horizontal winds.
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