Dynamics of the Low-Latitude Thermosphere and Ionosphere.

Abstract

Nighttime thermospheric neutral wind velocities are determined at Arecibo, Puerto Rico, by using a Fabry-Perot interferometer, which measures the Doppler shift of the O(('1)D) airglow emission. In summer, the winds are observed to flow toward the southeast between sunset and midnight. After midnight in summer, the meridional component of the wind usually slackens, while the zonal component may reverse. By contrast, in winter, the meridional wind is often small, and the predominant flow is eastward throughout the night. Vertical winds are inferred from the divergence of the horizontal flow. A maximum downward flow of about 5 m s('-1) is observed near midnight in summer. Incoherent scatter radar measurements are used to calculate the O('+) diffusion velocity and infer the vertical profile of the meridional wind. Horizontal temperature gradients and ion-drag forces are evaluated from radar measurements. I conclude that it may sometimes be necessary to include viscous forces to balance the meridional equation of motion for the neutral gas. Electrical conductivities of the E and F layers of the nighttime ionosphere are determined. The F layer Pedersen conductivity at the magnetically conjugate point is evaluated using ionosonde data. The F layer dynamo mechanism is found to be the source of most of the nighttime electric fields observed at Arecibo. The components of ion velocity perpendicular to the earth's magnetic field are generally consistent with well developed polarization electric fields, so long as the conjugate point is included in the analysis. The factors that determine the electron density at the F layer peak are investigated by evaluating the various terms in the O('+) continuity equation. I find that, at different times of night, chemical recombination, horizontal advection and ion velocity divergence may all be significant.