Measuring Thermospheric Winds and Temperatures Near the Equator: Evidence for the Development of an Inertial Instability?

Abstract:

Measurements of thermospheric winds and temperatures near the geomagnetic equator for the regions of northeastern Brazil and central Peru show evidence for the development of meridional wind gradients for each of the two regions. Also evident is the result that the zonal winds are higher in speed for the Peru measurements when compared with the zonal winds measured in Brazil. Within the 4 degree latitude separation of Arequipa and Jicamarca, the zonal winds observed are generally the same. The Brazil site shows a difference in the meridional winds across a spatial separation of 500 km of about 20 to 40 ms-1 decrease from south to north. This gradient is seen consistently during the local summer months. Comparison of Brazil and Peru zonal winds for simultaneous nightly measurements and for averaged monthly results shows the zonal winds in the mid-evening period (21-23 LT) are generally faster by about 30 to 75 ms-1 for the Peru site which is located near or at the geomagnetic equator. These results are interpreted in terms of an inertial instability mechanism that is activated as a result of a steady state unbalanced flow. The theory of fluid dynamics applied to the thermosphere indicates that near the geographic equator where the Coriolis parameter f approaches zero, the neutral medium becomes unstable developing wave structure that may transport momentum and energy into both zonal and meridional directions.