Vertical winds in the thermosphere

摘要

The neutral vertical circulation in the thermosphere plays an important role in redistributing heat and momentum and strongly affects the neutral composition, as well as plasma and electrodynamic processes. Vertical wind measurements in this altitude range are difficult, however, and the available data give limited temporal, spatial, and vertical coverage. The available ground-based techniques are generally limited to narrow altitude ranges but provide more extended temporal coverage. The in situ techniques provide more extended altitude range coverage but have very limited temporal coverage. The most extensive measurements have come from ground-based optical techniques using the 630.0-nm F-region red-line emission and to a lesser extent the 557.7-nm E-region green-line emission. A general characteristic of these measurements is that they often show large vertical wind speeds that persist for periods of several hours or longer at the same location. The large winds have been difficult to verify because of the difficulties associated with obtaining overlapping measurements and with obtaining a sufficiently comprehensive set of measurements to understand the dynamical drivers for the vertical winds. A surprising conclusion from examining all the available measurements, however, is that there is remarkable consistency between the characteristics of the thermospheric vertical velocities measured by different techniques and at different locations. We give an overview of the available data and find that, when considered together, the available observations all show the following patterns. Vertical velocities with magnitudes of 10-20 m s ~(-1) are common at high latitudes, but also at mid and low latitudes. There is a slight tendency for the winds to increase with height from the E region to the F region, but the increase with height is small, and in particular, is much less than expected from the decrease of background density with altitude. Another surprising characteristic that holds true across altitudes and latitudes is that the large vertical winds frequently persist for extended periods of an hour or longer. The drivers for the observed events of large vertical winds are unknown, but the winds clearly have important implications for the dynamics, electrodynamics, plasma physics, and chemistry of the thermosphere.