Upper-atmosphere dynamics

(meteorology) Motion of the atmosphere above 300 miles (500 kilometers); predominant dynamical phenomena are internal gravity waves, tides, sound waves, turbulence, and large-scale circulation.

The motion of the atmosphere above 50 km (30 mi). The predominant dynamical phenomena of the upper atmosphere are quite different from those encountered in the lower atmosphere. Among those encountered in the lower atmosphere are cyclones, anticyclones, tropical hurricanes, thunderstorms and shower clouds, tornadoes, and dust devils. Even the largest of these phenomena do not penetrate far into the upper atmosphere. Above an altitude of about 50 km (30 mi), the predominant dynamical phenomena are internal gravity waves, tides, sound waves (including infrasonic), turbulence, and large-scale circulation.

Except under meteorological conditions characterized by convection, the atmosphere is stable against small vertical displacements of small air parcels; this results from buoyancy forces that tend to restore displaced air parcels to their original levels. An air parcel therefore tends to oscillate around its undisturbed position at a frequency known as the Brunt-Vaisala frequency ω_B. If pressure waves are generated in the atmosphere with frequencies much greater than ω_B, they propagate as sound waves. For frequencies much less than ω_B, the waves propagate as internal gravity waves; in this case, the restoring forces for the wave motion are provided primarily by buoyancy (that is, gravity) rather than by compression.

Tides are internal gravity waves of particular frequencies. The term tidal usually implies that the exciting force is gravitational attraction by the Moon or Sun. However, it is conventional in the case of atmospheric tides to include also those waves that are excited by solar heating. One is therefore concerned with three separate excitation functions—lunar gravitation, solar gravitation, and solar heating.

Tidal wind patterns in the upper atmosphere generate electrical currents in the ionosphere through a dynamo action. These in turn give rise to diurnal variations in the geomagnetic field that can be observed at the Earth's surface. See also Atmospheric tides; Geomagnetic variations; Ionosphere.

Sound waves generated in the lower atmosphere may propagate upward; to maintain continuity of energy flow, the waves might be expected to grow in relative amplitude as they move into the more rarefied upper atmosphere. However, higher temperatures in the upper atmosphere refract most of the energy back toward the Earth's surface, giving rise to the phenomenon known as anomalous propagation. This involves the redirection of upward-moving sound waves back to the surface beyond the point where the source can be heard by waves propagating along the surface. Infrasonic waves with periods from 20 to 80 s have been observed occasionally with detectors at the Earth's surface in connection with auroral activity. See also Sound.

There is clear visual evidence of turbulence in the upper atmosphere; this evidence is obtained by examination of vapor trails released from rockets or of long-persisting meteor trails. The source of the turbulence is not clear. The atmosphere is thermodynamically stable against vertical displacements throughout the region above the troposphere, and work has to be done against buoyancy forces in order to produce and maintain turbulence. The only apparent source of energy is internal gravity waves, either tidal or of random period.

There are prevailing patterns of atmospheric circulation in the upper atmosphere, but they are very different from those that occur in the lower atmosphere, which are associated with weather systems and have complicated structures resulting from growth of instabilities. The upper atmospheric large-scale wind systems are mainly diurnal in nature and global in scale.

The main heat source that is responsible for the upper atmospheric circulation is ultraviolet radiation from the Sun, radiation that is mainly absorbed at altitudes between 100 and 200 km (60 and 120 mi). The atmosphere is not a good infrared radiator in this altitude region, so the temperature rises rapidly with altitude, providing a temperature gradient of such a magnitude that molecular conduction transfers the absorbed heat downward to altitudes below 100 km (60 mi) where the atmosphere does have the capability of radiating the energy back to space. Above about 300 km (180 mi), the temperature becomes roughly constant with altitude because very little energy is absorbed there (the gas is exceedingly rarefied) and the thermal conductivity is good enough under these circumstances to virtually eliminate vertical temperature gradients. See also Solar radiation.

The region of rising temperature above 80 km (48 mi) is known as the thermosphere. The exosphere is that region of the atmosphere that is so rarefied that for many purposes collisions between molecules can be neglected; it is roughly the region above 500 km (300 mi).

At high latitudes, the ionosphere moves in response to electric fields imposed as a consequence of interactions between the Earth's magnetic field and the solar wind. The imposed electric field causes the ionosphere to drift in a generally antisunward direction over the polar caps (regions at higher latitudes than the auroral zone, or magnetic latitudes greater than about 68°), with a return circulation (that is, generally sunward in direction) just outside the polar caps. See also Atmospheric general circulation; Solar wind; Wind.

Ultraviolet radiation of suitable wavelengths can photodissociate atmospheric molecules—something of great importance in the upper atmosphere. It is even important in the stratosphere, where ozone is formed as a result of absorption by molecular oxygen of ultraviolet radiation, the important wavelengths being below 242 nanometers. See also Ozone; Stratosphere.