Thermosphere

Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the thermosphere thus constitutes the larger part of the ionosphere.

Taking its name from the Greek θερμός (pronounced thermos) meaning heat, the thermosphere begins at about 80 km (50 mi) above sea level.

[1] At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass (see turbosphere).

Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation.

Radiation causes the atmospheric particles in this layer to become electrically charged, enabling radio waves to be refracted and thus be received beyond the horizon.

In the anacoustic zone above 160 kilometres (99 mi), the density is so low that molecular interactions are too infrequent to permit the transmission of sound.

The dynamics of the thermosphere are dominated by atmospheric tides, which are driven predominantly by diurnal heating.

Atmospheric waves dissipate above this level because of collisions between the neutral gas and the ionospheric plasma.

The thermosphere (or the upper atmosphere) is the height region above 85 kilometres (53 mi), while the region between the tropopause and the mesopause is the middle atmosphere (stratosphere and mesosphere) where absorption of solar UV radiation generates the temperature maximum near an altitude of 45 kilometres (28 mi) and causes the ozone layer.

Turbulence causes the air within the lower atmospheric regions below the turbopause at about 90 kilometres (56 mi) to be a mixture of gases that does not change its composition.

The lighter constituents atomic oxygen (O), helium (He), and hydrogen (H) successively dominate above an altitude of about 200 kilometres (124 mi) and vary with geographic location, time, and solar activity.

The ratio N2/O which is a measure of the electron density at the ionospheric F region is highly affected by these variations.

[2] These changes follow from the diffusion of the minor constituents through the major gas component during dynamic processes.

Astronomers have begun using this sodium band to create "guide stars" as part of the optical correction process in producing ultra-sharp ground-based observations.

The solar X-ray and extreme ultraviolet radiation (XUV) at wavelengths < 170  nm is almost completely absorbed within the thermosphere.

For instance, X-ray bursts associated with solar flares can dramatically increase their intensity over preflare levels by many orders of magnitude over some time of tens of minutes.

In the extreme ultraviolet, the Lyman α line at 121.6 nm represents an important source of ionization and dissociation at ionospheric D layer heights.

Quasi-periodic changes of the order of 100% or greater, with periods of 27 days and 11 years, belong to the prominent variations of solar XUV radiation.

That solar XUV energy input occurs only during daytime conditions, maximizing at the equator during equinox.

Solar wind particles penetrate the polar regions of the magnetosphere where the geomagnetic field lines are essentially vertically directed.

Along the last closed geomagnetic field lines with their footpoints within the auroral zones, field-aligned electric currents can flow into the ionospheric dynamo region where they are closed by electric Pedersen and Hall currents.

Ohmic losses of the Pedersen currents heat the lower thermosphere (see e.g., Magnetospheric electric convection field).

On the other hand, the fundamental diurnal tide labeled (1, −2) which is most efficiently excited by solar irradiance is an external wave and plays only a marginal role within the lower and middle atmosphere.

The variability of this heating depends on the meteorological conditions within the troposphere and middle atmosphere, and may not exceed about 50%.

If one considers very quiet magnetospheric disturbances and a constant mean exospheric temperature (averaged over the sphere), the observed temporal and spatial distribution of the exospheric temperature distribution can be described by a sum of spheric functions:[11] (3)

ta = June 21 is the date of northern summer solstice, and τd = 15:00 is the local time of maximum diurnal temperature.

During disturbed conditions, however, that term becomes dominant, changing sign so that now heat surplus is transported from the poles to the equator.

[5][12] In contrast to solar XUV radiation, magnetospheric disturbances, indicated on the ground by geomagnetic variations, show an unpredictable impulsive character, from short periodic disturbances of the order of hours to long-standing giant storms of several days' duration.

Also, due to the impulsive form of the disturbance, higher-order terms are generated which, however, possess short decay times and thus quickly disappear.

[18] ELVES is a whimsical acronym for "emissions of light and very low frequency perturbations due to electromagnetic pulse sources.

Earth's night-side upper atmosphere appearing from the bottom as bands of afterglow illuminating the troposphere in orange with silhouettes of clouds, and the stratosphere in white and blue. Next the mesosphere (pink area) extends to the orange and faintly green line of the lowest airglow , at about one hundred kilometers at the edge of space and the lower edge of the thermosphere (invisible). Continuing with green and red bands of aurorae stretching over several hundred kilometers.
A diagram of the layers of Earth's atmosphere
Figure 1. Nomenclature of atmospheric regions based on the profiles of electric conductivity (left), temperature (middle), and electron number density in m −3 (right)
Figure 2. Schematic meridian-height cross-section of circulation of (a) symmetric wind component (P 2 0 ), (b) of antisymmetric wind component (P 1 0 ), and (d) of symmetric diurnal wind component (P 1 1 ) at 3 h and 15 h local time. Upper right panel (c) shows the horizontal wind vectors of the diurnal component in the northern hemisphere depending on local time.