 |
| Earth atmosphere
diagram showing all the layers of the atmosphere
to scale. |
Thermosphere
The thermosphere is the layer in the Earth's atmosphere
directly above the mesosphere and below the exosphere.
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. 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. Temperatures are
highly dependent on solar activity, and can rise to
1,700 °C (3,100 °F) or more. Radiation causes the
atmosphere particles in this layer to become
electrically charged particles, enabling radio waves to
be refracted and thus be received beyond the horizon. In
the exosphere, beginning at about 600 km (375 mi) above
sea level, the atmosphere turns into space, although, by
the judging criteria set for the definition of the
Kármán line, the thermosphere itself is part of space.
The highly attenuated gas in this layer can reach 2,500
°C (4,530 °F) during the day. Despite the high
temperature, an observer or object will experience cold
temperatures in the thermosphere, because the extremely
low density of the gas (practically a hard vacuum) is
insufficient for the molecules to conduct heat. A normal
thermometer will read significantly below 0 °C (32 °F),
at least at night, because the energy lost by thermal
radiation would exceed the energy acquired from the
atmospheric gas by direct contact. 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 is uninhabited with the exception of
the International Space Station, which orbits the Earth
within the middle of the thermosphere, between 408 and
410 kilometres (254 and 255 mi). |
|
Thermospheric storms
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.
The reaction of the thermosphere to a large
magnetospheric storm is called a thermospheric storm.
Since the heat input into the thermosphere occurs at
high latitudes (mainly into the auroral regions), the
heat transport is represented by the term P20 in eq. is
reversed. Also, due to the impulsive form of the
disturbance, higher-order terms are generated which,
however, possess short decay times and thus quickly
disappear. The sum of these modes determines the "travel
time" of the disturbance to the lower latitudes, and
thus the response time of the thermosphere with respect
to the magnetospheric disturbance. Important for the
development of an ionospheric storm is the increase of
the ratio N2/O during a thermospheric storm at middle
and higher latitude. An increase of N2 increases the
loss process of the ionospheric plasma and causes
therefore a decrease of the electron density within the
ionospheric F-layer (negative ionospheric storm). |
|
