Thermosphere

The thermosphere (from Greek θερμός, thermos " hot, hot" and σφαίρα, sfära " ball " ) is the fourth and the second outermost layers of the atmosphere. It extends above the lying in 80-85 km altitude mesopause (upper boundary of the mesosphere ) to below the exosphere in about 500-600 kilometers above the earth's surface.

Despite its name, the " warmth" of the thermosphere is not " noticeable " because the air density is a million times smaller than near the ground. The temperature (300-1500 ° C) is expressed only in the rapid movement of the gas particles. Their mean free path is here several miles, so take place no more collisions or energy exchange between the particles.

If meteors occur in the Earth's atmosphere, its tracer usually begins in the lower thermosphere. Also during re-entry of space vehicles occur here first thermal loads, the highest temperatures are reached but usually only in the mesosphere.

The internationally accepted definition of the boundary between Earth's atmosphere and outer space is the Kármán line and lies at an altitude of 100 km, ie at the level of Homo break. Thus, most of the thermosphere would be already in space. This definition is more or less arbitrary and physically defined criterion no.

In the thermosphere including the Space Shuttle and International Space Station ( ISS) orbiting the Earth.

Temperature

The temperature initially increases strongly with altitude and can - also dependent on the solar activity - increase to 1700 ° C. But this feels because of the low density and the heat loss as a black body still cold to the touch.

Pressure

The air pressure decreases with increasing height in the lower part of the atmosphere. However, due to the influence of increasing temperature with altitude and the changing composition of the decrease is slower. In the upper part of the thermosphere, the pressure follows an exponential be roughly, the results from the barometric formula.

Although the atmosphere here is extremely thin, the air resistance for a long time but makes noticeable. The International Space Station (ISS ), which orbits the earth at about 350 km altitude, lose without regular raising its orbit by rocket engines within a few years, so much to level that she fell to the ground.

Chemical composition

Gas molecules are cleaved from the solar ray and extreme ultra-violet radiation into ions and electrons. Therefore, the ionosphere is also a part of the thermal sphere. Due to the low density of the ionosphere, these particles can exist long before they recombine.

The average mass of the individual gas particles increases with altitude shrinking. There is a reason for a fact that the upper parts of the thermosphere are more exposed to the solar wind and cosmic rays. Still existing molecules are therefore further decomposed above are more likely in their ingredients. Because of the lower pressure, these components also have lower probability to unite again to form molecules. A third reason is that light particles at the same temperature have a higher speed and are thus less affected by the gravity. In this way accumulate in the upper part of the thermosphere light atoms and ions. This accumulation can become particularly clear read on the molar mass of the gas.

History

In the period before the space research, the only indirect information about the altitude range above 70 km from the ionospheric and the Earth's magnetic field was coming. Electromagnetic waves below the VHF range (VHF = very high frequencies; 30-300 MHz), which are reflected on the different ionospheric layers and subdued, can be observed at the surface, depending on the frequency, day and year and the solar activity. The measured at the surface variations of the geomagnetic field are electric currents in the ionosphere and magnetosphere associated ( ionospheric dynamo layer).

With the launch of the Russian satellite Sputnik, it was possible for the first time, to systematically determine the deceleration of the orbital period of the Doppler effect measurements of the satellite signal and to derive the density of air in the upper atmosphere as well as their temporal and spatial variations. Mainly involved in these first measurements were LG Jacchia and J. W. Slowey (USA), Desmond King - Hele (England) and Wolfgang Priester and HK Paetzold ( Germany ). Today, a whole army of satellite directly measures the diverse components of the atmospheric gas in this height range.

Constituents of the neutral gas

It is customary, the atmosphere in the three regions: lower atmosphere (troposphere), middle atmosphere ( stratosphere and mesosphere ) and upper atmosphere to divide ( thermosphere ) (Fig. 1). The boundaries of these regions determine the two temperature minima around 12 km altitude ( tropopause ) and 85 km ( mesopause ) (Fig. 2). The density of the atmospheric gas increases nearly exponentially with the height (Fig. 3). The total mass of the atmosphere is M = ρA H ≃ 104 kg/m2 within a vertical column of a square cross-section above the earth's surface ( with ρA = 1.29 kg/m3 density of the atmosphere at the ground at z = 0 m and H ≃ 8 km the average scale height of the lower atmosphere ). 80 % of this mass is already within the troposphere, while the share of the thermosphere is only about 0.002 % of the total mass. It is therefore expected no measurable influence of the thermosphere to the lower layers of the atmosphere.

Turbulence is responsible for ensuring that the neutral gas in the area below the turbo break in about 110 km altitude, a gas mixture at constant molecular weight (Fig. 2). Above the turbo break, gas begins to segregate. As a result of dynamic processes try the different constituents continuously to arrive by diffusion in its equilibrium state. Your barometric formulas have scale heights that are inversely proportional to their molecular weights. The lighter constituents such as atomic oxygen (O), helium ( He) and hydrogen ( H) dominate therefore successively in the height range above about 200 km. Here, the average height scale is almost a factor of 10 larger than that in the lower layers of the atmosphere ( Fig. 2). The air composition varies here with the geographical location, time of day and season, but also with the solar activity and the geomagnetic fluctuations. The ratio N 2 / O is a measure of the electron density of the ionosphere F layer. This ratio may be due to dynamic processes change very quickly and can cause ionospheric storms.

Energy budget

The thermospheric temperature can be determined from observations of gas density, but also directly with the help of satellite measurements. The temperature profile obeys pretty well the law ( Bates profile ):

With T ∞ Globally averaged exospheric temperature above about 400 km, To = 355 K, and zo = 120 km reference temperature and height and s an empirical parameter that decreases with T ∞. From Equation 1, the heat above zo can determine = 120 km to qo ≃ 0.8 to 1.6 mW/m2 height. This heat is delivered to the lower layers of the atmosphere by conduction.

Constant with the height Exosphärentemperatur T ∞ is a measure of the solar Ultraviolet and X-ray ( EUV ). Now the solar radio emission F is a good indicator of solar activity at 10.7 cm. One can therefore derive an empirical formula, the F associated with T ∞ and is valid for geomagnetically quiet conditions:

W m- 2 Hz -1 ( the Covington index), a value for F, averaged with T ∞ in K, Fo 10-2 in over a month. Typically, the Covington index varies between about 70 and 250 in the course of the solar 11 -year cycle and is never less than 50 This means that T ∞ fluctuates between about 740 and 1350 K, even at quiet geomagnetic conditions. The residual temperature of 500 K in Equation 2 derives about half of energy input from the magnetosphere. Atmospheric waves from the troposphere, which are dissipated in the lower thermosphere, contribute to the other half of about 250 K.

Energy sources

Solar XUV radiation

The solar X-ray and extreme ultraviolet (XUV) radiation with wavelengths less than 170 nm is almost completely absorbed in the thermosphere, causing the high temperatures. A part of the neutral gas is ionized, and is responsible for the formation of different ionospheric layers. The visible solar radiation in the range of 380 to 780 nm, remains almost constant with a variation of less than 0.1 % ( solar constant ). The solar XUV radiation, however, is extremely variable in time. Thus, for example, solar X-rays associated with solar flares, increase dramatically within a period of a few minutes. Fluctuations with periods of 27 days or 11 years are among the most prominent variations of the solar XUV radiation. However, irregular fluctuations of all time periods are the norm. In magneto- spherical calm conditions the XUV radiation provides about half of the energy supply in the thermosphere. This extra heat is done during the day and has a maximum near the equator.

Solar Wind

A second source of energy is the energy input from the magnetosphere, which in turn owes its energy of interaction with the solar wind. The mechanism of the energy transport is not yet known in detail. One possibility would be a hydro- magnetic process. Solar wind particles penetrate into the polar regions of the magnetosphere, where the geomagnetic lines of force are directed substantially vertically. In this case, an electric field is produced that is directed from morning to evening. Along the last closed field lines of the geomagnetic field with their foot points in the auroral zones, electrical discharge currents flow in the ionospheric dynamo layer, where as electrical Pedersen and Hall currents in two narrow power bands ( DP1) to the evening side and from there back to the magnetosphere ( magneto- spherical electric convection field ). By ohmic losses of Pedersenströme the thermosphere is heated above all in the auroral zones. In addition urging disturbed magnetospheric conditions highly energetic electrically charged particles from the magnetosphere into the auroral zones, which can dramatically increase the electrical conductivity and thus enhance the electric currents. On the ground this phenomenon is observed as polar lights.

At low magnetospheric activity that energy supply is about one- quarter of the total energy budget in Equation 2, ie about 250 K. While strong magnetospheric activity, this proportion is growing considerably and may exceed the influence of the XUV radiation in extreme conditions far

Atmospheric waves

There are two types of large-scale atmospheric waves in the lower atmosphere: internal waves with a finite large vertical wavelengths, the wave energy upward to transport and their amplitudes grow exponentially with height, and external waves with infinitely large vertical wavelengths, the wave energy away from their source region decreases exponentially and can not transport wave energy .. Many atmospheric tidal waves and the atmospheric gravity waves, which are excited in the lower atmosphere are among the internal waves. Since their amplitudes grow exponentially, these waves are destroyed at the latest in the altitude range of 100 km through turbulence. Your wave energy is converted into heat. This is the proportion of about 250 K in Equation 2, the best fit of their Meridionalstruktur forth to the heat source in the troposphere all-day tidal wave (1, -2) is an external shaft and plays only a marginal role in the lower atmosphere. However, in the thermosphere, this wave developed into the dominant tidal wave. It drives the electric Sq - current in the height range between about 100 and 200 km ( ionospheric dynamo layer).

Spherical thermal warming, mainly by tidal waves, is preferably carried out on the day hemisphere in low and middle latitudes. Their variability depends on the meteorological conditions and rarely exceeds 50%.

Dynamics

Above about 150 km altitude degenerate all atmospheric waves to external waves, and it is hardly a vertical shaft structure visible. Your Meridionalstruktur is that of spherical functions Pnm m with a meridional wave number and n is the zonal wave number ( m = 0: zonally averaged waves, m = 1: all-day waves, m = 2: half-day waves, etc. ). The thermosphere behaves in first approximation as a damped oscillator system with low-pass filtering effect. That is, small- waves ( with great wave numbers n and m ) can be suppressed with respect to the large scale waves. In the case of low magnetospheric activity can describe the observed temporal and spatial varying Exosphärentemperatur by a sum of spherical harmonics:

It is φ the latitude, λ the longitude, t is time, ωa is the angular frequency of the year period τ, λ = ωdt the local time and? D is the angular frequency of a solar day. ta = June 21 is the time of the start of summer in the northern hemisphere? d = 15:00 and the local time of maximum wind.

The first term on the right in equation 3 is the average temperature of the global exosphere (of the order of 1000 K). The second term [( P20 = 0.5 (3 sin2φ - 1)] is generated by the difference in solar heating in low and high latitudes A thermal wind system is created with winds towards the poles in the upper Zirkulationsast and contrary winds in the lower branch. (Fig. 4a). It ensures a heat balance between low and high latitudes. coefficient ΔT20 ≈ 0.004 is small, because the Joule heating in the auroral zone the solar XUV -related excess heat at low latitudes partly compensated. the third term ( with P10 = sin φ ) is responsible for the transport of excess heat in the summer hemisphere to the winter hemisphere (Fig. 4b). Its relative amplitude is about ΔT10 ≃ 0.13. the fourth term finally ( with P11 = cos φ of the dominant tidal wave (1, -2) ) describes the transport of the excess heat from the day side to the night side (Fig. 4d). Its relative amplitude is about ΔT11 ≃ 0.15. Additional terms ( eg half-year, half-day waves etc. ) need to equation 3 be added. They are, however, of less importance. Such sums can be used for air pressure, air density, Gaskonstituenten, etc. derived.

Thermospheric storms

Far stronger than the solar XUV Stahlung vary the magnetospheric disturbances that can be observed on the ground as geomagnetic disturbances. They are difficult to predict and have fluctuations with periods from minutes to several days. The response of the thermosphere to a strong magnetospheric storm is called a thermospheric storm. Since the energy is supplied at higher latitudes (mainly in the auroral zones), changes the second term in Equation 3 P20 its sign. Heat is now transferred from the polar regions in the low latitudes. In addition to this term more higher-order terms are involved, however, the decay quickly. The sum of these terms determines the " maturity " of the interference from high to low latitudes, ie the response time of the thermosphere. At the same time a Ionosphärensturm can develop. Important for the formation of an ionospheric error is the change in the density ratio of nitrogen molecules (N2) to oxygen atoms (O). An increase in the N2 density increases the loss processes of the ionospheric plasma and is therefore a decrease in electron density in the F- layer responsible (negative Ionosphärensturm ).