History
Prior to the space age, the only indirect access to the height region above about 100 km altitudes came from ionospheric and geomagnetic research. Electromagnetic waves below the VHF-range (VHF = very high frequencies; 30 - 300 MHz) reflected and attenuated in the ionospheric D-, E-, and F- layers depending on frequency, time of day, geographic location, and solar activity can be observed on the ground. The geomagnetic activity, likewise observed on the ground, was attributed to upper atmospheric electric currents, known today as currents flowing within the ionospheric dynamo region and the magnetosphere. With the advent of the Russian satellite Sputnik in 1957, observations of the Doppler effect of the satellite signal overhead allowed for the first time to determine continuously the orbital decay of the satellite and thus the atmospheric drag from which the variations of the thermospheric density could be derived. Mainly involved in these early measurements were L.G. Jacchia and J.W. Slowey (USA), D.G. King-Hele (Great Britain), and W. Priester and H.K. Pätzold (Germany). They discovered for the first time the large daily variations of the atmospheric density, its reaction on geomagnetically disturbanced conditions, etc. Today, an array of satellites measures directly the various components of the atmospheric gas. A full summary of early and present observations was presented by G. W. Prölss in 2011 .
It is convenient to separate the atmospheric regions according to the two temperature minima at about 12 km altitude (the tropopause) and at about 80 km (the mesopause) (Figure 1). The thermosphere (or the upper atmosphere) is the height region above 80 km, while the region between the tropospause and the mesopause is the middle atmosphere (stratosphere and mesosphere) where absorption of solar UV radiation generates the temperature maximum near 45 km altitude and causes the ozone layer.
The density of the Earth's atmosphere decreases nearly exponentially with altitude. The total mass of the atmosphere is M = ρA H ≃ 1 kg/cm2 within a column of 1 square centimeter above the ground (with ρA = 1.29 kg/m3 the atmospheric density on the ground at z = 0 m altitude, and H ≃ 8 km the average atmospheric scale height). 80% of that mass is already concentrated within the troposphere. The mass of the thermosphere above about 80 km is only 0.005% of the total mass. Therefore, no significant energetic feedback from the thermosphere to the lower atmospheric regions can be expected.
Read more about this topic: Thermosphere
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