Thermosphere and Beyond
Thethermosphererises from the mesopause. The International Space Station (ISS) orbits
within the upper part of the thermosphere, at about 320 to 380 km above the Earth (Figure
15.7).
Figure 15.7: The International Space Station. ( 18 )What does an astronaut experience in the thermosphere? Temperatures in the thermosphere
can exceed 1000oC (1800oF) because oxygen molecules in the layer absorb short wavelength
solar energy. Yet despite these high temperatures, the atmosphere outside the ISS feels cold.
This is because gas molecules are so few and far between that they very rarely collide with
other atoms and so little energy is transferred. The density of molecules is so low that one
gas molecule can go about 1 km before it collides with another molecule.
Within the thermosphere is theionosphere. The ionosphere gets its name because nitro-
gen and oxygen molecules are ionized by solar radiation. In the process of ionization, the
neutrally-charged molecules absorb high-energy, short-wavelength energy from the Sun. This
causes the molecules to lose one or more electrons and become positively-charged ions. The
freed electrons travel within the ionosphere as electric currents. Because of the free ions, the
ionosphere has many interesting characteristics.
Have you ever been out on an open road and found a radio station on the AM dial that is
transmitted from hundreds of kilometers away? The reason radio waves can travel so far at
night involves the ionosphere. During the day, the lower part of the ionosphere absorbs some
of the energy from the radio waves and reflects some back to Earth. But at night the waves
bounce off of the ionosphere, go back down to the ground, and then bounce back up again.
This does not happen during the day due to ionization in the ionosphere. This bouncing up
and down allows radio waves to travel long distances.