Week 10 · 2-1 Components and Layers (B): ionosphere, aurora, exosphere, escape velocity and Mercury; 2-2 Chemical Reactions in the Atmosphere: ultraviolet rays, ozone formation, ozone depletion and its effects, protection measures, ozone in the troposphere · 3 lessons
1
The ionosphere, the aurora and the exosphere
Integrated Science Sec 1 Term 1, Unit 2: pp. 73–83
GoalDescribe the ionosphere, the aurora and the exosphere, and say why they matter.
New words
ionosphere · the lower part of the thermosphere, where atoms are ionised by solar radiationion · an atom or group of atoms that carries an electric chargeaurora · coloured light seen in the sky near the polesexosphere · the outermost, extremely thin layer, where atoms rarely collide
Explanation
The thermosphere begins at about 80 km. In the book its lower part (about 80 to 550 km) is the ionosphere. There the atoms of oxygen and nitrogen absorb X-rays and other high-energy radiation from the Sun and become ionised, which means they become charged. The ionosphere reflects radio waves, so wireless messages can travel round the curve of Earth over long distances. Near the poles the charged particles of the solar wind are steered by Earth’s magnetic field, hit the atoms of the ionosphere and excite them; when the atoms return to their normal state they give off light, the aurora (extra: oxygen gives green and red, nitrogen blue and violet). The exosphere is the outermost layer (about 700 to 1000 km in the book, though it really reaches much higher). It is extremely thin, atoms rarely collide, and some escape into space. Satellites orbit in the very thin air at great heights. The very high temperature quoted for the thermosphere (well over a thousand degrees) describes the fast motion of a few particles; a thermometer or a satellite there would not feel hot, because so few particles hit it.
solar X-rays and high-energy radiation→atoms of O and N lose electrons→charged layer: ionosphere→radio waves are reflected
Worked examples
A radio station sends a signal up to the ionosphere at about 300 km. It travels up and back, so the path is about 2 × 300 = 600 km, and at the speed of light (300 000 km/s) the time is 600 ÷ 300 000 = 0.002 s. The reflection is why a signal can reach places beyond the horizon.
Explain the aurora in order. Charged particles stream from the Sun; Earth’s magnetic field guides them to the poles; they hit atoms of the ionosphere and excite them; the atoms give off light as they calm down. That is why auroras are seen near the poles and not near the equator.
Region
Main feature
ionosphere
atoms are ionised
aurora
light near the poles
exosphere
extremely thin
Compare the three upper regions. Ionosphere: charged atoms, reflects radio waves. Aurora: light from excited atoms near the poles. Exosphere: extremely thin, atoms seldom collide, some escape.
Class activity
Draw a comic strip of four boxes with a grown-up: (1) the Sun sends charged particles, (2) the magnetic field guides them, (3) they meet atoms near the pole, (4) the atoms glow. Use colour pencils for the glow. Do not look at the Sun. Add a caption that says why an aurora is not seen in Egypt’s sky on a normal night.
Practice
1
What makes the ionosphere charged?
Atoms of oxygen and nitrogen absorb X-rays and other high-energy radiation and become ions.
2
Why can radio signals travel far round Earth?
The ionosphere reflects radio waves back toward the ground.
3
Why is the aurora seen near the poles?
Earth’s magnetic field guides the charged particles of the solar wind to the poles.
4
Why would a thermometer in the thermosphere not feel hot?
The air is so thin that very few particles hit it, even though they move fast.
