We are using steps to show something of the structure of the Earth’s atmosphere.
Radiation from the Sun (as light and particles) gives the atmosphere its form. Small variations in the Sun’s output can have significant effects on the measured temperatures and other properties.
Step 0 — Troposphere
Height: Sea Level
Temperature: Around 20°C
This is where we live, grow crops, have our factories, houses and most scientific laboratories.
Step 1 — Tropopause
Height: 9 km
Already we are higher than Mt. Everest (8.8 km). We are also above most of the world’s weather (rain, clouds). Intercontinental passenger aircraft fly at this height.
Step 2 — Stratosphere
Height: 18 km
You are now above 90% of the Earth’s atmosphere, measured by mass. The temperature is very low and the air is very thin; a human could not survive. This is also the bottom of the ozone layer.
The ozone Layer
Ozone is three oxygen atoms bonded together (O3). Ozone is important to us as it absorbs ultraviolet (high energy) light from the sun and prevents it from reaching the Earth.
Height: 27 km
At this height you find the largest concentration of ozone. The temperature has gone up slightly from step 2, as ozone absorbs sunlight and converts this to heat.
Also at around this height the chlorofluorocarbons (CFCs — chemicals produced for industrial and domestic use) do the most damage to the ozone layer.
The ‘ozone hole’ was discovered in this region from observations at a British Antarctic station. The hole is monitored by the US National Oceanic and Atmospheric Administration (NOAA), who provide up-to-date imagery and information on the ozone hole.
Height: 36 km
We are still in the ozone layer. Ozone is formed in the atmosphere mainly between heights of 30 and 60 km (between steps 3 and 7). Australian scientists have installed equipment in Antarctica (mainly at Davis station) to study the atmosphere at these heights. This includes the LIDAR and an ozone monitoring instrument.
Step 5 — Stratopause
Height: 45 km
The temperature has increased by a large amount from the step below, due to the absorption of solar ultra-violet light by ozone. The atmosphere is very thin here, so not much energy needs to be absorbed to produce a temperature rise.
Step 6 — Mesosphere
Height: 54 km
As there is less ozone here, the temperature is less than on the step below as not much solar radiation is absorbed.
Height: 63 km
As we go higher, the temperature continues to fall.
Height: 72 km
Again, as we go higher the temperature has fallen and is now as cold as it was back on steps 1 and 2, below the ‘warm’ ozone layer.
Step 9 — Mesopause
Height: 81 km
Temperature: −90° to −100°C
We are now at the bottom of a thin (~10 km) very cold region called the ‘mesopause'.
Height: 90 km
Temperature: down to −120°C
We are now near the top of the mesopause, which is the coldest part of the atmosphere. In fact, it is the coldest part of the Earth’s environment. (The coldest temperature recorded on the Earth’s surface was just −90°C at the Russian operated Vostok Antarctic station.)
Australian Antarctic scientists are studying the mesopause by measuring the light emitted by the naturally occurring OH (oxygen hydrogen) molecule. The light is emitted at an altitude of about 87 km and can be detected at ground level using very sensitive instruments. Our observations allow us to ‘take the temperature’ of the mesopause as a guide to the state of health of the Earth.
How do we make our observations?
Some atoms and molecules in the Earth’s atmosphere will give off small amounts of light under certain circumstances. Our instruments on the ground can measure this light. The frequency of the light contains information about the speed (winds) and temperature of the atoms and molecules. (In the same way, you can tell if a fire-engine is moving towards or away from you by the frequency of the sound of the siren). We have these instruments at Mawson and Davis stations.
The atoms and molecules emit light only at a few different heights and it can be very hard (if not impossible) to measure this light during the day. To overcome this we use a ‘LIDAR’ (light detection and ranging) instrument — by shining a powerful laser into the sky and measuring the reflection, we can measure winds, temperatures and density from about 10 km to the mesopause.
Step 12 — Thermosphere
Height: 110 km
We are now entering the upper atmosphere. At this height and above, the atmosphere is extremely thin (very low density). Solar radiation is able to split the electrons (negative charge carriers) away from the ions (positive charge carriers). This allows large electric currents to flow and also means that shortwave radio signals can ‘bounce off’ the atmosphere and return to Earth. The bottom of the green auroral light comes from near this height.
Height: 130 km
The temperature has started to increase rapidly but as the atmosphere is so thin the amount of heat energy involved is very small. At polar latitudes there are very large electric currents flowing near this height which are associated with the aurora.
Height: 150 km
The temperatures at this height and higher are very dependent on solar activity. Sunlight splits atoms into ions and electrons. At night the ions and electrons here recombine, so that there are about 100 times fewer free electrons and ions at night than by day.
(In Antarctica the ‘night’ can be many months long, so the atmosphere there will be different to the atmosphere above Australia.)
Height: 170 km
Temperature: 600–1200°C (approximately)
This is roughly the height of the top of the green aurora. The green glow comes from oxygen atoms, much like how orange light comes from sodium street lamps.
All four Australian Antarctic stations have auroral video cameras to monitor the aurora in green light every clear night.
Step 16 — Magnetosphere
Height: 190 km
We are entering the region known as the magnetosphere, where the Earth’s magnetic field plays an important part in many of the processes which occur here. The Earth’s magnetic field extends out for at least 40 000 km on the dayside of the Earth and many times that on the nightside.
The Earth’s magnetic field shields us from most of the high energy (fast moving) particles known as cosmic rays. Without this shield life as we know it would not be possible on Earth. Some cosmic rays can penetrate down to a height of about 20 km where they collide with molecules in the atmosphere. Some of the pieces from the collision reach ground level. (This radiation is not hazardous to life).
A cosmic ray observatory is located at Mawson.
Height: 210 km
This height is about the bottom of the red aurora. Like the green aurora, the red light is also produced by oxygen atoms. Electrons (negative charges) are ejected by the Sun at very high speeds. It takes them 1–3 days to reach the Earth. The Earth’s magnetic field guides these electrons to enter the night-side polar atmosphere which causes the aurora.
Height: 230 km
This is about the middle of the red aurora. The winds and temperatures here depend very much on how active the Sun is.
The Sun’s activity varies on an 11-year cycle. The cycle was at a high point about 1989 so the atmosphere at this height was warmer than at present, as we are now near the low point in the cycle.
The solar cycle may also affect temperatures lower in the atmosphere, including down to sea level. We have to understand these natural changes in addition to any changes taking place as a result of industrial and domestic pollution.
Step 19 — F2 layer
Height: 250 km
At this height we have more of the atoms split into electrons (negative charges) and ions (positive charges) than anywhere else in the atmosphere, this is called the F2 layer. The existence of these free electrons is very important for shortwave radio communication around the world, as radio signals reflect back off this layer rather than escaping into space.
We can also use radio waves to study this and other electron layers by sending a radio wave straight up and measuring the reflection.
An instrument at Casey research station can build up ‘radar maps’ of the sky from 80–800 km in height in this way.
Height: 270 km
Temperature: up to 2000°C
Only the thinnest trace of the atmosphere remains. The movement of electrons and ions is controlled mainly by the Earth’s magnetic field.
The magnetosphere (the region of the Earth’s magnetic field in space) contains a huge amount of power from the particles and magnetic field of the Sun, up to one million million watts of power. A large power station on Earth generates about one hundred million watts, meaning the magnetosphere is equivalent in this sense to ten thousand power stations.
Height: 290 km
This is the end of our tour but the Earth’s atmosphere doesn’t end here. It can be detected out to at least 1000 km above the Earth’s surface and the magnetic field of the Earth extends many times further again. The properties of our atmosphere depend on many complex interactions between processes at various heights, but ultimately the ‘driving engine’ is solar radiation, both light (including x-rays and ultra-violet light) and particles (cosmic rays and auroral electrons).
We hope we have helped you appreciate something of what goes on all the time in our atmosphere.
Many of the processes are very complex and will require many years of study before we fully understand what is happening.
One thing is certain however, for many reasons we hope the atmosphere remains above our heads for many years to come!
John Innis, Ray Morris, John French, Pene Greet, Pelham Williams, Damian Murphy, Andrew Klekociuk and Malcolm Lambert
Australian Antarctic Division