Primary Connections: Linking science with literacy
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The structure of the atmosphere

The envelope of gases that surround our Earth and allow life, as we know it, to exist is called "the atmosphere". The current atmosphere is the product of 4.6 billion years of changes in the geology of the Earth coupled with the evolution of life in its many forms. This topic investigates the physical structure and properties of the atmosphere, while the topic The composition of the Earth's atmosphere, investigates the origins and gases of our atmosphere.

Atmospheric layers
The atmosphere extends some 1000 km into space from the Earth's surface. It is made up of five physical layers - the troposphere, stratosphere, mesosphere, thermosphere and exosphere, each with distinctly individual properties. Two other regions - the ionosphere and magnetosphere - exist within these layers. Both are the product of interaction between the Sun and Earth's atmosphere.

One factor best distinguishes one layer from another - temperature. Each layer behaves differently from its neighbours with respect to the temperature and how temperature varies within the layer.

These layers and regions will now be investigated starting with the layer closest to the Earth's surface, the one we are most familiar with because we live in it - the troposphere.

The troposphere contains over half of the air in our atmosphere and extends up to 20 km above the Earth's surface at the equator. It is not the same depth in all places, being only about 8 km thick at the poles.

All life and most of the weather, clouds and pollution exist in this layer. The air temperature drops from an average of 20°C at the surface to a low of -55°C as you rise through the troposphere.

From the top of the troposphere to about 50 km, this layer contains the very important "ozone layer". The tops of large thunderstorms and dust from volcanoes can penetrate the lower part of the stratosphere, but generally there is little interaction with the lower troposphere. Highflying jets also travel in its lower reaches.

Temperature rises in this layer from -55°C at the interface with the troposphere to 0°C near the top of the stratosphere.

From 50 km to 90 km, this 40 km thick layer again decreases in temperature from 0°C to -90°C with altitude. Shooting stars or meteors, burn up in this layer.

From 90 km to about 300 km, the thermosphere dramatically increases in temperature with altitude, reaching 1200°C. The thermosphere is greatly affected by interactions with the solar wind from the Sun and is where the spectacular "Southern and Northern Lights" or auroras occur.

Reaching from 300 km to a 1000km or so, the exosphere is the less dense part of the Earth's atmosphere. In this layer, the air is so thin that satellites travel around the Earth, within its boundaries, without much effect on their motion.

The temperature in the exosphere drops off dramatically to that of near absolute zero in outer space.

The ionosphere
The ionosphere is a region within the upper mesosphere and thermosphere where solar radiation and particles in the "solar wind" strip electrons from atoms in the atmosphere and create an electrically charged zone of ions.

We use the ionosphere to communicate long distances by radio. Radio waves bounce off the ionosphere in much the same way that light is reflected by a mirror. The ionosphere changes its position from day to night, rising in the night and lowering in the day. This is why radio signals from far off places can be detected at night, while only close by transmissions are detectable during the day.

The magnetosphere
Contained within the Earth's thermosphere, the magnetosphere is the region where the Earth's magnetic field interacts with the charged particles coming from the Sun in the solar wind. These particles become trapped in the magnetic field of the Earth and circulate around the Earth following the Earth's magnetic field. Large bombardments of these charged particles cause the auroras in the ionosphere as they follow the Earth's magnetic field through the upper atmosphere towards the magnetic poles.