The Mesosphere and Thermosphere
Investigating the Outermost Layers of the Earth’s Atmosphere
Sep 21, 2009
The Earth’s atmosphere is layered. From the surface traveling upward the first layer one would encounter is the troposphere. This layer rests on the surface of the planet. Above the troposphere is the calm, stable stratosphere.
Beyond the stratosphere are the outer most layers of the atmosphere, the mesosphere and the thermosphere.
The Mesosphere is a Difficult Place to Study
The mesosphere is difficult to study. Weather balloons and aircraft cannot fly high enough to reach the mesosphere while weather satellites orbit above this layer.
Scientists are forced to study the mesosphere by sending instruments up into this layer aboard small sounding rockets. These small rockets can carry a very limited payload of instruments and since they travel straight up and fall straight back down, they spend very little actual time in the mesosphere. As a consequence, the mesosphere is still a somewhat mysterious place.
Characteristics of the Mesosphere
The mesosphere (literally “middle sphere”) is separated by the stratopause from stratosphere and below and by the mesopause from the thermosphere layer above. The mesosphere is located about 50 to 85 kilometers (30 to 50 miles) above the Earth's surface.
The mesosphere does not absorb any significant portion of solar radiation and as result, temperatures decrease with height throughout this layer. In fact, the mesopause at the top of the mesosphere may be the coldest place on Earth with frigid temperatures as low as -100 °C (-150 °F).
Millions of meteors plunge into the Earth’s atmosphere every day. Fortunately, most of this estimated 40 tons of meteoric material melts or vaporizes due to friction with the gases found in the mesosphere. Some of this vaporized material lingers in the mesosphere causing it to have an unusually high concentration of iron and other metallic elements.
While the stratosphere layer below the mesosphere is cloud-free, occasionally strange clouds known as noctilucent clouds or polar mesospheric clouds form in the mesosphere above the poles.
Strange lightning-like electrical discharges known as “sprites” and “ELVES” occasionally appear in mesosphere above the tops of thunderstorms embedded in the troposphere below.
Atmospheric waves and tides of several sorts carry energy from the stratosphere and troposphere upwards into the mesosphere. This energy drives the global circulation of the mesosphere.
The Characteristics of the Thermosphere
The thermosphere (literally “heat sphere”) is the outermost layer of the Earth’s atmosphere and it is situated above the mesopause. The thermosphere begins at about 90 km (56 mi) and extends on outward from there gradually blending into space. The outermost area of the thermosphere where the last vestiges of atmosphere give way to the nearly complete vacuum of space is often referred to as the exosphere.
The most commonly held demarcation of the beginning of space is 100 km (62 mi) above the Earth’s surface. Hence, satellites, the International Space Station and the space shuttle all operate within the outer regions of the thermosphere.
Temperatures rise sharply in the lower thermosphere and then level off at around 200 to 300 km (124 to 186 mi) above the Earth’s surface and hold fairly steady with increasing altitude above that height. Temperatures in the upper thermosphere can range from about 500°C (932°F) to 2,000°C (3,632°F) or higher when the Sun is very active than at other times.
These extremely high temperatures are caused by the fact that the thermosphere absorbs a most of the X-ray radiation and a great deal of the ultraviolet radiation streaming in from the Sun. However, even though the temperature is so high, one would not feel warm in the thermosphere because it is so near the vacuum of deep space due to the low density (number) of gas atoms there. Hence, there could not be enough contact with these few widely space atoms of gas to transfer much heat.
The Ionosphere and Auroras
The thermosphere is constantly bombarded by solar radiation and the solar wind. This intense solar energy tears apart atoms and molecules in the thermosphere creating electrically-charged ions. Areas of ions within the thermosphere are known as the ionosphere.
Radio waves which travel only in straight lines cannot normally travel beyond the curve of the Earth (the horizon). However, the ionosphere reflects radio signals and bounces them back to the surface. By bouncing back and forth off the ionosphere, radio signals can literally travel around the world, especially at night when the ionosphere is more active.
As the solar wind strikes the gases in the upper thermosphere it causes these gases to become energized and glow as shimmering veils of light known as auroras.
In the extreme outer reaches of the atmosphere lie the mysterious mesosphere and the electrically charged thermosphere. Although they are unbelievably thin, the shield that these layers form against incoming meteors and deadly solar radiation protects all surface life on the planet.
