The upper atmosphere: mesosphere, thermosphere, ionosphere and exosphere

Detail

The four upper layers at a glance

Layer	Height (NCERT)	How temperature changes with height	Key feature
Mesosphere	about 50–80 km	Falls, to about −100°C at 80 km	Most meteors burn up here; top is the mesopause
Thermosphere	80–700 km (Class 9)	Rises sharply	Holds the ionosphere; aurora forms here; ISS orbits here (~400 km)
Ionosphere	80–400 km (Class 11)	Part of the thermosphere	Charged particles (ions) reflect radio waves
Exosphere	Above the thermosphere	Very thin gas	Hydrogen and helium escape to space

• Order from the ground up: troposphere → stratosphere → mesosphere → thermosphere (with the ionosphere) → exosphere.
• Each boundary is a "pause": tropopause, stratopause, mesopause.
• Geographers study mainly the first two layers (troposphere and stratosphere), because weather and life happen there (Class 11).

Mesosphere — the coldest layer

• Mesosphere (the "middle sphere"): the layer above the stratosphere. It runs from about 50 km to 80 km.
• Temperature falls again with height:
• In the stratosphere, ozone takes in ultraviolet (UV) rays and warms the air.
• There is little ozone in the mesosphere, so nothing heats the air much.

• Temperature falls to about −100°C at 80 km. This is the coldest point in the whole atmosphere.

• Mesopause: the upper boundary of the mesosphere, at about 80 km.

• Meteors burn up here:
• Rocks from space (meteoroids) enter the air at very high speed.
• They rub against the air (friction) and get very hot.

• Most of them burn up in the mesosphere. We see them as "shooting stars".

• Noctilucent clouds ("night-shining" clouds): the highest clouds on the Earth.

• They form near the mesopause, about 76–85 km above the surface [3][4].
• Process: tiny bits of dust from burnt-up meteors float in the mesosphere → small amounts of water vapour freeze on this dust → thin ice clouds form [3][4].
• They are seen mainly in polar summer, just after sunset. The Sun is below the horizon for people on the ground, but it still lights up these very high clouds.
• NASA's AIM mission studies them. Its records show they are now seen more often [3].

Thermosphere — the hot layer

• Thermosphere ("heat sphere"): the layer above the mesopause. NCERT Class 9 gives it as 80–700 km.
• Temperature rises sharply with height:
• The gas here is very thin.
• Its few molecules take in the Sun's X-rays and short-wave UV rays.

• Each molecule gets very hot. But there are so few of them that the air would not feel hot to us.

• Satellites: the International Space Station (ISS) orbits in the thermosphere, at about 400 km. Many low-Earth-orbit (LEO) satellites also move through it.

• Satellite drag: when the Sun is very active, the thermosphere heats up and swells. Its air becomes denser at satellite height. This pulls on LEO satellites and slows them down. It affects orbit forecasts, collision warnings and re-entry [5].

Ionosphere — the electric layer inside the thermosphere

• Ionosphere: the part of the atmosphere at about 80–400 km (Class 11). It lies inside the thermosphere.
• Ion: an atom or molecule that has an electric charge because it has lost or gained an electron.
• How ions form:
• Strong solar radiation (X-rays and UV) hits gas atoms.
• The radiation knocks electrons off the atoms.

• Charged particles (ions and free electrons) are left behind.

• Reflects radio waves:

• Radio waves sent upward hit the ionosphere and bounce back to the Earth.
• They can bounce several times between the ground and the ionosphere.

• This makes long-distance (short-wave) radio possible, for example HF radio (high frequency radio) used by ships and aircraft.

• GPS: GPS signals from satellites pass through the ionosphere. Its charged layer bends the signal path, much as a lens bends light. This causes small errors in position [5].

• Solar storms disturb it:
• The Sun throws out a burst of charged particles (a solar flare or a CME, coronal mass ejection: a huge cloud of charged gas blasted out of the Sun).
• The ionosphere becomes uneven and unstable.
• Result: HF radio fades or is lost, and GPS signals are delayed, flicker (scintillation) or are lost [5].

Aurora — light shows near the poles

• Aurora: coloured light in the night sky, seen mostly near the poles. It forms in the thermosphere, not in the stratosphere (Class 9, Fig. 3.4).
• How an aurora forms (cause → process → result):
• Cause: the solar wind (a stream of charged particles thrown out by the Sun) reaches the Earth.
• Process: the Earth's magnetic field steers these particles towards the magnetic poles. There they hit gas atoms in the thermosphere and give them extra energy (excite them).
• Result: the excited gases give off light.

Gas	Colour of light
Oxygen	Green and red
Nitrogen	Blue and purple

• Names:
• Aurora Borealis (Northern Lights): northern hemisphere. Seen in places such as Norway, Alaska, Canada and Iceland.

• Aurora Australis (Southern Lights): southern hemisphere. Seen near Antarctica, and sometimes from Tasmania and New Zealand.

• The geomagnetic storm of May 2024:

• Geomagnetic storm: a strong disturbance of the Earth's magnetic field caused by a burst of solar wind or a CME.
• On 10–11 May 2024, a G5 ("extreme") storm hit the Earth. It was the strongest since October 2003 [2].
• Auroras were seen much closer to the equator than usual, down to about 26° magnetic latitude. This may be among the lowest-latitude aurora sightings of the last five centuries [2].
• In India, a red aurora was recorded from Hanle, Ladakh, home of the Indian Astronomical Observatory, in the high Himalaya (about 32.8°N). Auroras are very rare this far south.

Exosphere — the outermost layer

• Exosphere: the topmost layer of the atmosphere, above the thermosphere.
• The gas here is extremely thin. It is mainly hydrogen and helium, the lightest gases.
• Why gases escape: gravity is very weak this far from the Earth. Light, fast-moving atoms can fly off into space.
• It has no sharp top. It slowly merges into outer space.
• Little is known about it (Class 11).

Prelims Hooks

• Coldest layer of the atmosphere = mesosphere, about −100°C at 80 km (near the mesopause). It is not the tropopause.
• Meteors burn up mainly in the mesosphere, not the thermosphere or the stratosphere.
• Noctilucent clouds = highest clouds, about 76–85 km, near the mesopause, seen in polar summer, formed of ice on meteor dust [3][4].
• Ionosphere = 80–400 km (NCERT Class 11), lies inside the thermosphere, and reflects radio waves for long-distance short-wave radio.
• Aurora forms in the thermosphere. Trap: "aurora forms in the stratosphere" is wrong.
• Aurora colours: oxygen → green and red; nitrogen → blue and purple.
• Aurora Borealis = north; Aurora Australis = south.
• ISS orbits at about 400 km, in the thermosphere.
• Exosphere gases = hydrogen and helium. They escape because gravity is weak.
• May 2024 G5 storm, the strongest since 2003 [2]: aurora seen from Hanle (Ladakh).
• Order upward: mesosphere → thermosphere/ionosphere → exosphere. Temperature: falls in the mesosphere, rises in the thermosphere.

Mains Points

• Why temperature zig-zags with height: it depends on what absorbs sunlight in each layer. Ozone heats the stratosphere. The mesosphere has little absorbing gas, so it cools to about −100°C. In the thermosphere, thin gas absorbs X-rays and short-wave UV, so temperature rises sharply. This shows that the atmosphere is heated in layers, not just from the ground up.
• Space weather as a disaster risk (GS-III): solar storms disturb the ionosphere and thermosphere. They cut HF radio, cause GPS errors and scintillation, and increase drag on LEO satellites [5]. This matters for India's aviation, shipping, defence communication, power grids and satellite navigation. The May 2024 G5 storm, the strongest since 2003, showed the scale of the risk [2].
• The upper atmosphere as a useful resource: the ionosphere makes long-distance radio possible. The thermosphere hosts the ISS and LEO satellites. Noctilucent clouds and auroras help scientists track changes in the upper atmosphere, and noctilucent clouds are being seen more often [3]. Watching these layers supports communication, navigation and climate research.