Mesosphere

The mesosphere is the third layer of Earth's atmosphere.

It sits directly above the stratosphere and directly below the thermosphere. Scientists sometimes refer to this region as "near space." This is not a formal technical term. However, it describes the space between high-flying commercial airliners and low-orbiting satellites. The mesosphere is a vital part of our planet's atmospheric structure. It serves as a boundary where many different physical processes occur.

Temperature behaves in a unique way within this layer. As altitude increases, the temperature actually decreases. This happens because there is less solar radiation being absorbed. The atmosphere becomes more rarefied, or thin, as you go higher. There is also a diminishing concentration of ozone at higher altitudes. Ozone is the primary gas that absorbs UV wavelengths. Additionally, cooling is increased by radiative emission from CO2, or carbon dioxide. The top of this layer is called the mesopause.

This is the coldest part of Earth's entire atmosphere. Temperatures in the upper mesosphere can fall below -100 degrees Celsius. These temperatures vary depending on the season and the latitude.

Dynamic features drive the movement of air in this region. Strong zonal winds move from east to west. The mesosphere also experiences atmospheric tides and various waves. These include internal atmospheric gravity waves and planetary waves. Most of these waves begin in the troposphere or stratosphere. They then propagate upward into the mesosphere. In this layer, the amplitude of gravity waves can become very large. This makes the waves unstable, causing them to dissipate. This dissipation deposits momentum and helps drive global circulation.

Chemical layers also define the mesosphere. A deep sodium layer exists between 80 and 85 kilometers. This layer is made of unbound, non-ionized atoms of sodium. It has an average concentration of 400,000 atoms per cubic centimeter. This sodium is regularly replenished by meteors sublimating as they enter the atmosphere. This layer radiates weakly and contributes to a phenomenon called airglow. Astronomers even use this sodium band to create "guide stars." This helps them perform adaptive optical corrections for ground-based observations. Other metal layers, such as iron and potassium, also exist nearby.

The mesosphere is home to beautiful noctilucent clouds. These clouds are located in the upper mesosphere. They form from material called meteoric smoke. This smoke comes from the millions of meteors that enter our atmosphere. On average, about 40,000 tons of meteoric material enter each year. This ablated material serves as condensation nuclei for the clouds. Another interesting phenomenon is the D layer of the ionosphere. This layer is only present during the day. It occurs when nitric oxide is ionized by Lyman series-alpha hydrogen radiation.

Scientists have recently identified new types of light in this layer. In October 2018, a distinct type of aurora was discovered. These lights are often called "dunes" because they look like sandy ripples. These green undulating lights extend toward the equator. They originate at an altitude of about 90 kilometers. The green color comes from solar particles interacting with oxygen molecules. You can also find electrical discharges called red sprites and blue jets. These are forms of lightning that occur within the lower mesosphere.

Exploring the mesosphere is extremely difficult. It lies above the altitude records for all aircraft. It is also below the minimum altitude for orbital spacecraft. High atmospheric drag prevents satellites from staying in orbit here. Because it is so hard to reach, it is sometimes called the "ignorosphere." Scientists can only use sounding rockets to study it. These rockets can only take measurements for a few minutes at a time. This makes the mesosphere one of the least understood parts of our atmosphere. We are still discovering its many secrets.