Thermosphere

High levels of solar activity, including increases in extreme ultraviolet radiation and soft solar x-rays, influence the atmospheric chemistry of the thermosphere. Increasing concentrations of CO₂ in the atmosphere have decreased the thermosphere's density by at least 10 percent between the 1970s and the mid-twenty-first century, lessening drag and extending satellite lifetimes.

Background

The lower thermosphere is the region of the atmosphere above the mesopause–lower thermosphere boundary (MLT, about 90 kilometers above Earth’s surface). It extends to an altitude of about 550 kilometers. Atmospheric density decreases with altitude as the numbers of atoms and molecules decrease and temperatures rise. The lower thermosphere is sometimes called the ionosphere, because the atmosphere there is highly charged by energetic solar photons.

Solar radiation drives photochemical reactions and motions in Earth’s atmosphere. Careful monitoring of total solar output has revealed that it can vary by as much as 0.1 percent. This magnitude of variability would have little direct effect on Earth’s surface temperatures. However, bursts of activity on the Sun, such as flares and coronal mass ejections, release large amounts of extreme ultraviolet radiation (EUV), soft solar x-rays, and radio waves that cause ionization to strengthen in Earth’s thermosphere, creating visible auroras over the polar regions. Above altitudes of 100 kilometers, ambient temperatures may be 1,000° Celsius higher at solar maximum than at solar minimum.

The supposition that variations in solar activity could influence Earth’s weather and climate began several centuries ago. More than a millennium of solar observations have demonstrated that the appearance of sunspots follows irregular eleven-year and twenty-two-year cycles and that the appearance of auroras in the thermosphere closely follows the appearance of sunspots. Scientific attempts to link auroral appearance to weather patterns at Earth’s surface have proved negative. Conclusive scientific evidence that variations in solar activity have caused recent changes in Earth’s climate has been elusive.

Auroras occur when geomagnetic storms cause energetic particles to travel toward Earth along lines. The high-altitude (300 kilometers) red and middle green aurora result from the excitation of oxygen atoms, and the lower-altitude (75 kilometers) blue aurora is caused by the excitation of nitrogen atoms and molecules. The greater the number of molecules and atoms that are excited, the stronger the coloration of the aurora.

In the late twentieth century, scientific proponents of solar influences on climate began to claim that, when the Sun is very active over a long period of time, global warming occurs. During the period from about 1750 to the early twenty-first century, the Sun experienced more sunspots than it did during the period from 1645 to 1715, a time period referred to as the Maunder minimum because few sunspots were observed then. The Maunder minimum corresponds to the coldest portion of the Little Ice Age, which was observed in Europe and North America between the mid-thirteenth century and the mid-nineteenth century. Solar activity has increased since the Industrial Revolution, when anthropogenic greenhouse gas (GHG) emissions began to increase dramatically. Some scientists contend that up to half of the warming observed since the end of the Little Ice Age might be attributable to increased solar activity. This view has been met with skepticism by the international scientific community.

Space Weather and the Thermosphere

When the Sun is active, Earth’s magnetic field (which extends beyond the thermosphere to the magnetosphere) is strengthened, lessening the number of galactic cosmic rays entering Earth’s atmosphere. The solar magnetic affecting Earth has doubled in the last one hundred years. Protons in the inner Van Allen radiation belt (700 kilometers to 10,000 kilometers above Earth) having energy above 50 million electron volts are assumed to arise from the decay of neutrons produced by galactic cosmic-ray collisions in the high atmosphere.

Indirect Human Modifications of the Thermosphere

Carbon dioxide (CO₂) concentration continues to increase in Earth’s atmosphere, and increased CO₂ reaching the lower thermosphere has caused cooling. As the thermosphere cools, atmospheric gases settle, causing the air density at an altitude of 400 kilometers to decrease by about 2 percent. This lowered density has decreased drag on satellites, extending their useful lifetimes. Atmospheric drag causes unpowered objects below altitudes of 480 kilometers to fall back to Earth within months. Because drag above this altitude is lower, more debris remains in orbit at that height for extended periods of time.

Most satellites orbit Earth in the lower thermosphere, from communications satellites to the space shuttle and International Space Station (ISS) to weather surveillance satellites. They circle the globe in low Earth orbit (LEO), from about 160 kilometers to 1,600 kilometers above the Earth’s surface.

Pollution of the Thermosphere

Low Earth orbit altitudes are monitored for space junk by ground-based radar systems that track objects larger than 5 centimeters. The lower thermosphere is occupied by hundreds of thousands of old satellite parts; on March 11, 2009, the Space Station was temporarily evacuated because of a near miss with a piece of junk about 12 centimeters in diameter. The ISS was forced to alter its orbit once again in November of 2021 to avoid another potential collision with space junk. There have been thirty near misses with debris over the twenty-four-year lifetime of the ISS, with three occurring in 2020. A Russian spaceship docked at the ISS in 2024, firing its thrusters for more than five minutes to maneuver the ISS away from space debris.

On February 10, 2009, two LEO satellites (one defunct and one operational) collided over Siberia at an altitude of 750 kilometers, producing over 160,000 pieces of debris. Proliferation of debris by collisions of orbiting space junk is referred to as the Kessler syndrome: as more objects occupy the lower thermosphere, the chance of collision increases, and each collision creates even more debris, further increasing the chance of collisions.

Detonation of a nuclear weapon at an altitude of 420 kilometers above Johnson Island on July 8, 1962, generated an electromagnetic pulse (EMP) that disrupted electrical systems in Hawaii, 1,290 kilometers away. It also damaged most orbiting satellites. Detonation of a few nuclear weapons in the thermosphere at the start of a major war might cripple ground-based retaliation capabilities by destroying conventional communications that use integrated circuits, leaving intact only “hardened,” or specially shielded, communications systems. (This was the reason for proposed construction of the extremely low frequency grid in North America.) Speculation has been raised about weapons in space that could be used against selected ground targets, but this option would be very costly and dangerous, leading to another arms race.

Because spy satellites play an important role in information gathering by major countries, including the United States, Russia, and China, eliminating enemy satellites at the start of any major conflict would be advantageous. There is a consensus that this type of satellite warfare would render the lower thermosphere unusable to satellites for many years because of the Kessler syndrome.

Context

The thermosphere has undergone great change because of anthropogenic activity in the last fifty years. Increased anthropogenic CO₂ emissions have decreased drag at LEO, extending satellite lifetimes. Debris from satellites has rendered LEO more dangerous to piloted space missions and poses a steadily increasing hazard to functioning satellites. During the last century, the Sun has been more active, which may have caused solar influences on the Earth’s atmosphere to increase. During active solar periods in the past, Earth’s climate has apparently warmed, and when the Sun was less active, occurred. The mechanisms by which the small increases in total solar output might cause climate change on Earth are not known.

Key Concepts

• drag: friction caused by air as an object passes through it
• extreme ultraviolet radiation (EUV): electromagnetic radiation with wavelengths between 10 nanometers and 120 nanometers
• galactic cosmic rays: protons, electrons, and nuclei of light elements that originate outside the solar system and penetrate Earth’s atmosphere
• geomagnetic storm: magnetic activity caused when charged particles from solar flares strike Earth’s magnetic field
• low Earth orbit (LEO): an orbit situated between 160 kilometers and 1,600 kilometers above Earth’s surface
• soft solar x-rays: solar radiation with wavelengths between 0.1 nanometer and 10 nanometers
• space weather: conditions in outer space resulting from solar activity

Bibliography

Arnold, Neil. “Solar Variability, Coupling Between Atmospheric Layers and Climate Change.” Philosophical Transactions of the Royal Society of London Annual 360 (2002): 2787–2804.

Barth, Charles A., Scott M. Bailey, and Stanley Soloman. “Solar-Terrestrial Coupling: Soft Solar X-Rays and Thermospheric Nitric Oxide.” Geophysical Research Letters 26, no. 9 (1999): 1251–1254.

Glickman, Todd S., ed. Glossary of Meteorology. 2d ed. Boston: American Meteorological Society, 2000.

Hitchens, Theresa. “Space Wars: Coming to the Sky Near You?” Scientific American 238, no. 3 (March, 2008): 78–85.

Johnson, Mark R., et al. "The International Space Station Just Had a Near Miss with Space Junk - and the Problem Is Getting Millions of Times Worse." Startup Daily, 16 Nov. 2021, www.startupdaily.net/topic/space/the-international-space-station-just-had-a-near-miss-with-space-junk-and-the-problem-is-getting-millions-of-times-worse/. Accessed 18 Jan. 2023.

Latatta, Eric. "International Space Station Forced to Maneuver Out of Way of Approaching Space Debris." USA Today, 21 Nov. 2024, www.usatoday.com/story/news/nation/2024/11/21/international-space-station-junk-debris-nasa/76472858007/. Accessed 10 Dec. 2024.

Meinel, Aden, and Marjorie Meinel. Sunsets, Twilights, and Evening Skies. New York: Cambridge University Press, 1983.

Phillips, Tony. "The Thermosphere Is Warming Up." Spaceweather.com, 23 March 2022, spaceweatherarchive.com/2022/03/23/the-thermosphere-is-warming-up/. Accessed 17 Jan. 2023.