The discovery of "20 years late": The moon is actually in the earth's atmosphere
◎Reporter Tang Fang
The geocorona that covers the earth in a large area blocks the solar wind blowing to the earth, prevents the far ultraviolet radiation from directly reaching the ground, and protects the hydrosphere and biosphere of the azure blue planet of the earth.
In other words, the corona of this terrestrial planet with a magnetic field provides support for protecting the life environment and life itself that may exist on the planet's surface.
Ping Jinsong, Researcher, National Astronomical Observatory, Chinese Academy of Sciences
Can you think of it?
Maybe the moon has always been in the "embraces" of the earth.
In a study, the researchers analyzed the earth’s atmospheric dissipative layer data collected three times from 1996 to 1998 at the first Lagrangian point on the sun and the earth, and confirmed the observation results of the corona (the outermost layer of the earth’s atmosphere) : The Earth’s atmosphere extends to a height of approximately 630,000 kilometers, which is equivalent to 100 Earth’s radius.
This means that the moon is also wrapped in the earth’s atmosphere.
This conclusion has overturned people's understanding of the range of the geocorona: Previously, scientists estimated that the coronal layer was about 9-10 earth radii, and the moon was 320,000 to 340,000 kilometers away from the outermost layer of the earth's atmosphere.
Since 2020, in view of the arrival of a new peak year of solar activity, the research team has continued to monitor the solar wind and its impact on the earth's corona.
"The discovery of the large-scale geocorona has further expanded mankind's understanding of the composition and existence of planetary atmospheres, and also expanded the understanding of the interaction between the central star and the planets (such as the sun and the earth) atmosphere." Researcher at the National Astronomical Observatory of the Chinese Academy of Sciences Ping Jinsong told a reporter from Science and Technology Daily.
Unimaginable height of the corona can reach 100 Earth radii
The atmosphere surrounding the earth's surface is called the atmosphere, and from the inside to the outside are the troposphere, stratosphere, intermediate layer, warm layer and dissipation layer.
As part of the dissipative layer, the corona is located in the outermost layer of the Earth’s atmosphere, extending into interplanetary space.
"The research team's space observations in recent years have found that the height of the corona layer can extend to 100 radii of the earth, even the moon cannot be outside." Ping Jinsong pointed out that the key basis for this research conclusion is National Aviation The Solar and Heliospheric Observatory SOHO, jointly developed by NASA and the European Space Agency (ESA). The solar wind anisotropy probe SWAN load recorded the interaction data between the solar wind and the coronal hydrogen, and it was found at a distance of 630,000 kilometers from the earth’s surface. At the height of, there is still an interaction between the solar wind and the earth’s plasma.
He introduced that the corona layer mainly scatters far ultraviolet rays from the sun, and it also emits weak ultraviolet rays, but compared with solar radiation, the corona layer emits very little radiation.
The shape of the corona layer looks a bit like the tail of a comet flying near the sun.
In terms of composition, the corona layer is very different from other layers of the earth’s atmosphere.
For example, the corona layer is a low-density gas halo with hydrogen, helium atoms and ions as the main components, while the Earth’s atmosphere at an altitude of 1,000 kilometers is mainly composed of molecules such as nitrogen, oxygen, carbon dioxide, and water, as well as ions.
In addition, there is a plasma transition zone between the atmosphere and the corona in the lower part of the earth.
"The geocorona that covers the earth in a large area blocks the solar wind blowing to the earth, prevents the far ultraviolet radiation from directly reaching the ground, and protects the hydrosphere and biosphere of the azure blue planet of the earth. In other words, this kind of magnetic field The corona of the earth planet provides support for protecting the life environment and life itself that may exist on the planet's surface." Ping Jinsong said.
It is worth mentioning that the discovery of the large-scale corona is of great significance, and new research content has been added to planetary science.
"This discovery raises new questions to be explored for scientists. For example, in the process of planets gathering and collapsing from planetesimals into planets and then evolving, when did the star corona occur and how to stabilize? Another example is the corona of terrestrial planets. What are the similarities and differences with the corona of a gas planet in terms of composition and evolution?" Ping Jinsong said.
Very different from the stellar corona, the "life" of the corona is only a few dozen days
Not only the Earth has a corona, but Venus, Mars, Mercury and Jupiter in the solar system all have their own planetary coronas.
As a star, the sun also has its own corona, known as the corona.
There is a big difference between planetary corona and stellar corona.
Ping Jinsong explained that stars, especially young stars like the sun, have a thicker, thinner, and extremely low-density layer of atmosphere in their outermost layer, which is called stellar corona.
The thickness of the stellar corona can reach more than several million kilometers, and the temperature can reach several million degrees Celsius or higher. It can completely ionize the hydrogen and helium atoms in it to form plasma.
The star's corona is mainly composed of protons, highly ionized ions and high-speed free electrons.
These charged particles move so fast that the constantly charged particles break free from the gravitational restraint of the central star and shoot to the periphery, forming a stellar wind.
The gas in the stellar corona is continuously produced in the bottom photosphere, maintaining the existence of the stellar corona itself.
"The ions in the planetary corona will exchange charges with the stellar wind protons, resulting in a'lifetime' of only tens of days, which also limits the size range of the planetary corona." Ping Jinsong emphasized.
Due to the above difference, from the perspective of astronomical observation, planetary coronas are more difficult to be observed, while stellar coronas are easier to observe.
Ping Jinsong said that stellar corona not only emits radiation in the optical band, but also emits radiation in the radio band, so it can be observed by humans in multiple electromagnetic wave bands.
However, the planetary corona is not mysterious enough to be seen by the world. Scientists have also used a variety of detectors to get a glimpse of the "famous appearance" of the planetary corona.
"Shielding" the ultraviolet band geocorona increases the difficulty of astronomical observation
The earth's corona absorbs ultraviolet radiation from celestial bodies in the universe, blocking the opportunity for scientists to observe the stars in the universe by using electromagnetic waves from the ground or from planetary space, especially the mid-to-far ultraviolet band.
So scientists took another path.
"In these wavebands, scientists can only use ultraviolet telescopes flying in the middle and high levels of the earth's corona or beyond, such as the Lagrangian translational point of the solar-terrestrial system, to avoid the interference of the earth's corona on the ultraviolet waveband. Carry out astronomical observations." Ping Jinsong said.
In the process of human detection of extraterrestrial life, an important task is to find the "second earth."
Usually in the optical band, astronomers search for celestial bodies suitable for human habitation by blocking the luminosity changes of the larger central star by exoplanets.
Ping Jinsong said: "Because of the existence of the corona, scientists will monitor the absorption of ultraviolet electromagnetic radiation of specific wavelengths closely related to hydrogen and helium atoms in the ultraviolet band when the exoplanet obscures the central star to determine the existence and size of the corona. , And then infer the protection status of exoplanets and the probability of life on them."
In addition to the ultraviolet and optical bands, using the geocorona and terrestrial planetary coronas that can radiate radio-electromagnetic bursts with wavelengths of several kilometers to tens of meters, scientists can use very sensitive terrestrial radio devices to search for and monitor the presence of exoplanets. This wave of electromagnetic radiation is used to find more candidate celestial bodies for exo-terrestrial planets.
Although the existence of the corona has caused certain obstacles to astronomical observation, fortunately, the corona has left a window for humans to observe itself.
"They absorb the electromagnetic radiation of hydrogen and helium in the solar far ultraviolet band, and the hydrogen, helium atoms and ions excited by the radiation emit weak ultraviolet radiation, which can be seen by detectors far away from the earth." Ping Jinsong said.
The corona data for this study comes from the solar wind anisotropy probe SWAN carried by the SOHO launched in 1995.
The probe orbits the sun and conducts research on the sun.
In addition, it can measure light from the earth's corona.
Surprisingly, this batch of data was obtained by SOHO between 1996 and 1998.
As a result, the latest research findings have been jokingly called "twenty years late."
After my country's Chang'e-3 lunar probe was launched at the end of 2013 and successfully landed on the front of the moon, it also "seen" the earth's corona and confirmed the earth's "plasma layer" between the ionosphere and the magnetosphere. The presence.
Chang'e-3 carried an ultraviolet telescope for observing the plasma layer of the Earth’s outer atmosphere and monitored the “ghost shadow” of the earth’s corona over time.