Air glow, also known as "night glow", is a weak luminescence phenomenon in the upper atmosphere.It originates from the faint radiance emitted by some components of the atmosphere in the photochemical process after being exposed to sunlight.
In the earth's atmosphere, airglow is an optical phenomenon that causes the night sky facing away from the sun to be not completely dark even if starlight and scattered sunlight are excluded.It accounts for all observations except starlight, zodiacal light, galactic light andAtmospheric scatteringIn addition to the generated light, 40% of the remaining light energy.
The airglow can be seen on sunny nights all over the world, with various colors and strong at the horizon.There are seasonal and periodic changes.
(1) Classification
1) The airglow is traditionally divided into night airglow, twilight airglow and day airglow according to the observed time period:
The more obvious ones at night are called night airglow (or night glow), those appearing at dawn and dusk are called twilight airglow, and those appearing during the day but difficult to observe are called day airglow.Generally, the radiation of the corona is also included in the airglow.
2) The airglow can be divided into several types according to the different atmospheric components that generate radiation:
For example, OI558nm airglow of atomic oxygen, OI630nm airglow of atomic oxygen, NtwoAir glow, COtwoAirglow, OH airglow, OtwoAirglow, etc
(2) Similarities and differences with aurora
auroraAnd airglow are two important optical phenomena occurring in the middle and upper atmosphere, which are difficult to distinguish by visual inspection.The spectra of the two have some similarities, but are different in several other important aspects:
Auroral receptiongeomagnetic fieldIt can be observed at a limited latitude close to the geomagnetic pole, and its occurrence is related to solar activity;
The airglow has little relationship with the solar activity, and has seasonal and periodic changes.At the same time, the difference of their spectral characteristics also indicates that their excitation mechanisms are different.Compared with the aurora, the airglow is continuous, and has the characteristics of airglow radiation, free from latitude restrictions, weak light, and different production mechanisms, no matter day or night.
formation mechanism
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It is the product of photochemical processes in the middle and upper atmosphere.
In the daytime, the electromagnetic radiation of the sun excites atmospheric molecules or atoms in the atmosphere, making them lose one or more electrons. Molecules are separated into atoms, which excite the stable state of molecules or atoms to a higher energy state, or make molecules vibrate.At night, these molecules or atoms in the higher energy state will jump back to the ground state. At this time, the light of a certain wavelength released will cause emission and scattering, resulting in weak glow radiation.The most common and brightest airglow radiation characteristics are generally generated by chemical reactions, which often show different colors due to different components and processes involved in the action. Its radiation intensity is controlled by the atmospheric temperature and the density of relevant atmospheric components.
Due to the scattering of sunlight, this phenomenon will not be noticed in the daytime.
The airglow phenomenon was first confirmed by Swedish scientist Anders Egstrom in 1868.Since then, it has been observed that this is part of the electromagnetic energy process in the laboratory and in the study of various chemical reactions.Scientists have also found that these processes can occur in the Earth's atmosphere, and astronomers have also verified the existence of these processes and emissions.
The generation of airglow is mainly the reaction of oxygen and nitrogen with hydroxyl ions at a height of hundreds of kilometers.
The reaction mechanism of nitrogen and oxygen is the combination of nitrogen atoms and oxygen atoms to form the molecule of nitric oxide (NO).During this process, photons will be emitted, which can be any of several characteristic wavelengths of nitric oxide molecules, because when oxygen molecules and nitrogen molecules reach the top of the atmosphere and are dissociated by solar energy, these free atoms can carry out this process and encounter each other to form nitric oxide.
Other objects that can generate airglow in the atmosphere include hydroxyl (OH)[1], oxygen molecule (O2), sodium (Na) and lithium (Li).
application
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The middle and upper atmosphere (80~300km) is an atmosphere closely related to human living environment and vulnerable to solar activity. Its research occupies a special position in the field of solar terrestrial physics.The observation of airglow temperature is an important means to study the complex physical processes in the middle and upper atmosphere.
The temporal and spatial distribution of airglow is affected by such factors as atmospheric gravity waves[2]、Tidal waveandPlanetary waveTherefore, airglow has become an important tracer for atmospheric photochemical processes and atmospheric dynamic processes.The research on the distribution characteristics of airglow is helpful to promote people's in-depth understanding of atmospheric photochemical and dynamic processes.The study of airglow distribution is very important for the study of the middle and upper atmosphere.OH night glow radiation is very strong, and people have carried out observation and research work on OH night glow radiation for a long time. The distribution characteristics of OH night glow radiation by predecessors include height distribution characteristics, latitude distribution characteristics, diurnal variation characteristics, and seasonal variation characteristics.
observation
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The light of airglow is very dim, only through a special photometer, but it can be measured at any time and in any direction.The simplest photometer is to place the photographic film about 3 cm behind a small circular hole with a diameter of 1 cm, and expose it in a fixed direction for 10 minutes, then enough density will appear on the film, which can be measured physically.At present, photometers made of photocells or photomultiplier tubes are generally used for measurement[1]In this way, the sensitivity can be increased to one million times.The airglow observation station set up in South China is using such an instrument for observation.
