Aurora is the earliest space physics phenomenon that people observe, record and study. In ancient China, the aurora was called "Zhulong" and "DaDian" and so on. In the Nordic Lapland legend, the aurora is the tail of a fox that swept across the night sky.
Figure 1. The green aurora above Murmansk resembles the fox's tail. (Pictures are from the Internet)
As the name suggests, Aurora refers to some kind of light from the North Pole that people discovered in the early days, and later discovered that there were also Aurora in the South Pole. The north and south poles are extremely cold areas at the north and south ends of the earth, and the aurora is a luminous phenomenon that occurs over such cold areas. What causes this luminous phenomenon is a kind of warmth from distant space.
The atmosphere around us depends for survival is a transparent gas. Madberg hemisphere experiment proves that this gas has a powerful pressure, that is, atmospheric pressure. Under such atmospheric pressure conditions, there are about 270 billion molecules per milliliter of air at room temperature. People who like to climb mountains can often feel the changes in ear pressure. On high mountains like Mount Everest , the air is very thin, and the oxygen is also significantly reduced. This is because under the influence of the earth's gravity, the density of the atmosphere decreases with the increase of height. At an altitude of 100 kilometers, the atmosphere density is reduced to one millionth of the ground, which is almost equivalent to the "vacuum" environment that can be achieved in artificially manufactured vacuum tanks. At an altitude of 1,000 kilometers, the atmospheric pressure becomes thinner, with only about a few hundred thousand molecules per milliliter of gas (some molecules have been dismantled into separate atoms ).
In terms of ingredients, about 78% of the air molecules in the air on the ground are nitrogen molecules, about 21% of the air molecules are oxygen molecules, and a small amount of water vapor molecules and a smaller amount of inert gas. Due to sunlight, oxygen molecules will decompose into two oxygen atoms , and then the oxygen atoms will bind together with the oxygen molecules to form ozone molecules composed of three oxygen atoms. This is the ozone layer distributed at a height of 20-50 kilometers, which can absorb ultraviolet rays. It can protect life on the ground from ultraviolet damage. As the height increases, the sunlight is illuminating more and more oxygen atoms are dismantled; at the same time, the air is becoming thinner and thinner, and the oxygen molecules that can be dismantled are reduced, thus making the oxygen atoms largest at an altitude of 105 kilometers. Over 105 kilometers, the number of oxygen atoms has decreased, and the number of oxygen molecules and nitrogen molecules has decreased, so that oxygen atoms occupy the majority of the air components. The nitrogen molecule in the air is an diatomic molecule . The three chemical bonds between two nitrogen molecules are relatively firm and are not as easy to be disassembled like oxygen molecules. Therefore, they still exist in the form of diatomic molecules and gradually become thinner as the height increases. Nitrogen molecules in high altitude are susceptible to ultraviolet light or X-rays or lose an electron due to electron impact, becoming nitrogen molecular ions N2+.
With the help of spectral analysis and satellite observation, scientists have realized that the reason for the formation of aurora is the luminescence phenomenon caused by the charged particles from solar wind , mainly protons and electrons, which are guided and accelerated by Earth's magnetic field , and rushed into the atmosphere at extremely high speed, colliding with molecules and atoms in the atmosphere. If Aurora is a dazzling and colorful TV show, then the atmosphere is the screen of this TV show. The oxygen atoms, nitrogen molecules and nitrogen molecules in the atmosphere are the main active ingredients on this screen. It is precisely these three particles that are hit, and transitions to to a higher energy level, and to a lower energy level from a higher energy level, releasing specific energy, which releases a specific color of light, forming an aurora.
Figure 2. photon emitted by an oxygen atom after being hit by an electron.
Specifically, oxygen atoms are affected by electrons and gain energy, transition to the excited state, and then transition from the excited state to the low energy state, releasing green photons with a wavelength of 557.7 nanometers. They can also continue to jump from this low energy level to two lower energy levels close to the ground states that are very close to each other, releasing two red photons with a wavelength of 630.0 nanometers and 636.4 nanometers respectively.The density of oxygen atoms is relatively high in 90-150 kilometers, so the green aurora produced by oxygen atoms is generally 90-150 kilometers, while the red aurora produced by oxygen atoms is generally more than 150 kilometers. The nitrogen molecular ion N2+ is hit by electrons and obtains energy, transitions to the excited state, and then directly transitions from the excited state to the ground state, releasing purple photons with a wavelength of 391.4 nanometers, or from the excited state to the two low-energy states that are close to each other in the middle, releasing blue-purple photons with a wavelength of 427.7 nanometers and 470.8 nanometers respectively. Since these two low-energy states are already very close to the ground state, if they continue to jump to the ground state, the released photon energy is lower, and the corresponding wavelength exceeds the limit of red light that can be seen by the naked eye, and is in the range of infrared light . This part of the radiation is called the Menel line system. Since nitrogen molecules are distributed at the edge of the atmosphere at a height of about 1,000 kilometers, the purple and blue-purple aurora it emits are also at an altitude of 1,000 kilometers. After the nitrogen molecule is hit by electrons, it also jumps to the excited state because it obtains energy, and then jumps from the excited state to the low-energy state. However, most of the radiation is in the ultraviolet range because of its high energy. Only one 1-p line system is in the visible range, with typical wavelengths of 661.1 nanometers, 669.6 nanometers, 676.8 nanometers and 686.1 nanometers. These radiations are red or dark red visible light. Since nitrogen molecules are distributed in lower areas of the atmosphere, the red or dark red aurora emitted by nitrogen molecules are distributed at a height of 65-90 kilometers.
Figure 3. Photons emitted by nitrogen molecular ions after being hit by electrons.
If protons hit the nitrogen molecular ions, the same transition process will occur, releasing purple or blue-purple photons of the three wavelengths mentioned above. For oxygen atoms, if the proton is coming, the proton will "snatch" an electron from the oxygen atom during the impact, thus forming the excited state of hydrogen atom . The excited state atoms will then transition back to the low-energy state, and emit the famous Barmo line system Hα with a wavelength of 656.3 nanometers and Hβ photons with a wavelength of 486.1 nanometers, so they are red and blue-purple respectively. These excited hydrogen atoms continue to collide with oxygen atoms or nitrogen molecular ions, further participating in repeatable chain reactions until the energy of the hydrogen atom drops to the same as that of the hydrogen atom in the atmosphere, and its "temper" becomes much gentler. During this process, since hydrogen atoms do not show electrical properties, their movement will not be affected by the magnetic field, but will follow the laws of thermal motion during the collision. Therefore, the aurora forms in this case will not be arranged vertically like the candles described in the painting of the Edinburgh Observatory, but will spread throughout the sky. Because the earth's magnetic field is dragged by the solar wind and is pulled toward the night side, two funnel-shaped areas are formed at the north and south poles. The protons in the solar wind can enter the atmosphere unhindered along these two "horn mouths". The aurora in the polar gap area is diffuse, because of the excited hydrogen atoms generated in the middle. When the sun "losts its temper", the solar activity is relatively intense, and sometimes it produces high-energy proton flows with high energy, fast speed and high density. These high-energy protons enter the atmosphere of the north and south poles along the open magnetic line, and can produce these aurora that spreads throughout the sky.
electrons that collide with molecules, atoms and ions in the atmosphere mainly come from plasma sheets on the night side. This is a distant and magical area that can bring some warmth to the poles of the earth by "burning" magnetic lines. So where did the scarf come from? Let's listen to the next breakdown.
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中国大大, emotional 中国大大学 Physics, a front-line scientist who pays attention to popular science.
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