Do all planets have auroras?

Based on more than 2,000 years' observation and research on the aurora of the earth, it is now known that the formation of aurora must meet three preconditions: magnetic field, fairly thick atmosphere and solar wind.

According to the statistics of the formation conditions of auroras on planets in the solar system, we can predict that three terrestrial planets (Mercury, Venus and Mars, mainly composed of silicate stones) will not produce auroras, while four woody planets outside the earth (Jupiter, Saturn, Uranus and Neptune, mainly composed of hydrogen, helium, ice, methane and ammonia, and only a small proportion of rocks and iron) will produce auroras. Based on the latest observation and research data, we will introduce the typical auroras of Venus, Mars and Jupiter. Because mercury has almost no atmosphere, it is difficult to have the necessary conditions to produce aurora, and no signs of aurora have been found so far, so we will not discuss it.

The scattered and faint aurora on Venus.

Because the overall magnetic field of Venus is very weak, only 0.00 15% of the earth's magnetic field. So there should be no aurora on Venus. However, scientists have observed the aurora on Venus, but its aurora is a diffuse spot with changing shape and intensity, sometimes spanning the whole planet, which is obviously different from the aurora around the poles on the earth. Venus aurora is characterized by no magnetic field, so it is impossible to produce a magnetosphere similar to the earth. Therefore, the magnetic field of strong electrons in the solar wind cannot interact with the nonexistent Venus magnetosphere, thus producing auroras around the polar regions at high altitudes around the magnetosphere.

Scholars from the University of Los Angeles reported the aurora of Venus at night for the first time in 1986. They analyzed the ultraviolet radiation data on the back of Venus obtained by the ultraviolet spectrometer installed on Venus Pioneer satellite for eight years. Since then, many scholars have made theoretical simulation research on Venus Aurora. In 2008, scientists from Belgium, Russia and the United States used theoretical models to calculate the radiation ratio of oxygen atoms, and found that it was consistent with the actual measured values.

Since Venus has no inherent magnetic field, why can it produce aurora? This is because the interaction between the solar wind and Venus' atmosphere will produce current flowing through its atmosphere, thus producing a weak magnetic field similar to the tail of a comet. It is because of this induced magnetic field that the so-called induced magnetosphere will appear outside the upper atmosphere of Venus. In this way, there are conditions similar to the aurora produced by the earth. This induced magnetic field is very weak and widely distributed, and the main component of Venus' atmosphere is carbon dioxide, so the aurora produced is not bright and scattered, which is the source of Venus' diffuse mottled aurora.

We know that Venus is similar to the Earth in mass, volume and distance from the sun, but it is quite different from the Earth in all aspects. Therefore, the observation and research on Venus has never stopped. Since the launch of Mariner 2 in 1962, more than 30 spacecraft and satellites have been launched to Venus, including flying over Venus, orbiting Venus, entering the atmosphere and landing on the surface of Venus. Because of this, we have a solid understanding of Venus Aurora. However, there are still many unsolved problems about the aurora of Venus. For example, the intensity and source of aurora are still unknown. Scientists expect the Venus Express spacecraft launched by the European Union in 2005 to let us know more about it.

Aurora on Mars was discovered very late.

Similar to Venus, Mars has no overall magnetic field, only an uneven local magnetic field, and its atmosphere is very thin. There are few atoms such as oxygen and nitrogen that can excite the visible aurora, so it is impossible to have a circular or arc-shaped visible aurora like the earth. So are there auroras on Mars? In order to find out this problem, scientists have made a breakthrough in recent years after years of hard observation and research, including the close observation of Mars by the Mars Express spacecraft launched by the European Union in recent years. First, in June 2005, scientists from France, the Netherlands, the United States and Russia published an article in Nature, indicating that Mars does have aurora. They are based on the ultraviolet and infrared atmospheric spectrometers installed on the Mars Express, and the data obtained after detecting the characteristic spectrum of Mars atmosphere, thus confirming the existence of aurora on Mars. Then, in February 2006, 65,438+06 scientists from Sweden, the United States, France, Japan, Britain, Finland, Germany, Israel, Italy and Switzerland published a joint article in Science magazine. According to the experimental analyzer of space plasma and high-energy atoms installed on the Mars Express and the Mars probe MGS (Mars Global Surf) launched by the United States 1997, people have a deeper understanding of the aurora on Mars.

The first is the characteristics of the aurora on Mars. Unlike the aurora of the earth and other planets, the aurora of Mars is neither a ring or an arc near the top of the magnetic field lines, nor a dim ultraviolet aurora scattered or sometimes widely distributed like Venus. Mars aurora belongs to its own category, which is a highly concentrated and localized emission aurora controlled by the abnormal magnetic field of its shell. Aurora, which was first confirmed in 2005, is located in Terra Cimmeria region in the southern hemisphere of Mars. The radiation area of aurora is about 30 kilometers wide and 8 kilometers high, and there is a strong local abnormal magnetic field in this area. Aurora confirmed in 2006 is a discrete aurora, which is located at the interface between magnetized and unmagnetized areas of the Martian crust, and forms a complex pattern near the equator in the southern hemisphere of Mars.

Secondly, there are ultraviolet auroras invisible to the naked eye on Mars. Even if humans can land on Mars one day or fly over Mars at night, they can't see the visible aurora like the aurora of the earth, at least as far as we know. Third, it is the possible mechanism of the formation of aurora on Mars. In 2006, according to the analysis of observation data, scientists found that the local magnetic field on the surface of Mars can extend to 1 0,000 km wide and 10km deep, and extend upward from the surface to 1 0,300 km high. The local magnetic field is strong enough to resist the solar wind, thus forming the "Martian magnetosphere". There are many such local magnetic fields. In the interface area between magnetized regions of the earth's crust, the electron plasma in the solar wind accelerated by Mars at high altitude drops rapidly at night and collides with carbon dioxide molecules and/or atoms in the upper atmosphere of this area, thus exciting aurora. They found that the energy flux per second in this area is1~ 50mw/m2 (1MW =1000 MW), which is equivalent to the energy of producing bright and separated auroras over the earth. Scientists also found that there are many abnormal magnetic fields in the crust of Mars, so there are not just one or two auroras, but many places, which makes the aurora of Mars present a complex geographical distribution image.

These findings initially revealed the mystery of the aurora on Mars. However, there are still many unclear problems. For example, the strongest local magnetic field strength of Mars is only 1/50 of that of the Earth. How does the electron plasma accelerate to high enough energy to excite the aurora of Mars? (For the mechanism of aurora on the earth, please refer to the article "Beautiful, Spectacular and Charming Aurora" in the 3rd issue of 20 10). For another example, is it possible to observe the aurora of Mars like the aurora of the earth? For the latter point, Leblanc, a French scholar who first discovered the aurora of Mars, said: "We are not sure whether we can observe the bright aurora of Mars in the visible range." Therefore, observing and studying the aurora on Mars is just the beginning, and there is still a lot of work to be done.

Here's another anecdote. M. Holmstrom of the Swedish Institute of Space Physics (IRF) drew a so-called "green" aurora according to the observation data of many auroras of Mars, which was regarded by the viewer as the back of the sun of Mars. This picture is widely quoted by websites and related reports all over the world, which makes readers mistakenly think: "Ah! This is how the aurora on Mars is formed. " In fact, this is a half-true and half-false aurora map of Mars. Partly because it is based on many observations, not fabricated out of thin air; Half-fake is because it is pieced together with many results, so it doesn't match the real situation. This picture cannot be found in the official paper published by Holmstrom; It is also difficult to find this picture on the website of the Swedish Institute of Space Physics.

Omni-directional Jupiter aurora

Jupiter has a so-called self-contained aurora, that is, Jupiter's satellites emit particles to Jupiter, so they can excite Jupiter's aurora like particles in the solar wind. This formation mechanism is very unique.

The first discovery of Jupiter's aurora was on1March 5, 979. Six hours after approaching Jupiter, the spacecraft Voyager/KLOC-0 launched by NASA took a photo of Jupiter's ultraviolet aurora with a narrow-angle camera at an altitude of 320,000 kilometers from Jupiter. Its shape is similar to the aurora on the earth, but it can't be seen by human eyes.

In fact, as early as 10 years before the discovery of Jupiter's aurora, Schwiete of the University of Texas and Hunter of Yale University observed Jupiter's Ha line at 1968 and 1969 respectively, and predicted that Jupiter might have aurora. During the following 10 years, many scholars made continuous observation and theoretical research on Jupiter. Although Jupiter's aurora has not been officially discovered, it laid a good foundation for the discovery of 1979.

After the discovery of Jupiter's aurora, the observation and research on it can be said to be higher and higher. X-rays, ultraviolet light, near-infrared light and mid-infrared light, radio band and visible light Jupiter aurora have been observed. 1On September 20th, 997, NASA's Hubble Space Telescope (HST) took a panoramic view of the ultraviolet aurora in Jupiter's north and south poles, which was very similar to the aurora on Earth. 1998, Ingersoll and others of California Institute of Technology published the night visible light aurora images they took with Galileo solid-state camera system in 1996 and 1997.

What do we know about Jupiter's aurora so far? Let's talk about its main features and possible formation mechanism.

First of all, the X-ray, ultraviolet light, infrared light, radio band and visible light Jupiter aurora observed so far are all observed only on Jupiter. Generally speaking, the auroras of ultraviolet light, visible light and infrared light are produced by the collision of atoms and molecules in Jupiter's atmosphere with deposited charged particles, while the auroras of X-rays and radio wavelengths are produced by the radiation emitted by particles deposited in the atmosphere.

Secondly, the aurora of Jupiter, like the aurora of the earth, appears at the top of the north and south magnetic poles, in an oval or annular pie shape.

Thirdly, Jupiter's ultraviolet auroras can be divided into three categories: the stable main auroral ellipse, the auroral footprint of Saturn's satellites in the auroral ellipse, and the unstable scattered auroras (polar auroras) in the auroral ellipse. Take the Arctic ultraviolet aurora photographed by HST flying over Saturn in June, 5438+February, 2000 as an example. It's like an aurora cake covering Jupiter's North Pole. The main aurora ellipse is quite narrow, about as wide as Saturn's latitude 1, or hundreds of kilometers in Saturn's atmosphere. On the other hand, the tail of Enceladus' auroral footprint is very long, which surrounds almost half the circumference of Saturn. In fact, HST also observed the footprints of Enceladus and Enceladus. Because the interaction between Phobos and Saturn's magnetosphere is much weaker than the first three satellites, any aurora produced by Titan is immersed in the main aurora, which is difficult to distinguish. The polar aurora is located at a higher latitude than the main aurora, corresponding to the farther magnetosphere distance, and its variability is extremely high.

Third, Jupiter's aurora intensity is the strongest in planets of the solar system. For example, the total power of the above ultraviolet aurora reaches100tw (1tw =10/2tw), which is about 1000 times of the total power of the earth's aurora. This is because Jupiter's magnetic field is particularly strong and/or Jupiter has its own aurora, and there is no clear mechanism to explain it.

Finally, the mechanism of Jupiter's aurora production. When Jupiter can see the aurora, it is excited in two ways. One is that the atoms and molecules in Jupiter's atmosphere directly interact with the electrons collected by the solar wind lake. After the main hydrogen and helium in Jupiter's atmosphere are excited, they can produce optical radiation with a wide range of wavelengths, including visible light; The other is produced by fluorescence generated by high-energy photons. At present, there is a lot of evidence that the aurora of Jupiter, like the earth, is excited by the interaction between the magnetosphere and the solar wind. In 200 1 year, Little Waite of the University of Michigan reported in the journal Nature that they discovered the Arctic elliptical aurora closely related to the change of solar wind by using HST for the first time. In 2002, Mok and others of Hopkins University in the United States also clearly confirmed this point. 20 10 in September, a research team at UCLA found evidence that Jupiter's auroral excitation was related to the reconnection of Jupiter's magnetosphere tail. There are four ways to connect, deep into Jupiter's ionosphere, but it is also considered that the research is not thorough. For example, they don't know if it reconnects regularly. It seems that it will take time to really understand the mechanism of solar wind producing Jupiter's aurora.

Unlike the earth, Jupiter's aurora has its own unique way of generating, that is, a self-generating system without solar wind. Current research shows that Jupiter and Io form a unique system in the solar system. Michael of the Department of Astronomy at the University of Virginia in the United States believes that "they form a' small solar system one'. A very active volcano on Io spewed a lot of charged particles at Jupiter, which was spinning at a high speed. These charged particles are captured by Jupiter's strong magnetic field (Jupiter's magnetic field strength is 8.4 times that of the earth), forming a circle of plasma over the polar regions. These high-speed and high-energy charged particles collide with atoms or molecules in Jupiter's atmosphere, thus exciting aurora. Because Io constantly emits charged particles to Jupiter and constantly produces aurora, this aurora has always existed; And because this spontaneous aurora system rotates with Jupiter, this aurora also rotates with Jupiter. However, it should be pointed out that in order to emphasize the importance and particularity of Jupiter's autogenous system aurora, some reports and articles are written as if Jupiter has only this kind of aurora and no other types of aurora, which is of course a misunderstanding.

The strange shape of Saturn's aurora

According to the observation results of Pioneer 1 1 spacecraft and 1979 International Ultraviolet Detector (IUE) jointly launched by NASA and Britain, scientists speculate that Saturn has aurora. But it was not until 1980 65438+ 10 that Voyager really detected Saturn's aurora activity for the first time. 198 1 year, Voyager first detected Saturn's ultraviolet aurora. Broadford of the University of Southern California and others published these results in Science and Nature, and they found that Saturn's north and south poles were close to 80 degrees. The auroral radiation of hydrogen atoms and molecules exists in narrow polar regions at high latitudes. So far, people have observed Saturn's ultraviolet light, infrared light and radio wavelength range of aurora, and Saturn's visible light aurora was discovered at the end of 2009, but X-ray aurora has not been discovered. In 20 10, a group of scholars specializing in X-ray aurora from NASA, Britain, India and Spain wrote that "Saturn's X-ray aurora has not been observed so far, because its intensity is lower than the lower limit of all detection instruments at present, so it cannot be detected". They hope that the next generation and more powerful International X-ray Observatory (IXO) designed and built by the European Space Agency (ESA) and the National Aeronautics and Space Administration of the United States can reach the sensitivity of10-17 erg/(cm z sec), thus increasing the probability of detecting Saturn's X-ray aurora.

What do we know about Saturn's aurora so far? 1On June 5438+00, 997, NASA used the image spectrometer on the Hubble Space Telescope (HST) to take the first ultraviolet aurora image of Saturn's poles. At that time, Saturn was 65.438+03 billion kilometers away from the Earth. The strong solar wind sweeps across Saturn and collides violently with particles in its atmosphere to produce aurora, which is similar to the aurora mechanism on earth, but it is invisible to human eyes and can only be discovered by ultraviolet photography in space.

In order to find out whether Saturn's aurora is the same as Earth's aurora, on June 5438+1October, 2004, when Cassini approached Saturn's south pole, NASA commanded it to jointly monitor with HST, which took ultraviolet photos, while Casslm recorded radio signals to monitor the solar wind. Three ultraviolet aurora images of Saturn's Antarctic taken on 24th, 26th and 28th of October at 65438+/kloc-0 clearly recorded the dynamic changes of the aurora: the aurora gradually became stronger, but not rapidly. Scientists have found that Saturn is similar to the aurora of the earth in that Saturn's ultraviolet aurora is also an oval or circular line around Saturn's magnetic pole; The difference is that Saturn's aurora lasts for a few days, while Earth's aurora lasts only a few minutes. Moreover, compared with the aurora of the earth and Jupiter, Saturn's aurora is more easily regulated by the solar wind. In 2005, Clark of Boston University and others published an article in Nature. After comparing the shape of Saturn's ultraviolet aurora with that of the earth and Jupiter, they concluded that Saturn's ultraviolet aurora is basically different from the latter two.

On June 0, 2006,165438+1October 10 and June 5, 2008, Casslm took strange images of Saturn's infrared aurora and infrared cloud at a distance of 106 km and 600,000 km from Saturn's surface, respectively. Aurora is not only bright, but also covers a large area, obviously extending to the equator. Starard of the University of Leicester said: "We have never seen such an aurora anywhere else. It is not just an aurora ellipse similar to the aurora of the earth and Jupiter, but covers a huge area of the polar regions. According to the existing theory of Saturn's aurora formation, these areas should be empty, so it is extremely surprising to find such bright aurora in these areas. " This clearly tells people that there must be something special and unknown in Saturn's magnetosphere and its interaction with the solar wind and Saturn's atmosphere. On June 5438+065438+1October 13, 2008, Stallard and others published an article in Nature, pointing out that the existing theory cannot explain this special aurora, and more observation and research are needed to make it clear.

In the past, people always thought that there was no visible aurora on Saturn, but with the progress of observation instruments and research, new phenomena were constantly discovered and the understanding of aurora was deepening. June 5438+October 20091October, Casslm's visible light narrow-angle camera captured Saturn's visible aurora for the first time. At that time, Casslm was located at a height of 2.8 million meters from Saturn, and the images taken showed that the aurora moved from the back of Saturn to the sun. Aurora is screen-shaped, extending along Saturn's magnetic pole, with a height of 1200 km, which is the highest Arctic aurora in the known solar system. The image was originally black and white, but in order to clearly distinguish the background from noise, orange was artificially added. Therefore, scientists at NASA don't know what color Saturn's visible aurora is. Saturn's screen aurora also changes shape and brightness as it does on Earth, and becomes particularly bright when it moves from the back sun to the sun and passes the edge. Interestingly, at the end of the continuous visible aurora image, a bright serpentine aurora suddenly appeared, and the reason for its appearance is still unclear.

Scientists at NASA believe that these visible auroras should be caused by the rapid accumulation of charged particles in Saturn's upper atmosphere. The magnetosphere is the region where Saturn's magnetic field captures charged particles. The change of aurora on the screen shows that these charged particles are captured while flowing along the magnetic field between Saturn's magnetosphere and ionosphere.

Scientists at NASA believe that these visible auroras should be caused by the rapid accumulation of charged particles in Saturn's upper atmosphere. The magnetosphere is the region where Saturn's magnetic field captures charged particles. The change of aurora on the screen shows that these charged particles are captured while flowing along the magnetic field between Saturn's magnetosphere and ionosphere.