Aurora is the only spatial physical phenomenon visible to the naked eye, because it often appears near the north and south poles of the earth, so it is called aurora.
Do you know? Not only do you have auroras on Earth, but you also have them on other planets!
A long exploration of the aurora: from ancient times to the present, from the earth to the extraterrestrial
The aurora on Earth is caused by charged high-energy particles from the magnetosphere and the solar wind being guided into the Earth's atmosphere by the geomagnetic field and colliding with atoms in the upper atmosphere, and its forms are diverse, generally band, arc, curtain, radial, etc., these shapes are sometimes stable, sometimes continuous.
Aurora phenomenon (Image source: Veer Gallery)
The color of the aurora depends on the type of atmospheric atom at the height at which the aurora is generated and the energy it absorbs, most commonly green and red auroras, blue or purple auroras, and occasionally pink or yellow auroras that are proportionally mixed with green and red light.
As early as thousands of years ago, the magnificent aurora phenomenon was recorded by various civilizations around the world, witnessing human exploration of the natural world and the evolution of human civilization.
Atmospheric luminescence observed by the Visible and Infrared Astronomical Survey Telescope (VISTA) at the Paranal Observatory in Chile (Credit: (LCO)/ESO)
With the rapid development of modern technology, deep space observation technology provides us with a unique perspective, allowing us to observe and study the aurora phenomenon of other celestial bodies other than Earth.
As early as the 90s of the 20th century, the Hubble Space Telescope captured Jupiter's aurora. Cassini's narrow-angle camera captured the shimmering auroras over northern Saturn between October 5 and 8, 2009.
Auroras have also been observed on Venus and Mars. Since Venus and Mars do not have the same global dipolar magnetic field as Earth, Jupiter and Saturn, their auroras are produced in scattered locations and are not concentrated in the polar regions.
NASA's Hubble Space Telescope photograph of Jupiter's aurora in January 1998 (Credit: NASA)
In addition to these planets, are there other planets with auroras?
The answer is: yes.
A recent article published in Nature Communications, titled "Direct evidence of substorm-related impulsive injections of electrons at Mercury," takes us on the planet closest to the sun in the solar system, Mercury, revealing the planet's aurora-related discoveries and explorations.
Screenshot of the paper (Image source: Nature Communications)
This study may prove the possibility of auroras or aurora-like phenomena on Mercury.
Mercury: A new perspective on aurora generation
Mercury, the closest planet to the Sun (Mercury can reach temperatures of 420 degrees Celsius), has long been the focus of scientists' research.
Mercury has a weak magnetic field, interacting with the solar wind to form a small magnetosphere. Mercury's magnetosphere is a vibrant place that is influenced by solar wind interactions, producing a complex set of physical processes.
Compared to Earth, Mercury's magnetosphere is more compact, with the size of the magnetosphere about 5% of Earth's magnetosphere and the distance between the magnetosphere and the planet's surface about 1.45 times the radius of Mercury.
This has caused Mercury's magnetosphere to undergo rapid restructuring and change, with one of the key processes known as the Dungey cycle. In the Dungey cycle, plasma is accelerated, transported, lost, and circulated, resulting in various "mysterious" phenomena, such as the ring current outside Mercury, Mercury substorms, and so on.
Schematic diagram of Mercury's magnetospheric current system (Image source: (a) Shi Zhen, Institute of Geology and Earth, Chinese Academy of Sciences; (b) Zong Qiugang Team, Peking University)
However, we know very little about these processes on Mercury, and the existence of auroras on Mercury is also an unsolved mystery.
It wasn't until October 1, 2021 that the European Space Agency's (ESA) exploration mission "BepiColombo" in collaboration with the Japan Aerospace Administration (JAXA) flew by Mercury for the first time, providing new insights into the space environment of the mysterious planet Mercury.
During BepiColombo's first flyby of Mercury, scientists observed significant compression signatures in Mercury's magnetosphere.
THE INTERACTION OF MERCURY'S MAGNETOSPHERE WITH THE SOLAR WIND HAS COMPRESSED ITS MAGNETOSPHERE, WHICH IS ESTIMATED TO BE MORE THAN TWICE THE AVERAGE OBSERVED BY PREVIOUS MESSENGER MISSIONS.
This means that Mercury's magnetosphere changes rapidly under the influence of the solar wind, a phenomenon that provides important background information for subsequent observations.
BepiColombo observations of electrons precipitated toward Mercury's surface during its first flyby (Credit: Sae Aizawa)
Shortly after BepiColombo entered Mercury's magnetosphere, scientists observed energy-time dispersion of high-energy electrons (particles of different energies being separated), a feature that had never been observed on Mercury before. At the same time, electrons with energies ranging from 100 eV to several keV exhibit significant flux enhancement with clear periodic fluctuations with a period of 30-40 seconds.
Observed energy-dispersive electrons (red arrows indicate periodic fluctuations in electron flux) (Image source: Sae Aizawa)
These features suggest that these electrons are accelerated in the near-magnetotail region of Mercury's magnetosphere, then rapidly drift to the helical region, where they are injected into closed magnetic field lines and eventually settle to the surface.
This process is called "electron injection," which provides a "green channel" for auroras to occur.
On Earth, high-energy particles enter the channel, excite atoms in the atmosphere, eventually producing auroras. But unlike Earth, Mercury has almost no atmosphere, and electrons have nowhere to exert force after entering the channel, so Earth-like auroras have not been observed on Mercury.
Schematic diagram of electron injection of the earth (Image source: Yang Jian's team at Southern University of Science and Technology)
epilogue
Although this study did not directly observe auroras on Mercury, it did observe a series of physical phenomena closely related to aurora formation, which provides us with a new perspective on the mystery behind the wonders of planetary auroras, giving us a more comprehensive understanding of the formation mechanism, evolution process, and intricate interactions between different stars and the sun.
As space exploration technology continues to advance, we believe that Mercury will continue to bring us more surprises and discoveries, revealing the mysteries of the light of the stars in the solar system.
This article is produced by Popular Science China, produced by the Institute of Geology and Geophysics, Chinese Academy of Sciences, and supervised by China Science Popularization Expo, "Popular Science China" is an authoritative brand of science and technology in the use of information technology by China Association for Science and Technology to carry out scientific communication.
Author: Wang Yuqi
Author's institution: Institute of Geology and Geophysics, Chinese Academy of Sciences