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How can the candle-dragon glow there where the sun doesn't shine?
A New Research on Science Advances Casts Light on Jupiter’s Aurora Authors: Yao Zhonghua, Zhang Binzheng BEIJING—The successful Mars orbital insertion of Tianwen-1 in February 2021 represents one of the most important milestones in the history of China’s planetary explorations. Stemming from the homonymous lyric of the Chinese poet Qu Yuan (c. 340–278 BC), Tianwen represents the pursuit of ultimate scientific truth and address some key questions related to natural phenomena, as stated in the line “How can the candle-dragon glow there where the sun doesn’t shine?”. Standing for a luminescent appearance in the darkness of the night, the metaphor of the candle-dragon is used also in the Classic of Mountains and Seas, a Chinese text and a compilation of mythic geography and beasts from the 4th century BC, where its “...body extends to thousands of miles to the east of Wuqi”. Clearly figurative, the shape of this mythical beast is so long that it stretches out into space. Thanks to the images taken by modern optical instruments, we now know that these night lights are a poetic portrayal of the aurora borealis and aurora australis, also called northern and southern lights (Figure 1). The earliest record of aurora activity — accredited also by NASA—can be traced back to the Bamboo Annals, a chronicle of ancient China (2600 BC). Here we are told the story of the Yellow emperor’s (known as Huangdi) mother, Fu Bao, who noticed some rays of light stretching into the sky while taking a stroll, like “… a great accumulation of electricity around the Great Bear that illuminated the countryside”. Twenty-five months later, Fu Bao gave birth to the Yellow emperor. Only almost four thousand years later, the English physicist Henry Cavendish determined the origin of these lights at about 100 km in the upper atmosphere by applying trigonometry. Figure 1: Earth observation of aurora borealis. Credit: NASA/Terry Zaperach.In Figure 1, we can see the magnificent visual spectacle of the northern lights, prevalently green with some hints of violet. Located hundreds of thousands of kilometers up from the Earth, these colors are produced by the reaction with the two major gas species of the atmosphere, i.e. oxygen and nitrogen. As high-energy electrons emitted by the sun strike the Earth’s magnetic field and enter the Earth's polar atmosphere, they produce flashes of photons. Thus, aurora activity usually constitutes evidence of the existence of planets. If we compare the aurora activity on Earth and Jupiter, we could say it is like a small sorcerer in the presence of a great one, as the intensity of the Jovian aurora is hundreds of times stronger than the Earth’s. Nonetheless, one-third of the Jovian aurora produced near the magnetic axis is still labeled as ‘non-existent’ in modern textbooks. 
Figure 2: Image of Jupiter taken by a satellite located in the aurora region (left) and the Aurora Borealis from Earth. Credit: Zhang et al. 2021So far, the physics of planetary aurora activity has been explained by taking the results of Earth's observations as a reference. The mesmerizing phenomenon of northern and southern lights is visible only in high-altitude regions (above 65 degrees). This surrounds the Earth's magnetic pole, like the halo between the two imaginary circles in the right panel of Figure 2, which is commonly called the auroral oval. In the northern hemisphere, poleward of the auroral oval is the polar cap region that does not emit any light (as well as the area inside the internal imaginary circle). This is because the Earth magnetic field around the polar cap region is connected to the interplanetary magnetic field through the interaction with the solar wind, thus creating extremely tenuous, low-energy plasmas. As a result, in the polar cap, it’s generally not possible to produce enough high-energy particles to strike the Earth’s atmosphere. These beautiful lights do not occur appear in the sky of the polar cap. According to the images taken by the Hubble telescope and the Juno spacecraft, Jupiter does not only display auroral ovals, similar to the Earth’s ones, but it also presents auroral emission around the Jovian polar cap region close to the magnetic poles (see the left panel of Figure 2). These mysterious emissions near the polar cap region are not a distinctive feature of the Earth’s aurora activity, and thus, they couldn’t be explained through the existing theoretical model used for the Earth’s aurora. This mystery has puzzled the scientific community for over 20 years until now. In order to solve the riddle of the distribution of Jovian aurora, the scientific team of Dr. Zhang Binzheng (University of Hong Kong) and Dr. Yao Zhonghua (Institute of Geology and Geophysics, Chinese Academy of Sciences), who is also the leader of an ISSI-BJ International Team, has studied the interaction between the solar wind plasma, interplanetary magnetic fields, and Jupiter’s ionosphere and magnetosphere through supercomputer simulations. They used large-scale computer simulations to provide a new picture of the Jovian magnetosphere and modeled the spatial-temporal evolution of the Jovian magnetic field. In their computer simulations, the solar wind and Jupiter’s plasmas were treated as electrically conducting fluids, regulated by the Maxwell equations. Based on Juno’s measurements and supercomputer calculations, the team can now detailedly explain Jupiter’s aurora that significantly veers away from textbooks. In the framework of Jupiter's magnetic field, given the fast rotation of the celestial body and the relatively weak driving of the solar wind, great parts of the magnetic field near Jovian polar cap regions form the closed magnetic field lines that strike Jupiter’s northern and southern polar cap regions. Thus the plasma distribution in these areas and the one on Earth are very much different. The particles in the Jovian polar magnetosphere do not come from the thin, low-energy solar wind plasma but are from Jupiter’s high-energy particles. The structure of the Jovian system and the results of the measurements of the Jovian aurora activity match excellently, thus surprising us beyond our theoretical expectations. Zhang, Binzheng, Peter A. Delamere, Zhonghua Yao, Bertrand Bonfond, D. Lin Kareem A. Sorathia, Oliver J. Brambles, William Lotko, Jeff S. Garretson, Viacheslav G. Merkin, Denis Grodent, William R. Dunn, and John G. Lyon. How Jupiter’s unusual magnetospheric topology structures its aurora. Science Advances, Vol. 7(15). DOI: 10.1126/sciadv.abd1204The article is available at the journal website: https://advances.sciencemag.org/content/7/15/eabd1204/tab-article-info
The original version of this article (in Chinese) was written by Yao Zhonghua and Zhang Binzheng for Science AAAS China. Translator and editor: Laura Baldis ---
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