Mars Has Auroras Without a Global Magnetic Field, And We Finally Know How（附中文）

Earth’s auroras are a glorious wonder, but our planet isn’t the only place in the Solar System where these phenomena can be found.

An atmospheric glow, albeit sometimes in invisible wavelengths, has been spotted at every planet except Mercury, and even some moons of Jupiter… and even a comet. But Mars is where it gets interesting. The red planet is famous for its lost global magnetic field, an ingredient that plays a crucial role in the formation of aurora elsewhere.

But that doesn’t mean Mars is totally magnetism-free. Regions of localized magnetic fields sprout from some regions of the crust, particularly in the southern hemisphere. New analysis has confirmed that these small, local magnetic fields interact with the solar wind in interesting ways to produce Mars’s discrete (or structured) ultraviolet auroras.

“We have the first detailed study looking at how solar wind conditions affect auroras on Mars,” said physicist and astronomer Zachary Girazian of the University of Iowa.

“Our main finding is that inside the strong crustal field region, the aurora occurrence rate depends mostly on the orientation of the solar wind magnetic field, while outside the strong crustal field region, the occurrence rate depends mostly on the solar wind dynamic pressure.”

Here on Earth, we have a pretty good handle of how auroras – borealis and australis – happen. They appear when particles from the solar wind collide with Earth’s magnetosphere, and are then accelerated along the lines of the magnetic field to high latitudes, where they rain down into the upper atmosphere.

There, they interact with atmospheric particles to produce the shimmering lights that dance across the sky.

Evidence suggests that the phenomena form in similar ways on other bodies. For instance, Jupiter’s powerful, permanent auroras are also facilitated by the enormous planet’s complex magnetic field.

But Mars’s global magnetic field decayed fairly early on in the planet’s history, leaving behind only patches of magnetism preserved in magnetized minerals in the crust. Ultraviolet images of Mars at night have revealed that auroras tend to form near these crustal magnetic fields, which makes sense if magnetic field lines are required for particle acceleration.

Girazian and his team’s work also takes into account the solar wind conditions. They analyzed data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, which has been collecting ultraviolet images of the red planet since 2014. It’s also equipped with an instrument called the Solar Wind Ion Analyzer, which, unsurprisingly, analyzes the solar wind.

They compared data on the dynamic pressure of the solar wind, as well as the strength and angle of the interplanetary magnetic field, with ultraviolet data on the Martian auroras. They found that, outside the crustal magnetic field regions, the dynamic pressure of the solar wind plays a significant role in the detection frequency of auroras.

However, the pressure of the solar wind seems to play little role in the brightness of said auroras. This suggests that space weather events, such as coronal mass ejections, where masses of charged particles are ejected from the Sun and are associated with higher solar wind pressure, may trigger Martian auroras.

Inside the crustal magnetic field regions, the orientation of the magnetic field and the solar wind seems to play a significant role in the formation of auroras on Mars. At certain orientations, the solar wind seems to be favorable to the magnetic reconnection events or particle acceleration required to produce the ultraviolet glow.

These results, the researchers said, reveal new information on how interactions with the solar wind can generate auroras on a planet stripped of its global magnetic field. This information can be used to help better understand the formation of discrete auroras on very different worlds.

“Now is a very fruitful and exciting time for researching aurora at Mars,” Girazian said.

“The database of discrete aurora observations we have from MAVEN is the first of its kind, allowing us to understand basic features of the aurora for the first time.”

The research has been published in the Journal of Geophysical Research: Space Physics.

火星上的极光是如何形成的？ 科学家解谜-记者笛睿编译

科学家知道地球的极光是太阳风与磁场互动后产生的现象，并在其它天体上也看到了类似的现象，例如水星、木星的一些卫星、彗星，甚至是火星。

火星上没有一个像地球上这样覆盖全球范围的磁场，为什么也会产生极光（aurora）？多年来科学家都不知道这是为什么。一份新研究揭示了火星上极光可能的成因。

科学家认为火星在年轻的时候，上面曾有过像地球一样的覆盖全球的磁场，但是后来这个磁场逐渐减弱，到现在几乎消失。但是，火星南半球的地壳层存在一些区域性小范围的磁场。这份新研究认为，这些小范围的磁场与太阳风互相作用后，在火星上产生了离散型的极光。

主要研究者之一美国爱荷华大学（The University of Iowa）物理与天文学系研究员扎卡里·吉拉齐安（Zachary Girazian）说：“我们第一次详细研究了太阳风对火星上极光的影响。”

“我们的主要发现是，在地壳强磁场区域内，极光出现的频率取决于太阳风与磁场的朝向；在地壳强磁场区域以外，极光出现的频率取决于太阳风的动态压力。”

研究人员分析了火星大气与挥发物演化任务（Mars Atmosphere and Volatile Evolution Mission，缩写为MAVEN）收集的200多个离散极光的情形。MAVEN是美国宇航局（NASA）于2013年发射的太空探测器，也是第一个提供了火星极光数据的太空船。

研究人员发现，火星的极光多发生在南半球，而且他们认为是太阳风与局部区域的某种相互作用产生了极光的现象。

吉拉齐安在新闻稿中说：“现在是研究火星极光很有成果、令人兴奋的时刻。从MAVEN得到的火星极光的数据是同类中首例，帮助我们第一次了解了极光的基本特点。”

这份研究3月27日发表于《地球物理研究：太空物理学》（JGR Space Physics）期刊。（大紀元）

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