2019-02-27 Atmosphere
Earth's Atmosphere Is Bigger Than We Thought
We humans like to put labels and boundaries on things. For example, the boundary between Earth's atmosphere and space is the Kármán line, the point at 100 kilometres altitude where aeronautics end and astronautics take over. But Earth's atmosphere is way more complicated than that (there's even some debate about where the Kármán line should be). Now a team of astronomers has discovered that it's bigger than we even thought – extending all the way out to the Moon, and as far again. The region is called the geocorona, part of an atmospheric layer called the exosphere. It's a tenuous cloud of neutral hydrogen that glows in far-ultraviolet light. Because it's so thin, it's been hard to measure; previously, its upper limit was thought to be around 200,000 kilometres from Earth, because that's the point at which solar radiation pressure would override Earth's gravity.
地球大气层的范围远超我们的想象 人们总喜欢在一些事物面前贴上标签和边界。 例如，地球大气层和太空之间的边界称为卡门线，是一条海拔100千里的高度线，超过这条线，航空就该改称为航天了。 但是地球大气层并不是这么简单就可以定义的（甚至就卡门线到底应该设在哪里，还存在一些争议）。如今有一支天文学家团队发现，大气层的边界比我们原先想象的要大得多 - 一直延伸到了月球。 该区域被称为地冕，处在大气层的最外层，也被称作散逸层。 它是一层稀薄的中性氢云，在远紫外线照射下会发光。 因为它极稀薄，所以很难测量；之前它的上限被认为在距地球大约20万千米处，因为这是太阳辐射强度超过地球引力的一个临界点。 © 2019-02-27
label 标签 boundary 分界线 atmosphere 大气层 Kármán line 卡门线 complicate 复杂 debate 争议 glows in 发光 measure 测量 gravity 引力 aeronautics [ˌeərəˈnɔ:tɪks] n.航空学，飞行术; adj.航空（学）的; astronautics [ˌæstrə'nɔ:tɪks] n.航天学，太空航空学; astronomer [əˈstrɒnəmə(r)] n.天文学者，天文学家; geocorona [dʒɪ(:)əʊkə'rəʊnə] n.地冕（主要由氢组成的大气层的最外层）; atmospheric [ˌætməsˈferɪk] adj.大气的; 大气引起的; 有…气氛的; exosphere [ˈeksəʊsfɪə(r)] n.外大气层，散逸层/外逸层; （地冕也存在于这一层） tenuous [ˈtenjuəs] adj.稀薄的; 贫乏的; 薄的，细的; 精细的; neutral [ˈnju:trəl] adj.中立的; （化学中） 中性的; 暗淡的; 不带电的; hydrogen [ˈhaɪdrədʒən] n.<化>氢; ultraviolet [ˌʌltrəˈvaɪələt] adj.紫外线的; 紫外的; 产生紫外线的;n.紫外光; 紫外线辐射; previously ['pri:vɪəslɪ] adv.以前; 事先; 仓促; “previous”的派生; override [ˌəʊvəˈraɪd] vt.覆盖; 推翻，无视; 践踏; 优先于;n.佣金; 超驰控制装置; 撤消，推翻。
Well, according to data from the Solar and Heliospheric Observatory (SOHO) co-owned by the European Space Agency and NASA, that limit doesn’t even come close. The geocorona, scientists have found, extends out to as much as 630,000 kilometres (391,000 miles). Which means it engulfs our beloved satellite in an atmospheric hug. “The Moon flies through Earth’s atmosphere,” said physicist Igor Baliukin of Russia’s Space Research Institute. In fact, at an average distance of 384,400 kilometres (238,855 miles), it’s almost smack-bang in the middle of it. What makes it even more amazing is that SOHO made these observations over two decades ago, between 1996 and 1998. The data had just been sitting in an archive, waiting for someone to get around to analysing it.
The readings had been taken specifically to map the geocorona using the observatory’s SWAN instrument, a sensitive piece of equipment designed to measure far-ultraviolet emissions from hydrogen atoms, called Lyman-alpha photons. We can’t see these from Earth – they’re absorbed by the inner layers of the atmosphere – so we need instruments out there in space to look for them. Apollo 16 astronauts, for instance, were able to take a photograph of the geocorona in 1972 – not even knowing they were still inside it. The SWAN instrument can selectively measure light from the geocorona, filtering out Lyman-alpha emissions from farther out in space, and this is what allowed for the more accurate map.
As well as the surprising size of the thing, it revealed a strange effect of the Sun. On Earth’s day-side, the hydrogen atoms are compressed by sunlight, resulting in a density of 70 atoms per cubic centimetre, thinning out to 0.2 atoms at lunar orbit (that’s not very dense at all – it’s still effectively a vacuum). On the night side, the hydrogen density is higher due to solar radiation pressure – it kind of ends up looking a comet tail. While the hydrogen atoms do scatter ultraviolet radiation, the quantity is negligible, especially compared to the vast amounts being blasted out by the Sun, making space a hazardous radiation environment for astronauts.
So, knowing the full extent of the geocorona isn’t going to make much difference for space exploration. What the discovery does mean is that any space telescopes within the geocorona will likely need to adjust their Lyman-alpha baselines for deep-space observations. “Space telescopes observing the sky in ultraviolet wavelengths to study the chemical composition of stars and galaxies would need to take this into account,” said astronomer Jean-Loup Bertaux of the French National Centre for Scientific Research (CNRS), and former principal investigator of SWAN. You know what’s even more interesting? That means no human being has ever actually been outside of Earth’s atmosphere. Guess we’ve got some work to do…
Credit: Michelle Starr for Science Alert, 21 February 2019.
卡尔曼·托多尔（匈牙利语：Kármán Tódor，全名Szőllőskislaki Kármán Tódor），德文 西奥多·冯·卡门（Theodore von Kármán，1881年5月11日－1963年5月6日），匈牙利裔美国工程师和物理学家，主要从事航空航天力学方面的工作，是工程力学和航空技术的权威，对于二十世纪流体力学、空气动力学理论与应用的发展，尤其是在超声速和高超声速气流表征方面，以及亚声速与超声速航空、航天器的设计，产生了重大影响。他是喷射推进实验室（JPL）的创建人、首位主任，也曾是钱学森、胡宁、郭永怀、林家翘在加州理工学院时的导师。
Atmosphere 地球大氣層 Exosphere 散逸層（690 km～10,000 km） 大氣層過渡到星際空間，地冕也存在於這一層； Kármán line 卡門線 100 km 被國際航空聯合會所接受，定義為大氣層和太空的界線； Thermosphere 熱層（085 km～690 km） 熱成層/在太陽輻射作用下，溫度迅速增高=>1200℃； Mesosphere 中間層（050 km～085 km） 氣溫隨高度增高而迅速降低=>-100℃，對流運動強盛； Stratosphere 平流層（020 km～050 km） 同溫層/適於飛機航行，臭氧層在20~30公里高處； Troposphere 對流層（006 km～020 km） 集中了約75%的大氣品質和90%以上的水汽品質。
