The aurora borealis， or Northern Lights， could easily be described as Earths greatest light show. A phenomenon thats exclusive to the higher latitudes has had scientists in awe and wonder for centuries.

A group of physicists from the University of Iowa in the US have proven that the “most brilliant auroras are produced by powerful electromagnetic waves during geomagnetic storms，” according to a study.

The study shows that these phenomena， also known as Alfvén waves， accelerate electrons toward Earth， causing the particles to produce the light show we know as the Northern Lights.

“Measurements revealed this small population of electrons undergoes ‘resonant acceleration by the Alfvén waves electric field， similar to a surfer catching a wave and being continually accelerated as the surfer moves along with the wave，” said Greg Howes， associate professor in the Department of Physics and Astronomy at the University of Iowa and co-author of the study.

This idea of electrons “surfing” on the electric field is a theory first introduced in 1946 by a Soviet physicist， Lev Landau， that was named Landau damping. His theory has now been proven.

北極光經(jīng)常被描述為地球上最盛大的“燈光秀”。幾個(gè)世紀(jì)以來(lái)，這種高緯度地區(qū)獨(dú)有的現(xiàn)象讓科學(xué)家們大為敬畏和驚嘆。

根據(jù)一項(xiàng)研究，美國(guó)艾奧瓦大學(xué)的一群物理學(xué)家已證實(shí)“這絕妙的極光是由地磁暴期間強(qiáng)大的電磁波所產(chǎn)生”。

研究表明，這些現(xiàn)象也被稱為“阿爾文波”，它們可以使電子加速向地球移動(dòng)，導(dǎo)致粒子產(chǎn)生我們所了解的北極光。

“測(cè)量表明，這一小群電子受到阿爾文波電場(chǎng)的‘共振加速，這類似于沖浪者抓住一個(gè)浪潮，并隨著海浪的移動(dòng)而不斷加速?！卑瑠W瓦大學(xué)物理與天文學(xué)系副教授格雷格·豪斯表示，他是該研究的合著者。

1946年，蘇聯(lián)物理學(xué)家列夫·蘭道首次提出電子在電場(chǎng)上“沖浪”的理論，并將其命名為朗道阻尼，他的理論現(xiàn)已得到證實(shí)。

Recreating the Northern Lights

Scientists have understood for decades how the aurora most likely is created， but they have been able to simulate it in recent years， for the first time， in a lab at the Large Plasma Device （LPD） in UCLAs Basic Plasma Science Facility in the US.

Scientists used a 20-meter-long vacuum chamber to recreate Earths magnetic field using the powerful magnetic field coils on UCLAs LPD. Inside the chamber， scientists generated a plasma similar to what exists in space near the Earth.

“Using a specially designed antenna， we launched Alfvén waves down the machine， much like shaking a garden hose up and down quickly， and watching the wave travel along the hose，” said Howes. As they began to experience the electrons “surfing” along the wave， they used another specialized instrument to measure how those electrons were gaining energy from the wave.

Although the experiment didnt recreate the colorful shimmer we see in the sky， “our measurements in the laboratory clearly agreed with predictions from computer simulations and mathematical calculations， proving that electrons surfing on Alfvén waves can accelerate the electrons （up to speeds of 45 million mph） that cause the aurora，” said Howes.

“These experiments let us make the key measurements that show that the space measurements and theory do， indeed， explain a major way in which the auroras are created，” said Craig Kletzing， the study co-author.

重現(xiàn)北極光

幾十年來(lái)，科學(xué)家們已經(jīng)了解極光最有可能產(chǎn)生的原因。但最近幾年他們才在美國(guó)加州大學(xué)洛杉磯分校的基礎(chǔ)等離子體科學(xué)設(shè)施實(shí)驗(yàn)室的大型等離子體裝置上第一次模擬出極光。

科學(xué)家們利用加州大學(xué)洛杉磯分校的大型等離子體裝置上強(qiáng)大的磁場(chǎng)線圈，用一個(gè)20米長(zhǎng)的真空室重現(xiàn)地球磁場(chǎng)。在這個(gè)真空室內(nèi)，科學(xué)家們制造出一種類似于地球附近空間中存在的等離子體。

豪斯說(shuō)：“我們使用一種特殊設(shè)計(jì)的天線，讓阿爾文波沿著機(jī)器向下波動(dòng)，就像快速地上下?lián)u動(dòng)花園軟管那樣，然后觀察波浪沿著軟管傳播。”當(dāng)他們感受到電子沿著波“沖浪”時(shí)，便使用另一種專門的儀器來(lái)測(cè)量那些電子是如何從波中獲得能量的。

盡管實(shí)驗(yàn)沒(méi)有重現(xiàn)我們?cè)谔炜罩锌吹降牟噬⒐猓浪贡硎荆骸拔覀冊(cè)趯?shí)驗(yàn)室中的測(cè)量結(jié)果與通過(guò)計(jì)算機(jī)模擬和數(shù)學(xué)計(jì)算得出的預(yù)測(cè)結(jié)果明顯一致，證明電子在阿爾文波上‘沖浪可以使電子加速（速度高達(dá)4500萬(wàn)英里/時(shí)，約7242萬(wàn)千米/時(shí)），從而產(chǎn)生極光。”

“這些實(shí)驗(yàn)幫助我們進(jìn)行了關(guān)鍵的測(cè)量，由此表明空間測(cè)量和理論確實(shí)解釋了極光產(chǎn)生的主要方式?！痹撗芯康暮现呖死赘瘛た巳R津說(shuō)道。

Many space scientists were ecstatic to hear the news. “I was tremendously excited！ It is a very rare thing to see a laboratory experiment that validates a theory or model concerning the space environment，” said Patrick Koehn， a scientist in the Heliophysics Division of NASA. “Space is simply too big to easily simulate in the lab.”

“The electron acceleration mechanism verified by this project is at work elsewhere in the solar system， so it will find many applications in space physics. It will be of use in space weather forecasting as well，” Koehn said.

A long way to go

The theory of how the aurora is created has been proven， but theres still a long way to go in forecasting how strong each storm will be.

“Predicting how strong a particular geomagnetic storm will be， based on observations of the Sun and measurements from spacecraft between the Earth and the Sun， remains an unsolved challenge，” said Howes.

“We have established the link of electrons surfing on Alfvén waves about 10，000 miles above the Earths surface， and now we must learn how to predict the strength of those Alfvén waves using spacecraft observations，” he added.

很多太空科學(xué)家聽(tīng)到這個(gè)消息時(shí)都欣喜若狂。美國(guó)宇航局太陽(yáng)物理學(xué)部門的科學(xué)家帕特里克·科恩表示：“我興奮極了！通過(guò)實(shí)驗(yàn)室的實(shí)驗(yàn)驗(yàn)證有關(guān)太空環(huán)境的理論或模型是非常罕見(jiàn)的。太空明顯太龐大，不太容易在實(shí)驗(yàn)室中進(jìn)行模擬?！?/p>

“該項(xiàng)目驗(yàn)證的電子加速機(jī)制在太陽(yáng)系的其他地方也在發(fā)揮作用，因此它將在空間物理學(xué)中非常有用。它也將用于太空天氣預(yù)報(bào)?！笨贫髡f(shuō)。

未來(lái)的路還很長(zhǎng)

極光如何形成的理論已經(jīng)被證實(shí)，但要預(yù)測(cè)每次風(fēng)暴的強(qiáng)度還有很長(zhǎng)的路要走。

豪斯表示：“根據(jù)對(duì)太陽(yáng)的觀測(cè)和對(duì)地球和太陽(yáng)之間航天器的測(cè)量，預(yù)測(cè)特定的磁風(fēng)暴的強(qiáng)度仍然是一個(gè)懸而未決的挑戰(zhàn)。”

“我們?cè)陔x地球表面一萬(wàn)英里的地方搭建了電子在阿爾文波上‘沖浪的線路，現(xiàn)在我們必須學(xué)習(xí)如何通過(guò)航天器的觀測(cè)結(jié)果來(lái)預(yù)測(cè)阿爾文波的強(qiáng)度?！彼a(bǔ)充道。

Word Bank

exclusive /?k'sklu?s?v/ adj. 專有的；獨(dú)有的

accelerate /?k'sel?re?t/ v. 加速；加快

simulate /'s?mjule?t/ v. 模擬

validate /'v?l?de?t/ v. 證實(shí)；確認(rèn)