研究發現地球磁場比我們想得還要單純

原文網址：http://oregonstate.edu/ua/ncs/archives/2017/jul/study-finds-earth%E2%80%99s-magnetic-field-%E2%80%98simpler-we-thought%E2%80%99

研究發現地球磁場比我們想得還要單純

科學家找出了在千年尺度下地球磁場的演變模式，而呈現出有關磁場如何作用的新見解，並提供了新的測量方法來預測之前無法得知的磁場變化。

這項發現也讓研究人員能利用此地磁「指紋」來比較從大西洋和太平洋得到的岩芯，使他們能以更高的解析度來研究地球的過往。

由國家科學基金會支持的這項研究，成果近日刊登於《地球與行星科學通訊》(Earth and Planetary Science Letters)。

地球磁場對住在地球上的生物來說至關重要。科學家表示如果沒有地磁，從太陽來的帶電粒子(太陽風)會颳走大氣層使得生物無法生存。磁場同樣能幫助人類航海，還有動物的遷徙，雖然科學家對其運作方式僅有初步瞭解。數個世紀以來人類觀察結果和地質紀錄皆顯示隨著時間經過，地磁的結構和強度都會有相當驚人的變化。

儘管磁場是如此重要，對於它的變化原因和發生過程仍然有許多問題尚未明瞭。最簡單的磁場形式是由磁偶極產生：一對電量相等而極性相反的磁極，就像是棒狀磁鐵。

本篇研究的主要作者，奧勒岡州立大學的博士後研究員Maureen “Mo” Walczak表示：「我們早就知道地球並非是個完美的磁雙極，從歷史紀錄上我們可以看見其中缺陷。我們發現這種非磁雙極構造並非轉瞬即逝且無法預測。它們可以存在許久，並在數萬年之間重複發生――在整個全新世，非磁雙極構造都一直保持在同一位置。」

她補充說：「雖然並非十分完美，但這項發現在某種程度上就跟找到聖杯一樣。它是在瞭解地球磁場的進展中相當重要的第一步，也讓我們能用更細微的刻度來整合沉積物岩芯呈現的時間記錄。」

在大約80萬年前羅盤的指針會指向南邊，因為當時地球磁場是倒過來的。這類磁極倒轉通常每數十萬年就會發生一次。

雖然科學家對於地球磁場的倒轉模式已有很深的認識，但是在極性穩定期間發生的第二種地磁「搖擺」模式，稱為古地磁長期變化(paleomagnetic secular variation)或是PSV，也許才是解開為什麼某些地磁性質會發生變化的關鍵。

研究人員表示地球磁場並非總是跟地球自轉軸完美重合，這是「真北極」跟「磁北極」有所不同的原因。在北半球，現今磁場與真北極的差異顯然是以北美和亞洲為中心，下方有塊磁場強度高的區域所產生。

共同作者，奧勒岡州立大學的古地磁專家Joseph Stoner表示：「過去我們不知道這幅存在於當下的影像是否具有任何更為長久的重要性――而我們發現它確實有。」

當北美下方的磁場比較強時，也就是處於「北美型態」時，會造成北太平洋的磁場強度較高且磁傾角較陡，另一方面歐洲的磁場強度會較低而北大西洋的磁北極則會往西偏，跟歷史紀錄相當符合。

另一種「歐洲型態」在某種程度上來說則是剛好相反。它會造成北太平洋的磁傾角較緩且強度較低；而歐洲的磁場強度會較高，北大西洋的磁北極則會往東偏。

Stoner表示：「正如結果呈現的，地球磁場並沒有我們想像中的那麼複雜。這種單純的震盪模式似乎起源於地磁強度在某幾個地點反覆發生的週期性變化，而造成了大範圍影響。我們還尚未確定此種變化的成因為何，但有可能是好幾種因素的聯合作用，像是外核的對流方向或許受到地函最下層的影響而發生偏轉。」

研究人員研究從阿拉斯加灣取得的兩根高解析度沉積物岩芯，重建出此區域長達17400年間的PSV紀錄而辨識出前述的變化模式。然後他們將此紀錄跟從太平洋其他地方取得的沉積物岩芯互相比較，利用沉積物中可當作過往磁場紀錄的磁鐵礦指向，他們可以鑑定出磁場留下的指紋。

目前擁有同樣地磁訊號的沉積物岩芯涵蓋範圍從阿拉斯加至奧勒岡州，最遠可達夏威夷。

Walczak表示：「利用古地磁紀錄中的倒轉現象來重建出距離遙遠的環境之間的磁場對應關係，可以讓我們以數十萬年的尺度來觀察過去。若能發展出連貫的PSV地層，我們就能以短至數百年的時間尺度來觀察這些紀錄並比較不同海盆中發生的事件，使我們真正能以適合人類社會的時間尺度探討氣候異常究竟是如何蔓延到全球。」

地球液態外核中的鐵、鎳和其他金屬流動時會產生電流，進而製造出地球磁場，其強度足以保護地球免受太陽風和宇宙輻射帶來的傷害。儘管我們對它的變化是如此熟知，卻仍然尚未解開背後成因。

現在我們距解開這道謎題也許更靠近了一些。

Walczak和Stoner任職於奧勒岡州立大學的地球海洋與大氣科學院。其他作者包括同為奧勒岡州立大學的Alan Mix；佛羅里達大學的John Jaeger、Gillian Rosen 和James Channell；澳洲國立大學的David Heslop；以及南安普敦大學的Chuang Xuan。

Study finds Earth’s magnetic field ‘simpler than we thought’

Scientists have identified patterns in the Earth’s magnetic field that evolve on the order of 1,000 years, providing new insight into how the field works and adding a measure of predictability to changes in the field not previously known.

The discovery also will allow researchers to study the planet’s past with finer resolution by using this geomagnetic “fingerprint” to compare sediment cores taken from the Atlantic and Pacific oceans.

Results of the research, which was supported by the National Science Foundation, were recently published in Earth and Planetary Science Letters.

The geomagnetic field is critical to life on Earth. Without it, charged particles from the sun (the “solar wind”) would blow away the atmosphere, scientists say. The field also aids in human navigation and animal migrations in ways scientists are only beginning to understand. Centuries of human observation, as well as the geologic record, show our field changes dramatically in its strength and structure over time.

Yet in spite of its importance, many questions remain unanswered about why and how these changes occur. The simplest form of magnetic field comes from a dipole: a pair of equally and oppositely charged poles, like a bar magnet.

“We’ve known for some time that the Earth is not a perfect dipole, and we can see these imperfections in the historical record,” said Maureen “Mo” Walczak, a post-doctoral researcher at Oregon State University and lead author on the study. “We are finding that non-dipolar structures are not evanescent, unpredictable things. They are very long-lived, recurring over 10,000 years – persistent in their location throughout the Holocene.

“This is something of a Holy Grail discovery,” she added, “though it is not perfect. It is an important first step in better understanding the magnetic field, and synchronizing sediment core data at a finer scale.”

Some 800,000 years ago, a magnetic compass’ needle would have pointed south because the Earth’s magnetic field was reversed. These reversals typically happen every several hundred thousand years.

While scientists are well aware of the pattern of reversals in the Earth’s magnetic field, a secondary pattern of geomagnetic “wobble” within periods of stable polarity, known as paleomagnetic secular variation, or PSV, may be a key to understanding why some geomagnetic changes occur.

The Earth’s magnetic field does not align perfectly with the axis of rotation, which is why “true north” differs from “magnetic north,” the researchers say. In the Northern Hemisphere this disparity in the modern field is apparently driven by regions of high geomagnetic intensity that are centered beneath North America and Asia.

“What we have not known is whether this snapshot has any longer-term meaning – and what we have found out is that it does,” said Joseph Stoner, an Oregon State University paleomagnetic specialist and co-author on the study.

When the magnetic field is stronger beneath North America, or in the “North American Mode,” it drives steep inclinations and high intensities in the North Pacific, and low intensities in Europe with westward declinations in the North Atlantic. This is more consistent with the historical record.

The alternate “European mode” is in some ways the opposite, with shallow inclination and low intensity in North Pacific, and eastward declinations in the North Atlantic and high intensities in Europe.

“As it turns out, the magnetic field is somewhat less complicated than we thought,” Stoner said. “It is a fairly simple oscillation that appears to result from geomagnetic intensity variations at just a few recurrent locations with large spatial impacts. We’re not yet sure what drives this variation, though it is likely a combination of factors including convection of the outer core that may be biased in configuration by the lowermost mantle.”

The researchers were able to identify the pattern by studying two high-resolution sediment cores from the Gulf of Alaska that allowed them to develop a 17,400-year reconstruction of the PSV in that region. They then compared those records with sediment cores from other sites in the Pacific Ocean to capture a magnetic fingerprint, which is based on the orientation of the magnetite in the sediment, which acts as a magnetic recorder of the past.

The common magnetic signal found in the cores now covers an area spanning from Alaska to Oregon, and over to Hawaii.

“Magnetic alignment of distant environmental reconstructions using reversals in the paleomagnetic record provides insights into the past on a scale of hundreds of thousands of years,” Walczak said. “Development of the coherent PSV stratigraphy will let us look at the record on a scale possibly as short as a few centuries, compare events between ocean basins, and really get down to the nitty-gritty of how climate anomalies are propagated around the planet on a scale relevant to human society.”

The magnetic field is generated within the Earth by a fluid outer core of iron, nickel and other metals that creates electric currents, which in turn produce magnetic fields. The magnetic field is strong enough to shield the Earth from solar winds and cosmic radiation. The fact that it changes is well known; the reasons why have remained a mystery.

Now this mystery may be a little closer to being solved.

Walczak and Stoner are in Oregon State’s College of Earth, Ocean, and Atmospheric Sciences. Other authors on the study are Alan Mix, also of OSU; John Jaeger, Gillian Rosen and James Channell of the University of Florida; David Heslop of Australian National University; and Chuang Xuan of the University of Southampton.

原始論文：M.H. Walczak, J.S. Stoner, A.C. Mix, J. Jaeger, G.P. Rosen, J.E.T. Channell, D. Heslop, C. Xuan. A 17,000 yr paleomagnetic secular variation record from the southeast Alaskan margin: Regional and global correlations. Earth and Planetary Science Letters, 2017; 473: 177 DOI: 10.1016/j.epsl.2017.05.022

引用自：Oregon State University. "Earth's magnetic field 'simpler than we thought'."