藍色情懷

　　大部份的人都認為，磁針會指向北方是理所當然的。數千年來，水手靠著地球的磁場來導航；而鳥類和其他能感應磁場的動物，則已經運用這個方法有更長一段時間了。然而奇怪的是，地球的磁極並不是一直都朝著現在所指的方向。

　　Most of us take it for granted that compasses point north. Sailors have relied on the earth's magnetic field to navigate for thousands of years. Birds and other magnetically sensitive animals have done so for considerably longer. Strangely enough, however, the planet's magnetic poles have not always been oriented as they are today.

　 　有些礦物可以記錄地球磁場的方向；根據它們的記錄所顯示，在地球45億年的歷史中，地磁的方向已經反覆南北倒轉了好幾百次。不過，在最近的78萬年內都 未曾發生過倒轉──這比之前發生倒轉的平均間隔時間25萬年要長了許多。此外，地球的主要磁場自1830年首次測量至今，已經減弱了將近10％。這個速率 相當於在失去能量來源的情況下，磁場自然消退速率的20倍。難道是，下一次的地磁反轉即將來臨？

　　Minerals that record past orientations of the earth's magnetic field reveal that it has flipped from north to south and back again hundreds of times during the planet's 4.5-billion-year history. But a switch has not occurred for 780,000 years—considerably longer than the average time between reversals, about 250,000 years. What is more, the primary geomagnetic field has lessened by nearly 10 percent since it was first measured in the 1830s. That is about 20 times faster than the field would decline naturally were it to lose its power source. Could another reversal be on its way?

　　地球物理學家很早就知道，地球磁場變化的原因深藏於地球中心。地球就如同太陽系裡的一些其他天體，是利用內部的發電機來產生磁場。基本上，地球發電生磁的機制就和普通發電機一樣，藉由某個部份運動的 動能，產生電流和磁場。在一般的發電機裡，運動的部份是旋轉的線圈；而在行星或恆星裡，運動的則是導電的流體。在地球核心，有著體積相當於六個月球、處於 熔融狀態的鐵，形成不斷環繞流動的汪洋，構成了所謂的地球發電機（geodynamo）。

　　Geophysicists have long known that the source of the fluctuating magnetic field lies deep in the center of the earth. Our home planet, like several other bodies in the solar system, generates its own magnetic field through an internal dynamo. In principle, the earth's dynamo operates like the familiar electric generator, which creates electric and magnetic fields from the kinetic energy of its moving parts. In a generator, the moving parts are spinning coils of wire; in a planet or star, the motion occurs within an electrically conducting fluid. A vast sea of molten iron more than six times the volume of the moon circulates at the earth's core, constituting the so-called geodynamo.

　　一直以來，科學家主要是靠著簡單的理論來解釋地球發電機和其神秘的磁力。但最近10年，研究人員發展出新的方法來探索地球發電機的詳細運作機制。人造衛星能清楚拍攝地表磁場的圖像；而利用超級電腦來模擬地 球發電機，以及在實驗室裡建立的物理模型，則解釋了軌道上的觀測結果。這些工作，對於過去的磁極反轉提出了一套很有意思的解釋，更對下一次的反轉事件將如 何展開，提供了新的線索。

　　Until recently, scientists relied primarily on simple theories to explain the geodynamo and its magnetic mysteries. In the past 10 years, however, researchers have developed new ways to explore the detailed workings of the geodynamo. Satellites are providing clear snapshots of the geomagnetic field at the earth's surface, while new strategies for simulating earthlike dynamos on supercomputers and creating physical models in the laboratory are elucidating those orbital observations. These efforts are providing an intriguing explanation for how polarity reversals occurred in the past and clues to how the next such event may begin.

　　在我們開始探索磁場如何反轉之前，必須先瞭解，是什麼力量驅動著地球發電機。在1940年代前，物理學家便已明白，行星要能產生磁場，必須滿足三項基本條件；之後的發現都是建構在這個基礎上。第 一要件，要有大量的導電流體──地球的外核為液態，且富含鐵質。這關鍵性的外核包覆著幾乎是由純鐵所組成的固態內核，深埋在厚重的地函與極薄的大陸、海洋地殼之下，距離地表的深度約2900公里。地殼和地函的重量，使地核內的平均壓力高達地表壓力的200萬倍。此外，地核的溫度也十分極端──約為 5000℃，相當於太陽表面的溫度。

　　Before we explore how the magnetic field reverses, it helps to consider what drives the geodynamo. By the 1940s physicists had recognized that three basic conditions are necessary for generating any planet's magnetic field, and since then other findings have built on that knowledge. A large volume of electrically conducting fluid, the iron-rich liquid outer core of the earth, is the first of these conditions. This critical layer surrounds a solid inner core of nearly pure iron that underlies 2,900 kilometers of solid rock that form the massive mantle and the ultrathin crust of continents and ocean floors. The overlying burden of the crust and mantle creates average pressures in the core two million times that at the planet's surface. Core temperatures are similarly extreme—about 5,000 degrees Celsius, similar to the temperature at the surface of the sun.

　　這些極端的環境條件，構成了行星發電機的第二要件：流體運動的能量來源。驅動地球發電機的能量，部份是熱能，部份是化學能──兩者都在地球核心深處造成浮力。就像一鍋在爐上燒著的湯一樣，地核底部比頂部熱。（地核的高溫來自地球形成時囚禁在中心的熱能。）因 此地核底部那些較熱、密度較低的鐵會上升，就像熱湯裡的大氣泡那樣。當這些液體到達地核頂端，碰到上方的地函時，會喪失部份的熱。於是液態鐵會冷卻、密度 變得比周圍環境高，因而下沉。這個透過流體的上升及下沉，將熱能由下往上傳送的過程，稱為熱對流。

　　These extreme environmental conditions set the stage for the second requirement of planetary dynamos: a supply of energy to move the fluid. The energy driving the geodynamo is part thermal and part chemical—both create buoyancy deep within the core. Like a pot of soup simmering on a burner, the core is hotter at the bottom than at the top. (The core's high temperatures are the result of heat that was trapped at the center of the earth during its formation.) That means the hotter, less dense iron in the lower core tends to rise upward like blobs of hot soup. When the fluid reaches the top of the core, it loses some of its heat in the overlying mantle. The liquid iron then cools, becoming denser than the surrounding medium, and sinks. This process of transferring heat from bottom to top through rising and sinking fluid is called thermal convection.

　　現任職於美國加州大學洛杉磯分校的布拉金斯基（Stanislav Braginsky）曾於1960年代時指出，熱由地核的上部逃逸，同時也會使固態內核的體積增加，產生兩種額外的浮力來源，驅動對流：當液態鐵在固態內核的外緣凝固而形成晶體時，會釋放潛熱。這些熱可以加強熱浮力。此外，如硫化鐵和氧化鐵等密度較低的化合物被內核的晶體排出，上升通過外核，也會加強對流。

　　In the 1960s Stanislav Braginsky, now at the University of California at Los Angeles, suggested that heat escaping from the upper core also causes the solid inner core to grow larger, producing two extra sources of buoyancy to drive convection. As liquid iron solidifies into crystals onto the outside of the solid inner core, latent heat is released as a by-product. This heat contributes to thermal buoyancy. In addition, less dense chemical compounds, such as iron sulfide and iron oxide, are excluded from the inner core crystals and rise through the outer core, also enhancing convection.

　　行星要產生能自我維持的磁場，還有第三個條件：旋轉。地球自轉造成科氏力效應，會使地核內上升的流體偏向，就像造成洋流及熱帶風暴被扭轉成在氣象衛星影像中常見的漩渦狀那樣。在地核中，科氏力使湧升的流體沿著像開酒瓶器的螺旋路線上升，彷彿順著彈簧的螺旋線圈移動。

　　For a self-sustaining magnetic field to materialize from a planet, a third factor is necessary: rotation. The earth's rotation, through the Coriolis effect, deflects rising fluids inside the earth's core the same way it twists ocean currents and tropical storms into the familiar spirals we see in weather satellite images. In the core, Coriolis forces deflect the upwelling fluid along corkscrewlike, or helical, paths, as though it were following the spiraling wire of a loose spring.

　　地球擁有富含鐵的液態核心、有足夠的能量驅動對流，並具有科氏力可使對流的流體扭轉，這些是地球發電機之所以能自我維持數十億年的主要原因。但科學家需要更多證據來解釋令人困惑的問題，像是磁場的生成，以及為什麼磁極會隨時間而改變等。

　　That the earth has an iron-rich liquid core, sufficient energy to drive convection and a Coriolis force to twist the convecting fluid are primary reasons why the geodynamo has sustained itself for billions of years. But scientists need additional evidence to answer the puzzling questions about the magnetic field that emerges—and why it would change polarity over time.

　　過去五年裡，由於科學家終於能夠比對相隔 20年所觀測到地球磁場的準確分佈，因而有了重大發現。1980年，磁場衛星（Magsat）測量了地球表面上的磁場；另一枚衛星厄斯特 （Oersted）則是在1999年起進行同樣的測量（見第41頁圖）。假設地函的電流可以忽略，研究者可以利用衛星觀測到的這些結果，以數學方法推算出 磁場在地核頂部的分佈。地核內具有更為劇烈、複雜的磁場，而且也是磁場變動的真正發源處，但是研究人員可推算的極限是在地核–地函交界處；因為地核內的電 流極強，因此無法直接測量內部的磁場。儘管在此既有限制之下，研究人員仍然得到了許多重要的觀測結果，包括關於磁極可能開始反轉的線索。

　　A major discovery unfolded over the past five years as it became possible for scientists to compare accurate maps of the geomagnetic field taken 20 years apart. A satellite called Magsat measured the geomagnetic field above the earth's surface in 1980; a second satellite—Oersted—has been doing the same since 1999 [see illustration on page 55]. Investigators have mathematically projected these satellite measurements down to the top of the core using the assumption that the electric currents of the earth's mantle are negligible. The core-mantle boundary is the closest researchers can get to the much more intense and complicated magnetic field that exists within the core, where magnetic fluctuations actually originate; strong electric currents in the core prevent direct measurements of the magnetic field there. Despite the inherent limitations, several noteworthy observations came out of these efforts, including hints about the possible onset of a new polarity reversal.

　　重要 的發現之一，是地球的大部份磁場僅來自地核–地函交界面上的四個廣大區域。雖然地球發電機所產生的磁場非常強烈，但是磁場的能量只有約1%可以延伸到地核 外。在地表進行測量時，這個磁場最顯著的結構是偶極，多數時候與地球自轉軸大致平行。地磁就如同一根普通的磁鐵棒，而這個磁場的主要磁通量是在南半球由地核向上穿出，並在北半球向內進入地核。（指北針的磁針之所以會指向地球的地理北極，就是因為上述偶極的磁南極正好在那附近。）但人造衛星顯示，磁通量並非是均勻遍佈全球的。偶極磁場的強度大部份是來自北美洲、西伯利亞和南極洲沿海地表下方。

　 　One important finding was that most of the geomagnetic field originates at only four broad regions on the core-mantle boundary. Although the geodynamo produces a very intense magnetic field, only about 1 percent of the field's magnetic energy extends outside the core. When measured at the surface, the dominant structure of this field is called the dipole, which most of the time is roughly aligned with the earth's axis of rotation. Like a simple bar magnet, this field's primary magnetic flux is directed out from the core in the Southern Hemisphere and down toward the core in the Northern Hemisphere. (Compass needles point to the earth's north geographic pole because the dipole's south magnetic pole lies near it.) But the satellite missions revealed that the flux is not distributed evenly across the globe. Instead most of the dipole field's overall intensity originates beneath North America, Siberia and the coast of Antarctica.

　　任職於德國卡特倫堡–林島的馬克士普朗克太陽系研究所的克里斯坦森（Ulrich R. Christensen），推測這些大區塊是來自地核內部持續變化的對流結構，而且在幾千年間不斷改變。有沒有可能，類似的現象是造成磁偶極反轉的原因？ 由地質記錄得到的證據顯示，過去的反轉事件週期相當短，約4000~10000年。就算地球發電機停止運作，磁偶極也得要將近10萬年才會自行消失。因 此，這麼快速的變化，暗示了有某種不穩定性破壞了原來的極性，同時產生新的極性。

　 　Ulrich R. Christensen of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, suspects that these large patches come and go over thousands of years and stem from the ever evolving pattern of convection within the core. Might a similar phenomenon be the cause of dipole reversals? Evidence from the geologic record shows that past reversals occurred over relatively short periods, approximately 4,000 to 10,000 years. It would take the dipole nearly 100,000 years to disappear on its own if the geodynamo were to shut down. Such a quick transition implies that some kind of instability destroys the original polarity while generating the new polarity.

　　對個別的反轉事件而言，這神秘的不穩定性可能是流場結構的某 種混沌變化，只能偶爾成功的逆轉磁偶極。但發生反轉的頻率在過去1億2000萬年間穩定增加（見43頁圖），可能是有外在的控制因素。其中的一個可能，是地函底部的溫度變化能夠迫使地核改變其內部上升流動的結構。

　　In the case of individual reversals, this mysterious instability is probably some kind of chaotic change in the structure of the flow that only occasionally succeeds in reversing the global dipole. But the frequency of reversals, which has been increasing steadily for the past 120 million years [see illustration on page 57], may have an external control. One possible candidate is a change in temperature at the bottom of the mantle, which could force the core to change its upwelling patterns.

　　其他研究團隊分析了磁場衛星和厄斯特衛星觀測的分佈圖，發現可能引發磁極反轉的變 動跡象。余洛（Gauthier Hulot）和他在法國巴黎地球物理研究院的同事注意到，地球磁場的持續變異，是來自地核–地函交界面上某些磁通量方向與整個半球相反的區域。這些所謂的 反向通量斑塊（reversed flux patch）中，最大的一塊由非洲南端下向西延伸至南美洲南端下方。在這個斑塊裡，磁通量向內進入地核，然而南半球大部份磁通量是指向外的。

　　Symptoms of a possible reversal-inducing change came to light when another group analyzed the Magsat and Oersted satellite maps. Gauthier Hulot and his colleagues at the Geophysical Institute in Paris noticed that sustained variations of the geomagnetic field come from places on the core-mantle boundary where the direction of the flux is opposite of what is normal for that hemisphere. The largest of these so-called reversed flux patches stretches from under the southern tip of Africa westward to the southern tip of South America. In this patch, the magnetic flux is inward, toward the core, whereas most of the flux in the Southern Hemisphere is outward.

　　研究人員比較了最近由厄斯特衛星觀測到的 磁場以及1980年的觀測結果。所得到最重要的結論之一，是新的斑塊持續在北美東岸及南極等地區下的地核–地函交界處形成。更重要的是，較老的斑塊面積擴 大了且略向兩極方向靠近。1980年代晚期，英國裡茲大學的加賓斯（David Gubbins）研究較老舊、粗略的磁場圖，發現反向通量斑塊的增加、擴張和往兩極移動，可解釋磁偶極隨時間減弱的情形。

　　One of the most significant conclusions that investigators drew by comparing the recent Oersted magnetic measurements with those from 1980 was that new reversed flux patches continue to form on the core-mantle boundary, under the east coast of North America and the Arctic, for example. What is more, the older patches have grown and moved slightly toward the poles. In the late 1980s David Gubbins of the University of Leeds in England—using cruder, older maps of the magnetic field—noticed that the proliferation, growth and poleward migration of these reversed flux patches account for the historical decline of the dipole.

　　這些觀測結果可以利 用磁力線的概念解釋（實際上，磁場在空間中是連續的）。我們可以想像，這些磁力線「凍結」在液態鐵核中而隨之運動，就像是在水杯中的顏料線條被攪動時的樣 子。在地核中由於科氏力效應，流體中的渦流將磁力線扭結成團，看起來就像一團團的義大利麵條。這樣的糾結把更多的磁力線壓縮在地核內，因而增加了磁場的能 量。（如果這個過程不受抑制的話，磁場會無限制的增強。但是電阻會減弱、緩和磁力線的扭轉，適當阻止了磁場無限制的增強，但又不會破壞地球發電機的運 作。）

　　Such observations can be explained physically by using the concept of magnetic lines of force (in actuality, the field is continuous in space). We can think of these lines of force as being 「frozen」 in the fluid iron core so that they tend to follow its motion, like a filament of dye swirling in a glass of water when stirred. In the earth's core, because of the Coriolis effect, eddies and vortices in the fluid twist magnetic lines of force into bundles that look somewhat like piles of spaghetti. Each twist packs more lines of force into the core, thereby increasing the energy in the magnetic field. (If this process were to go on unchecked, the magnetic field would grow stronger indefinitely. But electrical resistance tends to diffuse and smooth out the twists in the magnetic field lines enough to suppress runaway growth of the magnetic field without killing the dynamo.)

　　具有強大磁通量的斑塊，不論方向是正是反，都是渦旋與地核內部的東–西向環狀磁場交互作用時，在地核–地函交界面上所形成。這些亂流般 的流體運動可以把環狀的磁力線彎曲、扭轉成小圈，形成極向磁場（poloidal field），方向為南–北指向。有時這種扭曲是由湧升流裡上升的流體造成的。如果湧升流夠強，極向磁場環圈的頂端會被排出到地核之外（見本頁〈反向通量斑塊〉）。這樣的過程會使小圈的兩端穿過地核–地函交界處，產生一對通量斑塊。斑塊之一具有正常方向的磁通量（與該半球整體的偶極方向相同）；另一個斑塊 的通量方向則是相反的。

　　Patches of intense magnetic flux, both normal and reversed, form on the core-mantle boundary when eddies and vortices interact with east-west-directed magnetic fields, described as toroidal, that are submerged within the core. These turbulent fluid motions can bend and twist the toroidal field lines into loops called poloidal fields, which have a north-south orientation. Sometimes the bending is caused by the rising fluid in an upwelling. If the upwelling is strong enough, the top of the poloidal loop is expelled from the core [see box on this page]. This expulsion creates a pair of flux patches where the ends of the loop cross the core-mantle boundary. One of these patches has normally directed flux (in the same direction as the overall dipole field in that hemisphere); the other has the opposite, or reversed, flux.

　　如果扭曲所造成的反向通量斑塊，比正常通量的斑塊更靠近地理極點，則會使磁偶極減弱，因為磁偶極對於在極點附近的變化 最敏感。這確實說明了目前位在非洲南端下的反向通量斑塊。如果整個行星的磁極要真的反轉，反向通量斑塊需擴大到涵蓋整個極區；同時，另一個地理極點附近也 會發生類似的區域極性全面變化。

　　When the twist causes the reversed flux patch to lie closer to the geographic pole than the normal flux patch, the result is a weakening of the dipole, which is most sensitive to changes near its poles. Indeed, this describes the current situation with the reversed flux patch below the southern tip of Africa. For an actual planetwide polarity reversal to occur, such a reversed flux patch would grow and engulf the entire polar region; at the same time, a similar change in overall regional magnetic polarity would take place near the other geographic pole.

 利用超級電腦模擬
Supercomputer Simulations

　　要增進對地球發電機的瞭解，有個好方法是 把電腦的發電機模型（缺乏亂流）與實驗室發電機（缺乏對流）加以比較。1960年代科學家首次證明了在實驗室中模擬地球發電機的可行性，但是成功之路仍十 分漫長。實驗室設備和真正行星核心在尺度上的巨大差異是重要關鍵。一個能自我維持的流體發電機，所需磁雷諾數（一個無因次參數）至少要超過10左右。

　　A good way to improve understanding of the geodynamo would be to compare computer dynamos (which lack turbulence) with laboratory dynamos (which lack convection). Scientists had first demonstrated the feasibility of lab-scale dynamos in the 1960s, but the road to success was long. The vast difference in size between a laboratory apparatus and the actual core of a planet was a vital factor. A self-sustaining fluid dynamo requires that a certain dimensionless parameter, called the magnetic Reynolds number, exceed a minimum numerical value, roughly 10.

　　地核的磁雷諾數非常大，可能約1000左右，主要是因為它的線性尺度非常大（地核的半徑約為3485公里）。簡單的說，要在體積很小的流體中產生很高的磁雷諾數非常困難，除非你能使流體以極高的速度運動。

　　The earth's core has a large magnetic Reynolds number, probably around 1,000, primarily because it has a large linear dimension (the radius of the core is about 3,485 kilometers). Simply put, it is exceedingly difficult to create a large magnetic Reynolds number in small volumes of fluid unless you can move the fluid at extremely high velocities.

　 　在實驗室的液態發電機裡產生自發磁場，這個夢想了10年之久的期望，終於在2000年成真；歐洲的兩個團隊，一個由拉脫維亞大學的蓋理提斯（Agris Gailitis）領軍，另一個則是由德國卡斯魯研究中心的史蒂格里茲（Robert Stieglitz）和繆勒（Ulrich Müller），以及拜律特大學的布瑟（Fritz Busse）共同率領，他們各自在大量液態鈉中達成自生磁場。（使用液態鈉是因為它的導電係數高且熔點低。）兩個團隊都找出方法，在1~2公尺長的螺旋形 管的系統中達成高速的流體運動，所產生的臨界磁雷諾數約為10。

　 　The decades-old dream of generating a spontaneous magnetic field in a laboratory fluid dynamo was first realized in 2000, when two groups in Europe—one led by Agris Gailitis of the University of Latvia and one by Robert Stieglitz and Ulrich Müller of the Karlsruhe Research Center and Fritz Busse of the University of Bayreuth, both in Germany—independently achieved self-generation in large volumes of liquid sodium. (Liquid sodium was used because of its high electrical conductivity and low melting point.) Both groups found ways to achieve high-speed fluid flow in a system of one- to two-meter-long helical pipes, resulting in the critical magnetic Reynolds number of about 10.

　　這些實驗結果印證了理論，讓我們對於把發電機的理論應用於地球和其他行星，有著某種程度的信心。如今世界各地許多團隊都忙於發展新一代的實驗室發電機。為了更真切模擬類似地球的幾何形狀，這些實驗將把液態鈉放入巨型球室內攪動，其中最大的直徑將近三公尺。

　 　These experimental results bear out theory, which gives us a measure of confidence when we apply our theoretical ideas about dynamos to the earth and other planets. Now many groups across the world are busy developing the next generation of lab dynamos. To better simulate earthlike geometry, these experiments will stir the liquid sodium inside massive spherical chambers—the largest nearly three meters in diameter.

　 　除了朝向更接近實際情形的實驗室發電機和三維電腦模擬等進行中的計畫，國際人造衛星CHAMP正在觀測地球磁場，其解析度足以即時、直接測量地核–地函 交界處的地磁變化。研究人員期望這枚衛星在為期五年的任務中，能提供地球磁場的連續圖像，讓他們能監測反向通量斑塊的持續增長，以及關於偶極場如何減弱的 其他線索。

　　Besides the ongoing plans for more realistic laboratory dynamos and 3-D computer simulations, the international satellite CHAMP (short for Challenging Minisatellite Payload) is charting the geomagnetic field with enough precision to directly measure its changes at the core-mantle boundary in real time. Investigators anticipate this satellite will provide a continuous image of the geomagnetic field over its five-year mission, allowing them to watch for continued growth of the reversed flux patches as well as other clues about how the dipole field is waning.

　　我們預期在未來10年或20年內，衛星觀測、電腦模擬及實驗室實驗這三種方法將整合為一。得到這奇特地球發電機更完整的圖像，我們將瞭解目前我們對於地球磁場以及磁極反轉的想法是否正確。

　　We anticipate that a synthesis of these three approaches—satellite observations, computer simulations and laboratory experiments—will occur in the next decade or two. With a more complete picture of the extraordinary geodynamo, we will learn whether our current ideas about the magnetic field and its reversals are on the right track.