重建千年來的太陽活動

 原文網址：https://ethz.ch/en/news-and-events/eth-news/news/2021/01/solar-activity-reconstructed-over-a-millennium.html

By Oliver Morsch

蘇黎世聯邦理工學院領導的國際研究團隊透過測量樹輪中的放射性碳，重建了自西元969年以來的太陽活動。這項結果有助於科學家更加了解太陽的動力學，並讓碳14定年法得出的有機物年代更加準確。

過去1000年的太陽活動(藍線，誤差範圍為白線)以及不到400年的太陽黑子紀錄(紅線)。背景代表太陽典型的11年活動周期。圖片來源：蘇黎世聯邦理工學院

太陽內部發生的事物只能透過間接的觀察方法來得知。比方說，太陽黑子可以呈現出太陽的活躍程度――太陽表面可以看到的黑子越多，太陽深處的活動就越劇烈。雖然人類在古代就已經知道太陽黑子的存在，但是開始對它們進行詳細的紀錄則要到大約400年前，望遠鏡發明出來之後。幸虧有這些紀錄，我們現在知道黑子的數量會以十一年為週期規律地變動；不只如此，太陽活動還有為時更長的強弱變化，並且反映在地球的氣候當中。

然而，要重建出在這些有系統的記錄開始之前，太陽的活動是如何變化，目前來說仍然相當困難。蘇黎世聯邦理工學院離子束物理實驗室的Hans-​Arno Synal與Lukas Wacker領導了一組國際研究團隊，成員包括德國哥廷根的馬克斯．普朗克太陽系研究所以及瑞典的隆德大學。他們透過測量樹輪裡放射性碳的濃度，而把太陽的十一年週期一路回推至西元969年。不只如此，他們建立起的這組資料，在利用碳14定年法得出更加準確的年代時也相當重要。成果最近發表於科學期刊《自然―地球科學》。

樹輪裡的太陽活動

為了重建過去一千年的太陽活動，並且達到以年為單位的超高時間解析度，研究人員利用了英國和瑞士的樹輪紀錄。樹輪裡每一兆顆碳原子只有一顆是帶有放射性的碳14，但是每一道生長輪含有的碳14比例會有微小的變化，同時透過生長輪的數目可以精確地算出其形成年代。接下來，由於已經知道碳14同位素的半衰期大概是5700年，因此可以回推生長輪形成的當下大氣中有多少放射性碳。放射性碳的主要來源是宇宙微粒，而太陽的磁場會阻擋其到達地球的量有多少――當太陽越活躍，遮蔽地球的效果就越強――因此我們可以從大氣中碳14濃度的變化來推導太陽活動的變化。

用現代的測量技術得到更好的結果

然而，要從濃度已經這麼低的碳14當中再精確測量其變化，就像是從大海裡找出一根針上面的一粒塵埃。「這類測量只在80年代和90年代做過而已，」Lukas Wacker表示，「但是他們只有測量最近400年的變化，而且用的計數方法非常耗費心力。」當時是利用蓋革計數器(Geiger counter)來直接測量樣品發生了多少次碳14放射性衰變的事件。這種方法需要較為大量的樣品，加上碳14的半衰期很長，因此還得耗費相當長的時間。「利用現代的加速器質譜儀，我們測量碳14濃度的誤差可以不到0.1%，而且時間只需要幾個小時，用的樹輪樣品量也小了幾千倍。」負責這些分析的博士生Nicolas Brehm補充。

在加速器質譜儀裡面，樹輪樣品中的碳14和碳12原子(碳12是「正常」沒有放射性的碳；另一方面，碳14的原子核則多帶了兩顆中子)會先被轉變成帶有電荷的型態，然後透過數千伏特的電壓加速，再讓它們通過一道磁場。由於這兩種碳同位素的質量不同，因此在磁場裡會往不同的角度偏折，使得它們的數量可以分開計算。研究人員得到這些初始數據之後，還得進行一些複雜的統計分析，再把結果用電腦模型來處理，才能得到他們想要的有關太陽活動的資訊。

一千年來皆以十一年為週期

這些程序讓研究人員成功把西元969到1933年的太陽活動逐年重建出來。他們從重建結果證實了太陽活動的週期是以十一年為規律，而且在長時間的太陽極小期期間，每次週期的振幅(太陽活動的起伏程度)也比較小。這些見解在深入了解太陽內部的動力學時非常重要。測量結果也證實了西元993年曾經發生太陽高能質子事件。在這類事件當中，被太陽閃焰加速到極快的質子到達地球之後，會讓碳14的產量稍微增加。此外，研究團隊也發現了另外兩起之前沒人知道的事件，分別發生在西元1052和1279年。代表這類會嚴重干擾地表電路以及衛星的事件，次數可能比之前認為的還要頻繁。

讓碳14定年法得到更準確的年代

由於現存的樹輪資料庫涵蓋了過去14000年，因此研究人員的近期目標是利用他們的方法，測定末次冰期結束以來每年的碳14濃度。這項新研究得到的數據還有另外一項附加價值：可以讓有機物用碳14定年法得到更加準確的年代，而國際公認的放射性碳定年校正曲線(radio carbon calibration curves ,IntCal)在最新版本也已經納入了他們的數據。「這個參考資料庫之前從未收錄蘇黎世聯邦理工學院的研究，」Lukas Wacke表示，「但是憑藉這份新的研究結果，我們一舉便貢獻了資料庫裡三分之一的數據。」

 

Solar activity reconstructed over a millennium

An international team of researchers led by ETH Zurich has reconstructed solar activity back to the year 969 using measurements of radioactive carbon in tree rings. Those results help scientists to better understand the dynamics of the sun and allow more precise dating of organic materials using the C14 method.

What goes on in the sun can only be observed indirectly. Sunspots, for instance, reveal the degree of solar activity – the more sunspots are visible on the surface of the sun, the more active is our central star deep inside. Even though sunspots have been known since antiquity, they have only been documented in detail since the invention of the telescope around 400 years ago. Thanks to that, we now know that the number of spots varies in regular eleven-​year cycles and that, moreover, there are long-​lasting periods of strong and weak solar activity, which is also reflected in the climate on Earth.

However, how solar activity developed before the start of systematic records has so far been difficult to reconstruct. An international research team led by Hans-​Arno Synal and Lukas Wacker of the Laboratory of Ion Beam Physics at ETH, which included the Max Planck Institute for Solar System Research in Göttingen and Lund University in Sweden, has now traced back the sun’s eleven-​year cycle all the way to the year 969 using measurements of the concentration of radioactive carbon in tree rings. At the same time, the researchers have thus created an important database for more precise age determination using the C14 method. Their results were recently published in the scientific journal Nature Geoscience.

Solar activity from tree rings

To reconstruct solar activity over a millennium with an extremely good time resolution of just one year, the researchers used tree-​ring archives from England and Switzerland. In those tree rings, whose ages can be precisely determined by counting the rings, there is a tiny fraction of radioactive carbon C14, with only one out of every 1000 billion atoms being radioactive. From the known half-​life of the C14 isotope – around 5700 years – one can then deduce the concentration of radioactive carbon present in the atmosphere when the growth ring was formed. As radioactive carbon is mainly produced by cosmic particles, which in turn are kept away from the Earth to a greater or lesser extent by the magnetic field of the sun – the more active the sun, the better it shields the Earth – it is possible to deduce solar activity from a change in the concentration of C14 in the atmosphere.

Better results through modern detection techniques

Precise measurements of a change in that already very small concentration, however, resemble the search for a grain of dust on a needle in a huge haystack. “The only measurements of that kind were made in the 80’s and 90’s”, says Lukas Wacker, “but only for the last 400 years and using the extremely laborious counting method”. In that method, radioactive decay events of C14 in a sample are directly counted using a Geiger counter, which requires a relatively large amount of material and, owing to the long half-​life of C14, even more time. “Using modern accelerator mass spectrometry we were now able to measure the C14 concentration to within 0.1 percent in just a few hours with tree-​ring samples that were a thousand times smaller”, adds PhD student Nicolas Brehm, who was responsible for those analyses.

In accelerator mass spectrometry, C14 and C12 atoms (the “normal”, non-​radioactive carbon; C14, by contrast, contains two additional neutrons in its nucleus) of the tree material are first electrically charged and then accelerated by an electric potential of several thousand volts, after which they are sent through a magnetic field. In that magnetic field the two carbon isotopes, which have different masses, are deflected to different degrees and can thus be counted separately. To eventually obtain the desired information on solar activity from that raw data, the researchers have to perform some intricate statistical analysis on it and further process the results using computer models. 

Regular eleven-​year cycle over a millennium

This procedure enabled the researchers to seamlessly reconstruct solar activity from 969 to 1933. From that reconstruction they could confirm the regularity of the eleven-​year cycle as well as the fact that the amplitude of that cycle (by how much the solar activity goes up and down) is also smaller during long-​lasting solar minima. Such insights are important for a better understanding of the internal dynamics of the sun. The measurement results also allowed a confirmation of the solar energetic proton event of 993. In such an event, highly accelerated protons that reach the Earth during a solar flare cause a slight overproduction of C14. Moreover, the research team also found evidence of two further, as yet unknown events in 1052 and 1279. This could indicate that such events – which can severely disturb electronic circuits on Earth and in satellites – happen more frequently than previously thought.

More precise dating by the C14 method

As tree ring archives exist for the past 14’000 years, in the near future the researchers want to use their method to determine the yearly C14 concentrations all the way back to the end of the last ice age. As a kind of “extra”, the data in the new study can be used for dating organic material much more precisely using the C14 method and have already been included in the latest edition of the internationally recognized radio carbon calibration curves (IntCal). “ETH had not been involved in that reference database before”, says Lukas Wacker, “but with our new results we have now contributed a third of the measurements in one go.”

原始論文：Nicolas Brehm, Alex Bayliss, Marcus Christl, Hans-Arno Synal, Florian Adolphi, Jürg Beer, Bernd Kromer, Raimund Muscheler, Sami K. Solanki, Ilya Usoskin, Niels Bleicher, Silvia Bollhalder, Cathy Tyers, Lukas Wacker. Eleven-year solar cycles over the last millennium revealed by radiocarbon in tree rings. Nature Geoscience, 2021; 14 (1): 10 DOI: 10.1038/s41561-020-00674-0

引用自：ETH Zurich. "Solar activity reconstructed over a millennium."