The Earth's inner core may have stopped rotating, or it may be rotating in the opposite direction - New research

Recently CNN.co.jp reported the following:

地球の内核が回転停止か、逆回転の可能性も　新研究

2023.01.26 Thu posted at 10:30 JST

（ＣＮＮ） 地球中心部にある内核の回転が止まり、さらに逆回転している可能性があると指摘する新たな論文が科学誌に掲載された。

地球は地殻とマントル、外核、内核から構成される。固体の内核は地殻の約５１５０キロ下にあり、半固体のマントルとの間には液体の外核がある。これにより内核は地球の自転とは異なる速度で回転できる。

地球の核は半径約３５４０キロで火星ほどの大きさ。鉄とニッケルが主成分で、地球の質量の約３分の１を占める。

北京大学の研究者イー・ヤン氏とシャオドン・ソン教授は２３日発行の科学誌「ネイチャー・ジオサイエンス」掲載の論文で、内核を通過する地震波を調査し、１９６０年代以降の記録も参照して内核の回転速度を推定した。

その結果、２００９年以降、それまで変動していた地震波にほとんど変化が見られなくなり、内核が回転を停止したことが示唆されるという。

論文は「内核がこの１０年間ほぼ停止し、逆戻りしている可能性もあることを示唆する驚くべき観測だ」と言及。ソン氏は「１９８０～９０年の１０年間は明確な変化があるが、２０１０～２０年には大きな変化がない」と述べた。

内核の回転は外核の磁場で推進され、マントルの重力効果でバランスをとっている。内核の回転する様子を把握できれば、地球内部の各層の相互作用についても解明が進む。

だが、回転速度や速度の変化については議論がある。

オーストラリア国立大学の地球物理学者フルボイエ・トカルチッチ氏は「内核は完全には止まっていない」と指摘する。今回の研究は「内核が１０年前より地球の残りの部分とより同期的になっていることを示すもの」とし「大変動は起きていない」との見方を示した。

ソン、ヤン両氏は、自分たちの計算によれば、電磁力と重力の小さなバランスの崩れが内角の回転を遅らせ、逆回転させる可能性があると主張する。これは７０年周期で起きる変化の一部であり、２００９、１０年付近で検知され、それ以前には１９７０年代前半に起きていたとしている。

トカルチッチ氏は今回の論文の「データ分析はしっかりしている」と認めつつ、そこから導かれる発見は「注意が必要だ」と言及。より多くのデータや革新的な方法がこの問題の解明には必要との認識を示した。

同氏は内核の周期を今回の論文とは異なる２０～３０年と推定する。このように予測に幅が出るのは地球最深部で起きる事象の把握が困難なためで、「我々は地球内部の性質を推定するのに地球物理学的な推論を使うが、学際的な発見で我々の仮定や概念フレームワークが確認されるまで注意深さが必要だ」という。

「地震学者は患者の臓器を不完全で制約のある器具で調べる医者のようなものだ。進歩はしているが地球中心部のイメージはまだはっきりしない」（トカルチッチ氏）

Translation

(CNN) A new paper in the scientific journal suggested that the inner core at the center of the Earth had stopped rotating and even reversed its rotation.

 The Earth consists of the crust, mantle, outer core, and inner core. The solid inner core lies about 5,150 kilometers below the crust, and is separated from the semi-solid mantle by a liquid outer core. This allowed the inner core to rotate at a speed different from the Earth's rotation.

The Earth's core had a radius of about 3,540 kilometers and was about the size of Mars. It was mainly composed of iron and nickel, and accounted for about one-third of the mass of the earth.

Researcher Yi Yang together with Professor Xiaodong Song of Peking University investigated seismic waves passing through the inner core, and also referred to records from the 1960s onwards to estimate the rotational velocity of the inner core. They published a paper on the 23rd in the scientific journal "Nature Geoscience".

In the result, it said that since 2009 that there had been almost no change in the seismic waves that had been changing until then, suggesting that the inner core had stopped rotating.

The paper said, "This is a startling observation which suggests that the inner core has nearly stopped over the last decade and may be reversing". Song said, “There was a clear change in the decade from 1980 to 1990, but no big change in 2010 to 2020.”

The rotation of the inner core was driven by the outer core's magnetic field and balanced by the gravitational effects of the mantle. If we could understand how the inner core rotated, we could be able to understand the interaction of each layer inside the earth.

However, there were controversies about the speed of rotation and changes in speed.

Hrvoje Tkalcic a geophysicist at the Australian National University pointed out that "The inner core hasn't completely stopped". About this study he held the view that “the inner core is more in sync with the rest of the Earth than it was a decade ago"; and also, that “no big changes has occurred'.

Song and Yang argued that, according to their calculations, small imbalances in the electromagnetic and gravitational forces could slow and delay interior angle rotation, and had a possibility in reversing the rotation. This was part of a 70-year cycle that was detected around 2009 and 2010; before that it happened in the early 1970s.

Tkalcic acknowledged that the paper's "data analysis is robust”, but also mentioned that the findings derived from it would 'need to be careful'. He acknowledged that more data and innovative methods might be needed to solve the problem.

He estimated that the cycle of the inner core would be 20 to 30 years, which was different from that in this paper. This range of predictions was due to the difficulties  in understanding events occurring at the deepest part of the Earth, and he said, “We use geophysical reasoning to infer properties of the Earth's interior, but caution is warranted until interdisciplinary findings confirm our assumptions and conceptual frameworks.”

"Seismologists are like doctors who examine a patient's organs with imperfect and limited instruments. Despite progress, the image of the Earth's center is still unclear" (Tkalcic)

              So, regarding the Chinese scientists’ suggestion in a research paper that the inner core at the center of the Earth has stopped rotating and even reversed its rotation, Tkalcic holds the opinion that we 'need to be careful' with the conclusion. He estimates that the cycle of the inner core is 20 to 30 years, which is different from the viewpoint in the paper.

Note:

1.Nature Geoscience is a monthly peer-reviewed scientific journal published by the Nature Publishing Group. The Chief Editor is Tamara Goldin, who took over from Heike Langenberg in February 2020. (Wikipedia)

2. Xiaodong Song (宋晓东) is the Peking University Chair Professor. His research focuses on seismology and studies of the Earth’s deep interior, including studies of structure and dynamics of the Earth’s core and lower most mantle, and lithosphere structure and tectonics of China and east Asia. (http://faculty.pku.edu.cn/SONGXiaodong/en/index.htm)

3. Professor Hrvoje Tkalcic is the Head of Seismology and Mathematical Geophysics of the Research School of Earth Sciences at the Australian National University. He uses seismology and mathematical geophysics tools to understand the internal workings of planets. (http://rses.anu.edu.au/~hrvoje/)