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The Forefront of Space Science

The 4.6-Billion-Year History of the Sun
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How did the Sun take shape?

A wind of plasma called solar wind flows out from the sun. The mass that this solar wind can carry away is now around 1/300 to 1/1,000 of the mass necessary for the solar weight reduction mentioned above. Thus, solar wind does not explain the mass loss. So, how did the Sun succeed in losing weight? Since, unfortunately, we cannot easily observe the Sun 4.6 to 3.5 billion years ago, we need to find an answer in young stars similar to the Sun.

There are numerous stars in the universe. If there is young star of the same spectral type as the Sun, the young Sun must have behaved similarly to the star. In many cases, these young stars rotate very fast. At present, the Sun rotates once a month. I believe the young Sun rotated once every few days. The young Sun must have been far brighter, blindingly bright, than the current one in X-rays and ultraviolet. The weight loss of these young stars has been calculated based on observations by the Hubble Space Telescope of the hydrogen Lyman-alpha line of young solar-type stars. The result shows the maximum mass loss was around 100 times the current Sun. Considering this fact, it may not ridiculous to find the cause of the Faint Young Sun paradox in the Sun.

Based on such observations, what was the juvenile Sun like? Due to its fast rotation, the dynamo becomes active, eventually producing vast magnetic fields. As a result, the Sun enters a state where large flare explosions and Coronal Mass Ejections (CME) occur almost continually (i.e. once per several minutes). CME is a phenomenon where massive mass and magnetic fields are released just like bubbles from the coronas because of the instability of the magnetic fields. The coronal temperature at that time was much higher than the current temperature, and the solar wind was much stronger as well. When the Sun loses its mass, its angular momentum is also lost. Eventually, solar rotation slows down as if the brake had suddenly been applied. The slowdown in rotation causes the solar dynamo to become inactive. Through this process, the Sun has gradually evolved to its current state. It is possible to say that the Sun successfully lost weight because of its active motion and has now entered the age of maturity. Because of the strong solar wind and CMEs, fast, high-temperature plasma must have rushed to the surface of the planets. Planets were also exposed to much stronger ultraviolet radiation than at present. If so, can we find the signature of the young, violent Sun on the surface of planets with no magnetosphere and atmosphere?

In addition, it is guessed that, as rotation becomes even faster, the mass loss of stars decreases and does not exceed around 100 times the current Sun. To explain this supposition, the following possibility is proposed: when solar rotation is too fast, the dynamo engine becomes extremely active, and consequently giant sunspots cover the North Pole and South Pole. The strong magnetic line connecting both poles plays the role of a lid, constraining mass loss. A Sun with giant sunspots on its polar regions is quite different from the current view observed by HINODE. It is interesting to imagine such a figure of the Sun.

To elucidate the earth’s history based on the history of the Sun

HINODE has made many discoveries related to the magnetic fields. They are, briefly: strong magnetic fields in the polar region; short-life horizontal magnetic fields; hydromagnetic waves; convection collapse; turbulent flow; and ubiquitous magnetic reconnection. Researchers around the world are continually excited by new results from HINODE that are published in academic journals almost every week. I believe that these elementary processes occurred in more violent form and larger scale on the young Sun.

The objective of HINODE and its follow-on satellite is mainly to elucidate the dynamo mechanism, and coronal and chromospheric activity. As we understand more about the basic processes of the phenomena associated with magnetic fields, short- and long-term forecasts of solar activity will be possible, eventually contributing to the earth’s environmental issues and the safe and affluent life of mankind. Research into the origin of the solar dynamo and magnetic field must uncover how the solar history of 4.6 billion years relates to a variety of phenomena occurring on earth, including the origin of life.


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