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The Forefront of Space Science

The 4.6-Billion-Year History of the Sun
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Here let me explain briefly the dynamo mechanism which creates sunspots (Fig. 3). The Sun rotates fastest at its equator, about one revolution per 25 days while the rotation of its North and South Pole areas is about once per 30 days. When there is a magnetic line along the meridian connecting the North and South Poles, it is stretched in its equatorial area and, accordingly, the magnetic filed is intensified. The amplified magnetic field then rises up to the solar surface to become sunspots. There are places where we can observe seeds of sunspots, i.e. magnetic fields, on the solar surface. These are the polar region of the Sun. HINODE is precisely observing them in the polar region for the first time in history (Fig. 4), providing us with various new findings. Since the polar magnetic fields are seeds of sunspots, we can forecast future solar activity trends by observing the polar area. Only HINODE allows us to observe accurately the polar regionís magnetic fields. So, in order not to miss any abnormal events, we routinely conduct intensive observation of the magnetic fields in the region.

Figure 3
Figure 3. Formation of solar magnetic field and sunspot
The magnetic line acts like rubber band. It stretches the north-south magnetic field to intensify it. The stretched magnetic line rises up to become a sunspot. Accordingly, we can observe the magnetic fields in the polar region that will become the seeds of the next cycle of sunspots.

Figure 4
Figure 4. Sunís magnetic fields in the polar region observed by HINODE (Sept 25, 2007)

Solar activity and birth of life could be connected

A little digression. When life emerged on the earth about 3.5 billion years ago, the temperature of the earth is thought to be a little higher than the present. The Sun is now 4.6 billion years old. According to the Standard Solar Model (SSM), the Sun has become brighter year by year since its zero age. The Sunís brightness 3.5 billion years ago was about 75% of its present level. If this estimate is true, the earth at the time must have been covered with thick ice in an entirely frozen state. This is a controversial ďFaint Young SunĀEparadox. In order to explain the birth of life in such an environment, many theories have been put forward, such as: life can be born even in frozen conditions; the earth was in fact not so cold since the ancient atmosphere contained much greenhouse gas or because of geothermal heat, although this last theory is suspect.

The SSM is a theory that has been verified in every detail. Therefore, no one has investigated the cause of the Faint Young Sun paradox on the Sun. To the earthís environmental system, the Sun is merely an environmental condition, which has slowly changed its brightness over 4.6 billion years. As a solar researcher, however, I would like to reexamine the solar origin theory, i.e., ďonce upon a time, the Sun was bright.ĀETo make the Sun brighter at that time, its weight at the age zero must be increased. As its weight increases, the Sun becomes brighter. Furthermore, the energy poured onto the earth increases because the distance between the Sun and earth becomes close. In conclusion, the heat amount that the earth receives from the Sun is proportional to about the solar mass to the power of six. If the Sunís mass was 5% greater than the current level, life can emerge on the earth without any other triggers.

To discuss this issue more accurately, my assumptions are: ďsolar mass was 5% greater than the present level for about one billion years after its birth. The energy poured on the earth was a little larger than the current level. After that, the Sun rapidly lost its excess mass and became its current size.ĀEIs this scenario possible? In fact, the answer to the question is found in the activity of the solar magnetic field. ISASís solar observation satellite series - evolving from HINOTORI to YOHKOH and HINODE - has continued to pursue the questions: ďhow are solar activity phenomena activated by the action of magnetic fields?ĀEand ďin the first place, what is the origin of the magnetic fields?ĀEI would like to point out a possibility that the Sun could have been closely linked to the birth of life on earth, an idea that has never been examined by research in relation to solar magnetohydrodynamic phenomena such as dynamo, coronal and chromospheric heating, and flare explosions.

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