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The Forefront of Space Science

The 4.6-Billion-Year History of the Sun
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Regarding the Sun, I should introduce observational results from HINODE, the latest solar observation satellite. However, since its findings have been reported several times in this “forefront of space science,EI would like to discuss the Sun from another viewpoint - the relationship between the Sun and the earth’s environment over a longer time-scale.

Is the Sun abnormal now?

Lately we often see newspapers article such as “Is an Anomaly happening on the Sun?EThe U.S.’s National Oceanic and Atmospheric Administration (NOAA) initially predicted that the next peak of solar activity would be around October, 2011. Lately, they moved the forecasted peak to May, 2013. This change seems to be in response to the current state of the Sun. The announcement means that the solar cycle is lengthening to nearly 13 years from its normal cycle of around 11 years.

In fact, looking at the daily observational data from HINODE, I have the impression that solar activity is unusually stagnant. The phenomena currently regarded as anomalous are: regarding the accumulation of no-sunspot days during the solar minimum, the Sun currently has few sunspots, almost equivalent to that of the minimum about 100 years ago; the total amount of energy coming from the Sun has decreased a little compared to that of the past two solar minimums; the magnetic field intensity in the solar polar region is almost half and the structure of the solar wind is largely different from that of past minimums: and the amount of cosmic rays poured on the earth from outer space is the highest over the 45-year observations. The earth is protected by the vesture of the solar magnetic field. Therefore, if the magnetic field extending from the solar polar region to planetary space weakens, the amount of cosmic rays would increase.

Is the Sun really in an abnormal state? For about 70 years from 1645, not so long ago, there was an age when almost no sunspots appeared. Since the earth experienced a minor ice age at that time (Fig. 1), the anomalies above provoke various thoughts and concerns. In fact, the apparent suspension of the generation mechanism of the magnetic field (called “dynamoE, the source of solar activities such as sunspots and flare explosions, has occurred many times over the past 6,000 years (Fig. 2). In addition, it seems that the earth was cold in such periods. Surprisingly, past data reveal that the solar activity cycle was extended just a little before the sunspots disappeared, elongating to a 13- to 14-year cycle. Currently, the usual 11-year solar cycle is lengthening. So, this is the reason for the view that solar activity has entered a stagnant period or that the Sun has returned to its normal state although it had been in a high state before. Even if sunspots have not been observed for a long time, however, we cannot conclude that the Sun is abnormal from the astronomical viewpoint, though we cannot explain the cause of the phenomenon with magnetohydrodynamics. Although the impact of the phenomena on the earth’s environment is of great concern, according to HINODE’s latest observations, fortunately or unfortunately, solar activity is gradually increasing. Therefore, it is likely the next maximum is due.

Figure 1
Figure 1. Change in sunspot number over the past 400 years
The bold line shows changes in average sunspot numbers. This is produced by modifying the data on Wikipedia.

Figure 2
Figure 2. Change in sunspot number over the past 6,000 years and the earth’s ice age (circled)
Modified from the Usoskin, I.G., G.A. Kovaltsov, Cosmic rays and climate of Earth: Possible connection, Compt. Rend. Geosci., 340, 441-450, 2008

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