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Solar aurorae create the Earth’s aurorae

--- Getting back to the basics, why do solar flares occur?

Solar flare (courtesy: SDO/NASA)
Solar flare (courtesy: SDO/NASA)

The mechanism of solar flares has not been fully explained, but it’s believed that they are part of the process of solar magnetic energy being converted into plasma energy. The Sun is a mass of hot gas, and the speed of its rotation is different at different latitudes. At its equator, the Sun rotates on its axis roughly once every 27 days, but at the poles it takes more than 30 days. Because magnetic field lines move together with plasma, the different rotational speeds at different latitudes cause distortion in the magnetic field. That means that tension is stored – and it is thought that a big explosion happens when the tension exceeds a certain limit.

By the way, the flares are called solar aurorae.

--- Solar aurorae?!

The magnetic field of the Earth’s magnetosphere gets distorted by solar wind particles and by the magnetic field of the sun. This was confirmed by satellites such as Japan’s magnetospheric observation satellite GEOTAIL, launched in 1992. And this distorted magnetotail causes aurorae. This is surprisingly similar to the mechanism of the generation of solar flares, which are also caused by the energy of distorted magnetic fields. This is why solar flares are called solar aurorae, and you can say that solar aurorae create Earth’s aurorae.

Something unexpected is happening to the Sun!?

--- Speaking of magnetic fields, the HINODE satellite confirmed the reversal of the Sun’s north polar magnetic field in 2012. Does that happen regularly?

The Sun’s typical magnetic field (above) and its magnetic field after May 2012 (bottom) (courtesy: National Astronomical Observatory of Japan/JAXA)
The Sun’s typical magnetic field (above) and its magnetic field after May 2012 (bottom) (courtesy: National Astronomical Observatory of Japan/JAXA)

The polar magnetic fields reverse at the peak of each 11-year solar cycle. However, the reversal does not happen at the same time at the south and north poles. This has not been explained either. In January 2012, HINODE found that the north magnetic field was in a zero state prior to completing the flip from negative to positive. HINODE also observed very little change in the south pole’s positive polarity. This has created a four-pole magnetic structure, with positive polarity at both the north and south poles, and with negative polarity in the vicinity of the equator. Some people say that something unexpected is happening at present on the Sun.

Observations by HINODE last January also found that there still weren’t any signs of a reversal of the southern magnetic field. The sun’s polar magnetic fields have been monitored for a while by Stanford University in the United States, for example, and it was already known that the magnetic fields reverse and that the reversal happens differently at the north and south poles. But HINODE captured the very moment of the flip in high resolution for the first time.

--- How could the Earth be affected by the four-pole magnetic structure?

We don’t know yet. The Sun is in this state right now, but no one knows how this will influence the Earth. Our group found and published the fact that solar activity reaches its peak at each pole in accordance with the flip at the north and south poles respectively. What is more complicated is that something unexpected is happening to the Sun. It has been quiet for some reason, regardless of the fact that it is supposed to be at its maximum this year according to its 11-year cycle.

--- What do you mean by “unexpected”? What’s happening to the Sun?

The current solar cycle is supposed to peak in 2013, but compared to the peak of the previous cycle, there have been fewer sunspots, which are an indicator of solar activity, and fewer powerful solar flares. This year, there have been no large geomagnetic storms, which are caused by intense solar flares. There were approximately 17 geomagnetic storms during the peak of each previous solar cycle, so it is strange that there has been none this year.

However, there are more sunspots than in 2010, so the 11-year solar cycle is tentatively taking shape, but again, the number at the peak is barely half that of past solar cycles. I have never seen anything like this in my research career. However, the existing records indicate that the Sun has a period of quiet activity about once every 100 years, and the Earth is cooled accordingly. So something normal may be happening to the Sun. It can be that we are only upset because our life expectancy is shorter than the solar cycle. The Sun may be laughing now because we are so surprised. (laugh).

There will be aurorae as long as the Sun exists

--- I have heard that this year is a good year to see an aurora because the Sun is active. Is the frequency of the occurrence of aurorae related to solar activity?

Aurora breakup (courtesy: Nanook Aurora Tours/Yoshifumi Otsuka)
Aurora breakup (courtesy: Nanook Aurora Tours/Yoshifumi Otsuka)

It is true that more aurorae appear when the Sun is active. However, this type of aurora represents only about 10 to 20 percent of the total, and the other 80 to 90 percent of aurorae occur randomly. Solar activity is more powerful when there are more sunspots, but even if there are no sunspots at all, there will be aurorae as long as there is the Sun. Because of the advertising that travel agencies are doing, many people think that they won’t be able to see an aurora for another 11 years if they miss this year’s solar maximum. That’s not really true.

--- Aurorae are sometimes seen in Hokkaido. Why is that?

It has a lot to do with solar activity. When an enormous solar flare emits hot solar wind, it causes a large geomagnetic storm, pushing aurorae that are usually seen over Alaska or Siberia to move southward to the Kamchatka Peninsula. In this case, you can see the red upper part of the aurora curtain from over Hokkaido to the region near the horizon in the north.

--- Is it possible to predict the occurrence of an aurora?

It is pretty much possible to make the prediction approximately two hours in advance. The prediction is made by comprehensively analyzing ground magnetic field data and satellite data. The satellite data are collected by NASA’s ACE (Advanced Composition Explorer). ACE observes solar wind, which is the source of aurorae. The prediction is made based on the time lag from the monitoring station to its arrival time on Earth. In Canada, there are about 20 monitoring stations to capture aurorae with super-wide-angle lenses, so it is possible to determine the type and location of an aurora in real time.

I like studying the unknown

--- What made you want to study aurorae?

Aurora photographed from the International Space Station (courtesy: NASA)
Aurora photographed from the International Space Station (courtesy: NASA)

When I was a schoolboy, there was an international project – the International Geophysical Year – from 1957 to 1958. The Earth’s atmosphere and magnetic field, as well as outer space, were observed on a global scale. At that time, the news about such events as the launch of the world’s first artificial satellite, Sputnik, and the construction of Japan’s Showa Station in Antarctica, was eagerly reported. However, TV was not yet popular, so I learned the news in science magazines, newspapers and on the radio.

And one day, I read a newspaper article about Prof. Takeshi Nagata, the then leader of the Japanese Antarctic research expedition. In the article, he was saying, “Aurorae are related to solar activity, but why do they appear on the night side of the Earth? Nobody knows the answer. I really hope that young people will be able to explain this.” And I said to myself, “Yes, I will do it!” This was how it all started.

--- Please tell us about your childhood.

I am from Hokkaido, and I was a precocious child who loved to interact with nature. Looking at nature, questions kept popping up in my head. For example, sitting on the beach, watching the sea, I wondered why there were waves despite there being no wind. And climbing a mountain, I wondered why the temperature went down in spite of the fact that I was getting closer to the Sun. When I had these kinds of questions, I tried to look up the answers and think about them as much as possible. And I loved the feeling of accomplishment when I later heard the answers at school. But if anything, I was even more thrilled when I did not understand than when I got it. I am a scientist now, but I am still the same way. When I find out about one thing, 10 new things that I don’t understand emerge. There is still no end to my intellectual curiosity.

--- You are thrilled by what you don’t know?!

Once you find the answer, it’s no longer interesting, is it? So, to tell you the truth, I am glad when an aurora forecast turns out wrong. Wrong forecasts suggest that there are still things to be solved, so we can leave them for younger scientists. So when aurora forecasts for tourists turn out wrong, I am secretly grinning.

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