The Sun is behind the cycle of sunspots. Although it is partly predicted by some models, its importance began to make astrophysicists puzzled. Some of them think they know the reason for the phenomenon. They propose a hypothesis supported by the observations of solar seismology.

Often presented as Galileo discovered sunspots but it now seems clear that, as with the first observations of the Moon with a telescope, it was Thomas Harriot who had preceded him. In fact, it seems that the observations of spots on the surface of the Sun are very old as well as in texts of ancient Greece than in other ethnic Chinese and Japanese, dating back over a thousand years, we find probable traces of observations of this phenomenon.

Since Galileo, astronomers have compiled the observations of sunspots and thus a cycle of 11 years was determined for the first time by the German amateur astronomer Heinrich Schwabe in 1843.

On this image of the Sun’s surface, the disc in the lower left gives a comparison of the size of sunspots with that of the Earth. Credit: NASA

Today we understand better about the nature of sunspots, their connections with the plasma convection and solar magnetohydrodynamics which develops with it. We distinguish, for example very clearly when sunspots makes a magnetogram of the Sun. The spots are indeed colder areas where convection is inhibited by a strong magnetic field.

A tube of intense magnetic flux between the two areas and that is why the spots are associated in pairs. Because they are cooler, they radiate less, and therefore appear darker in the optical domain. Their size easily exceeds that of the Earth and they only last two weeks on average.

A diagram of the internal structure of the Sun predicted by the theory of stellar structure and verified by solar seismology. In black, the propagation of rays associated with seismic waves p and g. Credit: NASA

Models of the solar interior and its atmosphere have existed for decades and they become increasingly complex and precise thanks to computers, with the observations of Soho and especially helioseismology.

Like Earth, the Sun is transversing seismic waves that occur on the surface of complex modes of vibration. In the same way that seismic waves are not the same in shape and mineralogical composition of the inner layers of the Earth, the modes of vibration of the Sun contain valuable information on the constitution and movements of our star.

Simply by measuring the Doppler movement of oscillations of the Sun’s surface for effective solar seismology and to learn a lot about our favorite star.

On 22 June 2009, the Sun’s surface in the visible shows that two shy sunspots. Credit: NASA

Response in the jet streams

However it had completed its last solar cycle, with a minimum spot, the Sun was slow to resume its activity. Some astrophysicists had expected a delay of one year for the first recurrence of sunspots, but their absence is extended beyond the stipulated period.

Should we conclude that starts another Maunder minimum solar like? Mentioned first in 1890 by E. W. Maunder, this event corresponds to a minimum in the number of sunspots between 1645 and 1715, with a strangely concomitant cooling of the Earth. The magnetic field of the Sun shows no signs of abating, this hypothesis was very probable.

Also on 22 June 2009, a magnetogramme shows more clearly the presence of very few sunspots. Credit: NASA

Rachel Howe and Frank Hill, two American astrophysicists specialists in  helioseismology and members of the National Solar Observatory (NSO) in Tucson (Arizona) have proposed an alternative explanation at a conference in Boulder (Colorado).

According to studies they have conducted every 11 years, jet streams similar to those existing on Earth, but spreading to thousands of kilometers into the Sun, begin at the poles. When they reach the critical latitude of 22 ° sunspots start to appear.

However, using networks of seismic observation of the Sun, the two researchers discovered that one of these jets streams migrated more slowly than usual since the poles and his arrival at the critical latitude seems to coincide with the beginning recovery of solar activity that should be more marked in the months and years to come.

If the model of the researchers is confirmed, it should greatly help to improve predictions of solar weather and the risks it runs on terrestrial satellites, with, for example, electrons killers.

This chart shows the number of sunspots observed in recent years and predicted for the near future. Curve, gives an average frame and other possible fluctuations most likely. Credit: NASA

The Sunspot Enigma Solar Cycle 2009