Minimum Inside: Reasons for the Fall

Collage showing the sun in a section. The lines of force of the magnetic field obtained as a result of theoretical calculations (in the center), as well as the visible solar corona at different phases of cyclic activity are shown — on the right with a minimum, on the left with a maximum

There was a quiet minimum of solar activity observed in recent years (read about it: “At the bottom”), expressed in the complete absence of even tiny spots on the star - for several months its surface remained completely unperturbed. Of the 366 days of 2008, 266 days in the sun were not just few spots - they were not at all. This is very rare even for deep recessions of the usual 11-year cyclical activity of the Sun, which made astronomers nervous and gave a lot of reasons for discussion about the nature of this phenomenon. And finally, the answer is found.

But to begin with, it is worth recalling that sunspots are dark areas with a lower (about 1.5 degrees) relative to the surrounding plasma temperature. It is in these areas that the lines of force of the star’s internal magnetic fields emerge on the surface of the Sun. It is from them that the most significant emissions of charged particles into space occur - powerful streams of the solar wind, which, when meeting with the upper layers of the earth's atmosphere, give rise to delightful auroras (read: “Sky TV”), and sometimes disable power plants and power lines, leaving entire cities without energy and communication (“Super Flash!”).

Sunspots have been constantly observed by astronomers since the time of Galileo - that is, from the beginning of the 17th century. Thanks to these observations, it was shown that the number of spots varies cyclically, with a period of 11 years, their number either increases (which corresponds to the maximum of solar activity), or sharply decreases (minimum). The peak of the 23rd since the beginning of observations of the activity cycle occurred in 2001, and in the region of 2008 the Sun went into the deepest minimum - the last time something like this was recorded only in 1913. Only in 2009 did the star begin to return to normal, and today we can observe a quite ordinary regular, already 24th, cycle.

So, what led to the fact that a few years ago the Sun so “lost” all its spots for so long? To understand this, Indian researchers, led by Dibyendu Nandi, created their own theoretical model of the Sun - and “drove” it through 210 activity cycles using a computer. At the same time, they changed one important parameter, the velocity of the meridian (directed from pole to pole) flows of hot plasma circulating in the upper layers of the star in its convective zone.

In this way, scientists have shown that fast meridian currents in the first half of the solar cycle, plus their significant slowdown in the second half, lead to a particularly deep minimum. In fact, under these conditions, the computer model showed activity that is fully consistent with the most recent minimum of the 23rd cycle. “It's amazing how very small changes in the dynamics of the solar plasma have a huge impact, ” says Dibendu Nandi.

It should be noted here that, in general, the mechanisms that determine plasma flows in the bowels of the sun and affect their velocities remain poorly understood. Meridian currents are guided by the energy of a rotating solar core in combination with a small temperature gradient from the hotter equator to the colder poles of the star. Most likely, changes in the nature of these currents should be associated with changes in these basic factors and / or with the influence of the dynamics of the solar magnetic field.

According to Harvard-Smithsonian Center for Astrophysics Press Release


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