What's down with the Sun? Major drop in solar activity predicted
As the current sunspot cycle, Cycle 24, begins to ramp up toward maximum, independent studies of the solar interior, visible surface, and the corona indicate that the next 11-year solar sunspot cycle, Cycle 25, will be greatly reduced or may not happen at all.
This is highly unusual and unexpected, Dr. Frank Hill, associate director of the NSOs Solar Synoptic Network, said of the results. But the fact that three completely different views of the Sun point in the same direction is a powerful indicator that the sunspot cycle may be going into hibernation.
Spot numbers and other solar activity rise and fall about every 11 years, which is half of the Suns 22-year magnetic interval since the Suns magnetic poles reverse with each cycle. An immediate question is whether this slowdown presages a second Maunder Minimum, a 70-year period with virtually no sunspots during 1645-1715.
Hill is the lead author on one of three papers on these results being presented this week. Using data from the Global Oscillation Network Group (GONG) of six observing stations around the world, the team translates surface pulsations caused by sound reverberating through the Sun into models of the internal structure. One of their discoveries is an east-west zonal wind flow inside the Sun, called the torsional oscillation, which starts at mid-latitudes and migrates towards the equator. The latitude of this wind stream matches the new spot formation in each cycle, and successfully predicted the late onset of the current Cycle 24.
In the second paper, Matt Penn and William Livingston see a long-term weakening trend in the strength of sunspots, and predict that by Cycle 25 magnetic fields erupting on the Sun will be so weak that few if any sunspots will be formed. Spots are formed when intense magnetic flux tubes erupt from the interior and keep cooled gas from circulating back to the interior. For typical sunspots this magnetism has a strength of 2,500 to 3,500 gauss (Earths magnetic field is less than 1 gauss at the surface); the field must reach at least 1,500 gauss to form a dark spot.
Using more than 13 years of sunspot data collected at the McMath-Pierce Telescope at Kitt Peak in Arizona, Penn and Livingston observed that the average field strength declined about 50 gauss per year during Cycle 23 and now in Cycle 24. They also observed that spot temperatures have risen exactly as expected for such changes in the magnetic field. If the trend continues, the field strength will drop below the 1,500 gauss threshold and spots will largely disappear as the magnetic field is no longer strong enough to overcome convective forces on the solar surface.
Moving outward, Richard Altrock, manager of the Air Forces coronal research program at NSOs Sunspot, NM, facilities has observed a slowing of the rush to the poles, the rapid poleward march of magnetic activity observed in the Suns faint corona. Altrock used four decades of observations with NSOs 40-cm (16-inch) coronagraphic telescope at Sunspot.
A key thing to understand is that those wonderful, delicate coronal features are actually powerful, robust magnetic structures rooted in the interior of the Sun, Altrock explained. Changes we see in the corona reflect changes deep inside the Sun.
Altrock used a photometer to map iron heated to 2 million degrees C (3.6 million F). Stripped of half of its electrons, it is easily concentrated by magnetism rising from the Sun. In a well-known pattern, new solar activity emerges first at about 70 degrees latitude at the start of a cycle, then towards the equator as the cycle ages. At the same time, the new magnetic fields push remnants of the older cycle as far as 85 degrees poleward.
In cycles 21 through 23, solar maximum occurred when this rush appeared at an average latitude of 76 degrees, Altrock said. Cycle 24 started out late and slow and may not be strong enough to create a rush to the poles, indicating well see a very weak solar maximum in 2013, if at all. If the rush to the poles fails to complete, this creates a tremendous dilemma for the theorists, as it would mean that Cycle 23s magnetic field will not completely disappear from the polar regions (the rush to the poles accomplishes this feat). No one knows what the Sun will do in that case.
All three of these lines of research to point to the familiar sunspot cycle shutting down for a while.
If we are right, Hill concluded, this could be the last solar maximum well see for a few decades. That would affect everything from space exploration to Earths climate.