Fall 2006 Issue

PROPAGATION
It’s All Backwards!

By Tomas Hood, NW7US/AAAŘWA

A view of the sun that reveals the magnetic orientation of several sunspots occurring on July 31, 2006. The darker areas indicate a south magnetic “pole,” while the lighter areas indicate a north magnetic “pole.” The sunspot that has a backward-oriented polarity is the likely signal that a new solar cycle is approaching. (Source: NASA/SOHO)

Since the start of 2006, we’ve been seeing a steady decline in the sun’s activity. Sunspots, one of the solar events that we use to gauge the activity level of the sun, are occurring less frequently and they are generally much weaker than they were during the peak of the current sunspot cycle. There are periods now when we don’t see any sunspots for days on end. This has signaled the end of solar Cycle 23, but how will we know when Cycle 24 is starting up?

On July 31st, the anticipated sign that Cycle 24 is possibly beginning was observed. The sign came in the form of a short-lived tiny sunspot that bubbled up from the sun’s interior, floated around a bit, and vanished again in a few hours. This particular sunspot was special: It was backward. However, it was too small and short-lived to be numbered as an official sunspot.

“We’ve been waiting for this,” says David Hathaway, a solar physicist at the Marshall Space Flight in Huntsville, Alabama. “A backward sunspot is a sign that the next solar cycle is beginning.”

“Backward” means magnetically backward. Sunspots are magnetic regions on the sun with magnetic field strengths thousands of times stronger than the Earth’s magnetic field. Plasma flows in these magnetic-field lines of the sun. Sunspots appear as dark spots on the surface of the sun. Temperatures in the dark centers of sunspots (the umbra) drop to about 3700°K, compared to 5700°K for the surrounding photosphere. This difference in temperatures makes the spots appear darker than elsewhere. Sunspots typically last for several days, although very large ones may live for several weeks. They are seen to rotate around the sun, since they are on the surface, and the sun rotates fully every 27.5 days.

Sunspots usually form in groups containing two sets of spots. One set will have a positive, or north, magnetic field while the other set will have a negative, or south, magnetic field. The magnetic field is strongest in the darker parts of the sunspot. The field is weaker and more horizontal in the lighter part (the penumbra).

During the course of a solar cycle, sunspots are magnetically oriented much the same way, sunspot after sunspot. However, when the sunspot of July 31st popped up at solar longitude 65 degrees west, latitude 13 degrees south, it was opposite the normal orientation for sunspots in that region of the sun. Sunspots in that area are normally oriented N-S. This sunspot was oriented S-N.

During the course of the average 11 years of a solar cycle, where solar activity rises and falls, swinging back and forth between times of quiet and storminess, the magnetic structure of the sun reverses itself. Right now the sun is quiet. During the peak of a solar cycle, the sun is very active and stormy. Right after the peak, the sun’s magnetic poles actually flip. At the end of a cycle, or at the start of a new cycle, sunspot magnetic poles flip.

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