Solar Activity Cycle

No one has yet fully explained the origin of the solar activity cycle. Astronomers have developed several possible scenarios, or models, that reproduce the general characteristics of the cycle, but the details remain elusive. One of the most well-known of these models was developed in the early 1960s by Horace Babcock.

Unlike Earth, the Sun is made of gas, and this makes a big difference in how these two bodies rotate. To see how Earth rotates, look at a spinning compact disc. Every part of the disc completes one rotation in the same amount of time. To see how the sun rotates, study the surface of a freshly made cup of instant coffee. The foam on the surface rotates at different speeds: the inner parts rotate faster, so that spiral patterns form on the surface of the coffee. This is called differential rotation, and it is how any liquid or gaseous body rotates. Therefore the sun, being gaseous, rotates differentially: the equator completes one rotation every 26 days, while regions near the poles rotate once every 36 days.

This is important because the sun's magnetic field, like Earth's magnetic field, gets carried along with the rotating material. When the magnetic field at the sun's equator has been carried through one complete rotation, the more slowly rotating field at higher latitudes has fallen behind. Over the course of many rotations, the field gets more and more twisted and tangled. And now the punch line: solar active features, like sunspots, are associated with regions of strong and complex magnetic fields. So the more twisted the magnetic field gets, the more activity there is. Finally, when the magnetic field gets tangled to a critical level, it rearranges itself into a simpler configuration, just as when you twist a rubber band too many times, it snaps. As the magnetic field's complexity decreases, so does the activity, and soon the cycle is complete. None of this happens on Earth, because Miami, Florida, and Fairbanks, Alaska, both rotate once every 24 hours. There is no differential rotation on Earth to tangle its magnetic field.

Coronal mass ejections (CMEs) are solar bursts that are as powerful as billions of nuclear explosions. These ejections are the largest explosions in the solar system, typically hurling up to 11 billion tons of ionized gas into space. CMEs produce geomagnetic storms that reach the earth in about four days. These storms can damage satellites, disrupt communication networks and cause power outages. The 1989 power blackout in the northeast portion of the United States and Canada was triggered by a geomagnetic storm that overloaded part of the power grid and caused a blackout to propagate through the system. Satellites have been disrupted, and on occasion destroyed, by the radiation accompanying CMEs. For these reasons, operators of satellites, power systems, pipelines, and other sensitive systems follow solar-terrestrial activities by monitoring data from ground and orbiting solar telescopes, magnetometers, and other instruments.

In 1999, scientists reported a strong correlation between an S-shaped pattern that is sometimes observed on the sun's surface and the probability that a coronal mass ejection will occur from that region within several days. These S-shaped regions are believed to be produced by the twisted solar magnetic fields. If the correlation holds up under closer examination, it may be possible to predict CMEs as routinely as meteorologists predict weather patterns.

The poles of the Sun's magnetic field change places each 11-year activity cycle. The north pole becomes the Figure 1. Annual mean sunspot numbers from 1610-1980. Illustration by Hans & Cassidy. Courtesy of Gale Group. south magnetic pole, and vice versa. Thus the 11-year cycle of sunspot frequency is actually half of a 22-year solar cycle in which the magnetic field reverses itself repeatedly. Actually, the length of the activity cycle isn't exactly 11 years; that's just an average value. Year 2000 saw the start of Cycle 23, i.e., the 23rd cycle since reliable data first became available.

In the course of each 11-year cycle, an increasing number of sunspots appear at high latitudes and then drift towards the equator. As already noted, sunspots are actually regions of intense magnetic activity where the solar atmosphere is slightly cooler than the surroundings. This is the reason sunspot regions appear black when viewed through viewing filters. Sunspots are formed when the magnetic field lines just below the sun's surface become twisted, and poke though the solar photosphere, i.e., the region of the Sun's surface that can be seen by viewers on Earth. The twisted magnetic field above sunspots are frequently found in the same places that solar flares appear.

Sunspots pump x rays, high-energy protons, and electrified gases into space. That is the reason sunspots can affect satellites and power and communications systems on Earth.

In 1998, scientists reported finding giant convective cells (red and blue blotches) on the face of the sun. Although evidence of the existence of these structures had been sought for more than 30 years, they had not been seen before because their movements were buried in the more violent, small-scale activities on the Sun. These blue and red shifts are believed to correspond to the rising and falling of gases and their spreading out across the solar surface.

The flow of solar gases is more powerful than the solar magnetic fields, so the gases can carry magnetic structures with them. The eruption of these magnetic structures from the surface and their looping into space and back coincides closely with the appearance of sunspots.