The Active Sun

Part I

The typical sunspot has structure and regularity that give us clues as to the nature of the spots. Spots are darker and thus cooler than the surrounding photosphere. A well-developed spot will have a central dark umbra and a lighter, striated penumbra. Within the region of the spot we see no granulation. Since granulation supplies the surface with fresh, hot gas. Stop the granulation and the surface should cool. Spots also like to occur in pairs.

The Sun has an 11-year sunspot cycle. Maunder found regularity in the location of the spots during the sunspot cycle. At the beginning of a cycle the spots (few in number) are appearing at higher latitude (about 30° from the Sun's equator). As the cycle progresses the spots form closer and closer to the equator. Sunspot maximum is reached as the spots are forming at about 15° from the equator. The numbers of spots then decrease but continue forming closer to the Sun's equator. By the end of the cycle the spots are forming very close to the equator as spots from the new cycle are beginning to appear at higher latitude.

Sunspots are regions of intense magnetic activity, as we can observe using the Zeeman Effect. There is regularity in the magnetic fields in the sunspots too. Magnetic fields must come in pairs of matching north and south poles. When sunspots occur in pairs, one is always a south pole while the other is a north pole. The magnetic polarities of the sunspots show the following trends:

• When pairs occur, they have opposite magnetic poles
• Pairs on the other side of the Sun's equator show opposite polarities
• All polarities are switched in the next sunspot cycle

One of the leading ideas of the origin of sunspots is the Babcock model. This model relies on two feature of the Sun that are required to produce sunspots:

• The Sun has a general magnetic field.
• The Sun is a differential rotator.

The weak magnetic field early in the sunspot cycle runs just under the photosphere and is very regular. But being weak the field can be pushed around by the gases. Because of the differential rotation the field lines begin to wrap around the Sun. As the lines get wrapped the lines move closer together and thus the field is strengthened. The flow of the lines becomes opposite in the Sun's two hemispheres. The field becomes most intense at 30° from the equator first.

In order for this strengthened magnetic field to produce sunspots, we require an instability. Just under the photosphere is the convective zone. Imagine a convective cell forcing the intense magnetic field toward the surface. As the field breaks through the surface the field now prohibits gas motion, granulation ceases, and the surface cools and darkens. A sunspot has formed. The strong field arches high over the Sun and reenters the surface in another point, where the same process produces another sunspot. The model shows that pairs of spots produced in this way have opposite magnetic poles. As the field line wrapping continues the spots form closer to the Sun's equator. After the maximum is reached, the lines unravel and the regular structure we had at the beginning is reestablished in the opposite direction. All major observations of sunspot activity are accounted for in the Babcock model.

Part II

Other stars should undergo spotting activity if them meet the requirements of general magnetic field, differential rotation, and a subsurface instability. These conditions seem to be met in stars cooler than the Sun.

Several kinds of solar activity seem to be related to sunspots. Plage regions are chromospheric brightenings that precede sunspots. Apparently the chromosphere can sense the rising magnetic field before it gets to the surface. Prominences are condensations in the chromosphere and corona that follow the magnetic field lines as they arch over the region of sunspots. The prominence shows us that the hot gases of the chromosphere and corona can be bottled up or trapped by the strong magnetic fields associated with sunspots. Occasionally, these magnetic bottles can rupture, producing solar flares. Particles jet out from the flare at speed approaching half the speed of light. In just an hour or two these particles arrive here at Earth and can cause communications disruptions and are potentially dangerous to unshielded humans in Earth orbit.

The Sun affects us in a number of ways, not least of which is the light and heat that the Sun provides. Charged particles from flares or solar wind can cause disruptions in communications and also the auroras. Charged particles cannot penetrate the magnetic field of the Earth, but they can follow the field lines along until the lines penetrate the Earth at the poles.

We are also discovering that the Sun has some long-term effects on the climate of the Earth due to solar activity. Studies of tree ring widths in the southwestern US has shown a 22-year cycle in step with the sunspot cycle for as long ago as we have samples to test. When the magnetic field effects are included, the length of the solar cycle is 22 years. This region of the world experiences a drought every 22 years caused by sunspot activity. We do not as yet know the mechanism.

Likewise, we know of several times when there have been very few sunspots for extended periods of time. The most famous is the Maunder Minimum extending from 1645 through 1715. These times correlate well with periods of cooler than average conditions. The Maunder Minimum itself correlates with the "Little Ice Age" in Europe. Again the mechanism is not clear, but the correlations are strong.