Sunspots

describe the appearance and explain the nature of sunspots
recall that the Sun’s rotation period varies from 25 days at the equator to 36 days at its poles
demonstrate an understanding of how astronomers use observations of sunspots to determine the Sun’s rotation period
interpret data (for example a Butterfly Diagram) in order to describe the long-term latitude drift of sunspots, determine the length of the solar cycle and predict the year of the next solar maximum

WARNING - never observe the Sun directly, either with or without an optical instrument. Your eyes are very delicate.

Sunspots are dark regions which appear on the surface of the Sun. They are darker because they are cooler than the regions surrounding them. (They are not actually dark at all but only appear so when compared with the rest of the Sun's surface. 5,000K instead of 5,800K.

They are huge, as large as planets. They consist of a darker region, the umbra, surrounded by a less dark region, the penumbra.

They usually appear in pairs, either side of the equator, and usually last from 50 to 100 days.

The Rotating Sun

When one observes the surface of the Sun over several days one sees that sunspots move as shown above, clear evidence that the Sun is rotating. Galileo was one of the first people to notice this. We also notice that the Sun rotates faster near its equator (period = 25 days) than it does near its poles (period = 36 days).

How could you measure how fast the Sun is rotating?

The Sunspot Cycle

The number of sunspots on the surface of the Sun seems to vary over an 11 year cycle from almost zero up to over 100.

Sunspots appear at high latitudes, 35 to 40 degrees, then as the cycle progresses they appear closer to the equator. At solar max they are around 15 degrees. As the cycle fades they move even closer to the equator to about 7 degrees.

The graph above shows how the number of sunspots has varied with time.

         pic NASA

The diagram above is called a butterfly diagram. It shows how the number of sunspots varies with time and how they drift towards the equator throughout the solar cycle.

Using the diagrams above can you predict how many sunspots there will be in 2010?

Scientists now think that this cycle is due to twisting of the Sun's magnetic field. Because the Sun rotates faster near its equator its magnetic field lines become twisted and distorted. Pairs of sunspots are the North and South poles of these disturbances. Charged particles from the Sun travel in huge loops due to these twists in the Sun's field.

(Where the magnetic field exits and enters the Sun there is less convection so it is cooler.)

Imagine twisting rubber bands more and more, eventually they will snap with a huge release of energy in the form of solar flares. This is what happens at "solar max" when the number of sunspots is  maximum. This is usually accompanied by a big increase in the amount of solar wind which arrives at Earth.

After this release of energy the Sun settles down and the cycle starts again.