Making sense of the solar cycle

THE NUMBER OF SUNSPOTS increases and decreases over time in a regular, approximately 11-year cycle, called the sunspot cycle. The exact length of the cycle can vary. It has been as short as eight years and as long as fourteen, but the number of sunspots always increases over time, and then returns to low again.

Sunspots look dark only because they’re a bit cooler than the surrounding solar surface. In reality, sunspots are still intensely hot—around 3,000 to 4,000 degrees Celsius.

More sunspots mean increased solar activity, when great blooms of radiation known as solar flares or bursts of solar material known as coronal mass ejections (CMEs) shoot off the Sun’s surface.

The highest number of sunspots in any given cycle is designated “solar maximum,” while the lowest number is designated “solar minimum.”

A sunspot group

Sunspot numbers climb and fall over an 11-year cycle.

Each cycle, varies dramatically in intensity, with some solar maxima being so low as to be almost indistinguishable from the preceding minimum.

The last solar maximum was in the year 2000, and the next is expected to occur in early 2013.

Solar cauldron

Sunspots are a magnetic phenomenon. The entire Sun is magnetised with a north and a south magnetic pole just like a bar magnet. The comparison to a simple bar magnet ends there, however, as the Sun’s interior is constantly on the move.

By tracking pressure waves that course through the centre of the Sun, an area of research known as helioseismology, scientists can gain an understanding of what’s deep inside the Sun.

They have found that the magnetic material inside the Sun is constantly stretching, twisting, and crossing as it bubbles up to the surface.

The exact pattern of movements is not conclusively mapped out, but over time they eventually lead to the poles reversing completely.

The Sun flips out

The sunspot cycle happens because the magnetic poles flip—north becomes south and south becomes north—approximately every 11 years. Some 11 years later, the poles reverse again back to where they started, making the full solar cycle actually a 22-year phenomenon.

The Sun behaves similarly over the course of each 11-year cycle no matter which pole is on top, however, so the shorter cycle tends to receive more attention.

Adapted from information issued by NASA / Goddard Space Flight Centre. Images courtesy NSO and SOHO (ESA & NASA).

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