Instant expert solar cycles

Everything you need to know about this phenomenon in five minutes

Sunspots, flares, the Sun’s hot corona and chromosphere, the solar wind and the Earth’s aurorae all result from magnetic fields generated by a dynamo operating inside the Sun. The dynamo is cyclic, with a solar cycle period of about 11 years. To understand the cycle we adopt a strategy similar to crime detectives, but instead of ‘follow the money’, we follow the energy. Our story begins in the solar core where nuclear fusion converts hydrogen to helium, releasing energy and heating the core to about 15 million degrees Kelvin. This energy is carried outwards by radiation for the first 70 per cent of the distance to the surface, after which it is carried by convective motions. These convective motions are driven by the temperature difference; because the viscosity inside the Sun is very low, the motions are turbulent.

The convective motions interact with the rotation of the Sun, which rotates once per 25 days at its equator. Importantly this interaction between the small-scale convective motions and the large-scale rotation leads to large-scale stresses and causes the poles to rotate slower than the equator at a rate of about once every 33.5 days.

To understand the solar dynamo we can think of a piece of string – a magnetic field line – threading from the north pole of the Sun through the convecting region to the south pole. The piece of string at the poles will rotate once every 33.5 days, however, the piece of string near the equator will rotate once every 25 days, and the string will, over the course of several years, get wound up. This winding up is half of the dynamo process.

The second half happens when the field becomes sufficiently strong to erupt to the surface. During the eruption process the rising flux tube is affected by the rotation of the Sun due to the Coriolis force, and undergoes a ‘tilt’ so that it emerges with a small north-south separation between its positive and negative magnetic polarity. The combined north-south separation from many active regions builds up over an additional several years to create a new polar field with the opposite polarity to that which was originally there. This is one solar cycle, which lasts for about 11 years.

Cycle 23 minimumAround the year 1997 the Sun had reached the minimum activity for the 23rd cycle. At this point the magnetic fields are relatively settled, and this is shown visibly with a low number of sunspots present of the face of the Sun. The magnetic field will slowly become more tangled as the unequal surface rotation takes its toll.1999200020012002199820031997 Cycle 23 maximumThe maximum sunspot activity for cycle 23, around the year 2001, was far greater than its succeeding cycle, which is a strong indication that the field lines are at their most tangled. This will subsequently cause more sunspots and more solar activity, for example more Coronal Mass Ejections (CME) will occur and the Sun will eject more highly energetic particles, powering Earth’s aurora and creating a marvellous spectacle.19962004