The sun’s magnetic field and the sunspot cycle
Sunspots
A photograph of the Sun’s surface will usually show some darker regions on the surface. These are called sunspots and often appear in pairs or groups — a sunspot group. Each spot has a central ‘umbra’ surrounded by a lighter ‘penumbra’. They appear dark because they are cooler than the average surface temperature. The umbra has a typical temperature 1000 K less than its surroundings. Around the outside of a sunspot group may be seen an area which is brighter than the normal surface. This is called a ‘plage’ — the French word for ‘beach’. (French beaches often have white sand!) The passage of sunspots across the Sun’s surface has been used to measure its rotation period. It has been found that at the equator the period is ∼28 days, but this increases with increasing latitude and is ∼35 days near the Sun’s poles, an effect called differential rotation. Sunspots are intimately linked to this differential rotation and how it affects the Sun’s magnetic field. The Sun’s field must be created by the movement of charged particles and its cause almost certainly lies in the convective zone, but no really good model yet exists. Imagine that at one particular moment the Sun has a uniform bipolar field just like that shown by iron filings under the influence of a bar magnet. Looking at the Sun’s face we could imagine a field line just below the photosphere directly down the centre of the Sun’s disc. The field and surrounding material remain locked together in what follows. Now move forwards in time by 35 days. The field close to the poles will have made one complete rotation and be in the same position as seen from Earth but close to the equator it will have rotated an additional amount ∼(35 28)/28 of one rotation or 1/4 360°, which is 90°. After three further rotations (as measured near the poles) the field near the equator will have one additional rotation. You can see that the field is being ‘wound up’ and becomes more intense. It gains ‘buoyancy’ and, in places, rises in a loop above the surface. Where it passes through the surface, the magnetic field inhibits the convective flow of energy to the surface so the localized region will be cooler than the surface in general — a sunspot appears. The energy that is inhibited from reaching the surface here will tend to reach the surface in the area surrounding a sunspot group making this region hotter and thus brighter — a plage.
It is possible to measure the polarity of the field across the Sun’s surfaces and we can examine the polarity of the field associated with sunspot pairs.
Consider a sunspot pair in the upper hemisphere. Assuming that the north pole is at the top then, as the field leaves the surface, it will be towards us and have positive polarity. As the fi eld re-enters the surface it will be away from us and have negative polarity. So the spots in a sunspot pair will show opposite polarity. In the upper hemisphere the left-hand spot would have positive polarity and the right-hand negative polarity. However, as the field reverses direction at the equator the sunspots in a pair observed in the lower hemisphere will have the opposite sense. The twisting of the magnetic field from its initial state gradually produces an increase in the number of sunspots observed across the Sun’s surface. This determines what is termed the sunspot number which reaches a peak after 3–4 years —
called sunspot maximum. The field then begins to reduce in strength and the sunspot number reduces for a further 7–8 years to a point when the Sun’s face can be totally devoid of spots — called sunspot minimum. (This was the case during 2007 close to sunspot minimum.)
derived from a SOHO space observatory image, ESA, NASA.
The sunspot cycle
The whole process then starts over again and is called the sunspot cycle. It is often said to be an 11 year cycle, though it can vary in length somewhat and the average length of the cycle over recent decades is 10.5 years. The following cycle has, however, one important difference; the field has the opposite polarity. Hence, perhaps we should call it the 21 year solar cycle instead. The cyclical variation is very apparent. The plot shows two interesting features: a complete lack of sunspots during the late 1600s, called the Maunder minimum; and increasing solar activity during the last 50 years. It is interesting to note that the River Thames regularly froze during the ‘mini ice age’ at the time of the Maunder minimum and the Earth’s temperature is now rising. Could it be that there is some connection between solar activity and global temperatures? However, it should be noted that though solar activity has fallen somewhat in the last 20 years the Earth’s temperature is still rising so the correlation has now been broken.
