Solar weather and magnetic field
So far in this chapter the Sun has been considered as a static ball of hot gas: energy is created in the core, and streams out to space through the convection zone, photosphere, and solar atmosphere. We might expect that the release of energy from the surface regions should be uniform over the solar disc. Although standard evolutionary models account for the Sun in terms of normal stellar processes, no account is taken of the weather, or activity, in the Sun’s outer layers, which we can see because of our proximity. Even violent solar storms only involve 10~6 of the solar luminosity, so that the weather has no significant effect on the standard models of evolution. Nonetheless the displays in the solar atmosphere are scientifically interesting, frequently of dramatic beauty, and often affect the Earth. The study of solar activity includes a description of solar magnetism, sunspots, prominences, and flares. The variety of phenomena is considerable, and it would be confusing to discuss all of them in detail here.

It is widely believed that solar activity is stirred up by the inter¬play between the solar MAGNETIC FIELD and the Sun’s DIFFERENTIAL ROTATION. At its surface the Sun has a general magnetic field that is approximately like a dipole and of strength 10-4 tesla (1 gauss). The Earth has a similar dipole field with a strength of 6 x 10-5 tesla (0.6 gauss). Symmetry and a precisely-defined axis are lacking. The weak surface field does not arise from a dipole inside the Sun; it results from many localized surface fields.

The Sun does not rotate rigidly like Earth; this is not surprising since we know that the Sun is gaseous throughout. The polar regions of the photosphere take 37 days to rotate once with respect to the distant stars, whereas the equator takes 26 days. As observed from Earth, which is moving around the Sun, the corresponding synodic periods are 41-27 days. Several authorities quote rotation periods which are slightly shot-tor than these. The periods given here are based on the observed Dopplor shifts of photospheric lines and not, as is more common, on the observed rotation rate of sun-spots, which rate is probably influenced by the Sun’s internal magnetic field. The faster rotation indicated by the magnetic field may be caused by a rapidly rotating core. The Sun is said to rotate differential because the material at progressively lower latitudes takes progressively less time to make one circuit

It is possible that the non-rigid rotation of our Sun is caused by rapid rotation of the solar interior, which would lead to a shear stress, and swifter rotation, at the equator of the visible surface. Extremely delicate measurements made in 1970 showed that the Sun is not precisely spherical; rather it is oblate, the deviation being of order 0.005 per cent, or about five times as large as one would predict for a core rotating in about 30 days. This observation may also be accounted for in terms of the centrifugal force present in a rapidly rotating solar core. However, this controversial measurement has not been confirmed independently so caution must be exercised in drawing any conclusion.

We now consider the relationship between the Sun’s magnetism and its differential rotation. First it is essential to emphasize that matter in the Sun is a good conductor of electricity: most of the particles, being free electrons or ions, are electrically charged. When the electrical conductivity is large, as it is below the photo¬sphere, it is not easy for matter to move relative to a magnetic field. If it attempts so to do, secondary magnetism results from induced currents; the resulting magnetic force opposes the motion (this is Lenz’s law of magnetism). Consequently the field m a plasma, which is a good conductor, becomes ‘frozen-in’, and is largely constrained to move with the plasma. This situation is pertinent to the solar interior. Because the Sun rotates differentially, the frozen-in field lines get progressively stretched out by
the shear in the rotation. The shearing action winds up the internal magnetic field, steadily increasing its strength.

If we imagine starting this process with a simple dipole field, we can visualize that it evidently gets twisted and amplified into an intense toroidal field, tightly encircling the Sun at lower latitudes. We can anticipate that eventually the density of field lines becomes sufficiently great for magnetism occasionally to burst out of the interior, on account of the repulsion between adjacent lines of the same polarity. Once the strength of the field exceeds about 1 tesla (104 gauss) it exerts a force on the surrounding plasma that exceeds the Sun’s gravitational attraction. Gas in the vicinity that contains charged particles becomes buoyant and therefore aids the process. Another effect is of adjacent magnetic field lines ‘reconnecting’, or short-circuiting the magnetism.

Observations show that solar activity is cyclic, with a characteristic time of 22 years. Over this period the internal magnetism rises to a crescendo and then collapses; this takes about 11 years. After the decrease the solar field reverses its polarity, and during the second 11-year period differential rotation winds the strength up once more, until the new field also short-circuits and a further reversal follows. It therefore takes 22 years for the Sun to return to its original magnetic state, although most of the observable effects of the process repeat at roughly 11-year intervals.

During each 11-year cycle, magnetic energy builds up as the rotation amplifies the field. This energy is released through sun-spots, solar active regions, and solar flares; the latter provide the most violent release. After the peak the field dies away and reverses, and the memory of the magnetic field during earlier cycles is soon erased. Superposition of weak fossil fields from old disturbed regions probably accounts for the weak general field of the solar surface.

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