Astronomy 162: Professor Barbara Ryden

Friday, January 10

THE ACTIVE SUN

``Busy old fool, unruly Sun,
Why dost thou thus,
Through windows and through curtains call on us?''
- John Donne, `The Sun Rising'

Key Concepts

Sunspots occur where the Sun's magnetic field is stronger than average.
The Sun's magnetic field varies on a 22-year cycle.
Magnetic activity can cause solar flares (`gusts' in the solar wind).

Okay, so why IS the corona (the outer layer of the Sun) so darn hot? The standard methods of energy transport (radiative diffusion, convection, and conduction) just don't work well enough to raise the corona's temperature to 2 million Kelvin. For a long time, astronomers are baffled. Now, however, they think that the Sun's magnetic field heats the corona. The Sun's magnetic field also causes energetic, but transient, phenomena such as

sunspots
prominences
solar flares

To see how magnetism can cause such a wide range of effects, it's necessary to look at the Sun's magnetic field in a little more detail.

An aside: Scientists find it useful to describe magnetic fields in terms of the field lines. To visualize magnetic field lines, consider placing a large number of compasses in the vicinity of the Sun. The compass needles will align themselves with the magnetic field lines. (You can also visualize magnetic field lines by scattering iron filings in the vicinity of a bar magnet, as shown in Figure 8-19 of the textbook.) Where the field lines are close together, the magnetic field is strong; where they are far apart, the field is weak. Charged particles, such as electrons and protons, tend to move along field lines rather than perpendicular to field lines. Thus, if a magnetic field exists within an ionized gas, the gas will flow along the field lines. Although the field lines are not physical objects, they do tend to behave in many ways like rubber bands. When they are stretched out of shape, for instance, they tend to snap back to their original shape when the force pulling on them is removed.

(1) Sunspots occur where the Sun's magnetic field is stronger than average

Sunspots are dark spots on the photosphere of the Sun. They are relatively dark compared to the rest of the photosphere because they are relatively cool compared to the rest of the photosphere. The average temperature of the photosphere is 5800 Kelvin; sunspots are at ``only'' 4300 Kelvin. In addition to being relatively cool, sunspots are also highly magnetized (in other words, the magnetic field lines within sunspots are very close together). The magnetic field in a sunspot can be 1000 times stronger than in the surrounding area. It is the presence of a strong magnetic field which keeps a sunspot cool. The closely packed field lines provide a barrier which prevents hot gas from being convected into the sunspots.

The current appearance of the Sun (at different wavelengths) is provided in images from the Big Bear Solar Observatory in California.

1) Every 11 years, on average, the number of sunspots visible on the Sun's surface reaches a maximum (the last sunspot maximum was in the year 2000).
2) Every 11 years, on average, the Sun's magnetic field reverses its polarity. (That is, for 11 years, the Sun's north magnetic pole would be near the Sun's north pole; for the following 11 years, it would be near the Sun's south pole. The Sun's direction of rotation doesn't change, just the orientation of it magnetic field.)

The 11-year sunspot cycle and the 11-year time span between magnetic reversals are linked together.

(2) The Sun's magnetic field varies on a 22-year cycle.

Note: since a compass near the Sun spends 11 years pointing north and 11 years pointing south, the TOTAL length of the cycle (before it returns to its starting point) is 22 years.

The Sun's magnetic field is varying with a 22 year cycle because the Sun is undergoing differential rotation; that is, different regions on the Sun's surface have different rotation periods. Near the equator, the rotation period is 25 days; near the poles, the rotation period is 35 days. The magnetic field lines rotate along with the ionized gas through which they pass in the Sun's outer layers. Since the gas rotates more rapidly near the equator than near the poles, the field lines become more and more tightly wrapped around the Sun as time goes on. (``Like twine wrapped around a ball'', as the textbook says - but they are really more like rubber bands wrapped around a ball.) Eventually, the field lines become so tightly twisted that they develop kinks like a twisted rubber band. Within these kinks, the magnetic field is very strong, and cool sunspots form.

Eventually (after 11 years or so), the field lines are extremely tightly wrapped around the Sun's equator. Kinks of opposite handedness can then bump into each other and cancel each other out. The sunspots disappear, and the whole process can begin again (this time, with the opposite orientation of the magnetic field).

(3) Magnetic activity can cause solar flares (`gusts' in the solar wind).

Sunspots generally occur in pairs, connected by a kink, or loop, of magnetic field lines. When hot ionized gas travels along the looped field lines, it forms an arch-shaped solar prominence. The top of the arch can be as much as 100,000 kilometers above the photosphere.

Sometimes, the field lines near a particularly large group of sunspots can suddenly snap, like overtwisted rubber bands. When this happens, it triggers a solar flare; that is, a sudden eruption of hot ionized gas away from the Sun. A solar flare represents a dramatic `gust' in the solar wind. If the material in the solar flare strikes the Earth, it can create a spectacular aurora borealis. It can also, by heating and expanding the upper atmosphere of the Earth, cause satellites in low Earth orbit to experience increased air resistance and rapid orbital decay.

Oh yes, you probably want to know more specifically how the magnetic field heats the corona. The corona is actually heated by `magnetic waves' traveling along field lines stretching upward from the photosphere into the corona. (Think of a field line as a long rubber band stretching from the photosphere up into the corona. If you waggle the photosphere end back and forth, a wave will travel along the rubber band up into the corona.) As the field lines wave back and forth, they accelerate the ions in the corona to very high speeds, corresponding to very high temperatures.

Prof. Barbara Ryden (ryden@astronomy.ohio-state.edu)

Updated: 2003 Jan 10