(S-3) The Magnetic Sun

Sunspots

Galileo and Christopher Scheiner observed dark spots on the face of the Sun, and from their motion they deduced that the Sun rotated, with a period of 27 days close to the equator, relative to the moving Earth (25 days, relative to the stars). The period increased to about 29.5 days at higher latitudes, showing the Sun's surface was not solid.

The sunspots, they guessed, were clouds floating above the surface, blocking some of the sunlight. We now know that sunspots are darker than their surroundings because they are moderately cooler, since their intense magnetic fields somehow slow down the local flow of heat from the Sun's interior. The process which causes this is still unclear.

            What is a "magnetic field," anyway?

What follows below is a brief summary of magnetism; more details can be found on the files linked below, all of them parts of the web site

Magnetism

The magnetism of rare natural "lodestones" was known in ancient Greece--supposedly first noted in the town of Magnesia, from which comes the name. The magnetic compass (a Chinese discovery) was used by Columbus and other early navigators, but it was not until 1820 that a Danish professor, Hans Christian Oersted (pictured on the left), found by accident that an electric current in a wire could deflect a nearby compass needle (click here for the full story). A Frenchman, André-Marie Ampere, showed soon afterwards that the basic magnetic phenomenon was the force between two electric currents in parallel wires; they attracted each other when they flowed in the same direction, and repelled when they were opposed (click here for a more detailed discussion).

Just as lines of latitude and longitude help us visualize positions on the Earth's globe, so magnetic field lines (originally named by Michael Faraday lines of force) help visualize the distribution of magnetic forces in 3-dimensional space. Imagine a compass needle which can freely turn in space to wherever the magnetic force tries to point it. Magnetic field lines are then imaginary lines which mark the direction in which such a needle would point.

A compass needle, for instance, has two magnetic poles at its ends, of equal strength, the north-seeking (N) pole and the south-seeking (S) pole, named for the directions on Earth to which they tend to point. Suppose the needle is free to point anywhere in 3 dimensions. If placed near the north pole, it would everywhere point towards the pole, and field lines therefore converge there (see drawing). If placed near the south pole, it would point away from it in all directions, and therefore field lines would diverge there, coming out of the Earth in a pattern that is a mirror-image of the pattern at the north pole. In between the lines form big arches above the Earth's equator, with their ends anchored in opposite hemispheres.

Any bar magnet has a pattern of field lines like that of the Earth, suggesting that the Earth acted as if a short but very powerful bar magnet was inside it. Actually such a magnet does not exist, and the pattern comes from electric currents in the Earth core, and slowly changes, year by year; still, the "terrestrial bar magnet" remains a useful visualization aid.

When two bar magnets are brought together, their (N,S) poles attract each other, their (S,S) and (N,N) poles repel: thus if a bar magnet were hidden inside the Earth, its S pole would be the one that pointed northwards, attracting the N pole of the compass needle. This strange mix-up of terminologies often confuses students: it is best to recognize the mix-up exists and then to ignore it.

  Michael Faraday who in the early 1800s introduced the concept of magnetic field lines, believed that space in which magnetic forces could be observed was somehow modified. His was a somewhat mystical view, but later mathematical developments found it quite useful, and today we refer to such a region of space as a magnetic field.

The Sunspot Cycle

Ever since then solar observers have carefully followed sunspot cycles, and have also reconstructed earlier cycles from available observations, defining a suitable "sunspot number" index to gauge the level of sunspot activity. The nature of sunspots remained unclear until 1908, when George Ellery Hale, using an instrument that observed the Sun in narrow ranges of color emitted by selected substances, reported that the light from sunspots was modified in ways that indicated it was produced in intense magnetic fields.

Sunspot fields turned out to be as intense as the ones we find near the poles of iron magnets--but extending across regions many thousands of kilometers wide. In conventional units, the magnetic intensity intensity in them reached about 1500 gauss (0.15 Tesla), while the field near the surface of Earth is typically 0.3-0.5 gauss, depending on location. In interplanetary space at the orbit of Earth, the magnetic field (carried from the Sun by the solar wind) is much weaker, typically 0.00006 gauss, while at the orbits of the outer planets, it is 20 times weaker still. Yet even there the instruments of spacecraft such as Voyager 2 still measure it reliably.

Sunspots display many interesting features. Generally (though not always) they appear in pairs, with opposite magnetic polarities. In half the solar cycles, the "leading" spot (in the direction of the Sun's rotation) will always have an N polarity, and the "following" spot an S polarity; then in the following cycle, polarities are always reversed. The general magnetic field, producing the Sun's north and south magnetic poles, also reverses polarity at each cycle, the reversal typically occuring 3 years after sunspot minimum. All these suggest that the 11-year cycle is a magnetic phenomenon. Astronomers believe that the electric currents which flow in the solar plasma and create those fields get their energy from the unequal rotation of the Sun--faster at the equator--which in its turn is driven by large-scale flows of the solar gas.

Solar Activity

The fastest and most significant among these was the solar flare--a brightening of the chromosphere near a sunspot group, rising within minutes and typically lasting 10-30 minutes.

Flares are usually observed in the red light emitted by hot hydrogen (Ha or "H-alpha line"), but it so happened that the first observation in 1859 was of a rare "white light flare" observable with an ordinary telescope (see here and here for the full story). This was followed 17 hours later by a huge magnetic storm, a world-wide disturbance of the Earth's magnetic field: something apparently was ejected from the Sun, and took that long to reach Earth.

We now know that "something" was probably a fast-moving plasma cloud, plowing through the ordinary solar wind, which takes 4-5 days to cover the same distance. The arrival at Earth of such clouds, with the shock wave that forms ahead of them, can be quite dramatic (see here for one story).

The most remarkable aspect of such activity is the speed with which it takes place. If a typical big flare spreads over 10,000 km in 10 minutes, it must propagate quite rapidly. Some of its features begin much more abruptly, e.g. the associated x-rays (observable from space) can rise in just a few seconds. All this suggests that the energy source is not the heat of the Sun, which spreads and changes rather gradually, but the intense magnetic fields of sunspots. (For more on these matters, see here.)