(S-3) The Magnetic Sun

Sunspots

    Around 1610, soon after the telescope first became available, three independent observers--Galileo, Galilei, Johann Fabricius and Christopher Scheiner--used it to observe dark spots on the face of the Sun. 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. (For a bmore detailed view of a sunspot, see

    The sunspots, Galileo guessed guessed, were clouds floating above the surface, blocking some of the sunlight from reaching us. 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?

    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 (such needles exist--see bottom of this web page). Magnetic field lines are then imaginary lines which mark the direction in which such a needle would point.

  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

    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 below the surface.

Variation of the solar constant, as observed by various spacecraft. Image provided by Davos Observatory, courtesy of Claus Fröhlich

Sunspots and the Sun's Energy Output

    Ever since the solar cycle was first discovered, people had tried to associate it with other periodic observations. The orbital period of Jupiter is close to 11 years, but unlike the sunspot cycle it has an exact value, and on the long run does not fit. Large magnetic storms do tend to occur near sunspot maximum, and so do energetic solar disturbances, which are being constantly monitored. Alas, we can only vaguely guess what goes on beneath the visible surface of the Sun.

    For a century and a half people looked for a correlation between sunspots and weather (also using tree-rings as evidence for the distant past) and failed to find any. However, sunlight energy-flow is notoriously hard to measure accurately. Even on clear mountain tops the observer is below the ozone layer which absorbs some, and the blue color of the sky shows some light is scattered, and of that, not all ends up going downwards. In the infra-red region, it is hard to separate radiation from the cooling Earth (scattered by greenhouse gases) from the one coming from the Sun.

    As example of this problem, have you ever noticed that we are 3% closer to the Sun in January than in July, because of the eccentricity of the Earth's orbit? The difference has been invoked in a theory of ice ages, and observations of the solar constant" in clear air have detected it. However, its variation during the solar cycle amounts to only 0.2%, and to observe it (see graph above) scientists needed observations from spacecraft outside the atmosphere.

    Such observations have now been conducted for about 30 years. Naively, one might expect less sunlight when some of the solar surface is darkened by sunspots. Actually, years of the most sunspots are also peaks of the solar energy output. Perhaps this is caused by increased output in ultra-violet and x-rays in such years, or perhaps heat from the regions beneath sunspots is diverted to neighboring areas of the Sun and radiated from them. Alas, we can only vaguely guess what goes on beneath the visible surface of the Sun.

    Note on Solar Magnetic Fields
    The fact that sunspots were intensely magnetic was evidence that motions in conducting fluids (like the Sun's plasma) could generate electric currents, whose magnetic field also maintain the same currents. This made Earth scientists realize that perhaps a similar "fluid dynamo" operated in the Earth's liquid core, and was the cause of the Earth's magnetic field (rather than some strange sort of permanent magnetism). Today the "dynamo theory" behind such processes is well developed, for both Earth and Sun; for more see "The Great Magnet, the Earth," home page http://www.phy6.org/earthmag/demagint.htm.

Solar Activity

    Flares are usually observed in the red light emitted by hot hydrogen (Hα 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.)

Exploring further

    A small bar magnet, on gimbals that allow it to point in any direction in space, can be procured from its manufacturer, Cochranes of Oxford, Ltd., Leafield, Oxford OX8 5NT, England, or from other sources. For more details search on the web for "Magnaprobe"
        Some spectacular sunspots are shown here.