5. A Magnetic Star

    • The visible photosphere, or sphere of light, is the part of the Sun we can watch each day. It is the level of the solar atmosphere from which we get our light and heat.

    • The sharp edge and apparently solid surface of the photosphere are illusions, caused by the high opacity of negative hydrogen ions in the photosphere. That is, the visible edge of the Sun is merely the level beyond which the gas in the solar atmosphere becomes thin enough to be transparent.

    • Being entirely gaseous, the Sun has no solid surface and no permanent features.

    • The photosphere contains dark, ephemeral spots of intense magnetism, called sunspots.

    • Solar magnetism is found in varying spots all over the Sun.

    • The largest sunspots are Earth-sized islands of magnetism, with magnetic fields thousands of times stronger than the Earthís magnetic field.

    • The intense magnetism in sunspots chokes off the outward flow of energy in them, making sunspots dark and cool in comparison to their surroundings.

    • The size of the Zeeman splitting of a spectral line gives the strength of the magnetic field where the line originated, and the polarization of the split lines gives the local magnetic field direction, in or out of the Sun.

    • Magnetograms are used to portray the strength and direction of the magnetic fields across the visible disk of the photosphere.

    • On a magnetogram, black is for magnetic fields pointing in, white for fields that point out, and the intensity of black or white portrays the strength of the local magnetic field.

    • The diaphanous solar atmosphere includes, from its deepest part outward, the photosphere, chromosphere and corona.

    • The chromosphere and corona are so rarefied that we look right through them, just as we see through the Earthís clear air.

    • Just above the photosphere lies the relatively thin chromosphere, first observed during a total solar eclipse as a narrow, rose-colored band at the Sunís apparent edge.

    • Helium was discovered as an emission line in the chromosphere during a solar eclipse in 1868.

    • The chromosphere has a temperature of about 10,000 kelvin.

    • The chromosphere is observed across the face of the Sun by tuning into the red light of the hydrogen alpha transition, revealing bright regions called plage, long dark filaments that snake across the Sun, and bright looping prominences at the Sun's apparent edge or limb.

    • A prominence and a filament are both magnetic loops filled with relatively cool, dense material. They appear dark against the bright solar disk, but they appear bright at the edge of the solar disk.

    • The chromosphere is also observed in the light of calcium emission lines, revealing a magnetic network, which coincides with the supergranulation convective cells that sweep magnetic fields to their edges.

    • Sunspots tend to travel in pairs of different magnetic polarity that point in opposite directions, in or out of the Sun.

    • Bipolar sunspot pairs are joined by magnetic loops that shape, mold and constrain the hot material above sunspots.

    • The magnetized realm in, around and above bipolar sunspot groups is a disturbed area called a solar active region, the seat of unrest on the Sun.

    • Magnetic fields that are generated inside the Sun loop through the photosphere and overlying solar atmosphere. These coronal loops constrain high-temperature, ionized gas that can be detected by its extreme-ultraviolet and X-ray radiation.

    • The number and position of sunspots varies over an 11-year cycle of solar magnetic activity, discovered by Samuel Heinrich Schwabe in 1843.

    • Active regions are formed in two belts parallel to the solar equator, one each in the northern and southern hemispheres.

    • According to Haleís law of polarity, the bipolar sunspots in one hemisphere have the same polarity alignment, with an opposite alignment in the northern and southern hemispheres.

    • The Sun switches its north and south magnetic poles every 11 years, returning to the same polarity every 22 years.

    • The Sunís 11-year magnetic activity cycle is attributed to the winding up of internal magnetic fields that emerge into the outer solar atmosphere to form varying magnetic loops.

    • The Sunís magnetism is generated by the moving charged particles and currents located at the tachocline just below the convective zone. This internal dynamo generates, amplifies and sustains the Sunís magnetic field.

    • Scientists still have difficulty in explaining precisely how the Sunís intense magnetism is generated, and why solar magnetic activity varies over an 11-year cycle.

Copyright 2010, Professor Kenneth R. Lang, Tufts University