The Structure of the Sun: The Convection Zone and the Photosphere.
Direct evidence for the size scales, velocities, and shapes of solar convective scales can be deduced from observations of convectionlike cellular motions at the photosphere. Small-scale cells called granules are about 1,000 km (620 mi) in diameter and are formed by hot up-flowing gases, surrounded by cooler down-flowing gases, moving about 1 km/sec (2,200 mph). Supergranules form a larger set of polygonal cells, of diameter roughly 30,000 km (18,600 mi), detected by their horizontal velocities of about 0.5 km/sec (1,100 mph).
In addition to transporting heat, convective motions of the Sun's gases are also thought to have important consequences for solar rotation, solar magnetism, and for the structure of the Sun's outer layers above the photosphere. Convection may help to explain the observation that the gases of the solar photosphere do not rotate rigidly - the angular rate at the equator is some 50% higher than the rate at latitudes of ± 75 degrees. Although a satisfactory theory of this basic solar property does not yet exist, models of the fluid mechanics of rotating, convecting shells indicate that such velocity differences might result from the forces exerted upon rising and falling convecting gases as the Sun rotates about its axis at the observed sidereal rate of about 25 days at the solar equator. The angular rotation rate also appears to increase inward, at least immediately below the photosphere, at a rate of 5% in the first 15,000 km (9,300 mi).
The Sun's magnetic field, observed at the photosphere, does not have the basic north-south dipole symmetry observed in the terrestrial magnetic field at the Earth's surface. The solar field lines seem to be wound around the Sun's rotation axis and roughly follow lines of constant latitude, rather than longitude. This property is inferred from the observed alternation of magnetic polarity in bipolar sunspot groups. The magnetic dipole axes of such groups tend to be oriented east-west, and within a given hemisphere (above or below the solar equator) the western half of all dipoles is generally of the same magnetic polarity. The polarity of dipoles in the northern and southern hemispheres is opposite. This law of alternation of polarities is called the Hale-Nicholson law.
The plasma of the solar convection zone is about as good a conductor as copper wire under room-temperature conditions. When a large volume of this material moves through a magnetic field, as in solar convection, it induces a large electric current that deforms the original field so as to displace it along with the motion. The mutual influence of magnetic fields and moving plasmas is known as magnetohydrodynamics (MHD). MHD studies show that the Sun's differential rotation will tend to stretch and pull out magnetic-field lines into the observed toroidal geometry.
Near the photosphere the known temperature, the mean molecular weight, and the acceleration of solar gravity indicate that the density decreases hydrostatically at the rapid rate of a factor of ten roughly every 1,000 km (620 mi) radially outward. This rapid decrease explains the sharp edge or limb of the Sun, even when seen with telescopes, because the shell in which the gas passes from being opaque to transparent is less than 1,000 km (620 mi) thick and subtends less than 1 arc second as viewed from the Earth. When looking at the center of the Sun's disk, it is possible to see deeper into the absorbing solar atmosphere than when looking toward the limb, where the line of sight is more nearly tangent to the photosphere. Because the temperature increases inward below the photosphere, the line of sight toward the center of the disk sees hotter, and thus brighter, layers. This phenomenon explains the prominent limb darkening seen in pictures of the photosphere.
A spectrogram of the solar light shows a bright background continuum traversed by many dark absorption lines. The continuum radiation that is visible to the eye, roughly between 4000 A and 7000 A, is emitted when electrons released from the relatively easily ionized heavy elements are captured by neutral hydrogen atoms. The dark Fraunhofer lines, such as the H and K lines of ionized calcium, are formed when light of certain discrete wavelengths is preferentially scattered by the particular species of neutral atoms or ions that are abundant at the density and temperature of the photosphere. The light emerging through the photosphere at these wavelengths is changed in frequency by multiple scattering of the photons from atoms and rapidly moving electrons, and is emitted instead in the continuum.