Introduction: some basic solar facts and terminology

Nearly 1,392,000 km in diameter and kg in mass, the Sun is the geometric and gravitational center of the planetary system. The Earth, third planet 149,600,000 kilometers away from center, remains to date the only planet in the solar system known to host life forms. The energy radiated by the Sun ultimately is the source of all life on Earth, a fact intuitively grasped by the majority of early civilizations, most of which accordingly granting the Sun a place of prominence in their respective religious practices. The Watt of energy radiated by the Sun originate in its deep interior, where thermonuclear fusion reactions combine hydrogen nuclei into helium nuclei. The energy released by these nuclear reactions is carried outward in about yr, first by radiation from the center to about 70 percent of the Sun's radius, then to the surface primarily via large-scale convective motions. This latter region, comprising the outer 30 percent in radius of the Sun, is known as the convection zone. Temperatures in the center of the Sun approach 15 million degrees Kelvin, falling to a mere degrees (!) at its surface. Because of these high temperatures, solar material is in a state called plasma, which refers to a gas of ionized atoms. In the case of the Sun, however, densities become high enough (particularly in the deeper solar interior) that the solar plasma behaves more like a fluid than a conventional gas. When speaking of the solar surface, one is then not referring to a solid surface such as provided by the Earth's crust, but rather to the photosphere, a fictitious spherical surface from which the bulk of solar radiation originates. The solar atmosphere refers to the region extending upward from and including the photosphere.

Sunlight passing through a glass prism or diffraction grating is decomposed into its constituent colors, which make up the solar spectrum. At first glance the solar spectrum appears characteristic of a body heated to a temperature of 5800 degrees Kelvin (5530 degrees Celcius). The portion of the spectrum visible to the human eye consists of the continuum of colors violet---blue---green---yellow---red, mapping onto the wavelength range 4000---7000Å (Åcm) collectively referred to as white light. The Sun also emits radiation at shorter (ultraviolet, X-ray) and longer (infrared, etc.) wavelengths, but the solar radiative output is most intense in the visible portion of the spectrum. Further scrutiny of the solar spectrum reveals the existence of narrow, dark bands where the intensity of sunlight is greatly reduced. These are known as spectral lines, and are associated with allowed electronic transitions in atoms present in the solar atmosphere; sunlight, traversing the atmosphere from below, is preferentially absorbed and scattered away from the line of sight at these wavelengths, leading to reduced intensities in the net outgoing spectrum. Because each chemical species is characterized by a different set of allowed electronic transitions, identification of spectral absorption lines in the solar spectrum allows the determination of the chemical composition of the Sun's atmosphere. Photographs of the Sun taken through narrow filters centered on strong spectral lines, because of the greatly reduced background brightness, often reveal details lost in white light. Other spectral lines are formed through collisional processes (rather than by radiative excitation, as in the case of absorption lines) processes, and so can be used as indicators of non-radiative effects in the solar atmosphere. This slide set includes many such images, taken with filters centered on the K line of neutral Calcium (Å) and of the first Balmer line of Hydrogen (H, Å).

The apparent daily path of the Sun in the Earth's sky is from East to West. In reality, of course, this motion of the Sun is only apparent and is due to the Earth spinning on its axis in the opposite direction. This direction of rotation ---counterclockwise for an observer in space looking down on the North pole--- is the same as the direction of the Earth's orbit around the Sun, and of the spin of the Sun around its axis. With the direction of rotation defined as East to West by convention, solar rotation produces a displacement of features on the solar disk that proceeds from left to right as seen from the Earth and with the solar North pole oriented towards the top of the field of view (which is the case for all slides in this set). Following the motion of features on the solar surface is complicated by the fact that the Sun does not rotate as a solid body; a fluid parcel located in the equatorial regions completes a revolution in days, while in polar regions a revolution requires some 30 days. Measurements of the solar rotation rate at high heliospheric latitudes are arduous at best, so that the derived polar values consequently carry significant uncertainties; that the solar poles rotate at least % slower than the equator is nevertheless well established. This pattern is known as differential rotation, and persists throughout the solar convection zone. Finally, one must realize that because the Earth moves through of its orbital path in 25 days, the equatorial solar rotation period as seen from the Earth (the synodic period) is longer than its true (sidereal) period by about two days. We are now ready to turn to the slides.