Solar Science Terms Active Region: A temporary area of the solar atmosphere in which plages, sunspots, faculae, flares, and other features of the sun can be observed. (go to first use in the text) Archaeoastronomy: The study of the astronomical practices, celestial lore, mythologies, religions and world views of all ancient cultures. Referred to, in essence, as the "anthropology of astronomy," to distinguish it from the "history of astronomy." (go to first use in the text) Aurora: A faint visual (optical) phenomenon on the earth associated with geomagnetic activity, which occurs mainly in the high-latitude night sky. Typical auroras are 100 to 250 km above the ground. The Aurora Borealis occurs in the Northern Hemisphere and the Aurora Australis occurs in the Southern Hemisphere. (go to first use in the text) Convection: The organized flow of large groups of molecules based on their relative densities or temperatures. A hot fluid or gas will move upward, and a cooler liquid or gas will sink downward.. (go to first use in the text) Convection zone: The solar layer just below the photosphere, in which plasmas circulate between the Sun's radiation zone and the solar atmosphere, carrying energy outward. (go to first use in the text) Core: Center of the sun, where gravitational pressure forces hydrogen ions (protons) to fuse into helium, releasing energy in the form of radiation. (go to first use in the text) Corona: The outermost layer of the solar atmosphere, characterized by low densities and high temperatures, often several million degrees Kelvin. (go to first use in the text) Coronal Mass Ejection (CME): Coronal mass ejections are explosions in the Sun's corona that spew out high-energy charged particles. CME's can seriously disrupt the Earth's environment through radiation, which arrives only 8 minutes after being released, and through very energetic particles pushed along by the shock wave of the CME. (go to first use in the text) Correlation: A mutual relationship between two quantities that can be represented in a mathematical or geometric form, such as a straight line or a curve in two-dimensional space. (go to first use in the text) Dynamo: Any mechanism that uses the physical motion of free electric charges to increase the strength of a magnetic field. (go to first use in the text) Flare: A sudden eruption in the vicinity of a sunspot, lasting minutes to hours, caused by the release of large amounts of magnetic energy in small volume above the solar surface. (go to first use in the text) Flux: A measure of the density of magnetic field lines over a surface area.   Flux/area is proportional to the average force on a charged particle on the surface. Gamma Rays: Extremely high-energy radiation observed during large, very energetic solar flares. Gamma rays are more energetic, and have shorter wavelengths than all other types of electromagnetic radiation. (go to first use in the text) Geomagnetic Storm: A worldwide magnetic disturbance. The term was coined by Alexander von Humboldt (1769-1859). After journeying the length of Siberia, Humboldt convinced the Czar to set up a network of magnetic observatories across the Russian lands, and additional stations were established throughout the British Empire, from Toronto to Tasmania. This network clearly showed that magnetic storms were essentially identical all over the world: a steep decrease of the field over twelve to twenty-four hours, followed by a gradual recovery, which lasted one to four days. The change in the magnetic field was small, but its world-wide scale suggested that something quite big was happening out in space. (go to first use in the text) Humbolt, Alexander von: A naturalist who gained attention by exploring the jungles of Venezuela, Humboldt devoted much of his life to the promotion of science. He produced five volumes of "Kosmos" (starting the modern usage of that term), an encyclopedic account covering the broad spectrum of the sciences. It was "Kosmos" that brought to the world's attention the discovery of the sunspot cycle by Heinrich Schwabe. Infrared (IR): The infrared includes electromagnetic radiation at wavelengths just beyond the visible spectrum. Infrared wavelengths are longer than visible radiation and shorter than microwave radiation. Humans perceive infrared radiation as "radiant heat." (go to first use in the text) Kelvin: A temperature scale with the same division as the Celsius (centigrade) scale and with the zero point at 0 degrees absolute. Room temperature is about 295 degrees Kelvin (295K). (go to first use in the text) Magnetic Field: A field of magnetic force lines, usually referred to here as the pattern of magnetic force emanating from and surrounding the sun or any of the planets. (go to first use in the text) Magnetic Pressure:  The resistance of magnetic field to being compressed in space, for instance by the presence of another field. Pressure on a magnetic field crowds field lines together, and the field pushes back with its own pressure. (go to first use in the text) Magnetosphere: The Earth's magnetic field and its area of influence above the atmosphere; Earth's "magnetic atmosphere." Specifically the outer region of Earth's ionosphere, starting at about 1000 km above Earth's surface and extending to about 60,000 km and to at least 100 Earth radii on the side away from the Sun. (go to first use in the text) Maunder Minimum: A period from 1645 to 1715, when the average number of sunspots was unusually low.  It was investigated by E. W. Maunder, and corresponds to a time called the "Little Ice Age", when temperatures were unusually cold. (go to first use in the text) Nuclear Fusion: The joining of atoms under tremendous temperatures and pressures to create atoms of a heavier element. In the Sun, four hydrogen atoms are fused to create each helium atom. Two of the hydrogen's protons become neutrons in the process. (go to first use in the text) Penumbra: The brighter area that surrounds the darker umbra or umbrae at the center of a sunspot. (go to first use in the text) Photosphere: The lowest layer of the solar atmosphere, where the sun's visible spectrum of light (electromagnetic radiation) is released. It is visible "surface" we see in white-light images of the sun. (go to first use in the text) Pixel: A shortened term for "picture element." It is the smallest element that can contain data in a detector or other two-dimensional representation, like an electronic image. (go to first use in the text) Plage: An extended bright area of an active region that exists from the emergence of the first magnetic flux until the widely scattered remnant magnetic fields merge with the background. (go to first use in the text) Plasma: A gas of charged particles, such as electrons and ionized (charge) nuclei, often hydrogen nuclei (protons). This occurs when atoms of a gas are torn apart by high temperatures, pressures, and/or electromagnetic fields. (go to first use in the text) Radiation zone:  Layer just outside the sun's core, where energy is transported mostly in the form of radiation.  This region, while too cool for fusion to occur, is still very dense and hot- about 4 million degrees Kelvin. (go to first use in the text) Solar Maximum: The month(s) during the Solar Cycle when the twelve-month mean (average) of monthly average sunspots numbers reaches a maximum. The most recent solar maximum occurred in July 1989. (go to first use in the text) Solar Minimum: The month(s) during the Solar Cycle when the twelve-month mean (average) of monthly average sunspot numbers reaches a minimum. The most recent minimum occurred in September 1986. (go to first use in the text) Solar Wind: The outward flow of plasma (high energy charged particles from the sun. Average speeds are about 350 km/sec. (go to first use in the text) Sun: One of the 100 billion stars in our galaxy. 1,390,000 km diameter. Temperature at the core: 15,600,000 K. Temperature at the surface: 5800 K. Sunspot: A temporary concentration in the magnetic field on the sun, where convection of hot matter from the sun's interior is inhibited, resulting in a cooler, darker area on the photosphere of the sun. The average sunspot is about the same diameter as the earth. ( go to first use in the text) Sunspot Cycle: Regular increase and decrease of sunspots and other solar activity, which are thought to be physically related. Sunspots go through one cycle of activity in approximately 11 years. The Sun's magnetic polarity reverses between every cycle. (go to first use in the text) Ultraviolet: Electromagnetic radiation at shorter wavelengths and higher energies than the violet part of visible light. (go to first use in the text) Umbra: The term for the dark area at the center of a sunspot. (go to first use in the text) X-rays: Electromagnetic radiation of very short wavelength, and very high energy. X-rays have shorter wavelengths than ultraviolet light, but longer wavelengths than gamma rays. (go to first use in the text) ©Copyright 2001 Regents of the University of California.