ACRIMSAT Science and Mission Overview
The ACRIMSAT spacecraft carrying the ACRIM III instrument will measure the sun's total energy output, continuing the database started in 1980 by ACRIM I on the Solar Maximum Mission (SMM). ACRIM I was the first instrument to clearly show that the energy from the sun is not a constant value but instead varies over time. These energy changes are small but significant and they cycle approximately every 11 years. ACRIMSAT is the third ACRIM mission to measure Total Solar Irradiance (TSI) and this vital data set will help climate scientists build more accurate climate models.
SUN SPOTS
The sun produces energy by converting hydrogen to helium in a process called nuclear fusion. The change in the sun's output of energy is related to the number and intensity of SUN SPOTS. Sunspots are a feature of the Sun that have been observed since ancient times. The ACRIM I instrument clearly demonstrated that the total irradiance that reaches the earth decreased during periods when sunspots are seen on the surface of the sun. When viewed through a telescope, they have a dark central region known as the umbra, surrounded by a somewhat lighter region called the penumbra. Sunspots are dark because they are cooler than the surrounding photosphere. They are the site of strong magnetic fields. The reason sunspots are cool is not entirely understood, but one possibility is that the magnetic field in the spots inhibits convection underneath them. Sunspots typically grow over a few days and last anywhere from a few days to a few months. Observations of sunspots first revealed that the sun rotates with a period of 27 days (as seen from Earth).
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SOLAR CYCLE
The number of sunspots on the Sun is not constant. In addition to the obvious variation caused by the Sun's rotation (sunspots disappear from view and then re-appear), over time new sunspot groups form and old ones decay and fade away. When viewed over short periods of time (a few weeks or months), this variation in the number of sunspots might seem to be random, but the long term data shows that the 11- year sunspot cycle is related to a 22-year cycle for the reversal of the Sun's magnetic field.
While the cycle has been relatively uniform this century, there have been large variations in the past. From about 1645 to 1715, a period known as the Maunder minimum, apparently few sunspots were present on the Sun and colder weather resulted on Earth.
Although the number of sunspots is the most easily observed feature, essentially all aspects of the Sun and solar activity are influenced by the solar cycle. Because solar activity (such as coronal mass ejections) is more frequent at solar maximum and less frequent at solar minimum, geomagnetic activity also follows the solar cycle.
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SOLAR FLARES
The Sun is not a quiet place, but one that exhibits sudden releases of energy. One of the most frequently observed events are SOLAR FLARES: sudden, localized transient increases in brightness that occur in active regions near sunspots. They are usually most easily seen in H-alpha and X-rays, but may have effects in the entire electromagnetic spectrum. The X-ray brightness from a large flare often exceeds the X-ray output from the rest of the Sun.
Another type of event, the CORONAL MASS EJECTION, typically disrupt helmet streamers in the solar corona. As much as 10 trillion kilograms of material can be ejected into the solar wind. Coronal mass ejections propagate out in the solar wind, where they may encounter the Earth and influence geomagnetic activity. Coronal mass ejections are often (but not always) accompanied by prominence eruptions, where the cool, dense prominence material also erupts outward.
All of these forms of solar activity are believed to be driven by energy release from the solar magnetic field. How this energy release occurs, and the relationship between different types of solar activity, is one of the many puzzles facing solar physicists today.
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