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Introduction For as long as man has gazed up at the skies and wondered about that brilliant orb that turns night into day, the Sun has been observed in visible wavelengths. This variety of solar astronomy is known as "White Light" observing. A host of delicate features may be seen or imaged through the telescope when protected with an approved ND5 front filter or Herschel wedge with a filtration reduction of 10,000x. The portion of the Suns atmosphere we are looking at when using this type of equipment is known as the "Photosphere". This translucent layer is conventionally considered as the "surface" of the Sun, and is where the majority of visible light is emitted. Contrary to popular belief, the sun appears completely white to our eyes (when protected by adaquate neutral filteration) and the yellow coloration is only seen when the sun is low on the horizon and viewed through dense dust layers in our atmosphere. Here are the White Light features I have both observed and imaged so far with my equipment. |
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Sunspots These are the primary features on the solar disk, however they are not always visible on any given day. Sunspots are the exit and entry points for tubes of magnetic fields which cause a localized cooling effect in the suns lower most atmosphere, the Photosphere. Although they look dark when seen on the disk through a sufficiently dark filter, they are actually brighter than the full moon in surface brightness. The photosphere is typically around 6000K in temperature, a bluish white color, while the sunspots can be as low as 4000K, more of a yellowish red color range. Sunspots consist of a dark inner region called the Umbra, surrounded by a warmer outer halo of radiating filaments called the Penumbra. Many sunspots occur in pairs of opposite magnetic polarity completing the circuit of the magnetic field. Spots typically last anywhere from a day to over a month before they decay. |
October 2, 2011 - Meade ETX prime
Thousand Oaks Filter, DMK3U camera|
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Sunspots II - Detailed Views In white light - which usually includes both full spectrum and partial visible spectrum views (such as a green filtered image) the phenomena known as Sunspots will show enormous details upon close examination. Sunspots grow from pores, and most of the time fade out at this stage. If the magnetic field is strong enough the immature sunpsot will develop a Penumbra and continue to grow into groups, usually surrounded by smaller pores and faculae. The interior of a sunspot is the dark Umbra. Looking essentially black in amateur scopes, the interiors can be imaged with care and special processing techniques to reveal a wealth of internal details. |
| Inside an umbra are "Dark Granules", tiny white points and grayish "Umbral Dots". Finally, a small core exists that is much darker and cooler and can be as low as 4000K in temperature. Because of this, sometimes the innermost umbra is a reddish hue.The Penumbra is the much lighter outer ring of filaments which surrounds the Umbra. Mature sunspots will have well developed penumbra which can show extensive details. The size of the penumbral filaments are around 1 arc second, and require good seeing to detect. You may also see at that level of resolution small white spots within the penumbral filaments called "Penumbral Grains". |
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Sunspot Groups Sunspots form from pores, often in small clumps creating a small group of features. Usually, they segregate into pairs, one called the "Leading" and the other the "Trailing" spots, which are opposite ends of the magnetic polarity. This polarity reverses on successive solar maximums creating a 22 year complete magnetic solar cycle as opposed to the popular 11 year cycle of spots commonly taught in astronomy classes. After a few weeks, the spot group begins to fall apart, and will usually break up into smaller pores and finally, all that is left is a pattern of faculae which dissipates over time. |
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Faculae These cloud like features of the suns photosphere are essentially magnetic disturbances on the sun which do not quite have the magnetic fields to become a full fledged sunspot. There are many more of these areas than sunspots and a large cluster of faculae can also proceed the growth of a sunspot group. Faculae hover in a region just over the surface of the photosphere, and can be seen to best advantage in Calcium K light which is near UV. But in white light these enigmatic features can be glimpsed best when on the darker limb of the sun, often accompanying sunspot groups.
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| Faculae (Plural for "Facula") are best viewed in blue or the near UV wavelengths. For this reason, many observers and imagers report excellent success using blue and green filters. Wratten 47 or 58 works well. A Baader "Continuum filter" also gives excellent results. The lifetime of faculae can be as long as sunspots themselves, typically facula groups around big sunspots will closely follow the spots across the disk as the sun rotates. There is also a class of this phenomena called "Polar Faculae", in which small groups of white areas appear on occasion near the polar regions of the Sun’s disk. Normally, faculae appear in the same lattitudes as sunspots, one north and the other south of the equator. |
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Limb Darkening The sun is a large ball of diffuse gas and plasma. Because of this, when you gaze at the center of the solar disk, it will appear brighter than the limb because you are looking through relatively translucent layers deeper into the brilliant core of the sun. On the edges, you are skimming the outer layers and it appears darker. This gives the Sun a three dimensional appearance like a sphere suspended in the blackness of space. |
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Light Bridge These occur in the later stages in a sunspot groups evolution, the trailing spot in a pair can start to appear to split in two, with the umbra crossed by a filament or break of plasma splitting it like a dividing cell. |
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Pores Pores are small dark areas typically 1 - 5 arcseconds in diameter with no penumbra. Pores can form and fade out in a matter of hours or minutes, and are not nearly as dark as fully formed sunspot umbra. Pores can continue to grow into sunspots, but usually they are noticed around sunspot groups formed by the same magnetic temperature damping that causes both faculae and sunspots. |
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Granulation Close examination of the Sun's photosphere in white light at high magnification reveals a granular pattern when the seeing is very good. This so called "Rice Grain Texture" are very small scale convective cells in the Suns upper most photospheric layers and average in size at around 2.5 arc seconds across. The center of the granule is the the up-welling of hot gas from the lower "Convective zone" deeper inside the Sun, and the dark lanes around them, known as intergranular walls or lanes are the down currents of the returning gas back to the convective zone. Granules last around 5 - 10 minutes then either fade out or merge with others. |
| Because of their tubular or mound like structure, granulation is easiest to see in the center of the disk, where you are essentially looking straight down a tube. As you approach the limb, they become nearly invisible as the domes of the granules hide the dark lanes of others and they become indistinct. Because of their small size, granulation is very difficult to both image and study visually. As a minimum, a 80mm refractor of the highest quality can make them out, however most texts recommend at least 125mm aperture for seeing them clearly. The image at left was with a Zeiss 80mm f/6 with a 5x Barlow. |
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