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RE: The Sun
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Title: Solar Spectroscopy at ARIES
Authors: K. Sinha

Identification of Fraunhofer lines with the known atomic and molecular absorbers and predictions leading to such an effort has been a challenging area of study crowned with occasional success. Such studies have also lead, amongst other things to (i) a determination of abundances of elements and that of their isotopes (ii) valuable information on model atmospheres and (iii) use of Sun as a laboratory source. We summarize and review here the work done in the last four decades in the area of solar spectroscopy at Aryabhatta Research Institute of observational sciencES (ARIES in short) with a view to pick up new and interesting areas for future investigations in the light of the tremendous progress made elsewhere in observations of the sun and in the laboratory studies.

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The Sun's activity
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A Finnish-led international team has used meteorites to investigate the sun's solar activity of past centuries.

Ilya Usoskin at Finland's Sodankyla Geophysical Observatory and colleagues compared the amount of Titanium 44 in 19 meteorites that have fallen to the Earth the past 240 years. They said their findings confirm that solar activity increased strongly during the 20th century. They also find the sun has been particularly active during the past few decades.
The scientists say studying the sun's activity is one of the oldest astrophysical projects, as astronomers began recording the number of sunspots to trace the sun's magnetic activity 400 years ago.
The team examined a set of 19 meteorites whose dates of fall are precisely known, measuring the amount of radioactive isotope Titanium 44 in each meteorite. Titanium 44 is produced by the cosmic rays in the meteorites while they are outside the Earth's atmosphere. After the meteorite has fallen, it stops producing the isotope.
By measuring the Titanium 44 in the meteorites, the scientists determined the level of solar activity at the time the meteorite fell.
The study appears in the journal Astronomy & Astrophysics Letters.

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Scientists studying sun spots believe the Sun could be entering a cooling period, which could help in the battle against global warming.

One study by scientists in Texas has predicted the Sun will cool to its lowest level for more than a century during the next few years.
The declining temperature of the Sun is part of a natural cycle, but some say the expected cool period could give us a better chance to combat global warming.

"If there was a period of low solar activity it could give us a little more time to combat global warming and to introduce the curbs on the carbon emissions that we need to limit climate change" - Professor Joanna Haigh, of Imperial College London.

Scientists have known for many years that the Sun's output varies over an 11-year cycle, but have recently discovered that there are also longer cycles of activity, characterised by drops in solar radiation and a fall in the numbers of sunspots.
The Texan research carried out by Leif Svalgaard suggests sunspot activity will drop to an "extremely small" level during the next decade and scientists predict a 0.2 per cent decline in global temperatures.
Cambridge solar physicist Nigel Weiss said predictions of a global fall in temperatures could well be accurate.

"Periods of high solar activity do not last long, perhaps 50 to 100 years, then you get a crash" - Nigel Weiss.

However, solar experts working in Boulder Colorado have predicted an increase in the numbers of sunspots over the same period, suggesting that global temperatures will rise still further.

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An increase in sunspot activity in early August and a large solar flare on August 17 are evidence the sun is on the verge of entering a new cycle of high activity, according to NASA scientists.

The sun follows a regular 11 year cycle of activity, with increased frequency of sunspots, solar flares and coronal mass ejections (CME) around the time of 'solar maximum', the last of which occurred in 2001.
The sun is currently in a time of minimum activity, although the appearance of sunspots, including a rare magnetically-reversed sunspot on July 31, indicate that the cycle of activity is due to begin to ramp up again towards the next peak, which is expected to occur in 2012.

NASA scientists observed the most recent sunspot, called Active Region 904, with intense interest using the Solar and Heliospheric Observatory (SOHO) satellite, which lingers in the gravity-neutral L1 Lagrange point between the Earth and the Sun.
After watching the sunspot rotate into view on August 9, it finally popped off a modest (C-class) flare and associated coronal mass ejection (CME) on August 17 when it had rotated into a location where it practically faced Earth.

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This composite image shows the Sun as viewed from the Cassini spaceprobe.

Cassinisun300606_sm

The images were taken on June30 using the CL1 and GRN filters.

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The Corona
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For the first time, researchers have developed a computer simulation that can accurately create a model of the sun's outer atmosphere, or corona. Funded by NASA and the National Science Foundation, the computer model marks the beginning of a new era in space weather prediction.

By accurately simulating the behaviour of the corona, scientists hope to eventually predict when it will produce flares and coronal mass ejections, huge clouds of hot plasma ejected from the sun. It's the same approach the National Weather Service uses to predict when the Earth's atmosphere will produce thunderstorms or hurricanes.
Such predictions will help protect astronauts against radiation from flares and coronal mass ejections, in addition to mitigating disruptions on orbiting satellites and land-based communications and power systems.

"This confirms that computer models can describe the physics of the solar corona" - Zoran Mikic of Science Applications International Corp, San Diego, California.

The turbulent corona is threaded with magnetic fields generated beneath the visible solar surface. The evolution of these magnetic fields causes violent eruptions and solar storms originating in the corona.
The computer model was based on spacecraft observations of magnetic activity on the sun's surface, which affects and shapes the corona. The observations were made with the Michelson Doppler Imager instrument on the Solar and Heliospheric Observatory (SOHO) spacecraft. The Science Applications International Corporation team released simulated "photographs" of the March 29 total solar eclipse 13 days before and again 5 days before the actual event.

Previous computer simulations were based on simplified models, so the calculations could be completed in a reasonable time. The new simulation is the first to base its calculations on the physics of how energy is transferred in the corona. Even using NASA and the National Science Foundation supercomputers, the calculations required four days to complete on about 700 computer processors.
During a total solar eclipse, the moon blocks direct light from the sun, so the much fainter corona is visible. This is the only time the corona is visible from Earth without special instruments, and it resembles a white, lacy veil surrounding the black disk of the moon. Because the corona is always changing, each eclipse looks different.
Since the physics of the corona is still not completely understood, the accuracy of the simulation will improve when our understanding of how energy flows through the corona improves. More detailed measurements of magnetic activity on the solar surface, like those expected from NASA's Solar Dynamics Observatory scheduled to launch in 2008, will also improve the accuracy of the simulation.

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Extreme Ultraviolet Imaging Telescope (EIT) image of a huge, handle-shaped prominence taken on Sept. 14, 1999. Prominences are huge clouds of relatively cool dense plasma suspended in the Sun's hot, thin corona. The hottest areas appear almost white, while the darker red areas indicate cooler temperatures.


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This is a real-time ultraviolet image of the Sun that measures a mid-temperature region of the Sun in a transition zone between the surface and the corona (the Sun's atmosphere).
The bright and dark features are all manifestations of the Sun's magnetic field (the brightest areas show the strongest magnetic fields)



The Dark filaments are areas cooler than the 1-million-degree corona; when seen protruding from the disk of the Sun, they are called prominences.
The fuzzy glow around the disk is ionised gas - relatively cool material that has had electrons stripped from the atoms.
The Mottled areas are known as the chromospheric network, where strong magnetic fields congregate.
Bright spots in the background of space represent cosmic rays or charged particles that have been accelerated when a coronal mass ejection collides with the ambient, but ever-changing solar wind.



Incidentally a total solar eclipse will occur on March 29, 2006.
People will be able to see the eclipse in Brazil, Egypt, Turkey, the Caucasus, the Caspian Sea coast, northern Kazakhstan and Altai (South Siberia).

A partial solar eclipse occurs on October 3, 2005. The shadow of the Moon will sweep a band through the Atlantic Ocean, Spain, Africa, the Indian Ocean, the North Caucasus, Ukraine and Southern Europe.

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