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Seeing the Sun – Photosphere, Chromosphere and Corona
It is the largest and brightest object in the Solar System, but observations of the Sun are not straightforward. Ground-based solar telescopes operate during the day, when the atmosphere is warmer than at night, and combined with the Sun’s heat, the resulting air turbulence can greatly reduce these instruments’ performance.
Poor seeing can be improved by careful climate control. The largest solar telescope is the 1.6 m New Solar Telescope (NST) at the Big Bear Solar Observatory in California. Alongside a sophisticated cooling system, the NST benefits from its location on a mountain lake, with a stable atmosphere and cold water to lower the air temperature.
As in other fields of astronomy, atmospheric turbulence is overcome altogether if the telescope is in space. The Solar and Heliospheric Observatory (SOHO) has been studying the Sun from a heliocentric orbit since 1996.
Most solar telescopes detect visible light emitted from the Sun’s “surface", the photosphere. Its atmosphere – the chromosphere and corona – is visible only during a total eclipse, which astronomers can mimic using an instrument called a coronograph. When attached to a telescope, this blanks out the Sun’s disk.
The solar atmosphere can also be explored at wavelengths at which it is brighter than the photosphere. The chromosphere is clearly visible through filters for the light emitted by excited hydrogen or calcium atoms, while the SOHO is investigating the corona with an ultraviolet telescope. Radio waves, X-rays and gamma rays all contribute to the solar picture. Such observations were largely impossible before the advent of space telescopes, as most of this radiation is absorbed by Earth’s atmosphere long before it reaches the ground.
Using a spectrometer to measure the absorption and emission of radiation at these different wavelengths, the temperature, density and chemical composition of the Sun's visible layers have also been determined.
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Exploring the Sun’s Hidden Interior
The interior of the Sun cannot be seen directly by any instrument. It was once described only by theoretical models, but theories can now be tested with observations.
Helioseismology is the study of vibrations in the photosphere, from which information about the Sun’s interior such as its density and temperature can be deduced. BISON, a ground-based project of the University of Birmingham, UK, uses spectrometers located at six sites around the world to continuously monitor changes in the Sun’s luminosity. From these data, the speed and magnitude of photospheric vibrations can be calculated.
The solar interior is also being explored with the help of tiny particles called neutrinos. Produced by nuclear reactions within the Sun’s core, neutrinos are difficult to detect because they hardly ever interact with matter. Facilities such as the Sudbury Neutrino Observatory (SNO) are designed to detect interactions when they do occur. The SNO is located 2 km underground inside a Canadian nickel mine and is famous for solving the “solar neutrino problem", using a large tank of heavy water as a neutrino detector.
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Beyond the Sun – Magnetic Field and Solar Wind
The Sun’s complex and ever-changing magnetic field is measured using an instrument called a magnetograph. The SOHO's Michelson Doppler Imager includes a magnetograph to study magnetic influences on the corona.
The solar wind is an important subject for research because it is responsible for geomagnetic storms – disturbances of Earth’s magnetic field that can disrupt communication systems. The solar wind’s speed, composition and relationship to the Sun’s magnetic field have been examined by probes such as Ulysses, while the SOHO's Ultraviolet Coronagraph Spectrometer is investigating how it is generated.
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New Tools for the Study of the Sun
In 2017, the NST will be overtaken as the world’s largest solar telescope by the planned 4 m Advanced Technology Solar Telescope in Hawaii. This will make a detailed study of the Sun’s magnetic field using visible and infrared observations. Meanwhile, NASA’s Solar Dynamics Observatory was launched in February 2010 and will focus on helioseismology, measuring the Sun’s magnetic field, and observing the corona at ultraviolet wavelengths.
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Big Bear Solar Observatory: Magi Media, http://commons.wikimedia.org/wiki/File:BBSO1.jpg
Coronagraph: NASA/Goddard Space Flight Center, http://commons.wikimedia.org/wiki/File:Solar_storm_2003-10-26_(SOHO-EIT_and_SOHO-LASCO).png
Sudbury Neutrino Detector: A.B. McDonald et al., http://commons.wikimedia.org/wiki/File:Sudbury_sno.jpg
Magnetic Field Lines: NASA, http://commons.wikimedia.org/wiki/File:Arcing_Active_Region.jpg
- “General Information & News" http://soi.stanford.edu/general/ (Accessed 06-10-11)
- “BiSON Background: An Overview" http://bison.ph.bham.ac.uk/index.php?page=bison (Accessed 06-10-11)
- “Big Bear Solar Observatory" http://www.bbso.njit.edu/ (Accessed 06-10-11)
- “Advanced Technology Solar Telescope" http://atst.nso.edu/ (Accessed 06-10-11)
- “The Sudbury Neutrino Detector" http://www.sno.phy.queensu.ca/ (Accessed 06-10-11)
- “SOHO Ultraviolet Coronograph Spectrometer" http://www.cfa.harvard.edu/uvcs/ (Accessed 06-11-10)
- “Solar Dynamics Observatory" http://sdo.gsfc.nasa.gov/ (Accessed 06-11-10)
- “Solar and Heliospheric Observatory" http://soho.esac.esa.int/ (Accessed 06-11-10)