Coronal loop: Difference between revisions

Revision as of 21:25, 29 January 2007

Revision as of 22:24, 29 January 2007

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solar body, protruding into the solar atmosphere. They are ideal structures to observe

when trying to understand the transfer of energy from the solar body, through the transition region and into the corona.

[[Image:Twistedflux.png|thumb|300px|left|A [[Cartoon]] demonstrating the configuration of solar magnetic flux during the solar cycle]]

Many scales of coronal loops exist, neighbouring open flux tubes that give way to the [[solar wind]] and reach far into the corona and heliosphere. Anchored in the photosphere (a rigid, [[magnetohydrodynamics|line-tied]], [[anchor]] is assumed where the [[magnetohydrodynamics|high-β]]; external plasma holds the loop ''footpoints'' in place), coronal loops project through the [[chromosphere]] and [[transition region]], extending high into the [[corona]].

[[Image:Cartoonloops.png|thumb|300px|left|[[Cartoon]] of the low [[corona]] and [[transition region]], where many scales of coronal loop can be [[observed]]]]

Also, coronal loops have a wide variety of temperatures along their lengths. Loops existing at temperatures below 1MK are generally known as cool loops, those existing at around 1MK are known as warm loops and those beyond 1MK are known as hot loops. Naturally these different categories radiate at different wavelengths.<ref>{{cite journal

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| year = 2001

}}</ref>

[[Image:Cartoonloops.png|thumb|300px|leftright|[[Cartoon]] of the low [[corona]] and [[transition region]], where many scales of coronal loop can be [[observed]]]]

Coronal loops populate both active and quiet regions of the solar surface. Active regions on the solar surface take up small areas but produce the majority of activity and are often the source of flares and [[Coronal Mass Ejection|Coronal Mass Ejections]] due to the [[Magnetic reconnection|intense magnetic field]] present. Active regions produce 82% of the total coronal heating energy.<ref>{{cite journal

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The solar community was rocked by the launch of ''[[Yohkoh]]'' (Solar A) from [[Kagoshima Space Centre]] (Southern Japan) in August [[1991]] It was lost on 14th December [[2001]] due to battery failure, but revolutionised X-ray observations in its decade of operations. Yohkoh (or ''Sunbeam'') orbited the Earth in an [[elliptical]] [[orbit]], observing X-ray and [[Gamma ray|γ-ray]] emissions from solar phenomena such as solar flares. Yohkoh carried four instruments. The Bragg Crystal Spectrometer (BCS), the Wide Band Spectrometer (WBS), the Soft X-Ray Telescope ([[Yohkoh|SXT]]) and the Hard X-Ray Telescope (HXT) were operated by a consortium of scientists from [[Japan]], [[USA]] and [[UK]]. Of particular interest is the [[Yohkoh|SXT]] instrument for observing X-ray emitting coronal loops.

[[Image:Yohkohimage.gif|thumb|200px|rightleft|X-ray solar coronal loops as viewed by the ''[[Yohkoh]]'' observatory]]

The SXT instrument observed X-rays in the 0.25-4.0[[SI units|keV]] range, resolving solar features to 2.5 arc seconds with a temporal resolution of 0.5-2 seconds. SXT was sensitive to

plasma in the 2-4MK temperature range, making it an ideal observational platform to compare with data collected from [[TRACE]] coronal loops radiating in the EUV wavelengths.<ref>{{cite journal

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The next major step in solar physics came at the launch of the [[Solar and Heliospheric Observatory]] (SOHO) in December [[1995]] from [[Cape Canaveral Air Force Station]] in [[Florida]], USA. SOHO originally had an operational lifetime of two years. The mission was extended to March [[2007]] due to its resounding success allowing SOHO to observe a complete 11 year solar cycle. SOHO continually faces the Sun holding a slow orbit around the First [[Lagrangian Point]] (L1) where the gravitational balance between the Sun and Earth provides a stable position for SOHO to orbit. SOHO is continually eclipsing the Sun from the Earth at a distance of approximately 1.5 million kilometres.

[[Image:SOHO solar flare sun large 20031026 0119 eit 304.png|thumb|200px|leftright|A typical [[Solar and Heliospheric Observatory|SOHO]] image of the chromosphere and magnetic structure of the Sun.]]

SOHO is managed by scientists from the [[European Space Agency]] (ESA) and NASA. Comprising of more instruments than both TRACE and Yohkoh, this large solar mission was designed to look at the chain from the solar interior, the solar corona to the solar wind. SOHO has 12 instruments on board including the Coronal Diagnostic Spectrometer (CDS), the Extreme ultraviolet Imaging Telescope (EIT), the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) and the UltraViolet Coronagraph Spectrometer (UVCS) which are all used extensively in the study of the transition region and corona.

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from [[Vandenberg Air Force Base]] as part of NASA’s Goddard Space Flight Center Small Explorer (SMEX) project. The small orbiting instrument has a 30×160cm, 8.66m focal length Cassegrain telescope with a 1200×1200px CCD detector. The timing of the launch was planned to coincide with the rising phase of the solar maximum. Observations of the transition region and lower corona could then be carried out in conjunction with SOHO to give an unprecedented view of the solar environment during this exciting phase of the solar cycle.

[[Image:Tracemosaic.jpg|thumb|200px|rightleft|Full-disk mosaic of the million-degree [[Sun]] by [[TRACE]]]]

Due to the high spatial (1 arc second) and temporal resolution (1-5sec), TRACE has been able to capture highly detailed images of coronal structures, whilst SOHO provides the global (lower resolution) picture of the Sun. This campaign demonstrates the observatory’s ability to track the evolution of steady-state (or [[quiescent]]) coronal loops. TRACE utilizes filters that are sensitive to electromagnetic radiation in the 171Å FeIX, 195Å FeXII, 284Å FeXV, 1216Å HI, 1550Å CIV and 1600Å range. Of particular interest are the 171Å, 195Å and 284Å band passes as they are sensitive to the radiation emitted by quiescent coronal loops.