Coronal loop: Difference between revisions

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{{Short description|Arch-like structure in the Sun's corona}}

[[Image:Traceimage.jpg|thumb|300px|right|Typical coronal loops observed by [[TRACE]]]]

{{More citations needed|date=March 2022}}

[[Image:Traceimage.jpg|thumb|Typical coronal loops observed by [[TRACE]]]]

[[File:AR1520 and Shimmering Coronal Loops.ogv|thumb|Dynamics of coronal loops observed by [[Solar Dynamics Observatory|SDO]]]]

In [[solar physics]], a '''coronal loop''' is a well-defined arch-like structure in the [[Sun]]'s [[Stellar atmosphere|atmosphere]] made up of relatively dense [[Plasma (physics)|plasma]] confined and isolated from the surrounding medium by [[magnetic flux tube]]s. Coronal loops begin and end at two footpoints on the [[photosphere]] and project into the [[Solar transition region|transition region]] and lower [[Solar corona|corona]]. They typically form and dissipate over periods of seconds to days<ref>{{Cite web |last=Loff |first=Sarah |date=2015-04-17 |title=Coronal Loops in an Active Region of the Sun |url=http://www.nasa.gov/content/coronal-loops-in-an-active-region-of-the-sun |access-date=2022-03-28 |website=NASA}}</ref> and may span anywhere from {{convert|1|to|1000|Mm|lk=in|sigfig=3}} in length.<ref name="Reale2014">{{cite journal |last1=Reale |first1=Fabio |title=Coronal Loops: Observations and Modeling of Confined Plasma |journal=Living Reviews in Solar Physics |date=July 2014 |volume=11 |issue=4 |page=4 |doi=10.12942/lrsp-2014-4 |doi-access=free |pmid=27194957 |pmc=4841190 |bibcode=2014LRSP...11....4R |url=https://link.springer.com/content/pdf/10.12942/lrsp-2014-4.pdf |access-date=16 March 2022}}</ref>

Coronal loops form the basic structure of the lower [[corona]] and [[transition region]] of the [[Sun]]. These highly structured and elegant loops are a direct consequence of the twisted solar [[magnetic]] flux within the solar body. The population of coronal loops can be directly linked with the [[solar cycle]], it is for this reason coronal loops are often found with [[sunspots]] at their footpoints. The [[upwelling]] magnetic flux pushes through the [[photosphere]], exposing the cooler plasma below. The contrast between the photosphere and solar interior gives the impression of dark spots, or ''[[sunspots]]''.

Coronal loops are often associated with the strong [[magnetic field]]s located within [[active region]]s and [[sunspot]]s. The number of coronal loops varies with the 11 year [[solar cycle]].

==Physical features==

==Origin and physical features==

A coronal loop is magnetic [[flux]] fixed at both ends, threading through the

Due to a natural process called the [[solar dynamo]] driven by heat produced in the Sun's core, [[convective]] motion of the [[electrically conductive]] [[plasma (physics)|plasma]] which makes up the Sun creates [[electric current]]s, which in turn create powerful [[magnetic field]]s in the Sun's interior. These magnetic fields are in the form of closed loops of [[magnetic flux]], which are twisted and tangled by [[solar differential rotation]] (the different rotation rates of the plasma at different latitudes of the solar sphere). A coronal loop occurs when a curved arc of the magnetic field projects through the visible surface of the Sun, the [[photosphere]], protruding into the solar atmosphere.

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.

Within a coronal loop, the paths of the moving [[electrically charged]] particles which make up its plasma—[[electron]]s and [[ion]]s—are sharply bent by the [[Lorentz force]] when moving transverse to the loop's magnetic field. As a result, they can only move freely parallel to the magnetic field lines, tending to spiral around these lines. Thus, the plasma within a coronal loop cannot escape sideways out of the loop and can only flow along its length. This is known as the ''[[Alfvén's theorem|frozen-in]]'' condition.<ref name="Malanushenko22">{{cite journal |last1=Malanushenko |first1=A. |last2=Cheung |first2=M. C. M. |last3=DeForest |first3=C. E. |last4=Klimchuk |first4=J. A. |last5=Rempel |first5=M. |title=The Coronal Veil |journal=The Astrophysical Journal |date=1 March 2022 |volume=927 |issue=1 |pages=1 |doi=10.3847/1538-4357/ac3df9|s2cid=235658491 |doi-access=free |arxiv=2106.14877 |bibcode=2022ApJ...927....1M }}</ref>

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

The strong interaction of the magnetic field with the dense plasma on and below the Sun's surface tends to tie the magnetic field lines to the motion of the Sun's plasma; thus, the two ''footpoints'' (the ___location where the loop enters the photosphere) are anchored to and rotate with the Sun's surface. Within each footpoint, the strong magnetic flux tends to inhibit the convection currents which carry hot plasma from the Sun's interior to the surface, so the footpoints are often (but not always) cooler than the surrounding photosphere. These appear as dark spots on the Sun's surface, known as [[sunspot]]s. Thus, sunspots tend to occur under coronal loops, and tend to come in pairs of opposite [[magnetic polarity]]; a point where the magnetic field loop emerges from the photosphere is a North [[magnet|magnetic pole]], and the other where the loop enters the surface again is a South magnetic pole.

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]].

AlsoCoronal loops form in a wide range of sizes, coronalfrom the minimum observable scale (< 100&nbsp;km) to 10,000&nbsp;km. There is currently no accepted theory of what defines the edge of a loop, which is embedded in a general corona that is itself strongly magnetized. Coronal loops have a wide variety of temperatures along their lengths. Loops existing at temperatures below 1MK1&nbsp;[[megakelvin]]&nbsp;(MK) are generally known as cool loops,; those existing at around 1MK1&nbsp;MK are known as warm loops; and those beyond 1MK1&nbsp;MK are known as hot loops. Naturally, these different categories radiate at different wavelengths.<ref>{{cite journal

| last = Vourlidas

| first = A.

| coauthors author2= J. A. Klimchuk, |author3=C. M. Korendyke, |author4=T. D. Tarbell, |author5=B. N. Handy

| title = On the correlation between coronal and lower transition region structures at arcsecond scales

| journal = Astrophysical Journal

| volume = 563

| issue = 1

| pages = 374–380

| year = 2001

| doi = 10.1086/323835

}}</ref>

| bibcode=2001ApJ...563..374V|citeseerx=10.1.1.512.1861| s2cid = 53124376

}}</ref>

A related phenomenon is the open [[flux tube]], in which magnetic fields extend from the surface far into the corona and heliosphere; these are the source of the Sun's large scale magnetic field ([[magnetosphere]]) and the [[solar wind]].

{{Gallery

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

|file:Twistedflux.png |A diagram showing the evolution of the solar magnetic flux over one solar cycle

|file:Cartoonloops.png |Diagram of the low [[solar corona|corona]] and [[solar transition region|transition region]], where many scales of coronal loops can be observed

|file:Energyfig.png |A modelled example of a quiescent coronal loop (energy contributions)

|align=center|width=200}}

===Location===

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

Coronal loops have been shown on both [[Active region|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 [[Solar flare|flares]] and [[coronal mass ejection]]s due to the intense magnetic field present. Active regions produce 82% of the total coronal heating energy.<ref>{{cite journal

| last = Aschwanden

| first = M. J.

| title = An evaluation of coronal heating models for Active Regions based on Yohkoh, SOHO, and TRACE observations

on Yohkoh, SOHO, and TRACE observations

| journal = Astrophysical Journal

| volume = 560

| issue = 2

| pages = 1035–1044

| year = 2001

| doi = 10.1086/323064

}}</ref> Coronal holes are open field lines located predominantly in the [[Geographical pole|polar]] regions of the Sun and are known to be the source of the fast [[solar wind]]. The quiet Sun makes up the rest of the solar surface. The quiet Sun, although less active than active regions, is awash with [[dynamic]] processes and [[transient]] events (bright points, nanoflares and jets).<ref>{{cite book

| bibcode=2001ApJ...560.1035A| s2cid = 121226839

}}</ref><ref>{{cite book

| last = Aschwanden

| first = M. J.

Line 41 ⟶ 55:

| publisher = Praxis Publishing Ltd.

| year = 2004

| isbn = 978-3-540-22321-54

}}</ref>

}}</ref> As a general rule, the quiet Sun exists in regions of [[coronal loop|closed magnetic structures]], active regions are highly dynamic sources of explosive events. It is important to note that observations suggest the whole corona is massively populated by open and closed magnetic fieldlines.

===Dynamic flows===

A closed fieldline does not constitute a coronal loop however, closed flux must be ''filled with plasma'' before it can be called a ''coronal loop''. With this in mind it becomes clear that coronal loops are a rarity on the solar surface as the majority of closed flux structures are '''empty'''. This means the mechanism that heats the corona and injects chromospheric plasma into the closed magnetic flux is highly localised.<ref>{{cite journal

Many solar observation missions have observed strong plasma flows and highly dynamic processes in coronal loops. For example, SUMER observations suggest flow velocities of 5–16&nbsp;km/s in the solar disk, and other joint SUMER/TRACE observations detect flows of 15–40&nbsp;km/s.<ref>{{cite journal

| last = Litwin

| firstlast = C.Spadaro

| coauthorfirst = RD. Rosner

|author2=A. C. Lanzafame |author3=L. Consoli |author4=E. Marsch |author5=D. H. Brooks |author6=J. Lang

| title = On the structure of solar and stellar coronae - Loops and loop

| title = Structure and dynamics of an active region loop system observed on the solar disc with SUMER on SOHO

heat transport

| journal = Astronomy ApJ& Astrophysics

| volume = 412359

| pages = 375-385716–728

| year = 19932000

| bibcode = 2000A&A...359..716S

}}</ref> The mechanism behind plasma filling, dynamic flows and coronal heating remains a mystery. The mechanism(s) must be stable enough to continue to feed the corona with chromospheric plasma and powerful enough to accelerate and therefore heat the plasma from 6000K to well over 1MK over the short distance from chromosphere, transition region to the corona. This is the very reason coronal loops are targeted for intense study. They are anchored to the photosphere, are fed by chromospheric plasma, protrude into the transition region and exist at coronal temperatures after undergoing intensive heating.

}}</ref><ref>{{cite journal

| last = Winebarger

The idea that the ''coronal heating problem'' is solely down to some coronal heating mechanism is misleading. Firstly, the plasma filling overdense loops is drained directly from the chromosphere. There is no coronal mechanism known that can compress coronal plasma and feed it into coronal loops at coronal altitudes. Secondly, observations of coronal upflows points to a chromospheric source of plasma. The plasma is therefore chromospheric in origin, there must be consideration of this when looking into coronal heating mechanisms. This is a ''chromospheric energization'' and ''coronal heating phenomenon'' possibly linked through a common mechanism.

| first = A. R.

{{unsolved|physics|Why is the Sun's Corona so much hotter than the Sun's surface?}}

|author2=H. Warren |author3=A. van Ballegooijen |author4=E. E. DeLuca |author5=L. Golub

| title = Steady flows detected in extreme-ultraviolet loops

| journal = Astrophysical Journal Letters

| volume = 567

| issue = 1

| pages = L89–L92

| year = 2002

| doi = 10.1086/339796

| bibcode=2002ApJ...567L..89W| doi-access = free}}</ref> Very high plasma velocities (in the range of 40–60&nbsp;km/s) have been detected by the Flat Crystal Spectrometer (FCS) on board the Solar Maximum Mission.

==History of observations==

{{Broader|Solar observation}}

Many strides have been made by ground-based telescopes (such as the [[Mauna Loa Solar Observatory]], MLSO, in [[Hawaii]]) and [[eclipse]] observations of the corona, but to escape the obscuring effect of the [[Earth]]’s atmosphere, space-based observations have have become a necessary [[evolution]] for solar physics. Beginning with the short (7 minute) [[Aerobee (rocket)|Aerobee]] rocket flights in [[1946]] and [[1952]], [[Spectrogram|spectrograms]] measured solar EUV and Lyman-α emissions. Basic [[X-ray]] observations were attained by [[1960]] using such rockets. The [[Skylark (rocket)|British Skylark rocket]] missions from [[1959]]-[[1978]] also returned mainly X-ray [[spectrometer]] data.<ref>{{cite journal

===Before 1991===

| last = Boland

Despite progress made by ground-based telescopes and [[eclipse]] observations of the corona, space-based observations became necessary to escape the obscuring effect of the Earth's atmosphere. Rocket missions such as the [[Aerobee (rocket)|Aerobee]] flights and [[Skylark (rocket)|Skylark rockets]] successfully measured solar [[extreme ultraviolet]] (EUV) and X-ray emissions. However, these rocket missions were limited in lifetime and payload. Later, satellites such as the [[Orbiting Solar Observatory]] series (OSO-1 to OSO-8), [[Skylab]], and the [[Solar Maximum Mission]] (the first observatory to last the majority of a [[solar cycle]]: from 1980 to 1989) were able to gain far more data across a much wider range of emission.<ref>{{cite journal |last=Vaiana |first=G. S. |author2=J. M. Davis |author3=R. Giacconi |author4=A. S. Krieger |author5=J. K. Silk |author6=A. F. Timothy |author7=M. Zombeck |year=1973 |title=X-Ray Observations of Characteristic Structures and Time Variations from the Solar Corona: Preliminary Results from SKYLAB |journal=Astrophysical Journal Letters |volume=185 |pages=L47–L51 |bibcode=1973ApJ...185L..47V |doi=10.1086/181318}}</ref><ref>{{cite book |last=Strong |first=K. T. |title=The many faces of the Sun: a summary of the results from NASA's Solar Maximum Mission |author2=J. L. R. Saba |author3=B. M. Haisch |author4=J. T. Schmelz |publisher=New York: Springer |year=1999}}</ref>

| first = B. C.

| coauthor = E. P. Dyer, J. G. Firth, A. H. Gabriel, B. B. Jones, C. Jordan, R.W. P.

McWhirter, P. Monk, R. F. Turner

| title = Further measurements of emission line profiles

in the solar ultraviolet spectrum

| journal = MNRAS

| volume = 171

| pages = 697–724

| year = 1975

}}</ref> Although successful, the rocket missions were very limited in lifetime and payload. During the period of [[1962]]-[[1975]], the satellite series [[Orbiting Solar Observatory]] (OSO-1 to OSO-8) were able to gain extended EUV and X-ray spectrometer observations. Then in [[1973]], [[Skylab]] was launched and began a new multi-wavelength campaign which typified future observatories.<ref>{{cite journal

| last = Vaiana

| first = G. S.

| coauthor = J. M. Davis, R. Giacconi, A. S. Krieger, J. K. Silk, A. F. Timothy &

M. Zombeck

| title = X-Ray Observations of Characteristic Structures and Time Variations

from the Solar Corona: Preliminary Results from SKYLAB

| journal = Astrophysical Journal Letters

| volume = 185

| pages = L47–L51

| year = 1973

}}</ref> This mission only lasted a year and was superceded by the [[Solar Maximum Mission]] which became the first observatory to last the majority of a [[solar cycle]] (from [[1980]]-[[1989]]).<ref>{{cite book

| last = Strong

| first = K. T.

| coauthor = J. L. R. Saba, B. M. Haisch, J. T. Schmelz

| title = The many faces of the Sun: a summary of the results from NASA’s Solar Maximum Mission

| publisher = New York: Springer

| year = 1999

}}</ref> A wealth of data was accumulated across the whole range of emission.

===1991–present day===

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:Tracemosaic.jpg|thumb|Full-disk mosaic of the million-degree corona by [[TRACE]]]]

In August 1991, the solar observatory spacecraft [[Yohkoh]] launched from the [[Kagoshima Space Centre|Kagoshima Space Center]]. During its 10 years of operation, it revolutionized X-ray observations. Yohkoh carried four instruments; of particular interest is the SXT instrument, which observed X-ray-emitting coronal loops. This instrument observed X-rays in the 0.25–4.0&nbsp;[[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 2–4&nbsp;MK temperature range, making its data ideal for comparison with data later collected by TRACE of coronal loops radiating in the extra ultraviolet (EUV) wavelengths.<ref>{{cite journal

[[Image:Yohkohimage.gif|thumb|200px|left|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

| last = Aschwanden

| first = M. J.

| title = Observations and models of coronal loops: From Yohkoh to TRACE, in Magnetic coupling of the solar atmosphere

| volume = 188

| pages = 1-91–9

| year = 2002

}}</ref>

The next major step in solar physics came atin December 1995, with 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 continuallyhas faces12 instruments on board, all of which are used to study the Suntransition holdingregion aand slowcorona. orbitIn aroundparticular, the FirstExtreme [[Lagrangianultraviolet Point]]Imaging Telescope (L1EIT) whereinstrument theis gravitationalused balanceextensively betweenin coronal loop observations. EIT images the Suntransition andregion Earththrough providesto athe stableinner positioncorona forby SOHOusing tofour orbit.[[Passband|band SOHOpasses]]—171&nbsp;Å&nbsp;FeIX, is195&nbsp;Å&nbsp;FeXII, continually284&nbsp;Å&nbsp;FeXV, eclipsingand the304&nbsp;Å&nbsp;HeII, Suneach fromcorresponding theto Earthdifferent atEUV atemperatures—to distanceprobe ofthe approximately[[chromospheric 1.5network]] to the millionlower kilometrescorona.

In April 1998, the [[TRACE|Transition Region and Coronal Explorer]] (TRACE) was launched from [[Vandenberg Air Force Base]]. Its observations of the transition region and lower corona, made in conjunction with SOHO, give an unprecedented view of the solar environment during the rising phase of the solar maximum, an active phase in the solar cycle. Due to the high spatial (1 arc second) and temporal resolution (1–5 seconds), 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 '[[wikt:quiescent|quiescent]]') coronal loops. TRACE uses filters sensitive to various types of electromagnetic radiation; in particular, the 171&nbsp;Å, 195&nbsp;Å, and 284&nbsp;Å band passes are sensitive to the radiation emitted by quiescent coronal loops.

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

==See also==

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.

* [[Solar spicule]]

* [[Solar prominence]]

* [[Coronal hole]]

==References==

The EIT instrument is used extensively in coronal loop observations. EIT images the transition region through to the inner corona by utilising four band passes, 171Å FeIX, 195Å FeXII, 284Å FeXV and 304Å HeII, each corresponding to different EUV temperatures, probing the [[chromospheric network]] to the lower corona.

{{reflist}}

==External links==

The [[TRACE|Transition Region And Coronal Explorer]] ([[TRACE]]) was launched in April, [[1998]]

{{Commons category}}

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.

*[http://trace.lmsal.com/ TRACE homepage]

*[http://sohowww.nascom.nasa.gov Solar and Heliospheric Observatory, including near-real-time images of the solar corona]

*[http://www.innovations-report.com/html/reports/physics_astronomy/report-33153.html Coronal heating problem at Innovation Reports]

*[http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/corona.html NASA/GSFC description of the coronal heating problem]

*[http://solar-center.stanford.edu/FAQ/Qcorona.html FAQ about coronal heating]

*[http://alienworlds.southwales.ac.uk/sunStructure.html#/photosphereflares Animated explanation of Coronal loops and their role in creating Prominences] {{Webarchive|url=https://web.archive.org/web/20151116133527/http://alienworlds.southwales.ac.uk/sunStructure.html#/photosphereflares |date=2015-11-16 }} (University of South Wales)

{{The Sun}}

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

{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}

{{Authority control}}

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.

All of the above space missions have been highly successful in observing strong plasma flows and highly dynamic processes in coronal loops. For example, SUMER observations suggest flow velocities of 5 - 16kms^-1 in the solar disk, other

joint SUMER/TRACE observations detect flows of 15-40kms^-1.<ref>{{cite journal

| last = Spadaro

| first = D.

| coauthors = A. C. Lanzafame, L. Consoli, E. Marsch, D. H. Brooks, J. Lang

| title = Structure and dynamics of an active region loop system observed on the solar disc

with SUMER on SOHO

| journal = Astronomy &amp; Astrophysics

| volume = 359

| pages = 716–728

| year = 2000

}}</ref><ref>{{cite journal

| last = Winebarger

| first = A. R.

| coauthors = H. Warren, A. van Ballegooijen, E. E. DeLuca, L. Golub

| title = Steady flows detected in extreme-ultraviolet loops

| journal = Astrophysical Journal Letters

| volume = 567

| pages = L89–L92

| year = 2002

}}</ref> Very high velocities have been detected by the Flat Crystal Spectrometer (FCS)

on board the Solar Maximum Mission where plasma velocities were found in the range

of 40 - 60kms^-1.

==Useful links==

* [[Hinode|The new solar observatory, ''Hinode'' (Solar-B)]]

* [[Yohkoh|The highly successful solar X-ray mission, ''Yohkoh'' (Solar-A)]]

* [http://trace.lmsal.com/ TRACE homepage]

* [http://sohowww.nascom.nasa.gov/ SOHO homepage]

==References==

<div class="references-small">

<references/>

[[Category:Sun]]

</div>

[[Category:Space plasmas]]

[[Category:Astrophysics]]

[[Category:Articles containing video clips]]