Coronal loop: Difference between revisions

Revision as of 20:36, 29 January 2007

Revision as of 21:13, 29 January 2007

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

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]].

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

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

| last = Vourlidas

| first = A.

Line 38 ⟶ 33:

| pages = 1035–1044

| year = 2001

}}</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

| last = Aschwanden

| first = M. J.

Line 45 ⟶ 40:

| year = 2004

| isbn = 3-540-22321-5

}}</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.

events. It is important to note that observations suggest the whole corona is massively populated by open and closed magnetic fieldlines.

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

Line 60 ⟶ 54:

}}</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.

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.

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

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.

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

==History of observations==