 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Coronal loop" – news ·newspapers ·books ·scholar ·JSTOR(March 2022) (Learn how and when to remove this message) |
Insolar physics, acoronal loop is a well-defined arch-like structure in theSun'satmosphere made up of relatively denseplasma confined and isolated from the surrounding medium bymagnetic flux tubes. Coronal loops begin and end at two footpoints on thephotosphere and project into thetransition region and lowercorona. They typically form and dissipate over periods of seconds to days[1] and may span anywhere from 1 to 1,000megametres (621 to 621,000 mi) in length.[2]
Coronal loops are often associated with the strongmagnetic fields located withinactive regions andsunspots. The number of coronal loops varies with the 11 yearsolar cycle.
Due to a natural process called thesolar dynamo driven by heat produced in the Sun's core,convective motion of theelectrically conductiveplasma which makes up the Sun createselectric currents, which in turn create powerfulmagnetic fields in the Sun's interior. These magnetic fields are in the form of closed loops ofmagnetic flux, which are twisted and tangled bysolar 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, thephotosphere, protruding into the solar atmosphere.
Within a coronal loop, the paths of the movingelectrically charged particles which make up its plasma—electrons andions—are sharply bent by theLorentz 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 thefrozen-in condition.[3]
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 twofootpoints (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 assunspots. Thus, sunspots tend to occur under coronal loops, and tend to come in pairs of oppositemagnetic polarity; a point where the magnetic field loop emerges from the photosphere is a Northmagnetic pole, and the other where the loop enters the surface again is a South magnetic pole.
Coronal loops form in a wide range of sizes, from the minimum observable scale (< 100 km) to 10,000 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 at temperatures below 1 megakelvin (MK) are generally known as cool loops; those existing at around 1 MK are known as warm loops; and those beyond 1 MK are known as hot loops. Naturally, these different categories radiate at different wavelengths.[4]
A related phenomenon is the openflux 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 thesolar wind.
Coronal loops have been shown on bothactive 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 offlares andcoronal mass ejections due to the intense magnetic field present. Active regions produce 82% of the total coronal heating energy.[5][6]
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 km/s in the solar disk, and other joint SUMER/TRACE observations detect flows of 15–40 km/s.[7][8] Very high plasma velocities (in the range of 40–60 km/s) have been detected by the Flat Crystal Spectrometer (FCS) on board the Solar Maximum Mission.
Despite progress made by ground-based telescopes andeclipse observations of the corona, space-based observations became necessary to escape the obscuring effect of the Earth's atmosphere. Rocket missions such as theAerobee flights andSkylark rockets successfully measured solarextreme ultraviolet (EUV) and X-ray emissions. However, these rocket missions were limited in lifetime and payload. Later, satellites such as theOrbiting Solar Observatory series (OSO-1 to OSO-8),Skylab, and theSolar Maximum Mission (the first observatory to last the majority of asolar cycle: from 1980 to 1989) were able to gain far more data across a much wider range of emission.[9][10]
In August 1991, the solar observatory spacecraftYohkoh launched from theKagoshima 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 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 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.[11]
The next major step in solar physics came in December 1995, with the launch of theSolar and Heliospheric Observatory (SOHO) fromCape Canaveral Air Force Station. 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 has 12 instruments on board, all of which are used to study the transition region and corona. In particular, the Extreme ultraviolet Imaging Telescope (EIT) instrument is used extensively in coronal loop observations. EIT images the transition region through to the inner corona by using fourband passes—171 Å FeIX, 195 Å FeXII, 284 Å FeXV, and 304 Å HeII, each corresponding to different EUV temperatures—to probe thechromospheric network to the lower corona.
In April 1998, theTransition Region and Coronal Explorer (TRACE) was launched fromVandenberg 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 'quiescent') coronal loops. TRACE uses filters sensitive to various types of electromagnetic radiation; in particular, the 171 Å, 195 Å, and 284 Å band passes are sensitive to the radiation emitted by quiescent coronal loops.
{{cite book}}: CS1 maint: publisher location (link){{cite journal}}:Cite journal requires|journal= (help)
| Internal structure | |||||||
|---|---|---|---|---|---|---|---|
| Atmosphere |
| ||||||
| Variation | |||||||
| Heliosphere | |||||||
| Related | |||||||
| Spectral class | |||||||
| Exploration | |||||||
