page_title: Stellar corona
section_title: Coronal loops
hierarchical_section_title: Stellar corona / Physical features / Active regions / Coronal loops
caption_attribution_description: Example of solar coronal loops observed by the Transition Region And Coronal Explorer (TRACE), 171&#160;Å filter. These loops have a temperature of approximately 106&#160;K. These loops contrast greatly with the cool chromosphere below.
context_page_description: A corona is an aura of plasma that surrounds the Sun and other stars. The Sun's corona extends millions of kilometres into outer space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. Spectroscopy measurements indicate strong ionization in the corona and a plasma temperature in excess of 1000000 kelvin, much hotter than the surface of the Sun.
  Light from the corona comes from three main sources, from the same volume of space:
  The K-corona is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines spreads them so greatly as to completely obscure them, giving the spectral appearance of a continuum with no absorption lines.
  The F-corona is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the zodiacal light.
context_section_description: Coronal loops are the basic structures of the magnetic solar corona. These loops are the closed-magnetic flux cousins of the open-magnetic flux that can be found in coronal hole (polar) regions and the solar wind. Loops of magnetic flux well-up from the solar body and fill with hot solar plasma. Due to the heightened magnetic activity in these coronal loop regions, coronal loops can often be the precursor to solar flares and coronal mass ejections (CMEs).
  The Solar plasma that feed these structures is heated from under 6000K to well over 10⁶ K from the photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one point and drain to another, called foot points (siphon flow due to a pressure difference, or asymmetric flow due to some other driver).
  When the plasma rises from the foot points towards the loop top, as always occurs during the initial phase of a compact flare, it is defined as chromospheric evaporation. When the plasma rapidly cools and falls toward the photosphere, it is called chromospheric condensation. There may also be symmetric flow from both loop foot points, causing a build-up of mass in the loop structure. The plasma may cool rapidly in this region (for a thermal instability), its dark filaments obvious against the solar disk or prominences off the Sun's limb.
  Coronal loops may have lifetimes in the order of seconds (in the case of flare events), minutes, hours or days. Where there is a balance in loop energy sources and sinks, coronal loops can last for long periods of time and are known as steady state or quiescent coronal loops.  (example).
  Coronal loops are very important to our understanding of the current coronal heating problem. Coronal loops are highly radiating sources of plasma and are therefore easy to observe by instruments such as TRACE. An explanation of the coronal heating problem remains as these structures are being observed remotely, where many ambiguities are present (i.e. radiation contributions along the LOS). In-situ measurements are required before a definitive answer can be had, but due to the high plasma temperatures in the corona, in-situ measurements are, at present, impossible. The next mission of the NASA, the Parker Solar Probe will approach the Sun very closely allowing more direct observations.
caption: TRACE 171Å coronal loops
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