Solar corona
The solar corona is the outermost layer of the Sun's atmosphere. It is a region filled with relatively hot, tenuous plasma that is structured by the solar magnetic field.
The solar corona lies above the photosphere and chromosphere and extends out to the edge of the solar atmosphere where it merges with the solar wind. The chromosphere and corona are separated by a thin, highly dynamic transition region. The outer edge of solar atmosphere where the corona transitions into the solar wind is defined by the Alfvén surface which forms an irregularly shaped boundary around the Sun at heights ranging from about above the photosphere.
Coronal light is typically obscured by diffuse sky radiation and glare from the solar disk, but can be easily seen by the naked eye during a total solar eclipse or with a specialized coronagraph. Spectroscopic measurements indicate strong ionization in the corona and a plasma temperature in excess of, much hotter than the surface of the Sun.
The solar corona is populated by structures such as prominences, coronal loops, and helmet streamers.
Observational history
In 1724, French-Italian astronomer Giacomo F. Maraldi recognized that the aura visible during a solar eclipse belongs to the Sun, not to the Moon. In 1809, Spanish astronomer José Joaquín de Ferrer coined the term 'corona'. Based on his own observations of the 1806 solar eclipse at Kinderhook, de Ferrer also proposed that the corona was part of the Sun and not of the Moon. English astronomer Norman Lockyer identified the first element unknown on Earth in the Sun's chromosphere, which was called helium. French astronomer Jules Jenssen noted, after comparing his readings between the 1871 and 1878 eclipses, that the size and shape of the corona changes with the sunspot cycle. In 1930, Bernard Lyot invented the "coronograph", which allows viewing the corona without a total eclipse. In 1952, American astronomer Eugene Parker proposed that the solar corona might be heated by myriad tiny nanoflares, miniature brightenings resembling solar flares that would occur all over the surface of the Sun.The high temperature of the Sun's corona gives it unusual spectral features, which led some in the 19th century to suggest that it contained a previously unknown element, "coronium". Instead, these spectral features have since been explained by highly ionized iron. Bengt Edlén, following the work of Walter Grotrian in 1939, first identified the coronal spectral lines in 1940 as transitions from low-lying metastable levels of the ground configuration of highly ionised metals.
General characteristics
Energy generated by nuclear fusion in the Sun's core heats the overlying interior and atmospheric layers as it is transferred outward. Temperatures in the interior decrease with increasing distance from the core and reach a minimum of 4400 K at the top of the photosphere, the Sun's surface layer. In the atmospheric layers above the photosphere, this trend reverses, and temperatures begin to increase with increasing altitude. This increase is most extreme at the top of the chromosphere—about 1600 km above the photosphere—where there is an irregular, approximately 100 km-thick transition region across which temperatures rise from about to more than with a corresponding increase in ionization and decrease in density. The hot, tenuous layer above this transition region is the corona, the outermost layer of the solar atmosphere.Temperatures in the corona typically range from to but can reach as high as in some active regions. The coronal plasma at these temperatures is almost completely ionized and highly rarefied. The particle number density is 10 particles per m3 at the base of the corona and generally decreases further with altitude due to gravitational stratification. These densities are low enough that collisions between particles are extremely rare, and the plasma is nearly collisionless.
The elemental composition of the corona is not uniform and varies between different coronal features. Compared to the uniform composition of the underlying photosphere, the coronal plasma generally has an overabundance of elements with low first ionization potential.
The solar magnetic field permeates the entire corona and influences the structure and dynamics of the coronal plasma. Charged particles in a magnetic field will be forced to spiral around the magnetic field lines and will not cross them except when scattered by collisions with other particles. As a result, the nearly collisionless coronal plasma only flows along the coronal field; closed field lines that start and end in the photosphere but project into the corona, such as in coronal loops, will confine the coronal plasma, while open field lines that reach interplanetary space, such as in coronal holes, will allow the coronal plasma to escape into the solar wind.
Radiation
is emitted, scattered, and absorbed by plasma and dust particles in the corona. Because of its low density, the corona is transparent to most wavelengths, and the majority of the radiation emitted in the corona, chromosphere, and photosphere can pass through without being scattered or absorbed. However, the small fraction of light from the photosphere that does get scattered forms the bulk of the total radiation from the corona in the visible range. Light scattered in the corona is typically divided into two components based on the scattering mechanism.- The K-corona is from photospheric light Thomson scattering off free electrons in the corona. Doppler broadening of the scattered 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 from photospheric light scattering off dust particles located beyond about one solar radius above the photosphere. These dust particles are much slower than the electrons, so Doppler broadening is negligible. As a result, the Fraunhofer absorption lines observed in the photospheric spectrum are also observed in the F-corona spectrum. The F-corona extends to very high elongation angles from the Sun and merges with the zodiacal light.
A small portion of the total visible light from the corona also originates from spectral lines emitted by ions in the hot, rarefied coronal plasma. The coronal spectral lines emitted from across the electromagnetic spectrum collectively form the E-corona. In the visible range, the integrated light from the continuous K- and F-corona far exceed that from the E-corona, but the isolated emission lines are strong relative to the continuous background and can be observed with narrow-band filters. Many of the E-corona emission lines are produced by forbidden transitions from metastable energy levels.
The corona also emits radiation in the radio, infrared, extreme ultraviolet, and X-ray ranges.
Structure
The upwelling of the solar magnetic field from the action of the solar dynamo constantly changes the structure of the solar corona. The corona is not always evenly distributed across the surface of the Sun. During periods of quiet, the corona is more or less confined to the equatorial regions, with coronal holes covering the polar regions. However, during the Sun's active periods, the corona is evenly distributed over the equatorial and polar regions, though it is most prominent in areas with sunspot activity. The solar cycle spans approximately 11 years, from one solar minimum to the following minimum. Since the solar magnetic field is continually wound up due to the faster rotation of mass at the Sun's equator, sunspot activity is more pronounced at solar maximum where the magnetic field is more twisted. Associated with sunspots are coronal loops, loops of magnetic flux, upwelling from the solar interior. The magnetic flux pushes the hotter photosphere aside, exposing the cooler plasma below, thus creating the relatively dark sun spots.High-resolution X-ray images of the Sun's corona photographed by Skylab in 1973, by Yohkoh in 1991–2001, and by subsequent space-based instruments revealed the structure of the corona to be quite varied and complex, leading astronomers to classify various zones on the coronal disc.
Astronomers usually distinguish several regions, as described below.
Coronal loops
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 holes and the solar wind. Loops of magnetic flux well up from the solar body and fill with hot solar plasma.The solar plasma that feeds these structures is heated from under 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.
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, 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, 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.
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. In-situ measurements are required before a definitive answer can be determined, but due to the high plasma temperatures in the corona, in-situ measurements are, at present, impossible. NASA's Parker Solar Probe approaches the Sun very closely, allowing more direct observations.