A corona (Latin, ‘crown’) is a type of plasma of the Sun or other celestial body, extending millions of kilometres into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. The word “corona” itself derived from the Latin, meaning crown, which in turn came from the Ancient Greek κορώνη (korōnē) meaning “garland” or “wreath”.
The high temperature of the corona gives it unusual spectral features, which led some to suggest, in the 19th century, that it contained a previously unknown element, “coronium”. These spectral features have since been traced to highly ionized iron (Fe-XIV) which indicates a plasma temperature in excess of 106 kelvin. The fact that the Sun has a million-degree corona was first discovered by Gotrian in 1939 and Bengt Edlén in 1941 by identifying the coronal lines (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green FeXIV line at 5303 Å, but also the red line FeX at 6374 Å).
Light from the corona comes from three primary sources, which are called by different names although all of them share the same volume of space. The K-corona (K forkontinuierlich, “continuous” in German) is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines completely obscures them, giving the spectral appearance of a continuum with no absorption lines. The F-corona (F for Fraunhofer) 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. The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona’s composition.
The sun’s corona is much hotter (by a factor of nearly 200) than the visible surface of the Sun: the photosphere‘s average temperature is 5800 kelvin compared to the corona’s one to three million kelvin. The corona is 10−12 times as dense as the photosphere, and so produces about one-millionth as much visible light. The corona is separated from the photosphere by the relatively shallow chromosphere. The exact mechanism by which the corona is heated is still the subject of some debate, but likely possibilities include induction by the Sun’s magnetic field and sonic pressure waves from below (the latter being less probable now that coronae are known to be present in massive, hot, highly magnetic stars). The outer edges of the Sun’s corona are constantly being transported away due to open magnetic flux generating thesolar wind.
Since the corona has been photographated at high resolution in the X-rays by the satellite Skylab in 1973, and then later by Yohkoh and the other following space instruments, it has been seen that the structure of the corona is very various and complex: different zones have been immediately classified on the coronal disc . The astronomers usually distinguish several regions, as described below.
The active regions are ensembles of loop structures connecting points of opposite magnetic polarity in the photosphere, the so-called coronal loops. They generally distribute in two zones of activity, which are parallel to the solar equator. The average temperature is between two and four million Kelvin, while the density goes from 109 to 1010 particle per cm3.
The active regions involve all the phenomena directly linked to the magnetic field, which occur at different heights on the Sun’s surface: sunspots and faculae, happening in the photosphere, spicules, Hα filaments and plages in the chromosphere, prominences in the chromosphere and transition region, and flares and coronal mass ejectionshappening in the corona and chromosphere, but if flares are very violent can perturb also the photosphere and generate a Moreton wave, as described by Uchida. On the contrary, quiescent prominences are large, cool dense structures which are observed as dark, “snake-like” Hα ribbons (filaments) on the solar disc. Their temperature is about 5000–8000 K, and so they are usually considered as chromospheric features.
In 2013, images from the High Resolution Coronal Imager revealed never-before-seen “magnetic braids” of plasma within the outer layers of these active regions.
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).
Solar plasma feeding these structures is heated from under 6000 K to well over 1×106 K from the photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one foot point and drain from the other (siphon flow due to a pressure difference, or asymmetric flow due to some other driver).
When the plasma goes upward from the footpoints towards the loop top, as it always occurs during the initial phase of a compact flare, it is defined as chromospheric evaporation. When the plasma rapidly cools falling down towards the photosphere, we have the chromospheric condensation. There may also be symmetric flow from both loop foot points, causing a buildup of mass in the loop structure. The plasma may cool rapidly in this region (for a thermal instability), creating dark filaments in the solar disk or prominences off the limb.
Coronal loops may have lifetimes in the order of seconds (in the case of flare events), minutes, hours or days. Usually coronal loops lasting for long periods of time are known as steady state or quiescent coronal loops, where there is a balance in loop energy sources and sinks (example).
Coronal loops have become very important when trying to understand the current coronal heating problem. Coronal loops are highly radiating sources of plasma and therefore easy to observe by instruments such as TRACE; they are highly observable laboratories to study phenomena such as solar oscillations, wave activity and nanoflares. However, it remains difficult to find a solution to the coronal heating problem 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 arrived at, but due to the high plasma temperatures in the corona, in-situ measurements are impossible (at least for the time being). The next mission of the Nasa Solar Probe Plus will approach the Sun very closely allowing more direct observations.
Large-scale structures are very long arcs which can cover over a quarter of the solar disk but contain plasma less dense than in the coronal loops of the active regions.
The large-scale structure of the corona changes over the 11-year solar cycle and becomes particularly simple during the minimum period, when the magnetic field of the Sun is almost similar to a dipolar configuration (plus a quadrupolar component).
Interconnections of active regions
The interconnections of active regions are arcs connecting zones of opposite magnetic field, in different active regions. Significant variations of these structures are often seen after a flare.
Some other features of this kind are helmet streamers—large cap-like coronal structures with long pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered as sources of the slow solar wind.
Filament cavities are zones which look dark in the X-rays and are above the regions where Hα filaments are observed in the chromosphere. They were first observed in the two 1970 rocket flights which also detected coronal holes.
Filament cavities are cooler clouds of gases(plasma) suspended above the Sun’s surface by magnetic forces. The regions of intense magnetic field look dark in the images, because they are empty of hot plasma. In fact, the sum of the magnetic pressure and plasma pressure must be constant everywhere on the heliosphere in order to have an equilibrium configuration: where the magnetic field is higher, the plasma must be cooler or less dense. The plasma pressure can be calculated by the state equation of a perfect gas , where is the particle number density, the Boltzmann constant and the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect makes sunspots dark in the photosphere.
The fraction of the solar surface covered by bright points varies with the solar cycle. They are associated with small bipolar regions of the magnetic field. Their average temperature ranges from 1.1 MK to 3.4 MK. The variations in temperature are often correlated with changes in the X-ray emission.
Coronal holes are the polar regions which look dark in the X-rays since they do not emit much radiation. These are wide zones of the Sun where the magnetic field is unipolar and opens towards the interplanetary space. The high speed solar wind arises mainly from these regions.
In the UV images of the coronal holes, some small structures, similar to elongated bubbles, are often seen as they were suspended in the solar wind. These are the coronal plumes. More exactly, they are long thin streamers that project outward from the Sun’s north and south poles.
The Quiet Sun
The solar regions which are not part of active regions and coronal holes are commonly identified as the quiet Sun.
The equatorial region has a faster velocity rotation than the polar zones. The result of the Sun’s differential rotation is that the active regions always arise in two bands parallel to the equator and their extension increases during the periods of maximum of the solar cycle, while they almost disappear during each minimum. Therefore the quiet Sun always coincides with the equatorial zone and its surface is lower during the maximum of the solar cycle. Approaching the minimum of the solar cycle (also named butterfly cycle), the extension of the quiet Sun increases until it covers the whole disk surface excluding some bright points on the hemisphere and the poles, where there are the coronal holes.
VARIABILITY OF THE CORONA
Flares take place in active regions and provoke a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they interest several zones of the solar atmosphere and involve many physical effects, thermal and not thermal, and sometimes wide reconnections of the magnetic field lines with material expulsion.
Flares are impulsive phenomena, of average duration of 15 minutes, even if the most energetic events can last several hours. Flares involve a high and rapid increase of the density and temperature.
An emission in white light is only seldom observed: usually, flares are only seen at EUV wavelengths and in the X-rays, typical of the chromospheric and coronal emission.
In the corona the morphology of flares, which can be grasped from the observations in the soft and hard X-rays, at the UV wavelengths and in Hα, is very complex. However, two kinds of basic structures can be distinguished:
As for temporal dynamics, three different phases are generally distinguished, whose duration are not comparable. These times, moreover, can depend on the range of wavelengths used to observe the event even considerably:
Sometimes also a phase preceding the flare can be observed, usually called as “pre-flare” phase.
Accompanying solar flares or large solar prominences, “coronal transients” (also called coronal mass ejections) are sometimes released. These are enormous loops of coronal material traveling outward from the Sun at over a million kilometers per hour, containing roughly 10 times the energy of the solar flare or prominence that accompanies them. Some larger ejections can propel hundreds of millions of tons of material into space at roughly 1.5 million kilometers an hour.
A solar storm
These movies have been taken by the satellite SOHO during two weeks in October and November 2003. The images have been taken at the same time by the different instruments on board SOHO: the MDI, producing magnetograms, the Extreme ultraviolet Imaging Telescope (EIT), which photographs the corona in the ultraviolets, and the Large Angle and Spectrometric Coronagraph (LASCO).
The first video at the top on the left (in grey) shows the magnetograms as they vary in time. At the top on the right (in yellow) the photosphere can be seen in white light as taken by the MDI.
Furthermore the EIT filmed the event in its four filters which are sensitive to different wavelengths, selecting plasma at different temperatures. The images in orange (on the left) refers to chromospheric plasma, while that one in green (on the right) to the corona.
In the last movie at the centre the Sun’s images taken in the ultraviolet filter by the EIT have been combined with those taken by the coronograph LASCO blue and white in this movie.
All the instruments registered the storm which is considered as one of the largest solar activity events observed by SOHO and maybe since the advent of space-based solar observations. The storm involved all the plasma of the solar atmosphere from the chromosphere to the corona, as can be seen from the movies, which are ordered from left to right, from top to bottom, in the outward direction of the increasing temperature on the Sun: photosphere (yellow), chromosphere-transition region (orange), low corona (green) and extended corona (blue).
The corona is visible to the SOHO/LASCO coronagraph instruments, which block the bright disk of the Sun so the significantly fainter corona can be seen. In this movie, the inner coronagraph (designated C2) is combined with the outer coronagraph (C3).
As the movie plays, we can observe a number of features of the active Sun. Long streamers radiate outward from the Sun and wave gently due to their interaction with the solar wind. The bright white regions are visible due to their high density of free electrons which scatter the light from the photosphere towards the observer. Protons and other ionized atoms are there as well, but are not as visible since they do not interact with photons as strongly as electrons. Coronal Mass Ejections (CMEs) are occasionally observed launching from the Sun. Some of these launch particle events can saturate the cameras with snow-like artifacts.
Also visible in the coronagraphs are stars and planets. Stars are seen to drift slowly to the right, carried by the relative motion of the Sun and the Earth. The planet Mercury is visible as the bright point moving left of the Sun.
The horizontal “extension” in the image is called blooming and is due to a charge leakage along the readout wires in the CCD imager in the camera.
Coronal stars are ubiquitous among the stars in the cool half of the Hertzsprung-Russell diagram. These coronae can be detected using X-ray telescopes. Some stellar coronae, particularly in young stars, are much more luminous than the Sun’s. For example, FK Comae Berenices is the prototype for the FK Com class of variable star. These are giants of spectral types G and K with an unusually rapid rotation and signs of extreme activity. Their X-ray coronae are among the most luminous (Lx ≥ 1032 erg·s−1 or 1025W) and the hottest known with dominant temperatures up to 40 MK.
The astronomical observations planned with the Einstein Observatory by Giuseppe Vaiana and his group showed that F-, G-, K- and M-stars have chromospheres and often coronae much like our Sun. The O-B stars, which do not have surface convection zones, have a strong X-ray emission. However these stars do not have a corona, but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs. Also A-stars do not have convection zones but they do not emit at the UV and X-ray wavelengths. Thus they appear to have neither chromospheres nor coronae.