Solar corona

The solar corona is the outermost, high-temperature portion of the solar atmosphere. This region of the solar atmosphere is the originating site of the solar wind, a flux of charged particles emitted by the Sun, and is considered to be the inner boundary of the interplanetary medium.

Overview

The solar corona is the outer atmosphere of the Sun. Well known from antiquity, it can be seen with the unaided eye during a total solar eclipse as a glowing white halo around the silhouette of the Moon. In all other instances, as the Moon is not blocking the bright light from the Sun’s photosphere, the corona is too faint at visible wavelengths to be observed without special instruments. The corona was not positively confirmed as a solar feature, rather than an artifact of the Earth’s atmosphere, until photographs of it were noted to look the same from widely separated sites on the Earth.

Spectroscopic studies of the corona show that its visible light emission is a continuous background of all visible wavelengths together with a few superimposed broad emission lines. Thus, its Spectrum is quite different from that of the photosphere, the visible solar disk, which has a visible light spectrum consisting of a continuous background with many dark absorption lines, produced by some of the elements found in the Sun. The coronal emission lines did not have wavelengths that matched those identified from any known chemical element, and some astronomers suggested that they were produced by a new element, dubbed “coronium.” In 1925, similar lines were observed in the spectrum of a Nova (an exploding star), RR Pectoris. In 1939, Walter Grotrian identified a red coronal line as being emitted by iron ions with nine electrons removed (called Fe X, X being the Roman numeral 10), and in 1942, Bengt Edlen identified a green coronal line as being emitted from iron ions with thirteen electrons removed (Fe XIV). It turned out that highly ionized atoms of familiar elements such as iron, calcium, and argon are the source of the coronal emission lines.

The high degree of ionization requires high temperatures in excess of one million kelvins. It is now thought that this high temperature is produced by the Sun’s strong magnetic field. Magnetic waves from the turbulent convective photosphere follow magnetic field lines upward into the lower density corona (about 10¹⁵ particles per cubic meter or less, compared to 10²² particles per cubic meter in the upper photosphere), becoming supersonic shock waves that transfer their energy to the coronal gas. In addition, the complex pattern of magnetic field lines may twist and reconnect, releasing energy. This heating causes the temperature to rise steeply from a minimum of about 4,400 kelvins in the upper part of the photosphere to about 1.5 to 2 million kelvins in the corona. In spite of its high temperature, because of its low density the corona is only about one-millionth as bright as the photosphere at visible wavelengths. This is why the corona usually cannot be seen with the human eye except when the Moon covers the much brighter photosphere during total solar eclipses.

In the early 1930s, a French scientist, Bernard Lyot, perfected the coronagraph, a telescope that artificially created an eclipse of the solar disk. The invention of this type of telescope allowed observation of the corona at times other than total solar eclipses, and for the first time it was possible to follow the evolution of the outer atmosphere of the Sun. The coronagraph is now standard equipment at many ground-based solar observatories, and similar instruments have also been used for studies of the corona performed from orbiting spacecraft. The development of this instrument has been vital to constantly monitoring the corona and studying how it evolves with time. Studies of the corona involving determination of its temperature, density, and chemical composition occupied researchers for the next three decades.

A major advance in solar physics was the publication in 1958 of a classic paper on the dynamics of interplanetary gas, by Eugene Parker of the University of Chicago. He deduced that, because of the high temperature and low density of the corona, there should be a continual outflow of ionized gas, which he called the solar wind, from the Sun's outer edges. Confirmation of the existence of the solar wind was quickly forthcoming from early satelliteexperiments and, with it, the recognition that Earth is actually surrounded by this flow of charged particles. Earth is exposed to a constant flux of charged particles—mostly electrons, protons (hydrogen nuclei), and alpha particles (helium nuclei)—from the Sun, moving at speeds of a few hundred kilometers per second. However, only a tiny fraction of the Sun’s mass is lost via the solar wind, about 10^-14 of its mass per year.

The corona may be observed not only at visible wavelengths but also in the ultraviolet and X-ray bands of the electromagnetic spectrum. A major advantage of observing at ultraviolet and X-ray wavelengths is that the corona is much brighter in these parts of the spectrum than the photosphere . This is the opposite of visible wavelengths. Therefore, the corona is easily observed against the disk of the Sun. However, Earth’s atmosphere effectively blocks ultraviolet and X-ray radiation from reaching Earth’s surface, so these wavelength regions must be observed above our atmosphere. Thus it was natural to turn to the brightest source in the sky, the Sun, as the first astronomical object to study with instruments carried above the absorbing atmosphere. The first ultraviolet observations of the Sun were obtained with a Spectrograph attached to a fin of a V-2 rocket, which was launched from the White Sands test facility on October 10, 1946. The results demonstrated conclusively the value of sending observational equipment above the absorbing blanket of air and opened a new field of ultraviolet astronomy.

As a step toward the exploration of the solar spectrum in these two wavelength regions, a series of sounding rocket experiments were conducted by the University of Colorado, the Air Force Cambridge Research Laboratory (AFCRL), and the Naval Research Laboratory (NRL) between 1958 and 1965. These experiments succeeded in characterizing the intensity and the wavelength distribution of the ultraviolet and soft X radiation from the solar corona. Many of these pioneering efforts were undertaken by Richard Tousey of NRL, a leader in this expanding field in the 1960s. The problem with using sounding rockets is that they are above Earth’s atmosphere for no more than a few minutes, so long-duration observations are impossible. The solution is to launch spacecraft into orbits around the Earth, and in some cases around the Sun, so the Sun may be observed for prolonged periods of time. Some of the first of these were the Orbiting Solar Observatories (OSOs), launched by the National Aeronautics and Space Administration (NASA). Nine of these spin-stabilized satellites were launched over a thirteen-year period, from March 1962 through June, 1975. The major research gains made through the use of these satellites included a definition of the differences between active solar regions and quiet regions as seen in the ultraviolet. Another positive aspect was evidence that material found in the corona is cooler over the solar poles than at the equator. Also, detected was the presence of coronal “hole” feature; large low-density regions of the corona that typically last for several of the Sun’s twenty-seven-day rotation periods. A white-light coronagraph flown on OSO 7 captured the ejection of a huge mass of material upward from the Sun’s disk, a mass that evidently had sufficient speed to escape the solar gravitational field.

Initially, NASA intended to conduct a second series of solar investigations from satellites. These were to be the Advanced Orbiting Solar Observatory (AOSO) series. These advanced spacecraft were to have improved pointing capabilities and were intended to employ higher spatial Resolution for studies of the solar atmosphere. This program was integrated into the crewed Skylab program. A set of high-resolution solar instruments, including a white-light coronagraph and several ultraviolet and X-ray imaging telescopes, was placed into Earth orbit with the Apollo Telescope Mount (ATM) system on Skylab. Of the many factors that made the ATM-Skylab mission uniquely valuable, two had special bearing on studies of the solar corona. First, the nine-month duration of the mission permitted uninterrupted observation of the evolution and activity of the Sun’s atmosphere over a very wide range of wavelengths that are not observable from Earth’s surface. Second, since this was a crewed mission, it was possible for the science teams to exploit interactive observing modes, where the data obtained suggested new observations, with the astronauts controlling the instrumentation to optimize the collection of scientific data.

The ATM-Skylab experiments, along with the data and knowledge gained from them about the solar corona, form a cornerstone for modern solar research concerning the outer portions of the Sun’s atmosphere. The ultraviolet and X-ray telescopes carried in this orbiting observatory were able to return new images of how the material in the corona is distributed over the disk of the Sun. For the first time, astronomers were able to specify the morphology and evolution of the distribution of mass in the outer atmosphere. Coronal holes and their relationship to the solar wind were investigated in detail with ATM data. It also became clear, using the images from the white-light coronagraph, that there is considerable transient activity in the corona, and numerous mass ejection events were detected.

Based on the ATM-Skylab experience, a second white-light coronagraph was launched on the NASA Solar Maximum Mission (SMM) spacecraft in February 1980. This instrument operated successfully for nine months until it was subject to an electronics failure. The equipment was later repaired in orbit by astronauts who were transported to the satellite by the space shuttle (STS-41C). The electronics package was successfully replaced, and the coronagraph experiment was subsequently operated for a number of years. Thus, the SMM coronagraph has been used to define the variations of the solar corona over a rather long period of time.

Knowledge Gained

The physical characteristics of the solar corona are now reasonably well known, as are its basic evolutionary characteristics. The corona is made up of a fully ionized mixture of the solar elements, mostly hydrogen and helium. Metals and other elements constitute a minor fraction of the total mass. The solar mixture is ionized by the high temperature of the outer atmosphere. The high ionization states of the elements identified by their emission lines in the spectrum of the corona require temperatures on the order of 1.5 to 2 million kelvins.

The solar surface exhibits numerous regions in which there is a concentration of magnetic flux. Magnetic forces are frequently strong enough to dominate the motion of the coronal plasma (ionized gas). Unlike Earth’s atmosphere, which is controlled by the interaction of pressure and gravitational force, the Sun’s outer atmosphere reacts to the interplay of three forces: pressure, gravity, and magnetism. Active regions, associated with sunspots, often show coronal material to be configured into loop or arch patterns, as if the material is confined to specific magnetic field structures. Coronal holes tend to form where the magnetic field lines are open and oriented radially outward into interplanetary space.

The white glow of the corona seen during total solar eclipses is produced by the scattering of photospheric light off the free electrons in the coronal plasma. On average, the electron density near the base of the corona is about 10¹⁴ to 10¹⁵ per cubic meter; it is greater near active regions and Sunspots and less in coronal hole regions. The major white-light structures tend to be long streamers, which extend from the limb (edge) of the Sun more or less radially, and loops, which are almost always associated with concentrations of surface magnetic fields. Streamers viewed at eclipse are often found to extend outward 4 to 6 solar radii. The nominal value of the solar radius is 700,000 kilometers, a length approximately equal to twice the distance from Earth to the Moon. Streamers occur over two kinds of solar disk features: long, linear streamers occur frequently over magnetically active regions and sunspots, and helmet streamers, which are shaped somewhat like bowling pins, are often seen over magnetic neutral lines, areas in which the magnetic polarity switches from one sign to the other.

The Sun has a twenty-two-year cycle of magnetic activity. Sunspots rise and fall in number with a period that is half of the magnetic cycle, so concentrations of magnetic flux have been observed to be at maxima in 1958, 1969, and 1980, for example. The total amount of material in the corona is modulated by this cyclic variation of magnetic flux, and the total mass of the corona varies by about a factor of 2 over the sunspot cycle. Also over the sunspot cycle, the number of coronal streamers occurring at any given time varies by a factor of 2.

By observing sunspots and other solar features, it is possible to establish the rotation period of the Sun. Near the equator, it is found to be about twenty-seven days. The basic rotation rate of the corona is approximately equal to the equatorial rotation rate, and the bright features used to follow coronal rotation have lifetimes that range from one to five months. By the late 1930s, it was known that some effects of the Sun dominated Earth’s upper atmosphere. Ionospheric disturbances and perturbations of Earth’s magnetic field were observed to follow a twenty-seven-day period (similar to the Sun’s rotation). These so-called M-regions could not, however, be correlated with distinct solar structures, such as an enhanced concentration of magnetic field. Following the discovery of the solar wind and its variation in space and time, the problem of locating the solar origins for Earth-perturbing wind streams began to receive much attention from scientists. During the 1970s, the origins of the high-speed solar wind streams were eventually identified with open magnetic field configurations and solar coronal holes, based on the OSO 7 and ATM-Skylab data sets. This correlation constituted a major breakthrough in the task of associating the interplanetary magnetic field and flux configuration at Earth with the physical conditions of the corona at the base of the solar wind.

Along with the solar wind, there are other sporadic ejections of solar material that escape the pull of solar gravity. First detected in the early 1970s, coronal mass ejections (CMEs) were finally explained using data from the white-light coronagraph carried in the ATM-Skylab observatory. Often appearing as huge loops or bubblelike structures having a dimension of half a solar radius in the lower corona, the ejections expand as they rise through the corona and may exceed the size of the Sun at heights above 5 solar radii. The average amount of mass ejected is on the order of 10¹² kilograms, and their mean outward velocity from the Sun’s surface is typically 300 to 400 kilometers per second.

Some CMEs are associated with flare activity, the catastrophic conversion of magnetic energy into thermal energy in or near sunspot regions. Occasionally, CME events occur in areas that have no obvious sunspots. CME events occur about once every three to five days during solar minimum, and the production of events can reach one to two per day during times near the maximum of the activity cycle. CME events constitute one of the most energetic phenomena detected on the Sun; typically, the Kinetic energy of such an event can exceed 10²⁵ joules.

Context

The corona provides scientists with a laboratory for studies of how low-density, high-temperature plasmas interact with magnetic fields, and it affords investigators a view of a situation in which pressure, gravity, and electromagnetic forces operate simultaneously. The study of such interactions is known as magnetohydrodynamics (MHD), and the solar corona offers investigators an example at close range where complete MHD processes, such as coronal mass ejections, may be observed over a wide variety of both temporal and spatial scales.

Recognition that the Earth is, in fact, subjected to constant bombardment by the solar wind flux has given new impetus to studies of how this wind is generated and how it is controlled by MHD processes. At the center of the solar system, there is a magnetic star, i.e. the Sun, that modulates the interplanetary space beyond it. The corona, reflecting the magnetic organization of the Sun over a great variety of spatial scales, is astronomers’ best clue to the initial organization of the interplanetary magnetic field and the structure of solar wind flow. Once the locations of coronal hole structures are identified, either by observing in the X-ray or ultraviolet regions or by using limb observations from coronagraphs, it is possible to predict when the sub-Earth point on the Sun is occupied by a coronal hole. This knowledge allows the prediction, with fair accuracy, of when Earth will be subjected to a high-speed stream of solar wind.

The geophysical significance of coronal observations has led to international collaboration in satellite investigations of the Sun. The Solar and Heliospheric Observatory (SOHO) satellite, launched in 1995 and operated cooperatively by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA), provides continuous monitoring of the Sun from a stable orbit around the Sun at the Earth-Sun L1 Lagrangian point. This is the point, located 1.5 million kilometers from Earth along the line between the Earth and Sun, where the gravitational pull of the Sun and Earth are equal. Among SOHO’s many devices for studying various aspects of the Sun, it carries plasma diagnostic instruments for investigation of the solar wind along with several telescopes for solar coronal studies.

Studies of the Sun’s corona have provided insights into observed features of other stars. In the late 1970s and early 1980s, X-ray observations from the High-Energy Astronomical Observatories (HEAOs) were interpreted to show, to the surprise of many investigators, that almost all types of stars have outer coronas that radiate at the short wavelengths characteristic of high temperatures. It is now accepted that coronas are a standard feature of stellar atmospheric structure.

Similarly, spectroscopic studies performed in the 1980s with the International Ultraviolet Explorer (IUE) satellite demonstrated that stellar winds, outflows of ionized gas like the solar wind, are common from other stars. However, unlike the Sun, which is a star of moderate Luminosity and surface temperature, luminous hot stars have stellar winds driven by the radiation pressure of the intense Ultraviolet radiation they emit. Again, insight gained from Earth’s own star has aided in the identification and interpretation of a common stellar process.

In December 2021, NASA flew, for the first time, a spacecraft that entered the upper atmosphere of the Sun. This was the Parker Solar Probe mission. Launched in 2018, from 2021-2028 the mission is scheduled to make 24 orbits around the Sun. The probe is unique in that it is shielded by a carbon-composite shield that is 11.43 centimeters (4.5 inches) thick. Researchers hope Parker can provide insights into why the Sun’s corona is much greater in temperature than its photosphere. Parker may also compile new data on the nature of the solar wind.

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