The Corona

At the center of our solar system there is a magnetic variable star, our Sun, which drives every cubic centimeter of interplanetary space. The upper atmosphere of the Sun, the solar corona extends from the visible disk of the Sun outward, eventually enveloping the earth. The earth, our home planet, is located at a distance of about 200 solar radii from the visible surface of the Sun. The dimension of a solar radius is roughly 700,000 km, approximately twice the distance from the earth to the Moon, and the solar radius is a convenient scale for discussing the solar corona, and the heliosphere, the extension of the solar atmosphere into interplanetary and interstellar space. Astronomers feel comfortable using the solar radius as a measure of length when discussing the corona, the interplanetary medium, and the sizes of other stars.

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Total solar eclipse images of 1980 February (above) and 1988 March (below) taken from sites located in India (1980) and the Philippines (1988) by expeditions from the High Altitude Observatory of Boulder, Colorado. Note that the 1980 image, taken near the maximum of the solar activity cycle shows many streamers located at all azimuths around the occulted disk of the Sun. Taken later in the cycle, about a year past the minimum, the 1988 image shows several large (bottle-shaped) helmet streamers which are restricted to latitudes between N45 and S45. The helmet streamers, which are large scale, dense structures, have measured lifetimes from less than one to more than several solar rotations.

A special telescope, known as the White Light Coronal Camera, was used for both of these observations. Half of the diameter of the dark central image of the moon is equal to a distance of one solar radius.

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Through most of history, coronal research has been dominated by the simple fact that observation was possible only during the special astronomical circumstance of a total solar eclipse. There are between two and five solar eclipses each year, but many occur over the oceans and are not easily documented. Some are not total, being only partial or annular, and a good opportunity for eclipse observation comes along every only two or three years. Solar eclipses are also brief, the average duration of totality being only two to three minutes, limiting efforts to study evolution of the corona to following changes in the corona from one eclipse observation to another.

There are a number of natural timescales operating on the Sun. The Sun rotates on its own axis once every 27 days (as viewed from the earth), and the period of the magnetic variation most often detected using sunspots is an 11-year fluctuation. Other types of changes in the structure of the corona take place on a variety of time scales ranging from minutes to a fraction of a day. Thus progress in investigating the solar corona was paced by the availability to investigate the changes of the solar corona by following ground-based observations of a series of total solar eclipses. The white light corona seen at the time of a total eclipse is the result of scattering of sunlight by electrons in the corona.

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A combination of two coronal images, one taken from the ground and one from space. The central image was made in soft X-rays by an instrument on the Yohkoh ("Sunbeam") satellite (Japan); it shows the very hot plasma in primarily closed magnetic structures in the magnetically dominated lower corona. The blue-white image was made at the same time with a white light (electron scattering) coronagraph based at Mauna Loa, Hawaii, and operated by the High Altitude Observatory of Boulder, Colorado. In this case the large scale, relatively weak magnetic field structures of the solar corona are seen extending upward for roughly a solar radius in altitude.

In the 1930's, a French astronomer, Bernard Lyot, solved the technical problem of creating an artificial eclipse of the Sun within a telescope system, and since that time it has been possible to view the solar corona on regular basis. Even with this development, there are practical limitations to ground-based observing of the solar corona imposed by the scattering of light by both dust and molecules in the earth's atmosphere, as the brightness of the white light corona ranges from one millionth to one billionth of the central solar disk brightness. The coronagraph flown on the 1973 Skylab mission solved this problem by observing from a location on the Apollo Telescope Mount, a cluster of instruments used to view the solar atmosphere from this early version of a space station. By using a coronagraph in space, it became possible to made eclipse like observations as often as one wished for an extended period of time. In the case of Skylab, the mission lasted almost nine months, about nine solar rotations, but only one fifteenth of the duration of the solar magnetic variability cycle. The Solar Maximum Mission spacecraft, launched in 1980 and operated until 1989, represented a further refinement of the use of a coronagraph on a satellite observatory platform for the investigation of the nature of the solar corona since it was possible to accumulate thousands of images of the solar corona over this nine-year period.
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The lower solar corona as seen in soft X-rays on 1993 February 25. The bright regions of this image indicate the magnetic complexity found in the corona above sunspots and active regions. The base of a helmet streamer structure is seen in the lower right, and the dark lane at the lower, central portion of the disk is a coronal hole structure. Coronal holes are large scale features of reduced density (and are therefore dark in soft X-ray images, since the soft X-ray intensity is proportional to the square of the electron density in the emitting region) and are identified as being open magnetic field regions which are sources for high speed streams of solar particles (electrons, protons, and ions).

By using a combination of eclipse and coronagraph observations, a picture of the solar corona has emerged which suggests that the solar corona is a place where unique physical conditions and processes exist. Spectroscopy of the corona suggests that, by some not fully understood mechanism, the Sun has the ability to create very high temperature material in the corona. Radiation characteristic of one to two million degrees are regularly observed with coronagraph instruments. Images of the corona made from satellites in low earth orbit in the soft X-ray region of the spectrum demonstrate a highly structured corona where besides the forces of pressure and gravity, magnetic fields play a role in the determination of the Sun's outer atmosphere. Occasionally observations of flare regions in the corona demonstrate radiation which is interpreted to originate at very high temperatures between 10 to 40 million degrees C. These situations arise in areas where coronal magnetic fields are relatively strong and it is believed that the Sun has an effective mechanism for converting magnetic field energy into thermal energy. Current research indicates that in regions of relatively high magnetic field strength in the solar corona, corresponding to structures of small scale size (a few hundredths of a solar radius in length), some of the most energetic radiative processes originate in these small scale, high magnetic field regions of the corona.
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A coronal mass ejection (CME) event in progress. These two images were made with the coronagraph flown on the Solar Maximum mission spacecraft and demonstrate the scale and speed of a CME event. The occulting disk image is about 1.8 solar radii in diameter and the images are taken a few minutes apart. The large loop-shaped CME structure is roughly the size of the Sun in the second image, and the velocities estimated for this type of event range from several hundred to a thousand kilometers per second (well over a million miles an hour), a velocity that would take a space traveler from the earth to the Moon in twenty minutes.

In contrast to solar flares, which occur in small scale structures with relatively high magnetic field strength, there is a second kind of energetic phenomenon detected in the solar corona. These are the huge mass ejection events which were discovered and first studied in detail in the early 1970's with data collected with the Skylab and OSO-7 coronagraphs; a much larger data set was amassed with the later P78-1 and Solar Maximum Mission instruments. Evidently, some of the largest scale structures of the corona, which are governed by large scale, weak magnetic fields, become unstable and huge amounts of mass are occasionally discharged from the solar atmosphere out into the heliosphere. Particle detectors carried on research satellites operating between Venus and Jupiter have confirmed that these ejections are detected far from the Sun, and must sometimes impact the earth. At the time of peak solar magnetic activity near the maximum in the sunspot cycle, there are two or three such events per day. Near the minimum of the magnetic activity cycle this rate falls to approximately one or two mass ejection events every ten days. The size scales of such events are typically seen to be a large fraction of a solar radius, and the speed of ejection averages to a value of about 400 km/s. The detection, analysis, physical mechanisms and consequences of coronal mass ejections remains a topic of concentrated scientific research at this time.
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Composite of a SPARTAN 201, ground-based coronagraph, and Yohkoh soft X-ray image obtained during the first flight of the SPARTAN 201 system. Images such as this have been used to:

Three forces are active in the solar corona at the base of the heliosphere; these are gas pressure and gravity forces similar to those experienced by humans near the earth's surface, and a third force produced by solar magnetic fields. As a consequence of these forces, a continuous flux of material is ejected from the Sun and blows outward through the heliosphere: the solar wind of charged particles.

Within a few solar radii of the Sun's visible surface, magnetic forces are thought to be the cause of the structuring seen at the times of total solar eclipse, such as helmet streamers and coronal holes. Coronal holes are now known to be regions where the density of the corona is considerably reduced, causing a relatively dark region to appear in soft X-ray and EUV (extreme ultraviolet) images. During much of the magnetic activity cycle there are semi-permanent polar coronal holes, and it has been known since the Skylab era that coronal hole structures seen in the solar corona are associated with the detection of high speed solar wind streams which sweep past the earth. The physical mechanisms for the acceleration of the solar wind and the conditions of interplanetary space, which slowly evolve in step with the change of the Sun's periodic variation of magnetic field, are also the subject of intense interest to the international research community.

Text provided by Dr. Richard R. Fisher, NASA Goddard Space Flight Center
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