The Sun, which is the source of life on Earth since its origin is certainly the star which was the most observed since the appearance of mankind. Nevertheless, its complexity is such that scientists are still far from understanding all of the its behavior. Despite the privileged position we have, the Sun is, nevertheless, a common star among many other stars, but the only one that we see closely and that we can study with sufficient precision.
- Distance Earth-Sun : about 150 millions kilometers
- Diameter : about 1.4 million kilometers (Earth = 12 700 km)
- Mass : about 2 .1030 kg (Earth = 6 . 1024 kg)
- Density : 1.41 g/cm3 (water = 1 g/cm3, Earth = 5,5 g/cm3)
- Power radiated by the Sun : 4 . 1023 kW
- Solar energy received by the Earth : 1353 J/m2/s
- Temperature at the centre of the Sun : about 14 millions Kelvins (0 K = -273.15 °C)
- Temperature at the "surface" of the Sun : about 5 800 K
- Composition : hydrogen (94 %), helium (6 %), traces of oxygen, carbon, nitrogen, silicium, neon, iron, sulfur essentially but all atoms are present.
The first estimation (wrong) of the distance from the Earth to the Sun was made by Aristarchus, during the IIth century BC. He found about 20 times the Earth-Moon distance (actually, 390 times). But the first valid result was obtained in 1671 by Jean Richer and Jean -Dominique Cassini at the Paris Observatory. Aristarchus was also the first to estimate the diameter of the Sun finding 5 to 10 times that of the Earth (about 100 times, in fact). p>
Thanks to Newton, during the XVIIth century, we could estimate the mass of the Sun.
Sunspots were highlighted by Fabricius in 1611, Scheiner in 1612 and Galileo in 1613, the latter from observations he made in 1610 , when he projected the image of the Sun through his telescope on a white sheet, like his contemporaries.
In 1672, Isaac Newton separated sunlight through a prism and obtained to the first solar spectrum. The first full spectrum of absorption of the Sun (chemical signatures of the elements) was performed by Fraunhofer in 1814, which marks the real beginning of astrophysics.
Originally, the observations of the Sun were made without instrument or by means of refractors, since the time of Galileo. It was therefore an observation in visible sunlight, but it emits a spectrum that extends beyond the visible: gamma rays (γ), X rays, ultraviolet (UV) (after the violet end of the arc -en-ciel ) infrared (beyond the red of the rainbow), and radio waves. Each "slice" of wavelength requires its own type of observational instrument more or less complex.
Earth's atmosphere absorbs a large part of solar radiation, and only some wavelength bands reach the ground. It is, of course, visible light, until some radio wavelengths.
Tools for observing the Sun from the ground in the radio wavelengths are radio telescopes. These are large antennas looking to the sky, the invention of which date of the fifties, as radioheliograph of Nançay in Sologne, France. Considering the visible range, mainly two types of instruments coexist.
- solar telescopes with spectrograph, as the Pic du Midi de Bigorre, as the Solar Tower in Meudon Observatory or the Franco-Italian THEMIS telescope in the Canary Islands allow to make images of the Sun, or to break the light to obtain the spectrum
- coronagraphs as the Pic du Midi de Bigorre, which, if they work on the principle of the telescope, have the particularity to obscure the solar disk thus revealing the corona the light of which being normally masked by the too high surface brightness of the Sun. The first coronagraph was invented in 1930 by the French Bernard Lyot.
As for other wavelengths not observable from the ground, some instruments -often telescope types- embedded aboard satellites or balloons are sent over the Earth's atmosphere. This is for example the case of the US-European SOHO satellite, operating a battery of telescopes and coronagraphs observing the Sun in ultraviolet wavelengths, or the Japanese satellite YOHKOH which gives images of the Sun in X-rays
Understanding the structure of the Sun need to reproduce the observations made by means of laws of physics. Physical and mathematical representations, with powerful computers can simulate what happens in the Sun's atmosphere. The comparison of these simulations and models developed with the observation allows to test the accuracy of our understanding of the daytime star.
The Sun, like all stars, is a ball of gas emitting its own light. This gas is very dense in the heart of the Sun and becomes very thin as it moves away of the center. However, we can separate this star in successive layers, as onion skins, each of which playing a special role. Already two parts may be isolated: the layers that can be "seen" (the word "seen" is used to mean with a telescope, regardless of wavelength), and those we can not "see". This allows to define a kind of arbitrary surface, and thus an inside and an outside.
Although the Sun is a ball of gas with no real surface we use this word to designate the part of the Sun whose radiation does not reach us directly.
1.1. Nuclear reactions
The Sun is a star. This means that it emits its own light (unlike a planet like Earth, that simply reflects a large part of the sunlight). Hence, from where this radiation is coming? Many theories were proposed , for example the conversion of gravitational energy into light : the Sun would shrink regularly and the increase of energy associated with this contraction would serve to maintain solar radiation, but the life of the Sun could not exceed a few tens of millions of years, whereas it is billions of years which must be considered, or even a model of bombardment by meteorites whose impact energy is transformed into light energy, but the life of the Sun would be extremely short. In 1920 Eddington suggested that the energy of the stars was nuclear. We now know that this is the fusion of hydrogen into helium which provides energy to the Sun : four hydrogen nuclei fuse to form a helium nucleus :
4 H1 ÷> He4 + energy
Every second, 633 millions tons of hydrogen are converted therefore into 628 millions tons of helium. That is to say, every second, nearly 5 millions tons of solar material is converted into energy (based on equivalence mass-energy: E = mc2), which is radiated into space. This reaction is accompanied by the emission of other particles called neutrinos, which are particularly difficult to detect.
The region where nuclear reactions take place is named the core. It is estimated that extends from the center to about 0.25 solar radius. P>
Up 0.70 solar radius extends the radiative zone. The material is relatively opaque, so the radiation is there constantly absorbed to be reissued in a random direction. These permanent "bounces" make energy taking about 1 million years to cross this area. p>
The outermost layer of the solar interior, occupying 0.30 solar radius, is named the convective zone. The material, very opaque, causes an accumulation of the energy from the zone radiative, creating convection currents - similar to a whirlpool of water boiling in a pot - the hot material ascending to the surface, cold material falling. The radiation put here two months to cross the convective zone.
The outer layers of the Sun are the only ones we can observe directly. The choice of the wavelength allows direct observation targeting one or the other of the outer layers (the higher the temperature is, the wavelength decreases, from the visible to X-ray). Photosphere and chromosphere are viewed in visible light, the transition zone in the ultraviolet and the corona in X-rays, in radio waves and visible light.
This is the most famous part of the Sun, because it is what we see to the naked eye (although you should never do), or when projected solar image through binoculars or a telescope. What is named the solar disk, or the surface of the Sun is the photosphere. Until the middle of this century, it was the main part of the Sun that was observed. The material is sufficiently thin to allow the radiation to pass through. The average temperature of the surface of this layer of a few hundred kilometers thickness is of 5800 K. The density will drop rapidly outwardly, and its temperature, after passed through a minimum of about 4000 K starts to increase.
The average temperature is around 10 000 K, increasing from the Sun outwards. The thickness of this layer is a few thousand kilometers. Its reddish color appearance is due to the emission of the hydrogen and it has given its name. As it is much less dense than the photosphere, the emission from the chromosphere is much lower than the photospheric emission. This is why, normally, the chromosphere is invisible, the sensitivity of our eye being saturated by the light of the solar disk. However, the radiation of the chromosphere is more intense than the full moon! During solar eclipses one can observe the chromosphere, where the brightest part of the disk Solar is hidden. Observations at specific wavelengths make possible to observe it on the disk, as is the case with the spectroheliograms made at Meudon Observatory.
This zone is very thin, but very complicated to study because the temperature varies from a few tens of thousands of degrees to several million degrees within a few hundred kilometers.
The corona extends as a thin halo around the Sun and is visible during eclipses or through instruments in space. The temperature reaches several million degrees. It has no precise limit and is mixed with the interplanetary medium. A flow of material escapes continuously and sweeps the entire solar system.
Although it seems to always present the same aspect, watching carefully, it is evident that the Sun is changing : changing aspect of its "surface", sudden ejections of material in the space, explosive phenomena, ...
Sunspots, discovered by Galileo, follow the rotation of the Sun, appear as dark regions because they are cooler than the surrounding material - but still about 4,000 degrees. These are regions where the solar magnetic field is very intense, therefore the interest of their study that tells us about it and solar activity that is related. Their size can reach tens of thousands of kilometers in diameter for the largest.
These are sudden release of a large amount of energy, accompanied by the ejection of particles accelerated in space. The energy released by a small part of the solar surface may approach the energy released by the whole disk.
The prominences are impressive phenomena which can sometimes be observed during solar eclipses. They are seen under the aspect of large pipes forming an immense arch on the edge the Sun. The dimensions of these loops can be enormous, an object point of the dimension of the Earth could easily pass under the greatest. It is believed that this is the summit of huge loops of magnetic field inside which the material from the low solar atmosphere is trapped. As this material is colder, when a prominence is located above the solar disk, we can see it under the form of a dark filament.
The most visible sign of solar activity is the number of spots on its surface. In times of minimum activity, there can be no spot, whereas at the maximum period, there may be several hundreds. The duration of the cycle is 11 years during which the Sun therefore passes successively through a maximum and a minimum.
The contraction of a huge mass of gas itself, causing the appearance of nuclear fusion reactions , gave birth to the Sun, 4.5 billion years ago. The amount of material that "burns" every second allows to estimate the duration of its life to about 10 billion years. After this time, after a brief phase of contraction, it will begin a new process of nuclear fusion with heavier chemical elements, swelling at the same time to absorb the orbit of the Earth, while ejecting its outermost layers, and, having no more usable nuclear fuel , it will shrink again to become an inert dense object that will cool slowly.
Besides the heat it provides us, the day-night cycle due to the rotation of the Earth and the seasons caused by the tilt of the Earth relative to the axis of its orbit, the Sun influences other ways the terrestrial environment. During its moments of highest activity, it sends in space clouds of high energy particles at high speed, coming hit the upper atmosphere of the Earth, organized according to Earth's magnetic field, which, fortunately, protects us. An obvious consequence of this is the phenomenon known under the name of aurora, but other effects are felt, highlighted in recent decades : disturbance of radio communications, destruction of the electronic board of some satellites, and mainly the danger to astronauts in space at that time.
Finally, at the present time, researchers are attempting to find a relationship between the times when the maximum of solar activity is the lowest, and mini-ice age that took place on Earth.
Since we succeeded to observe the solar corona, the question that arose was to know from where came the phenomenon, from the Sun or from the Moon. It was not until the mid-nineteenth century that it was possible to decided in favor of a phenomenon related to the Sun. Nevertheless it remained that long, the study of the solar corona was paired with the opportunity to observe a total solar eclipse, a phenomenon relatively rare, at least in areas easily accessible to scientists . Travel difficulties of the time, insecurity of countries where to go, and, of course, uncertainty to make the observation because of clouds made very random the attempts to study this corona. It was not until 1930 and the invention by the French Bernard Lyot, of the Paris Observatory, of the coronagraph for the study of the solar corona that the scientific studies really make a giant leap. It was possible to monitor the outer layer of the Sun almost continuously without waiting for a total eclipse. It was not before the second half of XXth century that it was possible to observe in radio astronomy, just after the war, and, a fifteen years later, the beginnings of astronautics, even if the first experiences with balloons occurred earlier. Indeed, if the coronagraph was a way to limit the brightness of the Sun in visible wavelengths in order to observe the corona, the radio waves, ultraviolet and X-rays, were emitted only by the corona and therefore there was no risk of glare. But the UV and X-rays are stopped by the atmosphere , and therefore it was necessary to be able to move above it to fully benefit of such radiation. However, at present, coronagraphs continue to be based on the principle developed by Lyot.
This is in the middle of the XIXth century , that really started the study of the physics of the solar corona. The idea of applying spectrometric observation to the solar corona returns to the French astronomer Jules Janssen, from the Observatory of Paris. In 1868, using spectrometry, he succeeded to identify an element unknown he named helium (from Helios, the Sun god) which will be identified on Earth at the end of this century. In subsequent eclipses other unidentified spectral lines were found in the spectra made, including one called "Coronium" which turned out to be -many years later- iron heated to a temperature of a few million degrees. We did not imagine at that time, that the solar corona could be extended to such temperatures. Although the physical interpretation of observations became much later, the contribution of these new methods was considerable, both for the study of the corona for the prominences , huge arches of cold material trapped by the solar magnetic field rising in the corona. The prominences can also be observed on the disk of the Sun, in the form of dark filaments, absorbing radiation from the deeper layers of the solar atmosphere. The next major step for the physical knowledge of the solar corona was therefore the invention of the coronagraph in 1930, which allowed to do not need eclipses to observe the solar corona . However, only "real" solar eclipses allowed to observe the weak radiations from the high crown. In 1944, the astrophysicist Joseph Shklovsky demonstrated theoretically that the region of transition between the high chromosphere (temperature of the order of tens of thousands of degrees) and the corona (at more than one million degrees) could only be very thin. Many observations were needed to confirm this deduction. During the solar eclipses of 1952 and 1970 American and Japanese astronomers, showed that the corona was down to a few thousand miles away from the "surface" of the Sun. The eclipse of 1991 allowed to demonstrate certain behaviors at large scale of the corona, especially the origin of some major projections of material into space, that astronomers hoped to confirm through international campaigns of observation during the eclipse of August 11, 1999.
Meanwhile, the Ulysses probe studies the behavior of the particles emitted by the Sun, the satellite YOHKOH observes the Sun in X-rays, and the SOHO satellite carries two coronagraphs, all contributing to provide the necessary information in the development of theories for understanding the solar corona.
More generally, the observations of the corona provided the opportunity to study the plasma (ionized matter and magnetic field) and its properties. The corona, as the Sun itself, is a macrophysical laboratory as it is not possible to build it on Earth.
However two major questions remain about the solar corona : first of all what is the mechanism which heats several million degrees (whereas it was more logical that the temperature decreases from the center to the periphery of the Sun). Second, where is the origin of the solar wind, which physical processes cause the acceleration of particles at different speeds from the Sun? One can reasonably expect that the treatment of many SOHO satellite observations will allow in the near decades to answer these questions.
During a total solar eclipse, any telescope or binoculars - with the proper filters ! - allows detailing the solar corona. This simplicity (which is actually apparent) of the equipment necessary for the observation of the corona greatly facilitates the work of both professional astronomers and amateurs, the difficulty however remaning in the place of observation : eclipses have the bad taste to occur often in places not easy to access (in the middle of the oceans...). Another "defect" of the eclipses of the Sun : they last only a short time (maximum duration of an eclipse at the equator is less than 8 minutes !) and the observer is dependent on the state of the sky at the time of the totality phase : the lower clouds will currently nullify all efforts often made during the several months of preparation.
However, if all the right conditions are met, observation of a natural solar eclipse is of great benefit. First, the sky darkens considerably during it, which greatly reduces the diffusion problems of the sky brightness in the instruments. On the other hand, the apparent diameter of the Moon, very close to that of the Sun, mask precisely the solar disk, thus allowing the observation of the very low corona, something impossible if a cache system is used, as it is in the case of coronagraphs. This access to the base of the corona allows to make the links with the lower layers of the solar atmosphere.
The principle of the coronagraph consists in - in a simplified manner - placing an occulting disc having a diameter close to the apparent diameter of the Sun, which hides the solar disk from which comes most of the solar light, avoiding to be dazzled and allowing to highlight faint areas (the corona is about one million times less brighter than the solar disk). This revolutionary invention allowed to overcome the stress of waiting for a total eclipse in an area accessible to the observation allowing to advance the study of the corona. However, the need to minimize scattered light from the solar surface needed to install an occulting disk slightly larger than the apparent diameter of the Sun. We can no longer observe the lowest region of the corona but understanding the interface between the lower atmosphere and the crown is fundamental to understand how to explain it. p>
Despite this drawback, the coronagraph is a fundamental tool to the study of the phenomena of the middle and the high corona, and for its large scale behavior.
The observation of radio waves coming from the corona provides information about the behavior of the particles that compose the corona. Indeed, at temperatures of a few million degrees, many atoms (which of course hydrogen, which is the essential component of the solar material) lost all their electrons. Thus, electric currents are induced by the magnetic field, they generate a specific behavior of the electrons, which result in the emission of radio waves. These waves, usually having wavelengths between a few centimeters and several tens meters, allow to observe the signature of the motion of electrons of the corona, each wavelength being emitted at a different altitude of the solar corona, which allows to obtain a kind of information "in relief" on the high solar atmosphere. However, the spatial resolution does not allow to have information on the small-scale behavior of the corona. p>
Balloons and rockets are ways to get out of the area of the atmosphere which absorbs the major part of the ultraviolet or X radiation. It is possible to install more classical coronagraphes or spectrographs. However, the difficulty of stabilizing a balloon and the brevity of a rocket flight are relatively marginal methods, although useful for observing the corona.
Satellites can take complex instrumentation for periods of several years, which makes significant strengths for this type of study. For example spectrographs for X-rays, as the Japanese satellite YOHKOH. The good stability of satellites offers unparalleled opportunities which, however, must be tempered by the difficulties to manipulate remote instruments and process observations taken by cameras from which we can not control the accuracy so simple.
The Ulysses probe carries radio instruments on board, flew at large distances from the poles of the Sun, and provided information on a region which is not easily accessible from Earth.
The SOHO satellite, meanwhile, is stationary with respect to the Sun. He took twelve instruments up to 1.5 million kilometers from the Earth (only one hundredth of the distance Sun-Earth), at the Lagrange point L1 (where gravitational attractions of the Earth and the Sun are in equilibrium) much farther than the Moon, and remains permanently, 24 hours pointed on the Sun. Among the instruments on board are two coronagraphes: LASCO formed of a battery of three coronagraphes in "cascade " showing the entire corona from 1.1 solar radius to 30 solar radii and observes various wavelengths in the visible range, and UVCS, watching portions of the corona from its base up to a distance 12 solar radii, in the ultraviolet and visible wavelengths.
However, technical observations from space have the default to limittheir lifespan.
The corona is not uniformly bright: the brightness decreases from the edge of the Sun outwards. It is therefore difficult to simultaneously obtain good images either for the lower and upper corona. If the low corona is overexposed, the high corona is underexposed. To overcome this problem, it can be used more special absorbent filters, towards the internal corona than outwardly. < /p>
As we saw in the previous chapter, each technique of observation has its own advantages and its own disadvantages, none is sufficient to study everything, in fact, observations at different wavelengths are complementary: some are the signature of regions at different temperatures, so different parts of the orona, other provides information on the motion of particles in the corona and the magnetic field. All tell us little after little about the items that can be used to reconstruct the complicated puzzle that form the solar corona, the behavior of the solar wind and the reason why the temperature of the corona is so high.
Yet , beyond the astrophysical observation of a "natural" solar eclipse, the beauty and strangeness of such an event is an unforgettable experience in the life of an individual.
Credit : J. Aboudarham/observatoire de Paris