A corona is the very hot outermost layer of a star's atmosphere. Our Sun's corona can only be seen during a total solar eclipse. (Source: NASA's "Starchild" Glossary.)
One of the most puzzling features of the Sun is what has been dubbed "the solar corona problem." There is a region around the Sun, extending more than one million kilometers from its surface, where the temperature can reach two million degrees. This region, called the Solar Corona, is where the solar wind originates. The corona has been found to emit X-rayradiation (the corona is a plasma; at temperatures greater than a million degrees a plasma will radiate a lot of X-rays). The corona can be seen during solar eclipses, when the main radiation from the Sun's surface is blocked by the passage of the Moon.
The problem is, no one can really explain how this corona exists. Even if the temperature in the core of the Sun does reach 15 million degrees, it drops to a mere 5000 degrees at the surface. The temperature should be even lower farther away from the sun. But the temperature of the corona is measured at more than a million degrees. This incredibly hot temperature requires a permanent heating mechanism, or the plasma would cool down in about an hour. There are many mechanisms which could heat some gas above the surface of the Sun, but none of those mechanisms could account for the large rate of heating necessary to heat the corona. This phenomenon remained a mystery for more than 50 years. Things have changed. Although the details of the answer are not completely known, it does seem that the solution is near.
Using data from instruments onboard the SOlar and Heliospheric Observatory (SOHO) and from the more recent Transition Region And Coronal Explorer (TRACE), solar physicists have identified small patches of magnetic field covering the entire surface of the Sun. Contrary to the bright, large magnetic field loops which are linked to the "active regions" during periods of solar maxima, these patches seem to appear and disappear randomly in time scales on the order of 40 hours. Scientists now think that this magnetic carpet is probably a source of the corona's heat. (Source: "The Sun's Corona" - NASA's Imagine the Universe.)
Solar radiationLast Updated on 2010-01-24 19:11:07
Almost all of the energy that drives the various systems (climate systems, ecosystems, hydrologic systems, etc.) found on the Earth originates from the sun (Figure 1). Solar energy is created at the core of the sun when hydrogen atoms are fused into helium by nuclear fusion (Figure 2). The core occupies an area from the sun’s center to about a quarter of the star’s radius. At the core, gravity pulls all of the mass of the sun inward and creates intense pressure. This pressure is high enough to force the fusion of atomic masses.
For each second of the solar nuclear fusion process, 700 million tons of hydrogen is converted into the heavier atom helium. Since its formation 4.5 billion years ago, the sun has used up about half of the hydrogen found in its core. The solar nuclear process also creates immense heat that causes atoms to discharge photons.... More »
Solar Corona: CoronagraphsLast Updated on 2007-12-26 00:00:00
For thousands of years, humans had stared at the sun during total eclipses, marveling and even trembling at the halo of greenish light that surrounded the blackness like the iris of some malevolent eye. The likeness to a human eye was not lost on civilizations as ancient as the Egyptians, and for whom the vengeful Eye of Horus may have been its antecedent. But to study this ineffable glow in more detail, only the rare total solar eclipse provided the means. By the late 1800’s astronomers and solar observers realized that the corona was the key to understanding many solar phenomena, perhaps even the mysterious aurora borealis themselves.
Bernard Lyot sitting at his Coronagraph at the Pic du Midi observatory in France ca 1939. (Courtesy: American Museum of Natural History.)
In 1930, Bernard Lyot (1897-1952) invented an instrument that allowed total solar eclipses to be recreated at... More »
Solar Eclipses: Ancient GreeceLast Updated on 2007-11-27 00:00:00
By 450 BC, the Greek civilization was in its ascendancy. The historian Herodotus (ca 460 BC) mentions that Thales was able to predict the year when a total solar eclipse would occur. Details of how this prediction was made do not survive. The eclipse occurred in either 610 BC or 585 BC. Apparently the method used worked only once because what is known of Greek scientific history does not suggest that the method was ever reliably used again.
Thales is said to have visited Egypt, and from the empirical rules in use there for land surveying, brought back to Greece the ideas of deductive geometry later codified by Euclid. Before 450 BC, Meton realized that a single period of 235 lunar months (19 years) would cause the popular lunar calendar to return to synchrony with the solar, seasonal calendar. At this time, the same lunar phase would be recorded at the same time of the solar calendar... More »
Solar Corona: First ObservationsLast Updated on 2007-11-26 00:00:00
It’s amazing to think that in the thousands of years that humans have admired total solar eclipses, virtually no ancient sketches or descriptions of the details of such an event have survived. These eclipses were regarded with great interest by dozens of civilizations spanning 3000 BC to 1000 AD. Among the most famous were the Ancient Egyptians. One would imagine that a total solar eclipse of their chief god Ra would have been cause for comment on at least one monument.
The most often quoted account of an eclipse (possibly on March 20, 71 AD) is that provided by the Greek philosopher Plutarch (45-120 AD), found in sections 931D-E of his book, De Facie in Orbe Lunae (On the face which appears in the Moon). Although the description is not believed to be written in Plutarch’s own voice, but is an account narrated by a character named Lucius, it notes that, 'Now, grant me that nothing... More »
Solar Coronal Mass Ejections: First ObservationsLast Updated on 2007-11-26 00:00:00
Since the 1970’s, astronomers have studied brief explosions of matter in the solar corona which were at one time called ‘coronal transients’. Thanks to satellite observatories such as SMM and Skylab, these transients were quickly found to be complex and spectacular eruptions of matter, now called coronal mass ejections or CMEs. We commonly mark the advent of the discovery of these events with the observations made during the modern satellite era, but this may not be a fair assessment of past historical events.
"1860: First observations of a coronal mass ejection." drawing by G. Tempel.
See "Preview Image Source below, for further description.
The total solar eclipse of 18 July 1860 was probably the most thoroughly observed eclipse up to that time. What is unusual about this eclipse is that, unlike most drawings of the solar corona up until that time, the drawings of the... More »
Drag and drop the content to change the order of featured content. The top nine will be displayed.