The corona is the very hot layer of the solar atmosphere above the chromosphere. It extends to Earth and beyond as the solar wind. The Sunıs temperature rises to 2 million degrees C (4 million degrees F) at the bottom of the corona, and remains almost that hot as it reaches Earth.
The high temperature of the corona presents one of the most puzzling problems of solar physics. The chromosphere and photosphere are closer to the Sunıs core than is the corona, but the corona is several hundred times hotter than the chromosphere and photosphere. According to the laws of thermodynamics (the branch of physics that deals with the movement and transfer of heat), heat cannot move from a cooler area to a warmer area. Scientists believe that the temperature of the corona results from effects of the Sunıs magnetic fields instead of radiation from the Sunıs core.
Comparisons of the corona and the Sun's magnetic fields have shown that the corona is hottest where the magnetic fields are strongest. The entire corona is stitched together by thin, bright, magnetized loops of material that constrain the hot, dense gas of the corona and shine brightly at X-ray wavelengths. These loops are in a continuous state of change - they can rise from inside the Sun, sink back down into it, or expand into space.
They often come together, sometimes merging with each other and sometimes destroying each other. The magnetic loops store magnetic energy. When they interact, the magnetic loops release their stored energy into the corona, providing the energy that keeps the corona so hot. The coronaıs magnetic field also has gaps in it, called coronal holes. When astronomers use X-ray telescopes to look at the corona, coronal holes appear as large dark areas, because they are cooler and contain less material than the rest of the corona.
Spectral lines come from atoms emitting and absorbing light when their electrons gain or lose energy. The corona is so hot that atoms in the corona are stripped of some of their electrons. These atoms then have different numbers and arrangements of electrons from atoms in the rest of the atmosphere and thus produce different spectral lines.
The corona emits most of its radiation at very short ultraviolet and X-ray wavelengths. The underlying photosphere emits very little radiation in these parts of the spectrum, so an image of the Sun in short ultraviolet and X-ray wavelengths produces an accurate picture of the corona. Much of the ultraviolet and X-ray radiation that hits Earthıs atmosphere is absorbed by atoms and molecules in the atmosphere, so scientists use instruments in space to study the corona.
Explosions in the CoronaSolar Flares and Coronal Mass Ejections
Studies of the corona reveal dramatic, violent events called solar flares and coronal mass ejections (CMEs). Solar flares release energy from magnetic loops in the corona, heating the gases of the corona and sending particles and radiation out into the solar system. A coronal mass ejection occurs when an explosion in the corona pushes millions or billions of metric tons of material out into space. The frequency of occurrence of both solar flares and CMEs follows the pattern of the 11-year sunspot cycle (as the number of sunspots increases, so does the number of solar flares and CMEs). Both kinds of solar explosions seem to result from the sudden release of energy stored in coronal magnetic fields.
The Sunıs ever-changing magnetism produces unrest on an awesome scale. The sudden, brief, intense outbursts called solar flares can rip through the Sunıs atmosphere with tremendous violence. They release energy equivalent to that of billions of hydrogen bombs in a just few minutes, increasing the temperature of Earth-sized regions of the corona by ten times and flooding the solar system with intense radiation.
During a solar flare, the tops of magnetized coronal loops release energy. In less than a second, electrons and positive ions within these loops accelerate to nearly the speed of light. The explosion hurls the electrons and ions out into space and down into the Sun. The particles strike the dense chromosphere below and produce high-energy X rays and gamma rays.
Solar flares are probably triggered when oppositely directed magnetic fields come together in the corona, releasing their stored magnetic energy in a manner similar to that of a tightly twisted rubber band that suddenly snaps. After releasing their pent-up energy, the magnetic fields reconnect and relax to a stable configuration.
Coronal mass ejections are giant magnetic bubbles that expand to nearly the size of the Sun itself as they leave the low corona. The CMEs move outward at speeds from 200 to 1,000 km/s (100 to 600 mi/s). They carry up to 10 billion metric tons of coronal material into the space of the solar system. They accelerate and propel ahead of them vast quantities of high-speed particles.
CMEs sometimes occur when part of the coronal magnetic field becomes sheared and twisted, often disrupting a filament (a loop of material in the chromosphere, also called a prominence). The filament shoots through the chromosphere into the corona, carrying material with it.
Coronal Explosions and Earth
Earth is affected by the radiation and particles that solar flares and coronal mass ejections release. Intense radiation from a solar flare reaches Earth's atmosphere in just eight minutes. The X-ray radiation of flares strips electrons from atoms and molecules in Earth's atmosphere, changing the electrical properties of the atmosphere.
This change can disrupt radio communications and make the atmosphere expand farther into space than usual. Friction can develop between the expanded atmosphere and satellites that orbit near Earth, slowing down the satellites. Frequent solar flares can also increase levels of ultraviolet radiation in the atmosphere, which in turn changes oxygen molecules into ozone (oxygen made up of molecules containing three oxygen atoms instead of the usual two). This added ozone actually helps block harmful radiation from the Sun.
Particles that solar flares and CMEs release take a day or more to reach Earth. Blasts of these particles can compress Earth's magnetic field. Disruptions in Earthıs magnetic field can cause geomagnetic storms. Geomagnetic storms occur when Earthıs magnetic field compresses and intensifies, then relaxes back to its normal intensity.
The increased intensity of the magnetic field can interfere with signals passing through the atmosphere and cause power surges on wires that carry electricity. CMEs can also trigger intense auroras, colorful displays of light that occur in the atmosphere near Earthıs poles when energetic particles enter the atmosphere. In this case, energetic charged particles collide with atoms and molecules of the atmosphere. This boosts the atoms and molecules to higher energies and forces them to glow. Particles released by a CME can damage or destroy Earth-orbiting satellites and may endanger astronauts in space.
Solar flares and CMEs have such a large potential for affecting Earth that space weather forecasters continuously monitor the Sun from ground and space to warn of threatening solar activity. If humans can learn to predict these violent events by pinpointing magnetic changes on the Sun, these predictions will provide very useful early warnings. Flares and CMEs are tied to the cycle of solar activity. The most recent maximum of solar activity occurred in 2001, and the next should occur in 2012. Forecasters study the Sun carefully during these periods.
The Sun's Wind
The outermost part of the Sun is a stream of particles that flows from the Sun into the solar system. This part of the Sun, called the solar wind, is the corona expanding into space. The solar wind extends all the way to the heliopause, far past the orbit of Pluto. The corona is so hot that it cannot stand still. It is expanding outward in all directions, filling the solar system with a ceaseless flow of electrons, ions, and magnetic fields.
The solar wind has two components. The fast part of the wind pours out of the regions near the poles of the Sun at speeds around 750 km/s (around 470 mi/s). The slower component of the solar wind gusts unevenly from the Sun's equatorial regions at speeds from 300 to 400 km/s (190 to 250 mi/s).
Scientists believe that the fastest part of the solar wind leaves the Sun through coronal holes, cool spots in the corona. The magnetic field of the Sun is relatively weak around coronal holes and thus allows particles in the solar wind to escape. Heavier particles seem to move more quickly than lighter particles in the same stream within coronal holes. The intermittent gusts from nearer the equator come from solar flares and coronal mass ejections.
Both components of the solar wind gain speed as they spread out and leave the Sun. The fast component reaches its top speed close to the Sun, but the slow solar wind continues gaining speed much farther out.
The Sun rotates as it emits the solar wind, so the solar wind spirals around the solar system. The solar wind carries the Sunıs magnetic field with it and sets up a spiral magnetic field throughout the solar system. The solar wind and its magnetic field affect the magnetic fields of the planets, the direction of the tails of comets, and even the flight paths of spacecraft.
History of Studying the Sun
The Sun is so important to life on Earth that humans have always paid special attention to it. The movement of the Sun across the sky helps mark time. The change throughout the year in the Sun's daily path helps mark the seasons. Many cultures attach special significance to solar events, such as eclipses. The brightness of the Sun made studying it closely difficult for humans for many years. Looking at the Sun directly is dangerous, and even thick clouds do little to protect human eyes from the damage that direct sunlight causes. Astronomers could not make true scientific studies of the Sun until they developed techniques to observe the Sun indirectly.
The study of the Sun has both pushed and been pushed by revolutionary scientific discoveries. Early indirect observations of the Sun, using a telescope, allowed scientific study of the Sun to begin, showing that the Sun is a dynamic, changing body. The development of spectroscopy and the discovery of elementary particles and nuclear fusion allowed scientists to begin to understand the composition of the Sun and the processes that fuel it. The development of artificial satellites and other spacecraft finally allowed scientists to study the Sun from space, allowing a full view of all of the Sun's radiation and a continuous study of the Sun.
A Beginning of Scientific Study
Greek philosopher Aristotle was the first known person to use a device that allowed indirect observation of the Sun. Sometime between 384 bc and 322 BC Aristotle noticed that a hole in a screen would create an image of the Sun on the ground, if the screen were between the Sun and the ground. He made a simple version of a device called a camera obscura to take advantage of this effect. A camera obscura is still a popular way to observe solar eclipses.
Italian scientist Galileo observed the Sun with a telescope for the first time in 1610. Looking through a telescope directly at the Sun is even more dangerous than looking at the Sun with the naked eye, so Galileo turned the telescope into a camera obscura. He pointed it at the Sun and then set up a screen behind the eyepiece. The eyepiece projected the image of the Sun onto the screen. Galileo observed sunspots with his telescope. He saw that sunspots rotate with the Sun and change in size and shape. Galileo's work showed that the Sun is a changing and active body.
Spectroscopy
The next major breakthrough in the study of the Sun was the development of ways to study sunlight. In the mid-17th century English scientist Isaac Newton used a prism - a specially cut chunk of glass - to break sunlight down into its different colors. This range of colors is called the Sunıs spectrum, and the study of spectra is called spectroscopy.
In 1802 British scientist William Wollaston found that the solar spectrum was cut by several dark gaps.
By 1815 German physicist Joseph von Fraunhofer had cataloged the wavelengths of more than 300 of the gaps, called absorption lines. Fraunhofer assigned letters to the most prominent absorption lines. In the mid-19th century German scientists Gustav Kirchhoff and Robert Bunsen related the absorption lines in the Sunıs spectrum to chemical elements. In 1925 English-born American astronomer Cecilia Payne (later Cecilia Payne-Gaposchkin) compared the spectrum of the Sun to that of other stars to show that virtually all bright, middle-aged stars have the same composition.
The spectrum of the Sun's corona was studied for the first time in the mid-19th century. During the solar eclipse of August 7, 1869, American astronomers Charles A. Young and William Harkness independently discovered that the coronaıs spectrum featured an especially bright line of green light. Bright lines in a spectrum are called emission lines. They are the fingerprints of elements in the substance producing the light. The corona's bright green emission line comes from highly ionized iron, indicating that the corona has very high temperatures.
Studying the Sun's Photosphere and Sunspots
Detailed studies of the Sun's photosphere and the sunspots began with Galileo's telescopic camera obscura of the 17th century. The next revolution in this area occurred in the 1840s, when German scientist Heinrich Schwabe discovered that the number and positions of sunspots vary over an 11-year period. In 1859 British astronomer Richard Carrington discovered solar flares. Carringtonıs discovery helped explain that geomagnetic storms (increased intensity of Earthıs magnetic field) are related to events on the Sun. In 1908 American astronomer George Ellery Hale showed that sunspots contain magnetic fields that are thousands of times stronger than Earthıs magnetic field.
Study of the Sunıs Energy
The Sun produces an enormous amount of energy. Scientists could not explain how something with the mass of the Sun could produce so much energy until they discovered nuclear fusion. The details of just how nuclear fusion changes hydrogen into helium nuclei were not known until discoveries in the field of elementary particles were made.
Elementary particles are the tiny particles that make up all matter. The most familiar particles, the particles that make up atoms, are protons, neutrons, and electrons. Protons and neutrons are the main particles involved in nuclear fusion. Both types of particles are about the same size and mass, but protons have a positive electric charge, while neutrons are electrically neutral. New Zealand-born British physicist Ernest Rutherford discovered the proton in 1918. British physicist Sir James Chadwick discovered the neutron in 1932, and was awarded the 1935 Nobel Prize in physics for his discovery.
The first fusion reaction in a laboratory occurred in the early 1930s. In 1938 German-born American physicist Hans A. Bethe and American physicist Charles L. Critchfield demonstrated how a sequence of nuclear reactions, called the proton-proton chain, makes the Sun shine. Bethe was awarded the 1967 Nobel Prize in physics for his discoveries concerning energy production in stars.
Discovering the Structure of the Sun
After scientists understood how the Sun produces its energy, they began developing theories to explain how the Sunıs energy travels from the core to the Sun's atmosphere. For the first few decades after the discovery that fusion powers the Sun, scientists deduced the Sun's structure by comparing the theoretical output of the Sun's core to the energy actually released at the Sun's atmosphere.
In the 1960s American physicist Robert Leighton developed a camera that could record Doppler shifts in light at the Sunıs surface. A Doppler shift is a change in the wavelength of light caused by the movement of the object that is emitting the light. If the object is moving away from the observer, each wave will have to travel farther to reach the observer, making the distance between waves (the wavelength) longer. An object moving toward the observer will seem to emit light with a shorter wavelength. Leighton used this device to discover that the Sun seemed to pulsate in and out, making a complete cycle about every five minutes. See the Oscillating Sun section of this article.
Leighton's discovery launched the field of helioseismology, or the study of the Sun's interior by observing the vibrations of the Sun and how sound waves move through it. In the 1970s scientists demonstrated that the entire Sun is vibrating with ponderous, organized rhythms that can extend to its very core. Scientists developed models of the interior of the Sun based on vibrations at its surface.
In 1995 six observatories around the world coordinated with each other to begin observing the oscillations of the Sun as a team. This project, a collaboration of 20 nations, is called the Global Oscillation Network Group (GONG). GONG can keep constant watch on the Sun because, at any given time, daytime is being experienced by at least one of the observatories. GONG has allowed scientists to get a better idea of the interior structure of the Sun through helioseismology.
Solar Research from Space
Studying the Sun from space has revolutionized solar physics. Since the first observations from space began to be made, scientists have made great advances in the study of the Sun's surface, energy production, structure, and relationship to Earth and the solar system. Study from space began in 1957 when the Soviet satellite Sputnik 2, the second satellite to go into space, carried instruments to study the Sun. Since then, many missions have been devoted to studying the Sun.
The series of missions by the Pioneer spacecraft of the United States included several experiments to study the Sun and its relationship to Earth. The Pioneer program lasted from the late 1950s to the 1970s. The U.S. Mariner 2 spacecraft, launched in 1962, used data obtained from its voyage to Venus to demonstrate that a low-speed solar wind is continuously emitted from the Sun, and also discovered high-speed streams in the Sun's winds.
In the 1960s and 1970s the U.S. Orbiting Solar Observatory (OSO) series studied the Sun over an entire cycle of solar activity. One of the satellites in the OSO series was the piloted Skylab space station, launched in 1973. Skylab astronauts used X-ray telescopes to transform our knowledge of the Sunıs corona. In the early 1980s the United States launched the Solar Maximum Mission spacecraft to study the Sun during its most active period. The joint Japanese, U.S., and British probe Yohkoh studied solar flares through the 1990s.
Two of the most productive solar spacecraft of the late 1990s and early 2000s were the Ulysses spacecraft and the Solar and Heliospheric Observatory (SOHO). Both spacecraft are joint projects of the United States and the European Space Agency (ESA).
Ulysses was launched in 1990 and SOHO was launched in 1995. Ulysses's orbit takes it over the poles of the Sun, then back to the planet Jupiter to get a gravitational boost that sends the spacecraft back to the Sun. By 2001 Ulysses had passed over the Sunıs north and south poles twice. Ulysses's mission has contributed much knowledge about the solar wind in regions above the Sun's poles.
Findings from this mission conclusively demonstrated that the fast component of the solar wind pours out at high solar latitudes, including from polar coronal holes. The slow component of the solar wind is constrained to low latitudes near the solar equator. Ulysses also found that the Sun's magnetic field does not warp at the poles as much as scientists expected.
SOHO is at a point in space where the Sun's gravitational pull balances Earth's gravitational pull, so the satellite orbits the Sun with Earth. SOHO always faces the Sun. This probe has allowed scientists to make great leaps forward in their understanding of the structure and dynamics of the solar interior, the heating mechanisms of the solar corona, and the origin and acceleration of the solar wind. SOHO has returned amazing images of the Sun, including comets hitting the Sun and features on the Sunıs surface that scientists compare to tidal waves, tornadoes, and rivers.
- Encyclopedia Encarta
Solar maximum or solar max is the period of greatest solar activity in the solar cycle of the sun. During solar maximum, sunspots appear.
Solar maximum is contrasted with solar minimum. Solar maximum is the period when the suns magnetic field lines are the most distorted due to the magnetic field on the solar equator rotating at a slightly faster pace than at the solar poles. The sun takes about 11 years to go from one solar maximum to another and 22 years to complete a full cycle (where the magnetic charge on the poles is the same).
Historic maximums
The last solar maximum was in 2001, and on March 10, 2006 NASA researchers announced that the next cycle would be the strongest since the historic maximum in 1958 in which northern lights could be seen as far south as Mexico. This projection was based on research done by Mausumi Dikpati of the National Center for Atmospheric Research (NCAR).
Solar Max and Earth Magnetics
Ongoing massive solar flares ---->solar mass ejections reach Earth ---->effect planetary magnetics ---->shifting ocean and jet stream currents in Pacific area - Ring of Fire ----> unusual and extreme weather patterns around the world ----> resulting in Earth Changes.
There appears to be a correlation between the rise and fall of civilizations with the rise and fall of radiation from the sun. The graph shows a long-term envelope of sunspot activity derived from the center graph of Carbon 14. More carbon 14 is absorbed in the growth rings of tress during the sunspot minima. Sunspot minima also correlcate with mini-ice ages and a winter severity index based on a mean for Paris and London - for the period shown. The Maya disappeared during a sunspot minimum.
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