VI.

The Sun’s Atmosphere

The material in the Sun farther out from the center than the photosphere makes up the Sun’s atmosphere. The atmosphere extends far beyond the disk we see in the sky. Very diffuse solar gases extend all the way to Earth and beyond.

The solar atmosphere consists of, from the innermost part outward, the photosphere, the chromosphere, the corona, and the expanding outer layers of the corona that astronomers call the solar wind. The photosphere is the visible part of the Sun. We look right through the chromosphere, the corona, and the solar wind, just as we see through Earth’s atmosphere at night.

The chromosphere and corona are visible during total solar eclipses, when the Moon lines up between the Sun and Earth, completely blocking the main disk of the Sun from view. The thin chromosphere becomes visible a few seconds before or after a solar eclipse, creating a narrow pink, rose, or ruby-colored band at the edge of the Sun. For up to eight minutes during an eclipse, the corona is visible to the unaided eye as a faint, shimmering halo of pearl-white light spreading out from the lunar silhouette. Although the light of the chromosphere and corona is not bright enough to be dangerous, and can be viewed safely without filters during the total phase of an eclipse, the partial phases of a solar eclipse are very hazardous to human eyes and can only be viewed indirectly or through special filters. Scientists can study all layers of the Sun’s atmosphere at any time using special instruments.

A.

The Photosphere

The photosphere is the lowest, densest level of the solar atmosphere. The visible light that reaches Earth from the Sun originates in the photosphere. That light comes from a thin, bright shell about 300 km (about 200 mi) thick, a thickness of less than 0.05 percent of the Sun’s radius.

The photosphere has a temperature of 5510°C (9950°F). It is a diffuse, tenuous gas with a pressure that is only a small fraction, 0.0001, of the amount of pressure in Earth’s atmosphere at sea level. The photosphere is opaque (not transparent), because it contains negative hydrogen ions (a hydrogen atom with two electrons, instead of the usual one). Hydrogen ions block, absorb, and emit light, all of which prevent light from passing directly through a cloud of hydrogen ions.

A.1.

Granulation and Supergranulation

Some images of the Sun suggest that its white-hot disk is perfectly round and smooth, without a blemish. This uniform appearance is misleading. Under close inspection with a telescope, the photosphere breaks into a million tiny bright points, called granules, with a strongly textured and varying pattern. The hot granules are about 1,500 km (about 900 mi) across, and they are grouped into much larger supergranules about 30,000 km (about 20,000 mi) in diameter.

Granules are places within the photosphere where hot, and therefore bright, material reaches the surface. The granules are in constant turmoil and change. Hot gas rises up, liberating its energy. After the gas cools, it sinks downward along the dark lanes between the granules. Each bright cell lasts only a few minutes before it is replaced by another. This honeycomb of rising and falling gas marks the top of the convective zone.

A.2.

Sunspots

Large, dark spots, called sunspots, are often visible in the photosphere. The biggest sunspots exceed Earth in size and are easily visible with a telescope. Sunspots rotate with the Sun and change in size and shape. They come and go, with lifetimes lasting from hours to months.

The number of sunspots increases, then decreases, over an 11-year cycle. The position of sunspots changes as the number changes. Sunspots are concentrated in two belts, one north and one south of the solar equator. When the number of sunspots is at a minimum, the belts are near the equator. When the number of sunspots is at its maximum, the belts are at higher latitudes, nearer the poles.

Sunspots are places in the Sun’s photosphere that contain magnetic fields thousands of times stronger than Earth’s magnetic field. Sunspots appear dark, because they are much cooler than their bright surroundings. The concentrated magnetism in sunspots keeps them cold. The strong magnetic field of a sunspot acts as a valve, choking off the heat, light, and energy flowing outward from the solar interior. This valvelike action keeps sunspots at a temperature of 3230°C (5850°F), or just over half the temperature of the surrounding photospheric gas.

While sunspots are darker than their surroundings, they still radiate light. A sunspot is about ten times brighter than the full Moon. Scientists were perplexed for decades over what holds sunspots together. Scientists believed that the outward pressure of the strong, localized magnetic fields that are concentrated in sunspots should make the sunspots expand and disperse. By examining motions beneath sunspots, helioseismologists have shown that flows of gas converge below sunspots. The converging flows force the surface magnetic fields together and concentrate them to form sunspots.

A.3.

The Sun’s Spectrum

Sunlight appears yellowish, but it is actually a combination of a rainbow of colors. Scientists use special instruments called spectrographs to separate sunlight out into its different colors. These instruments do the same thing that water molecules in the atmosphere do when the molecules produce a rainbow. Each color corresponds to a different wavelength of light. Red has the longest wavelength of visible light, and violet has the shortest. The range of wavelengths of sunlight and the intensity at each wavelength are called the Sun’s spectrum. The study of the spectra of the Sun and other objects or materials is called spectroscopy.

When sunlight is spread out like a rainbow in the Sun’s spectrum, many dark gaps separate one color from another in the row of colors. These gaps are called absorption lines. Each absorption line is created when sunlight passes through the gases in the Sun’s photosphere. Atoms and ions of each element in the gas absorb light at certain wavelengths, creating dark gaps in the Sun’s spectrum.

The dark absorption lines in the spectra of the Sun and other stars fingerprint the ingredients of these stars. Each chemical element produces a unique set of lines, and the presence of these lines shows that a particular element is present in the stellar photosphere. Darker absorption lines indicate greater absorption and therefore larger amounts of the element.

Absorption lines in the Sun’s spectrum show that hydrogen is by far the most abundant element in the Sun. Other prominent absorption lines are produced by helium, sodium, calcium, and iron. Altogether, 92.1 percent of the atoms in the Sun are hydrogen atoms, 7.8 percent are helium atoms, and the other, heavier elements—sodium, calcium, iron, and other elements—make up only 0.1 percent of the atoms in the Sun. The Sun’s absorption lines are called Fraunhofer lines, named after German physicist Joseph von Fraunhofer, who cataloged them in the 1800s. The most common Fraunhofer lines are listed below, by the letter Fraunhofer gave them, the color that they block, and the element that causes them.

The Fraunhofer lines designated A and B actually have nothing to do with the composition of the Sun. They only appear on spectra gathered within Earth’s atmosphere. Earth’s atmosphere absorbs sunlight at the wavelengths of the A and B Fraunhofer lines, creating dark lines on the Sun’s spectrum. A spectrum gathered above Earth’s atmosphere would not have these lines.

B.

The Chromosphere

The chromosphere is a thin layer about 2,000 to 3,000 km (about 1,200 to 1,900 mi) thick, just above the visible photosphere. The chromosphere’s temperature rises from 5510°C (9950°F) near the photosphere to about 9700°C (17,500°F) near the corona. At temperatures such as those in the chromosphere, hydrogen emits a distinctive deep red color. Scientists often study the chromosphere by filtering out all sunlight except the light that has the wavelength produced by hydrogen in the chromosphere. Calcium ions (calcium atoms with one electron missing) also produce distinctive radiation in the chromosphere. Calcium ions emit ultraviolet light, or radiation with a wavelength just shorter than visible light. The radiation released by calcium ions is also useful for examining details in the chromosphere.

Hydrogen and calcium emissions reveal huge regions of cool, dense gas suspended above the photosphere by powerful magnetic fields. The cool gas looks dark against the brightness of the Sun beneath it. At the edge of the disk of the Sun, where the chromosphere extends beyond the lower layers of the Sun, the gas of the chromosphere creates bright loops called prominences against the dark sky. Against the surface of the Sun, however, the prominences look dark. Prominences are often called filaments when they appear against the background of the hot Sun. Sunspots extend from the photosphere into the chromosphere, creating even darker spots on the chromosphere. Hot gas from the photosphere penetrates the chromosphere around the sunspots, creating bright regions called plages.

C.

The Corona

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 Sun’s 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 Sun’s 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 Sun’s magnetic fields instead of radiation from the Sun’s 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 corona’s 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 Earth’s atmosphere is absorbed by atoms and molecules in the atmosphere, so scientists use instruments in space to study the corona.

C.1.

Explosions in the Corona—Solar 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 Sun’s ever-changing magnetism produces unrest on an awesome scale. The sudden, brief, intense outbursts called solar flares can rip through the Sun’s 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.

C.2.

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 Earth’s magnetic field can cause geomagnetic storms. Geomagnetic storms occur when Earth’s 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 Earth’s 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.

D.

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 Sun’s 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.