Solar Structure - A Closer Look

Solar Structure Courtesy: McREL

The interior structure of the sun is still somewhat of a mystery. The opaque mass of the sun is traditionally thought of as being organized in layers around a central core. From the core outward, these layers are called: the radiative layer (or zone), the convection layer, and the photosphere, which is the surface layer we observe. Surrounding the sun is its atmosphere, just like the air around the Earth. The lower layer of the sun's atmosphere is called the chromosphere, and the outer layer of its atmosphere is the corona.

The Core of the Sun

The extremely dense solar core holds about 50% of the sun's total mass, but takes up only about 1.5% of its total volume. Physical conditions inside the sun's core are extreme. The temperature is thought to be around 15 million degrees Kelvin, a temperature so extreme that atoms are stripped of their electrons. Thus the sun's core is a mixture of protons, neutrons, nuclei, and free electrons.

The pressure at the core is perhaps 250 billion times the pressure of the atmosphere at the Earth's surface. The sun's huge mass does not collapse inward by gravitational attraction only because of this enormous outward pressure generated by the heat of the core. Also, the sun does not explode like a hydrogen bomb, because the stupendous mass of the gases above the core contains its explosiveness.

The density at the sun's core is likewise extremely high. At the core is the nuclear furnace that produces the sun's energy. If we could see into the core, it would appear black, since none of the energy produced there lies in the visible part of the spectrum. The sun mainly produces short-wavelength gamma rays. Some of these gamma rays collide with each other, losing energy. As they lose energy they transform to x-rays, still not visible to the human eye.

Visible Color Spectrum

The x-rays produced in the sun's core gradually work their way to the surface of the sun, following a path through bubbles of reduced temperature, pressure, and density. The radiative layer extends from the core about 70% of the way up to the surface. Hydrogen and helium nuclei and unattached electrons populate the radiative layer. In this layer, some of the loose electrons are captured by helium nuclei $(He2+)$ to form ionized helium atoms $(He+)$. The radiative layer becomes full of ionized hydrogen and helium atoms. This mixture of ionized hot gases and electrons is called plasma and is sometimes regarded as a fourth state of matter.

Deep in the radiative layer, the x-rays collide with particles and change direction in random ways. Each x-ray may travel only a few millimeters before it suffers another collision and is set off in a different direction. Nevertheless, the x-rays continue to move toward the surface. The time to complete this journey to the surface is measured in millions of years, an incredible fact given that x-rays travel at the speed of light! To put it in more personal terms, the sunlight that gave you your most recent sunburn resulted from a nuclear reaction that took place perhaps 1,000,000 years ago deep within the core of the sun.

The bumping together of the x-rays in the radiative layer robs them of their energy. Consequently, their wavelengths gradually increase as they move upward toward the convection zone, finally becoming visible light.

The Convection Layer

Gas Movement in the Convection Layer Courtesy: McREL

Finally, the photons arrive at the convection layer, 150,000 km below the sun's surface, where temperatures are slightly less than 1 million degrees Kelvin. Here, nuclei are able to hold onto electrons, and undamaged atoms exist. With reduced energy, the light is absorbed by gaseous atoms in the convection zone which hold onto it rather than bouncing it off or absorbing and then re-radiating it. These atoms effectively block the outward flow of the sun's energy by absorbing it and becoming enormously hot. Convection currents, like those observable in warming liquids and air, carry the sun's energy to the photosphere on seething rivers of hot gases.

As the temperature of the gas that has absorbed energy at the bottom of the convection zone increases, the gas expands, becoming less dense than its surroundings. These bundles of hot gas, because they are less dense, float up toward the surface of the convection layer like hot air balloons through a cold morning. At the top of this layer, they radiate away their excess energy, becoming cooler and denser, and then sinking down again through the convection layer. The effect is of "conveyor belts" of hot gas moving up and cooler gas moving down.

At the surface between the convection layer and the photosphere, the gas is very turbulent, rising up in the center of structures called convection cells (supergranules), flowing to the cell boundaries and then sinking. The processes going on at these cell boundaries, where plasmas with oppositely-oriented magnetic fields collide and where magnetic energy is converted into motion, are probably responsible for the heating of the sun's corona and the acceleration of the solar wind particles which will be collected by the Genesis spacecraft.

Interestingly, although it may have taken the radiation millions of years to reach the bottom of the convection zone, its energy moves up through the entire layer in about three months. All of the energy sent into space from the surface of the sun is transported there by convection.

The Photosphere

Photosphere with Sunspots Courtesy: NASA

Above the convection zone is the photosphere, the visible bright surface of the sun. Since the photosphere is a gas, its outer limit is somewhat difficult to define. The photosphere is probably several hundred kilometers thick.

Temperatures in the photosphere are even lower than in the convection layer, and the gas densities are quite small (estimated to be one-millionth the density of water or less). The gaseous atoms no longer block radiative energy flow. As the hot atoms cool, they release their excess energy once again as radiation that streams unimpeded into space and ultimately supports life on Earth.

Sunspot Detail Courtesy: NASA

The pebbly, granular visible surface of the sun, the photosphere, is where early astronomers focused most of their attention. It is here that we find the easily observed blotches that are now called sunspots. Sunspots come and go in a regular pattern of about 11 years, but there is uncertainty about the driving force behind their appearance and disappearance. They vary in size and often occur in groups that sprawl over hundreds of millions of square kilometers on the sun's surface. They look dark because they are cooler than the surrounding surface of the sun. Sunspots are thought to arise from the temporary inhibition of convection currents by strong localized magnetic fields. In other words, if a convection current is prevented from carrying its load of thermal energy to the surface, the surface served by that current will cool and a sunspot will appear. Intervals of high sunspot activity usually coincide with a wide range of other dramatic solar events such as coronal mass ejections (CMEs) and flares, which often disrupt electronic communications, and possibly even weather patterns, on Earth.

The Chromosphere

Chromosphere Structures Courtesy: McREL

The lower atmosphere of the sun—the chromosphere—escaped scrutiny by early astronomers because it is invisible in contrast to the bright photosphere below it. The relatively miniscule amount of light emitted by the chromosphere is only momentarily visible to the unaided eye during a total solar eclipse when the moon blocks light from the sun's photosphere. The chromosphere appears transiently under these conditions as a thin, bright red ribbon that encircles the silhouette of the moon. Modern astronomers have been able to study the chromosphere at their convenience, owing to the wide variety of instruments that are available to them. Amateur astronomers should never attempt to view the chromosphere unaided.

Solar Prominence Courtesy: NASA

The chromosphere is an exciting and unique feature of the solar landscape. Solar astronomers have found there a host of transient exotic structures, including spicules, prominences, and plages. The spicules are abundant but short-lived, evanescent streams of hot gases that vault high into the chromosphere. More impressive and photogenic are the prominences, some of which are spectacular bright loops of hot gas that arch high above the top of the chromosphere and often extend into the corona. Some of them have widths the size of Earth while others may approach half the diameter of the sun itself. Prominences often are associated with sunspots and some of them-the quiescent prominences- may hold their shape for months before collapsing. Others-called eruptive prominences-erupt from the chromosphere as gaseous streamers. Plages are bright, cloud-like structures that are found in the vicinity of sunspots.

Coronal Mass Ejections (CMEs) Courtesy: NASA

Closely related to prominences are monstrously energetic coronal mass ejections (CMEs) and flares, which typically begin as a loop that explodes within a few hours and spews all sorts of solar material into space, including a strong blast of x-rays and ultraviolet rays. This radiation arrives at Earth eight minutes later and can severely disrupt the ionization of the Earth's upper atmosphere. This in turn can cause major problems with communications and power systems everywhere on Earth. After about twenty-four minutes, the next wave hits. This consists of very high-energy protons that could be extremely harmful to any astronauts who happened to be in the way. Finally, after one or two days, the Earth is slammed with a magnetic shock wave travelling at more than 600 miles per second. In 1989, one of the strongest flares ever observed erupted, causing a power failure all across the province of Quebec and creating an aurora borealis that was seen as far south as Key West, Florida.

The power of coronal mass ejections (CMEs) and flares cannot be overestimated. Flare temperatures may reach 50 million degrees Kelvin, which is several times hotter than the core of the sun. If the power of a single CME or flare could be harnessed, it would be sufficient to provide the energy needs of the inhabitants of Earth for millions of years.

The chromosphere seems to derive its spectacular behavior from the dominant force in the solar atmosphere: magnetism. In contrast to the dense plasmas in the sun's lower regions, the plasmas of the sun's atmosphere are dilute and unable to contain the immense magnetic field of the sun. Rather, the magnetic fields dictate the behavior of the plasma in the sun's atmosphere, giving rise to the bizarre features of this region. Loop prominences, for example, are observed when plasma is captured by magnetic fields and bent back into the chromosphere.

The Corona

Corona During Eclipse Courtesy: NASA

The outermost layer of the solar atmosphere is the corona, in some ways the most mysterious layer of all. Contrary to expectations and seemingly to the laws of thermodynamics, the temperature rises steadily from a minimum of around 4000 degrees Kelvin in the chromosphere to more than a million degrees Kelvin in the corona. This makes the corona the second hottest part of the sun, after the sun's core. How is it possible for heat to be transported from a cooler body (the chromosphere) to a hotter body (the corona)? Although astrophysicists are uncertain about the mechanism of energy transfer to the corona, it is thought by many to be the result of magnetic waves transported along magnetic field lines emerging from the sun.

The incredibly dilute, superheated gases of the corona reach millions of miles into space. A total eclipse shows the corona as a luminous white halo surrounding the glowing disk of the sun. The corona glows during an eclipse because light from the photosphere bounces off free electrons in the coronal plasma. The shape of the corona is synchronized with the solar activity cycle, changing from a jagged ring around the sun during the peak of the cycle to wispy plumes and coronal streamers reaching millions of miles into space at the end of the cycle. The streamers are the origin of the dense, lower-speed component of the solar wind.

Mysteries remain about the structure of the sun. The Genesis mission's study of the solar wind will aid in astrophysicists' understanding of structures inside the nearest star, the solar source of Earth's light and heat.