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| The Interior |
| The Photosphere |
| The Chromosphere |
| The Transition Region |
| The Corona |
| The Solar Wind |
| The Heliosphere |
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The solar interior is separated into four regions by the different processes that occur there. Energy is generated in the core.
This energy diffuses outward by radiation (mostly gamma-rays and x-rays) through the radiative zone and by convective fluid
flows (boiling motion) through the outermost convection zone. The thin interface layer between the radiative zone and the
convection zone is where the Sun's magnetic field is thought to be generated.
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In the process of fusing hydrogen to form helium, the nuclear reactions also produce elementary particles called neutrinos. These elusive particles pass right through the overlying layers of the Sun and, with some effort, can be detected here on Earth. The number of neutrinos we detect is but a fraction of the number we expect. This problem of the missing neutrinos is one of the great mysteries of solar astronomy.
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The photosphere is the visible surface of the Sun that we are most familiar with. Since the Sun is a ball of gas, this is not a solid surface but is actually a layer about 100 km thick (very, very, thin compared to the 700,000 km radius of the Sun). When we look at the center of the disk of the Sun we look straight in and see somewhat hotter and brighter regions. When we look at the limb, or edge, of the solar disk we see light that has taken a slanting path through this layer and we only see through the upper, cooler and dimmer regions. This explains the "limb darkening" that appears as a darkening of the solar disk near the limb. up |
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A number of features can be observed in the photosphere with a simple telescope (along with a good filter to reduce the intensity of sunlight to safely observable levels). These features include the dark sunspots, the bright faculae, and granules. We can also measure the flow of material in the photosphere using the Doppler effect. These measurements reveal additional features such as supergranules as well as large scale flows and a pattern of waves and oscillations. The Sun rotates on its axis once in about 27 days. This rotation was first detected by observing the motion of sunspots in the photosphere. The Sun's rotation axis is tilted by about 7.25 degrees from the axis fo the Earth's orbit so we see more of the Sun's north pole in September of each year and more of its south pole in March. Since the Sun is a ball of gas it does not have to rotate rigidly like the solid planets and moons do. In fact, the Sun's equatorial regions rotate faster (taking about 24 days) than the polar regions (which rotate once in more than 30 days). The source of this "differential rotation" is an area of current research in solar astronomy. A 4.3 Mb MPEG movie showing magnetic features in the photosphere over a 36 day period is available here. These data were obtained with the GONG solar telescope network. The movie illustrates the rotation of the Sun as well as the evolution of the Sun's magnetic features - including sunspots. up |
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The chromosphere is an irregular layer above the photosphere where the temperature rises from 6000° C to about 20,000° C. At these higher temperatures hydrogen emits light that gives off a reddish color (H-alpha emission). This colorful emission can be seen in prominences that project above the limb of the sun during total solar eclipses. This is what gives the chromosphere its name (color-sphere). up |
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When the Sun is viewed through a spectrograph or a filter that isolates the H-alpha emission, a wealth of new features can be seen. These features include the chromospheric network of magnetic field elements, bright plage around sunspots, dark filaments across the disk and prominences above the limb. up |
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The chromosphere is the site of activity as well. Changes in solar flares, prominence and filament eruptions, and the flow of material in post-flare loops can all be observed over the course of just a few minutes. The chromosphere is also visible in the light emitted by ionized calcium, Ca II, in the violet part of the solar spectrum at a wavelength of 393.4 nanometers (the Calcium K-line). This emission is seen in other solar-type stars where it provides important information about the chromospheres and activity cycles in those stars. up |
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The transition region is a thin and very irregular layer of the Sun's atmosphere that separates the hot corona from the much cooler chromosphere. Heat flows down from the corona into the chromosphere and in the process produces this thin region where the temperature changes rapidly from 1,000,000ºC (1,800,000ºF) down to about 20,000ºC (40,000ºF). Hydrogen is ionized (stripped of its electron) at these temperatures and is therefore difficult to see. Instead of hydrogen, the light emitted by the transition region is dominated by such ions as C IV, O IV, and Si IV (carbon, oxygen, and silicon each with three electrons stripped off). These ions emit light in the ultraviolet region of the solar spectrum that is only accessible from space. up |
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The transition region has been studied from space using instruments on several spacecraft including the Solar Maximum Mission and the Solar and Heliospheric Observatory. The Transition Region and Coronal Explorer (TRACE) mission is now actively acquiring data on the structure and dynamics of the transition region. The images to the left are from the SUMER instrument on the SOHO Mission. The top image is emission from Carbon IV at temperatures of about 100,000ºC. The bottom image is emission from Sulfur VI at temperatures of about 200,000ºC. up |
1.The White-Light CoronaThe Corona is the Sun's outer atmosphere. It is visible during total eclipses of the Sun as a pearly white crown surrounding the Sun. The corona displays a variety of features including streamers, plumes, and loops. These features change from eclipse to eclipse and the overall shape of the corona changes with the sunspot cycle. However, during the few fleeting minutes of totality few, if any, changes are seen in these coronal features. up |
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2.The Emission Line CoronaEarly observations of the visible spectrum of the corona revealed bright emission lines at wavelengths that did not correspond to any known materials. This led astronomers to propose the existence of "coronium" as the principal gas in the corona. The true nature of the corona remained a mystery until it was determined that the coronal gases are super-heated to temperatures greater than 1,000,000ºC (1,800,000ºF). At these high temperatures both hydrogen and helium (the two dominant elements) are completely stripped of their electrons. Even minor elements like carbon, nitrogen, and oxygen are stripped down to bare nuclei. Only the heavier trace elements like iron and calcium are able to retain a few of their electrons in this intense heat. It is emission from these highly ionized elements that produces the spectral emission lines that were so mysterious to early astronomers. We can now produce artificial eclipses in coronagraphs that cover the disk of the Sun and filter out everything except the emission due to these coronal ions. These coronagraphs produce images of the "emission line corona." Examples of these observations can be seen at the National Solar Observatory's Coronal Data page. up |
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3.The X-Ray CoronaThe corona shines brightly in x-rays because of its high temperature. On the other hand, the "cool" solar photosphere emits very few x-rays. This allows us to view the corona across the disk of the Sun when we observe the Sun in X-rays. To do this we must first design optics that can image x-rays and then we must get above the Earth's atmosphere. In the early 70's Skylab carried an x-ray telescope that revealed coronal holes and coronal bright points for the first time. Today we have the Yohkoh, Yohkoh, SOHO, and TRACE satellites obtaining new and exciting observations of the Sun's corona, its features, and its dynamic character. up |
1.The Solar WindThe solar wind streams off of the Sun in all directions at speeds of about 400 km/s (about 1 million miles per hour). The source of the solar wind is the Sun's hot corona. The temperature of the corona is so high that the Sun's gravity cannot hold on to it. Although we understand why this happens we do not understand the details about how and where the coronal gases are accelerated to these high velocities. This question is related to the question of coronal heating. up |
The Ulysses spacecraft has now completed one orbit through the solar system during which it passed over the Sun's south and north poles. Its measurements of the solar wind speed, magnetic field strength and direction, and composition have provided us with a new view of the solar wind.
The Advanced Composition Explorer (ACE) satellite was launched in August of 1997 and placed into an orbit about the L1 point between the Earth and the Sun. The L1 point is one of several points in space where the gravitational attraction of the Sun and Earth are equal and opposite. This particular point is located about 1.5 million km (1 million miles) from the Earth in the direction of the Sun. ACE has a number of instruments that monitor the solar wind and the spacecraft team provides real-time information on solar wind conditions at the spacecraft.
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The heliosphere is a bubble in space produced by the solar wind. Although electrically neutral atoms from interstellar space can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself. The solar wind streams off of the Sun in all directions at speeds of several hundred km/s (about 1,000,000 mph in the Earth's vicinity). At some distance from the Sun, well beyond the orbit of Pluto, this supersonic wind must slow down to meet the gases in the interstellar medium. It must first pass through a shock, the termination shock, to become subsonic. It then slows down and gets turned in the direction of the ambient flow of the interstellar medium to form a comet-like tail behind the Sun. This subsonic flow region is called the helio-sheath. The outer surface of the helio-sheath, where the heliosphere meets the interstellar medium, is called the heliopause. The precise distance to, and shape of, the heliopause is still uncertain. Interplanetary spacecraft such as Pioneer 10 and 11 and Voyager 1 and 2 are traveling outward through the solar system and will eventually pass through the heliopause. The solar wind consists of particles, ionized atoms from the solar corona, and fields, in particular magnetic fields. As the Sun rotates once in about 27 days, the magnetic field transported by the solar wind gets wrapped into a spiral. Variations in the Sun's magnetic field are carried outward by the solar wind and can produce magnetic storms in the Earth's own magnetosphere. up |
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