Department of Physics Montana State University Bozeman, Montana 59717 U.S.A.
(Note: The following article appeared in the
October 1998 issue of The Physics Teacher
magazine, a publication of the American Association
of Physics Teachers. Figure captions are included
at the bottom.)
"Give me the splendid silent Sun, with all his beams full dazzling." -- Walt Whitman
For thousands of years the Sun has held the fascination of skyward-looking humans. Over the millenia its beauty and seeming constancy have captured the imagination of artists and poets. However careful observations even as early as those of Galileo revealed that the Sun is not constant: the granulated visible surface is blemished with spots that march across the disk, their numbers waxing and waning with the years. The part of the Sun which is visible to our eyes is but one layer, called the photosphere. With the exception of the glimpses gained during infrequent solar eclipses, this is all that early astronomers could see of the Sun. Above the photosphere there is much more --- an atmosphere of hot, ionized gas. During a solar eclipse the bright photosphere is blocked out by the Moon and the much fainter upper parts can be seen, in particular the layers known as the chromosphere and corona. In the last 50 years, developments in astronomical techniques have allowed us to view the solar atmosphere directly and to view it every day (without the requirement of a solar eclipse). This has revealed fascinating and beautiful physics, which we will try to share in this article.
The development of narrow bandpass optical filters in the last half of the twentieth century has provided a means to image the chromosphere. With temperatures between 5000 and 20000 K, the structure of the chromosphere is best revealed in the red light of the first hydrogen Balmer line (wavelength 6562.8 Angstroms), called "H-alpha" by solar astronomers. These images reveal structures unlike those seen in unfiltered "white light". The thread-like "filaments" seen in Figure 1 are condensations of dense cool gas, suspended by a harness of magnetic field lines. On the Sun's disk they appear as dark light-absorbing features; when seen above the Sun's edge, or "limb", they are called "prominences" and their emission can be seen against the dark background of space. Images of the chromosphere reveal motions and dynamism that are not as evident in the white light photospheric observations. Filaments have been seen to twist, undulate, and even to wink in and out of visibility as their motion Doppler-shifts the absorption of radiation out of the narrow passband (typically only 0.5 Angstroms wide) of the telescope filter.
Chromospheric movies have demonstrated waves, or "disturbances", travelling through the chromosphere, perturbing any structures that get in the way. First noticed in the 1960s and explored more fully in the last two years, these disturbances seem to be compressive waves whose restoring force is the magnetic pressure between field lines. Such waves would travel at the Alfven speed1, which in the solar corona is typically thousands of kilometers per second. An example is demonstrated in Figures 2 and 3, showing the events of May 12, 1997. Figure 2 is an image made by the Extreme Ultraviolet Imaging Telescope (EIT), one of 12 scientific instruments on board the Solar and Heliospheric Observatory spacecraft (SOHO)2. This image shows emission in the high corona at a wavelength of 195 Angstroms, primarily from eleven-times ionized iron. By digitally subtracting this image from another taken a short time later, emphasis can be placed on changes that occur in the interval. A sequence of these "difference images" appears in Figure 3; the sequence spans the time 04:30 to 06:00 UT. In the color palette used here, white pixels represent areas where brightening has occurred, and black pixels represent areas of diminished emission. Thus the images in Figure 3 show a travelling wave of emission enhancement emanating from a source region. These disturbances have come to be known as "Moreton waves", after the astronomer who first reported their detection3. EIT has detected more than a hundred "Moreton wave" incidents4.
Even more exotic are the observations of the highest, hottest, parts of the Sun's extended atmosphere, the corona. Until relatively recently, our only observations of the corona have been during rare solar eclipses. The corona visible in white light during a solar eclipse is viewable not because of light emitted by the coronal gas, but because of light from the photosphere being scattered by electrons in the corona. Another twentieth-century invention allows these "eclipse" observations to be made year-round. A coronagraph mimics a solar eclipse by placing a metal disk in the light path of the telescope, to block out the bright photosphere. However, the design of a coronagraph is more complicated than simply placing a metal disk in front of a camera. The major obstacle is scattered light -- in the Earth's atmosphere, and in the telescope itself. This scattered light can be orders of magnitude brighter than the corona one is trying to view. It wasn't until about 1930 that French astronomer B. F. Lyot (1897-1952) hit upon a design that worked reliably. Not just a single innovation, Lyot's design incorporated several methods which individually could not completely dampen the scattered light within the telescope, but taken as a combination significantly enhance viewing of the corona. These steps (see footnote 5) help reduce the amount of scattered light within the telescope and thus make the telescope as effective as possible, but none can address the scattering caused by the Earth's atmosphere. To get above as much of the atmosphere as possible, coronagraphs have been built in carefully selected mountain sites, launched on high-altitude balloons, and more recently carried into space. Two notable examples of space-based coronagraphs are the Spartan mission, and the Large Angle Spectrometric Coronagraph (LASCO) on board the SOHO spacecraft. Coronagraphs at several observatories around the world and in orbit show beautiful streamers and the catastrophic ejections of plasma clouds. These "coronal mass ejections" typically occur when a filament (like the filaments shown in Figure 1) breaks free of its magnetic harness and travels out into interplanetary space. Figure 4 shows an example of a quiescent streamer on the left-hand side as well as a large coronal mass ejection (CME) captured by SOHO/LASCO. The interaction of these ejected particles with the Earth's magnetosphere gives rise to "space weather" --- aurorae (the Northern and Southern Lights), ionospheric disturbances, bulk charging of spacecraft components (leading in some cases to electronic failures), and induced currents in long conductors (e.g., oil pipelines and electric power lines). These bombardments also can pose a radiation risk to astronauts.
Because of the high temperature of coronal plasma (greater than one million Kelvins), much of the emitted radiation is in the X-ray regime. X-rays do not penetrate Earth's atmosphere, so the Sun's most dynamic face was not revealed until the dawning of the space age. Some of the very first scientific experiments to be lofted by rockets were designed to detect solar X-rays. Modern experiments actually focus these "hard" photons into images. Focusing X-rays into an image is tricky business and involves some interesting physics. There are two ways in which this is done, both involving mirrors. As we might imagine, very high precision is necessary because to make good images the roughness of the mirror surface needs to be smaller than the wavelength of the incident light, which is very small indeed for X-rays.
In the first method, the mirrors have shapes similar to those in visible-light reflecting telescopes, but the surfaces have multiple layers of thin coatings, and the spacing between the layers is carefully tailored to create an interference effect that allows a specific wavelength (chosen by the multilayer designer) to be reflected. This "normal incidence multilayer" method allows for the reflection of very narrow ranges wavelengths. The second method is "grazing incidence", in which the photons strike the reflecting surface at a very shallow angle and glance off, much like a thrown rock skipping of the surface of a pond. To focus X-rays in this manner, the reflective surface (a very smooth metal) is machined into a complex shape, a hyperboloid cylinder. The mirror is thus not at all like the mirrors in common optical telescopes. Missions using this design include Yohkoh, the (Japan/USA/UK cooperative) mission 6 that produced our cover photo.
Blocking out the photosphere, on the other hand, is pretty easy for X-ray pictures because the photosphere is far too cool to make appreciable contribution to the X-ray luminosity. The catch is that camera detectors which are sensitive to X-ray light are also sensitive to visible light. A thin sheet of metal (aluminum, magnesium, and beryllium are typical) placed in front of the telescope can act as a filter, to reject visible light from the cool photosphere while transmitting the X-rays from the high-temperature corona. The images produced by these X-ray telescopes reveal that X-ray production is far from isotropic: the hot plasma is collected into regions of activity with strange shapes. Nor is the production rate constant, contradicting Walt Whitman's description at the beginning of this article --- the Sun is anything but silent. Sudden flares on timescales of just a few minutes (or less!) occasionally raise the X-ray flux above the background level, which itself varies from day to day.
The nature of these strange-shaped features can be understood by comparing an X-ray (coronal) image with a simultaneous white-light (photospheric) image. In a white-light image (right-hand side of Figure 5) dark sunspots mark locations of concentrated intense magnetic fields. The corona (left half of Figure 5) displays collections of hot plasma into looplike features in active regions at the same locations. This is no coincidence: the explanation is given by the high degree of ionization in the million-degree plasma. Lorentz forces due to strong magnetic fields cause the gyroradius of ions to be very small, effectively constraining the plasma to move along field lines. This again is pretty remarkable physics --- natural magnetic bottling.
The temporal evolution and sudden flaring can also be explained as a result of the Sun's magnetism, although there are still many unanswered questions. The reconnection of field lines can release energy by rearranging the magnetic topology. A prevailing mystery is explaining how reconnection can proceed on the observed time scales (minutes), since the high conductivity of the plasma and the typical length scales of coronal loops makes the magnetic diffusion timescale very long, on the order of thousands of years. Cadres of plasma theorists are working to solve these mysteries, trying to understand the mechanisms that can shorten the pertinent length scales or locally enhance the effective resistivity of the plasma.
A beautiful example of a class of time-evolving X-ray feature are the arcades associated with coronal mass ejections and filament eruptions. The general scenario, constructed in increments by a number of researchers, goes as follows: As a filament breaks free of its magnetic harness, it lifts and "erupts" to form a CME. The magnetic field lines reconnect underneath the rising filament, so the theory goes, and the sudden release of magnetic energy heats the plasma along the reconnected field lines. The result is a row of aligned magnetic arches, or "arcade", that suddenly appears in X-ray images --- so suddenly that they are sometimes called "zipper" arcades (Figure 6).
The high temperatures found in the corona form a paradox that is one of the most puzzling mysteries in the field of solar physics. Since astronomers firmly believe that the Sun's energy source is nuclear fusion in its core, it seems obvious that the core should be the hottest part of the Sun with the plasma gradually becoming cooler closer to the surface. Consistent with this expectation, the temperature in the region above the photosphere drops from 5700 to a chilly 4500 Kelvins. Surprisingly, above this minimum-temperature region, the temperature dramatically increases to as high as 20000 Kelvins in the chromosphere. Even more strange, the temperature then skyrockets upwards into the corona where temperatures exceed a million Kelvins. Clearly we cannot explain the corona's temperature via radiation, convection, or conduction from the photosphere, because these thermal mechanisms would tend to transport the heat from the hotter corona to the cooler photosphere, and the corona would disappear rather quickly. There must be some nonthermal energy-transfer mechanism responsible for this astounding temperature increase. The three leading, and competing, theories are: (a) a combination of numerous smaller flares convert magnetic energy into heat; (b) acoustic waves are generated by the turbulent convection in the zone directly underneath the photosphere and then propagate up into the higher atmosphere as shock waves and deposit their energy; and (c) energy is carried upward by magnetic waves into the higher atmosphere where they somehow deposit their energy. The solar physics community is currently divided on this issue, with scientists from all three camps reporting progress. It is quite likely that some combination of mechanisms may be responsible for heating the corona.
These scientific problems are not confined to the study of our Sun. After all, solar physics is really just stellar astrophysics with "a better view". More distant stars provoke many of the same questions as does the Sun, and some are even more profound. For example, some distant stars have very strong winds, some much faster and denser than the Sun's expanding atmosphere. Enormous spots and flares have been observed on distant stars that make the Sun appear relatively quiet. Moreover, the coronal heating mystery is even more pronounced in some dwarf M-class stars. Although M dwarfs have photospheric temperatures that are much cooler than that of our Sun, some of them have the highest X-ray luminosities of any single stars known. The hope is that the mechanism responsible for heating the corona of a cooler M star is similar to that which heats the corona of our G-class Sun.
We understand that much of the Sun's activity is related to its dynamic magnetism. An interesting additional wrinkle is that because the Sun is not a rigid body, its magnetic field is constantly evolving. Magnetic field lines are distorted and stretched by plasma convection and differential rotation. This brings a forever-changing picture of the Sun to our telescopes. The level of activity in the Sun's atmosphere changes from day to day (and often from hour to hour!) and follows a general cyclic trend. At the root of this cyclic trend, the Sun's global magnetic field "flips" after approximately 11 years, so that the magnetic polarity of, say, the extreme northern latitudes in 1995 is the opposite of the polarity of the same region in 1984. As this pole-swapping progresses, magnetic fields at the middle latitudes become distorted, giving rise to increased activity. This means more frequent and more energetic flares, CMEs, and X-ray production, along with the accompanying terrestrial effects. The last maximum was between 1989 and 1991, whereas the years 1995-1996 saw a period of minimal activity.
The study of the Sun's activity provides an exciting challenge. The picture is always new, the problems always fresh, and the puzzles always interesting. There is no shortage of fascinating physical problems, and with the current constellation of observatories on the ground and in orbit, the tools we have to attack these mysteries are better than at any time in the past. As we head into the new solar cycle --- maximum activity is expected in 2001-2002 --- be assured that solar astronomers will be watching the skies.
1. Alfven speed scales as the magnetic field strength divided by the
root of the plasma density.
2. SOHO is an ESA/NASA mission of international cooperation. More images and information can be gotten from the world wide web at http://sohowww.nascom.nasa.gov/
3. R. Athay, & Gail E. Moreton, Astrophysical Journal, 133, 935 (1961).
4. Many more images are on the world wide web at http://umbra.nascom.nasa.gov/eit/cme/
5. A fairly complete description of the several parts of a coronagraph can be found in C. R. Kitchin's book, Astrophysical Techniques, 1984 (Bristol, Philadelphia, and New York: Adam Hilger Ltd.).
6. More images and information is available on the web: at http://www.lmsal.com/SXT/ for technical information and images; and http://solar.physics.montana.edu/YPOP/ for images, movies, and classroom-tested lesson plans.
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