The first total solar eclipse to cross the U. S. from coast to coast in 99 years is not only a must-see spectacle but also a valuable scientific opportunity
I love to be outdoors during solar eclipses, enjoying the universe appearing to darken around me while my research observations get under way. Long ago I used to suggest that people make a pinhole projector or even use cheese graters from their kitchens to watch these events. But in recent years the availability of partial-eclipse filters for only a dollar or so has made such advice obsolete. Now anyone can glance up at the sun through such a filter starting more than an hour prior to totality and see an apparent bite being taken out of the solar disk. During the last few minutes before totality, you will notice that the ambient light changes in quality, becoming eerie. Shadows sharpen because they result from a thin crescent of sunlight rather than the full disk of the sun. The air cools, and a wind stirs. Shadow bands may sweep rapidly over the ground.
With seconds to go, as the moon moves completely in front of the sun, just a few shafts of sunlight leak through valleys on the moon’s edge, reducing the sun to an arc of bright beads. These fade out until only one is left—so bright that it looks like the diamond on a ring, perhaps with a narrow, reddish rim to its sides and a whitish band all around the lunar silhouette. Then the diamond, too, disappears. You can and should drop your filters and look straight at what is left of the sun, a region of its atmosphere that had been hidden by the blue sky.
This is the inner and middle solar corona, the plume of plasma that flies out from the sun’s surface. It is about as bright as the full moon—a million times as faint as the everyday sun—and equally safe to look at with the naked eye. You first glimpse the corona as the band of the diamond ring, and then you see it in all its glory: a pearly white halo of gas that extends outward to several times the sun’s radius. If you are lucky, you might see a mighty eruption of plasma into interplanetary space.
But what, really, is the point of my trying to describe a total solar eclipse in words? It is so astonishingly moving and beautiful that nobody has ever described it adequately. People routinely come up to me after eclipses to say that they know how I had tried to convey the excitement but that I had nonetheless fallen short. Television and computer screens do not do it justice. Photographs flatten the dynamic range and lose the dazzling contrast. To be outdoors as the universe apparently darkens, gradually at first and then by an additional factor of 10,000 within seconds, is completely discombobulating. It conjures up primal fears of the sun being taken away.
I saw my first eclipse as a first-year college student, and I was hooked. Starting then, I have been all over the world to see 65 solar eclipses (including 33 total eclipses) . I look forward to my 66th on August 21, when the path of totality traverses from the U. S. West Coast to the East Coast for the first time since 1918.
And I do not catch these events just for the fun of it—eclipses offer scientists viewing conditions that routine observations cannot replicate. Although terrestrial telescopes can be equipped with a small metal cone or disk—making a so-called coronagraph—to blot out the sun on demand, their artificial eclipses are not as good as the real ones. The ambient air molecules leave the sky too blue and bright, even from pristine and high mountain sites. And space coronagraphs have to blot out not only the everyday solar disk but also a wide band around it, or else too much light would scatter inside the instrument. Furthermore, any telescope has a limited resolution and smears out incoming light a bit. Natural eclipses do not have this problem, because the “telescope” is, in effect, the entire Earth-moon system, with an exceptionally high resolution. We link our ground-based observations with spacecraft views to get a complete picture of the sun. Only in the crisp shadow of the moon are we able to see the inner and middle part of the corona in visible light.
It is in those inner expanses that we seek an answer to one of the most nagging puzzles in astrophysics: Why does the sun’s temperature increase as you move away from its surface? Usually things cool down as you retreat from a hot object, such as a campfire or a steam radiator. Within the sun, the temperature starts at 15 million degrees Celsius at the center and steadily falls as you move outward, dropping to 5,500 degrees C at the solar photosphere, the surface that emits sunlight into space. But then the trend reverses. The tenuous gas just above the visible surface climbs back up to over 10,000 degrees C and abruptly leaps to millions of degrees. Scientists still debate the details of how that occurs.
We have made tremendous observational and theoretical advances since I first described the science of the solar corona in Scientific American in 1973. A flotilla of spacecraft now monitor the sun in ultraviolet light and x-rays, which we cannot view from the ground, and researchers have developed sophisticated tools to link all our observations together. We know the outline of the solution of the coronal-heating problem—that it involves the sun’s magnetic field—but the details remain murky. And this is hardly the only problem that the corona presents to us. Observations during the upcoming eclipse should help tackle these questions.
Scientists already understand much about the solar corona. For one thing, it looks like a giant porcupine. It is drawn into fine streamers, some of which are wider at their base and come to a peak at higher altitudes, like pointy helmets. The shape they form varies with the sunspot cycle.
When spots proliferate, as in the years 2012 through 2014, streamers burst out even from latitudes as high as 30 degrees north and south so that the corona appears round overall. During sunspot minimum periods, such as the one we are in, the corona is squat, and the streamers we see are limited to regions nearer the sun’s equator, and thin, straight coronal plumes appear at the poles. From the open regions between streamers, a flow of charged particles called the solar wind escapes outward into the solar system at hundreds of kilometers per second, perhaps twice the rate of the solar wind from other regions. At the base of the corona, anchored to the solar photosphere, are small loops of gas, perhaps made up of multiple threads too fine for our current observations to discern. These coronal loops may pulse as waves bounce back and forth along or through them.
All this delicate intricacy is the product of the solar magnetic field, which arises from churning gas deep within the sun. What researchers do not know, however, is exactly how the dynamics of the magnetic field are responsible for the bizarrely high temperature of the corona. We know the magnetic field is involved because magnetic processes are not subject to the same thermodynamic restrictions that prevent energy from flowing by heat conduction from the hot surface to the even hotter corona.
Scientists have two main ideas for how the sun’s magnetic field could transfer some of its energy into the corona to heat it up. One way is through extremely tiny solar flares. These explosions occur when the magnetic field undergoes an abrupt change in its configuration, within seconds. When you map out the field at the sun’s surface, you occasionally see the north and south polarities in sunspot regions become jumbled. This condition puts the magnetic field under enormous stress, and to relieve it, the two polarities suddenly connect in a new way, emitting tremendous amounts of stored energy. Such a reconnection heats the corona locally to 10 million degrees C or higher, gives off a bright flash, and sometimes ejects plasma into space. The flare can zap spacecraft orbiting Earth and could pose a serious risk to astronauts journeying to Mars.
The flares we observe are too intermittent to explain the baseline temperature of the solar atmosphere, but might explosions too small to detect individually also wrack the corona? James Klimchuk of NASA’s Goddard Space Flight Center has especially championed the idea of such nanoflares. Millions of small explosions going off in the corona every second, each with a billionth as much energy as a large flare, would keep it broiling hot.
The main competing set of theories is that oscillations in the magnetic field heat the corona. Vibrating loops in the lower corona could shake the surrounding gas, thereby raising its temperature. These waves could take multiple forms. Scientists have ruled out sound waves, driven by gas pressure, but Alfvén waves, driven by magnetism or by a hybrid of the two, called magnetoacoustic waves, are still viable. Could magnetic waves of some kind be enough to raise the coronal temperature to millions of degrees?
In principle, researchers should be able to distinguish between the nanoflare and wave mechanisms by measuring oscillations of coronal gas. Fluctuations with periods from about 10 seconds to minutes would betray the passage of standard Alfvén waves along coronal loops. Observations of vibrations of the sun’s surface using a technique known as helioseismology suggest that the sun is capable of triggering such waves. Although its strongest oscillations occur with a comparatively languid period of about five minutes, those are only one type among many undulations that the surface undergoes.
Eclipse observations could be crucial to measuring fluctuations in coronal loops. The logistic advantages of observing from Earth allow us to use equipment that has higher temporal resolution than exists on any current spacecraft. My team uses rapid-readout charge-coupled devices (CCDs) that capture images numerous times a second. By comparison, the Atmospheric Imaging Assembly cameras on NASA’s Solar Dynamics Observatory (SDO) have been taking observations through several of their range of 10 filters every 12 seconds, and the Solar Ultraviolet Imager on the National Oceanic and Atmospheric Administration’s new Geostationary Operational Environmental Satellite (GOES-16) has a 10-second cadence at best for its six filters.
What we have found so far extends the realm of possibilities. Some oscillations may have periods shorter than one second, matching a theoretical prediction of a special mode of Alfvén waves that travels along the surfaces of loops rather than through their interiors. But our data are scanty: only a few minutes of such high-cadence observations from a pair of prior total solar eclipses. This year we will be using our complicated CCD apparatus, with filters of astonishingly pure color, to isolate the hot coronal gas to search for the time spectrum of waves again. We hope that our results will help researchers choose between the different theories of coronal heating or even lead them to the conclusion that several mechanisms are at work simultaneously. In the active regions above sunspots, the conditions for flaring are auspicious, and waves are comparatively weak. In quiet regions, however, we may have either waves on small loops or trillions of nanoflares all the time.
Scientists have devised some tricks for making the most of the exceptional opportunities eclipses offer. Eclipse observations enable us to scrutinize the shape of the corona in high spatial and temporal resolution. Our ground-based eclipse images show detail about eight times finer in each dimension than the best space coronagraph.
