Info

Jupsat Pro Astronomy Software

Secrets of the Deep Sky

Get Instant Access

Pacific Ocean, Mexico, United States (Texas, Oklahoma, Arkansas, Missouri, Illinois, Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire, Maine), southeastern Canada, Atlantic Ocean

During such a solar eclipse, the Sun's normally invisible atmosphere becomes as bright as the full Moon, a shimmering halo of pearl-white light that extends outward from the lunar silhouette against the blackened sky (Fig. 6.2). A representative description of the spectacular crown of light, observed during the eclipse of 1842, was provided by Francis Bailey (1774-1844), a stockbroker and enthusiastic amateur astronomer in England:

I was astounded by a tremendous burst of applause from the streets below and at the same moment was electrified at the sight of one of the most brilliant and splendid phenomena that can be imagined. For at that instant the dark body of the Moon was suddenly surrounded with a corona, a kind ofbright glory. ...I had anticipated a luminous circle round the Moon during the time of total obscurity..., but the most remarkable circumstance attending this phenomenon was the appearance of three large protuberances apparently emanating from the circumference of the Moon, but evidently forming a portion of the corona.25

tte protuberances that Bailey observed were arches of incandescent gas, which loop up into the solar corona and are held there by strong magnetic fields. Astronomers now call the looping features prominences, the French word for "protuberances."

tte reddened prominences can rise up to a few hundred thousand kilometers (Fig. 6.3), large enough to stretch from the Earth to the Moon, hanging there for weeks and months at a time. Powerful magnetic forces hold these quiescent prominences together and suspend them against the force of the Sun's gravity for so long.

FIG. 6.2 Eclipse corona

The million-degree solar atmosphere, known as the corona, is seen around the black disk of the Moon, photographed in white light from atop Mauna Kea, Hawaii during the solar eclipse on 11 July 1991. The million-degree, electrically charged gas is concentrated in numerous fine rays as well as larger helmet streamers. (Courtesy of the High Altitude Observatory, National Center for Atmospheric Research.)

FIG. 6.3 Quiescent prominence Dense, cool gas, seen in the red light of hydrogen alpha at the rim of the Sun, outlines magnetic arches that are silhouetted against the dark background. The prominence material, appearing as a flaming curtain up to 65,000 kilometers above the photosphere, is probably injected into the base of the magnetic loops in the chromosphere. (Courtesy of the Big Bear Solar Observatory.)

FIG. 6.3 Quiescent prominence Dense, cool gas, seen in the red light of hydrogen alpha at the rim of the Sun, outlines magnetic arches that are silhouetted against the dark background. The prominence material, appearing as a flaming curtain up to 65,000 kilometers above the photosphere, is probably injected into the base of the magnetic loops in the chromosphere. (Courtesy of the Big Bear Solar Observatory.)

Other active, eruptive prominences, smaller but still large enough to girdle the Earth, might last only a few minutes or hours before their magnetism becomes unhinged, and the prominence breaks out and away from the Sun.

During a total solar eclipse, we are seeing patterns of free electrons in the corona made visible because they scatter the light that strikes them, like motes in a sunbeam. And because these electrons are constrained and molded by magnetic fields, the corona's form varies as the Sun's variable magnetism changes and shifts its shape, tte electrons are densely concentrated within magnetized loops close to the Sun, creating bubble-like, or arch-like, structures known as helmet streamers (Fig. 6.4), which are peaked like old-fashioned, spiked helmets once used in Europe.

A low-lying prominence is often found near the bottom of a helmet streamer. A cavity, visible as a region of reduced brightness, separates the prominence and the arcades of magnetized loops that seemingly support the arched helmet streamer.

In the outer part of the corona, far from the Sun, the streamers become narrower and surmounted or prolonged by long, straight, tenuous stalks that extend far into interplanetary space, tte long, graceful stalks can extend at least ten million kilometers, or 14 solar radii, into interplanetary space, as if some invisible agents were pulling or pushing them out like stretched salt-water taffy.

Eclipse Corona Radius

FIG. 6.4 Coronal structures near sunspot minimum The Sun's corona becomes visible to the unaided eye during a total solar eclipse, such as this one observed from Oranjestad, Aruba on 26 February 1998, close to a minimum in the Sun's 11-year cycle of magnetic activity. Several individual photographs, made with different exposure times, were combined and processed electronically in a computer to produce this composite image, which shows the solar corona approximately as it appears to the human eye during totality. Note the fine rays and helmet streamers that extend far from the Sun along the equatorial regions (left and right), and the polar rays (top and bottom) that suggest a global, dipolar magnetic field. (Courtesy of Fred Espenak.)

FIG. 6.4 Coronal structures near sunspot minimum The Sun's corona becomes visible to the unaided eye during a total solar eclipse, such as this one observed from Oranjestad, Aruba on 26 February 1998, close to a minimum in the Sun's 11-year cycle of magnetic activity. Several individual photographs, made with different exposure times, were combined and processed electronically in a computer to produce this composite image, which shows the solar corona approximately as it appears to the human eye during totality. Note the fine rays and helmet streamers that extend far from the Sun along the equatorial regions (left and right), and the polar rays (top and bottom) that suggest a global, dipolar magnetic field. (Courtesy of Fred Espenak.)

Helmet streamers are rooted within magnetic loops that sometimes straddle active regions and connect regions of opposite magnetic polarity. Streamers also often rise above long-lived, quiescent prominences that are commonly embedded in the closed-magnetic loops at the bottom of streamers, ttese bright, curved, low-lying magnetic loops constrain the densest coronal material close to the photosphere, within one or two solar radii.

tte shape of the corona is molded by magnetic forces that vary with the number of sunspots and the amount of solar activity. At sunspot maximum, when magnetic activity is strongest, the streamers are distributed all around the solar limb and presumably all over the Sun (Fig. 6.5). When the number of sunspots is low, the relatively weak magnetic activity is largely confined to the Sun's equatorial regions, where the sunspots and streamers are localized, ttus, the eclipse observations indicate that the overall shape of the corona changes in synchronism with the 11-year solar activity cycle; near maximum the coronal structures are stretched out in all directions outside the equatorial plane, and near minimum the corona is considerably flattened toward the equatorial regions.

tte width and radial extension of the streamers is also related to the solar activity cycle. At the time of maximum activity, streamers are narrower and shorter, near minimum; they are wider and more extended.

FIG. 6.5 Corona at sunspot maximum The dim, ghostly light of the Sun's outer atmosphere, the corona, during a total solar eclipse on 16 February 1980, observed from Yellapur, India. This was a time near sunspot maximum, when the sunspots are most numerous. The bright helmet streamers were then distributed about the entire solar limb, resembling the petals on a flower. The stalks of the streamers stretch out 4 million kilometers, or about six solar radii, from the center of the Sun in this photograph. It was taken through a radially graded filter to compensate for the sharp decrease in the electron density and coronal brightness with distance from the Sun. (Courtesy of Johannes Durst and Antoine Zelenka, Swiss Federal Observatory.)

tte corona can be routinely observed in broad daylight using a special telescope, called the coronagraph that has a small occulting disk to mask the Sun's face and block out the photosphere's light (Fig. 6.6). tte first coronagraph was invented in 1930 by the French astronomer, Bernard Lyot (1897-1952), and soon installed by him at the Pic du Midi observatory in the Pyrenees. As Lyot realized, such observations are limited by the bright sky to high-altitude sites where the thin, dust-free air scatters less sunlight, tte higher and cleaner the air, the darker the sky, and the better we can detect the faint corona around the miniature moon in the coronagraph.

tte best coronagraph images with the finest detail are obtained from high-flying satellites where almost no air is left and where the daytime sky is truly and starkly black. Initial discoveries were made from a coronagraph aboard NASAs Seventh Orbiting Solar Observatory, abbreviated OSO 7, which circled the Earth from 1971 to 1974. tte clear, nearly continuous, edge-on views of the corona from NASAs Skylab satellite in 1973-74 resulted in the full realization of long-lived holes in the corona, as well as the giant Sun-sized, expanding bubbles dubbed coronal transients or Coronal Mass Ejections, abbreviated CMEs.

Objective Lens, Lj

FIG. 6.6 Coronagraph Sunlight enters from the left, and is focused by an objective lens, Lj, on an occulting disk, blocking the intense glare of the photosphere. Light from the corona, which is outside the photosphere, bypasses this occulting disk and is focused by a second lens, L2, forming an image of the corona. Other optical devices are placed along the light path to divert and remove excess light. The French astronomer Bernard Lyot (1897-1952) constructed the first coronagraph in 1930, and the best coronagraphs are now employed in satellites orbiting above the Earth's obscuring atmosphere.

Objective Lens, Lj

FIG. 6.6 Coronagraph Sunlight enters from the left, and is focused by an objective lens, Lj, on an occulting disk, blocking the intense glare of the photosphere. Light from the corona, which is outside the photosphere, bypasses this occulting disk and is focused by a second lens, L2, forming an image of the corona. Other optical devices are placed along the light path to divert and remove excess light. The French astronomer Bernard Lyot (1897-1952) constructed the first coronagraph in 1930, and the best coronagraphs are now employed in satellites orbiting above the Earth's obscuring atmosphere.

tte Solwind coronagraph aboard the U.S. Air Force's P78-1 satellite continued investigations of the CMEs from 1979 to 1985, as did the coronagraph aboard NASA's Solar Maximum Mission in 1980 and from 1984 to 1989. More might have been learned if the Air Force had not decided to shoot down the P78-1 satellite during a test of an anti-satellite missile in 1985.

tte joint NASA-ESA SOlar and Heliospheric Observatory took up the space coronagraph investigations of the Sun from 1996 to 2005, with a remarkable series of discoveries. Nevertheless, all coronagraphs provide only a limited, edge-on view of the corona, just a flat projection against the sky.

Was this article helpful?

0 0
Champion Flash Photography

Champion Flash Photography

Here Is How You Can Use Flash Wisely! A Hands-on Guide On Flash Photography For Camera Friendly People!. Learn Flash Photography Essentials By Following Simple Tips.

Get My Free Ebook


Post a comment