Artificial Eclipses

There are many aspects of the Sun and interplanetary space that may only be studied during a solar eclipse, and yet such a natural event occurs only every year and a half or so, and often then in inhospitable places from the perspective of astronomical observations. It is therefore natural to wonder whether it is feasible to create an artificial eclipse, by using a circular baffle to imitate the action of the Moon when it passes in front of our nearby star.

A telescope designed to do this is called a coronagraph; that is, it is used to study the corona. It is equipped with an obscuration to

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FIGURE 5-2. This four-hour sequence of images obtained with the coro-nagraph on the SOHO satellite (see page 133) shows Iwo comets falling into the Sun in June 1998. Several distinct coronal streamers are obvious in these images, their forms altering over these several hours. A coronagraph produces an artificial eclipse, allowing the solar atmosphere to be studied continually, rather than only during the few minutes of a natural eclipse. (There are many other advantages of data collection from space, such as access to wavelengths that are absorbed by the terrestrial atmosphere. An example is the ultraviolet image shown in Figure 1-5.)

fit over the image of the luminous solar disk. Astronomers have been using coronagraphs for some time, but they are of limited utility down here on Earth because the atmosphere scatters so much sunlight. The innermost corona is about one-millionth the brightness of the photosphere. Stepping out by a distance equivalent to the solar radius, the coronal brightness drops by about a factor of a thousand, to be about a billionth that of the solar disk. The light of the sky is greater than this value, meaning that the studies that may be tackled using ground-based coronagraphs are limited. By launching a coronagraph on a satellite, however, astronomers can get their instrument above the atmosphere and avoid such drawbacks. It just costs a lot more. Far above the Earth, a coronagraph can be guided so as to keep its baffle over the Sun, allowing perpetual monitoring of the corona, prominences, and other solar phenomena. Over the past couple of decades several such telescopes have been launched, returning invaluable data.

The most advanced satellite of the type is the Solar and Heliospheric Observatory (or SOHO), a joint project of NASA and the European Space Agency. On board are 11 separate instruments, one of which is called LASCO (Large Angle Spectroscopic Coronagraph). This has allowed the experimenters to study the corona out to 30 times the radius of the Sun, watching how vast bodies of hot plasma (bubbles of highly ionized, or charged, gases) are thrown out into space. Such spasmodic events, termed coronal mass ejections, sometimes result in a hundred billion tons of sun-stuff being launched outwards. These disperse somewhat as they move away from the Sun, but they can affect us on the Earth in various ways, because our ionosphere (the layer of the upper atmosphere consisting of gases ionized largely by the solar ultraviolet radiation) is disturbed as we run through streams of this plasma.

FIGURE 5-3. This image obtained by the Clementine satellite in 1994 shows the solar corona shining above the limb of the Moon. The bright disk is Venus. The part of the Moon towards the spacecraft obviously cannot be illuminated directly by the Sun, as that is on the far side of the Moon. In fact the lunar surface can be seen clearly here owing to Earthshine: sunlight that has been reflected by the Earth, which is off the field of this image to the right.

FIGURE 5-3. This image obtained by the Clementine satellite in 1994 shows the solar corona shining above the limb of the Moon. The bright disk is Venus. The part of the Moon towards the spacecraft obviously cannot be illuminated directly by the Sun, as that is on the far side of the Moon. In fact the lunar surface can be seen clearly here owing to Earthshine: sunlight that has been reflected by the Earth, which is off the field of this image to the right.

These types of solar gas ejection can be seen in Figure 5-2. The frames shown there also display a quite distinct phenomenon: comets falling into the Sun.

Another way to achieve a type of artificial eclipse is to use the Moon as the baffle and fly a satellite into its shadow. An example of this is shown in Figure 5-3, which is an image obtained by the Clementine spacecraft in 1994 while it was in orbit around the Moon.

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