A total solar eclipse will sweep across parts of Mexico, the US and Canada today. This is one of the greatest natural phenomena on Earth: the sun will disappear under the shadow of the moon, the sky will turn to night and stars will shine out, the temperature of the air will drop and puffy cumulus clouds could vanish.
Just before and after the eclipse, intense rays of light called Baily’s beads beam out as the last sunlight pierces out over mountains and valleys on the moon’s surface. Soon afterwards all sunlight disappears but the corona — the sun’s faint outer atmosphere — emerges, normally not visible because the sun itself is some million times brighter and swamps the view.
A small reddish-pink rim called the chromosphere also appears around the sun, a thin layer of plasma just above the solar surface that is home to prominences, worm-like filaments of plasma protruding into the corona. And today’s eclipse is particularly exciting because the sun is highly active as it approaches solar maximum, the peak in its roughly 11-year cycle of solar activity. As a result, plenty of streamers of plasma are expected to fly out from the corona, and such prominences can sometimes snap explosively to form a coronal mass ejection, an eruption of billions of tonnes of magnetised plasma blasting out from the sun into space.
Scientists are also going to study how birds, bats and flying insects may react to the darkness of the eclipse, and especially interesting will be any effect on the spring migrations of birds currently under way. A team of scientists will measure the animal movements during the eclipse, helped by 13 radar stations near its path that can help to track any changes in behaviour.
Views of the eclipse are likely to be spoilt by overcast skies in Texas and some other parts along its path. However, one eclipse science project will be unaffected by the weather. Nasa is launching three rockets high into the eclipse shadow to help to understand how the sudden drop in sunlight disturbs the Earth’s upper atmosphere along with radio and satellite communications.
