The tail of comet Ikeya-Zhang took a beating from a solar explosion.© Gerald Rhemann / JPL / NASAEruptions of matter from the surface of the Sun can create chaos in a comet's tail, astronomers have found.
They have shown that three different coronal mass ejections in 2002 caused wobbles in the tail of comet Ikeya-Zhang. It is the first time the Sun's ejections have been shown to affect a comet in this way.
Astronomers Geraint Jones of NASA's Jet Propulsion Laboratory, Pasadena, and John Brandt, from the University of New Mexico, Albuquerque, used pictures taken by amateur astronomers at the time of the ejections to examine the tail. They publish their results online in Geophysical Research Letters1.
Jones and Brandt believe that by analysing distortions in the tails of other comets, they will be able to pinpoint exactly where short-lived solar eruptions are coming from, and how far their effects reach into space.
Although several satellites already monitor solar activity in our region of the Solar System, the researchers say that comets could provide useful information about the effects of coronal mass ejections farther away from the Sun.
Gale force
The stream of energetic particles thrown off by the Sun is known as the solar wind, and is responsible for comets' tails. As a comet approaches the Sun, the particles warm up its surface and push a stream of ionized material outwards, forming a distinctive tail that can be 200 million kilometres long.
Solar flares frequently burst from around sunspots, releasing extra energetic particles into the wind. But coronal mass ejections are the most impressive of the Sun's emissions.
These ejections boost the solar wind to gale force, as the star's magnetic field opens up and allows huge amounts of hot plasma to escape into bubbles that are sometimes as large as the Sun itself. If these outbursts reach Earth, they can damage communication satellites and even cause power grids to fail.
Delayed impact
After seeing photographs of the comet Ikeya-Zhang in which its tail was disturbed, Jones and Brandt looked back over observations made by the Solar and Heliospheric Observatory (SOHO), a satellite operated jointly by NASA and the European Space Agency. The researchers found that massive eruptions from the Sun on 2 March, 9 and 10 March, and 17 April 2002 all preceded disturbances in the comet's tail by one or two days. Each tail disturbance lasted for less than an hour.
Over those six weeks, comet Ikeya-Zhang moved from being about 75 million kilometres away from the Sun to almost 150 million kilometres away (the Earth is about 150 million kilometres from the Sun).
From these observations, the astronomers worked out that the eruptions must have thrown out matter at up to 1,131 kilometres per second. By comparison, the fastest known solar winds only reach about 800 kilometres per second.
Astro-archaeologists
Robert Walsh, a solar physicist from the University of Central Lancashire in Preston, UK, says that tying together the tail pictures and the SOHO data on mass ejections was "unique".
Jones and Brandt have also delved into records of Halley's comet from 1910 and 1985, and found patterns in its tail that could be explained in a similar way. "This is almost archaeological evidence for coronal mass ejections that we could go back and interpret," says Walsh.
Astronomers might use old comet photographs to reconstruct the frequency of these solar eruptions over time, Walsh says. Jones and Brandt hope to attract more photographs of comet distortions from dedicated amateur astronomers.
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