Issue 60

17 March - 27 April 1998

Science insight

The sun behaving badly

When you look at the sun through a normal telescope it looks well behaved enough, says Dr Peter Cargill, but when observed from space at ultraviolet or xray wavelengths, the sun can look incredibly violent, with solar flares and large eruptions.

Dr Peter Cargill.

“What I am most interested in studying is the behaviour of these eruptions as they travel from the sun to the earth, and then what happens as the earth’s magnetic field interacts with these big eruptions,” he explains. Dr Cargill has been a reader in the space and atmospheric physics group in the Blackett Laboratory at Imperial since September 1996, after spending five years as a researcher with the US Navy.

Solar eruptions take about three days to travel from the sun to the earth and can incapacitate satellites, sometimes permanently. “The amount of material expelled can be as large as one million million tonnes and its speed up to two million miles an hour. This packs quite a powerful punch!”

It is only since 1980 that scientists have rigorously established a chain of cause and effect between solar eruptions and disturbances in space around the earth which not surprisingly has attracted plenty of commercial and military as well as academic interest. For example, when magnetic storms occur the highest part of the atmosphere heats up and expands outwards. Satellites, which typically skim through the edge of the atmosphere about 200 miles up and are normally systematically decelerated by the earth’s atmosphere, undergo even more friction than usual - with the result that they fall back to earth more quickly, shortening their lifespan.

“There is considerable interest in assessing how many of these storms occur and how many of them expand the atmosphere. What you are trying to work out is how high to put your satellites,” says Dr Cargill, who studied for his PhD at St Andrews in Scotland. He then spent 15 years in the United States, partly as a postdoc at the National Center for Atmospheric Research in Colorado and as a researcher at the University of Maryland.

“The sun’s magnetic field is critical in understanding this violent activity,” he explains. “Every so often, a large part of the sun’s outer atmosphere, called the corona, is blown out into space by changes in the solar magnetic field.” The eruptions are called CMEs, coronal mass ejections, and when they interact with the earth’s magnetic field they create geomagnetic storms.

A coronal mass ejection observed from the Solar Maximum Mission satellite in August 1980.The sun, upper right, is blocked from view by a small disc in front of the telescope. Note the gradual inflation of the bright structure in the seven o’clock position and its subsequent eruption as first a bright rim, and then a more elliptical structure. Figure courtesy of High Altitude Observatory, Boulder, Colorado.

“In very large storms the induced currents can flow along conducting structures such as trans-oceanic cables, pipelines and power lines. In the latter case when the rock beneath the lines is resistive, the current flows above ground instead. In a massive geomagnetic storm in March 1989 this effect led to a catastrophic failure of the power system in Quebec, Canada.”

It is now known that the sun’s violent activity peaks every 11 years, when CMEs are likely to be much more frequent but do not necessarily produce bigger storms. The next cycle peaks at the end of this century.

“One of the holy grails of space science is the prediction of the storms,” explains Dr Cargill. For a forecasting scheme to be successful, it is vital to continually monitor the sun and its outer atmosphere. “This can be accomplished by placing a satellite at a point sunward of the earth where the solar and terrestrial gravities cancel.”

At present, the ESA/NASA satellite SOHO (Solar and Heliospheric Observatory) is placed at that point. On board is an instrument designed to observe earthward-bound eruptions for the very first time, by enabling scientists to see a ‘halo’ of hot gas associated with such eruptions as they head for earth. Before, they were only able to see CMEs that were moving sideways and would miss earth.

Even so, forecasting remains inaccurate. “An attractive alternative that is actively being discussed in the UK and elsewhere is a pair of spacecraft, viewing the sun from different angles to the side of a line between the sun and earth. Such a ‘stereo’ image would remove many of the present uncertainties.”

Discussions for a stereo mission are already taking place between Imperial’s André Balogh, Peter Cargill and others, and could be launched around 2007, perhaps in time for the 11-year cycle of solar eruptions peaking in 2010.