January 01, 2001

Article at Newton Magazine

Liquid Fireball

Sun in ultraviolet taken by the orbiting space probe Solar and Heliospheric Observatory

By Wilson da Silva

OUR SUN is a seething, broiling mass of superhot gases. Rivers of fire pour across its surface, hydrogen burns at its core and long geysers of plasma rise thousands of kilometres into space before cascading back onto the Sun’s surface.

The ‘geysers’ are known as solar flares, and every 11 years or so, the Sun undergoes a particularly chaotic sequence of these convulsions. This picture in ultraviolet was taken by the orbiting space probe SOHO, or Solar and Heliospheric Observatory, and shows one of the most powerful flares seen in the last decade.

Flares are brilliant explosions of pent-up magnetic energy and radiation that originate deep within the Sun’s chromosphere (one of our star’s many layers of superhot gases). Twisted magnetic fields beneath the Sun’s surface, in areas known as sunspots, hold back this energy until it finally breaks, triggering a huge electric current – like a massive arc welder – that erupts into space.

They can be very powerful, heating their gaseous contents to millions of degrees then releasing energy equivalent to billions of nuclear bombs in streams of energised particles that hurtle from the Sun. Physicists who study our nearest star regularly catalogue these events; this flare was detected on 14 July 2000 erupting from a large sunspot zone known as Group 9077. 

Solar flares are often associated with another cataclysmic solar phenomenon – the coronal mass ejection. These are gigantic bubbles of electrified gas containing up to 10 billion tonnes of the Sun’s gaseous material. A coronal mass ejection that came after this flare was clocked heading towards Earth at 1300 to 1800 km per second. 

These streams of charged particles, whether from a solar flare or a coronal mass ejection, can take from a few days to as little as 15 minutes to reach Earth’s magnetosphere, a protective buffer around the Earth created by the planet’s magnetic field. Luckily, it deflects most of these particles – but not always.

If the coronal mass ejection is powerful enough, it can drive a shock wave into the magnetosphere and squeeze it, triggering a magnetic storm high above Earth that can damage the electronics aboard satellites. Both coronal mass ejections and solar flares can at times even penetrate as far as the ionosphere (about 100 km up) and interfere with electricity transmission equipment on the ground.

In 1989, the whole power grid in Canada’s second most populous province, Québec, was knocked out by a solar flare. In such events, even air travellers at high altitudes can be exposed to increased levels of radiation – the equivalent of a brief chest X-ray.

But flares can also result in beauty: they generate the luminous sheets, arcs and bands of light in the night sky we know as the aurora australis in the Southern Hemisphere and the aurora borealis, or northern lights, near the North Pole. These are often an eerie green and blue, edged in red borders that ripple like a glowing curtain of soft light.