Giant solar flare

Halloween 2003 will live forever in the annals of solar history. In the space of two weeks centred around the spooky celebration, solar physicists witnessed the most sustained bout of solar activity since satellites took to the skies.

The Solar and Heliospheric Observatory (SOHO) was monitoring it all. The ultraviolet telescope captured the climax of activity on 4 November 2003, showing a blistering solar flare bursting from active region 10486 at 19:29 GMT. Solar flares are the near-instantaneous release of energy caused by a loop of magnetism snapping into a more stable configuration.

In this process, the energy of up to a thousand billion Hiroshima-sized atomic bombs can be released in just a few minutes. That release is seen here. The horizontal white streak is where the camera has been blinded by the brightness of the flare.

Things began when a giant sunspot, fully ten times the diameter of Earth, hove into view around the western limb of the Sun in late October. It was followed by another, equally large, spot and together they moved across the face of the Sun generating flares on an almost daily basis. This image shows the second spot’s parting volley.

Solar flares are classed according to the energy they release at X-ray wavelengths. There are three major categories: C, M and X, further divided into 10 subclasses. M1 flares are ten times more powerful than C1, and X1 flares are ten times more powerful than M1 flares, or 100 times more powerful than C1.

This 2003 flare was so powerful that it broke right through the top of the X-class range, which is usually given as X10. Analysis showed that it clocked in at X28, making it 28 times more powerful than an X1.

A billion tonnes or so of the solar atmosphere was propelled into space at a speed of 2300 km/s – a staggering 8.2 million km/h.