The great aurorae of 29 and 30 October 2003

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Three massive solar storms erupted during the second half of October 2003. This was somewhat surprising because solar activity was well on a downturn from its previous peak in May 2000, as part of its 11-year cycle. There were three simultaneous naked-eye sunspots – usually individually quite rare events – and the active (magnetically dis- turbed) region linked in with one of these covered an area of 2600 millionths of the solar hemisphere. These remained visible on the solar disc for about two weeks (due to the Sun’s relatively slow rotation). Regions of opposed and twisted magnetic field were forced together above the active regions to form solar flares (rapid releases of magnetic energy, electromagnetic radiation and ener- getic particles from active regions in the solar corona – the outer atmosphere of the Sun). There were several dozen medium to strong flares in late October. On 28 October at 1110 GMT an X17.2 flare (the third largest in the last 25 years) was associated with a ‘halo’ coronal mass ejection (a vast magnet- ic bubble of plasma) aimed directly at the Earth. This hurtled along with a velocity of about 1000 km s –1 , creating a bow shock in the solar wind (a background stream of charged particles from the Sun, travelling at typically 400 km s –1 ), and reaching our plan- et about 20 hours later. The result was huge numbers of energetic particles cascading down the Earth’s magnetic field lines and generating intense geomagnetic storm con- ditions of strength normally seen only once per sunspot cycle.