The science of nature’s spectacular light shows

sir – Having spent many years actively engaged in auroral physics, I was pleased to see the photographs accompanying your report (“Shock and aurora as Northern Lights head south”, February 28).

However, you suggest that this event is caused by the direct entry into the atmosphere of particles from solar flares. In fact, the phenomenon known as an aurora is produced by charged particles (primarily electrons) precipitating into the atmosphere, not from the sun but from the Earth’s magnetosphere. In the case of the brightest and most dynamic aurorae, such as auroral arcs, the electrons are energised en route.

It is true that some weak forms of aurora, primarily those occurring on the dayside of the Earth and across the magnetic polar caps, are caused by the direct entry of solar wind, and there are occasionally aurorae over the polar caps caused by direct entry of solar-flare protons.

However, the iconic, spectacular aurorae are of a different class entirely.

Duncan Bryant

The sky at night: an aurora reflected on tranquil water in Keflavik, south-west Iceland