Madam, - In Weather Eye (October 27th) Brendan McWilliams discusses the mysterious hissing sound which is said to accompany the aurora borealis,otherwise known as the "Northern Lights", and he implies that the scientific community is baffled as to its origin. As a practising geophysicist I would like to contribute an explanation.
Auroral phenomena are accompanied by the emission of natural radio noise in the very low frequency (VLF) band of 3 to 30 kiloHertz and it is not impossible that any electrically charged material in the environment could resonate to these frequencies, just as electrical equipment can resonate or "hum" to the 50 Hz mains frequency.
As audio frequencies above 10 kiloHertz are in the upper range of hearing, this would also explain why some people claim to hear the hiss while others present do not. - Yours, etc.,
GEORGE REYNOLDS, Annamoe, Co Wicklow.