What, I asked myself, would actually happen in a close [cometary] approach? I sought out Gerry McCormac, a colleague who had trained and worked as an atmospheric physicist. [...] When asked what he thought would happen if a comet approached very close to the earth, his reply was startling:

If it came within the earth's magnetosphere it would probably be spectacular ... the sky would go purple or green, particles from the comet would spiral down the lines of force and it is likely that you would have amazing auroral displays and coloured streamers...

Suddenly a new series of possibilities had opened up. One had to try to imagine a cometary body, or associated debris, passing within the Earth's magnetosphere and possibly producing a fabulous, moving, coloured display. So I asked if there would be any other associated phenomena - noise maybe? He replied: "Well, the Eskimos say that at the time of aurora they sometimes hear a hissing noise ... but! ...scientists who took up sensitive listening equipment did not manage to record anything." [...]

There is a whole literature on auroral sounds and, indeed, sounds heard directly from bolides, even comets. Colin Keay has written extensively on these phenomena. [...]

People have claimed that they have heard an incoming fireball and in fact heard it and turned around to see it. There is the contradiction: the flash of light from the fireball is traveling much faster than any related sound.

Traditional wisdom holds that we cannot actually hear fireballs coming in, we may hear the rumble and explosion only some time later, usually after we have seen them. Keay has accumulated information showing that some people genuinely do hear fireballs as they come in and before they see them: how? The plasma trail from a large fireball may generate Extra Low or Very Low Frequency radio emissions; if an observer happens to be standing beside a suitable object (or perhaps if he is wearing a suitable object like glasses or headgear, that object can act as a transducer for the electromagnetic signal - thus the observer actually 'hears' the incoming fireball as it enters the atmosphere, before seeing it.

The technical name for this phenomenon is 'geophysical electrophonics.'