Actually, I work in a lab where I'm investigating ELF 'whistlers' that we've detected. It's really interesting stuff - we're not sure what causes them, but we've found some correlation between events and certain limits on the solar elevation angle. We think it might be ions getting trapped and moving along a field line.
Reading back my post I seem to imply that SDR enthusiasts discovered it, that wasn't my intention. This phenomenon was probably well documented before SDR.
When I first saw it, my first thought was "what a strange technique to run a radar, I wonder what the benefit is."
In ELF, would it be possible to set up three omnidirectional antennas and triangulate the source to get an idea in what layer of the atmosphere it's generated? I'd also love to read peer-reviewed publications, please post some references if you can.
I'm not quite sure if it would be possible to triangulate the position of the events - our sensors only detect relative changes in the magnetic field in the X and Y direction. As it stands we have only a handful of sensors: one that we operate at South Pole Station, and data that is made available to us from Taiwan. We are thinking, however, of placing a new sensor at McMurdo station so that we can see how latitude affects our findings.
Something that we've just found in the past few months shows that the occurrence of events can be limited by the elevation of the sun - we've never seen events happen when the sun was below ten degrees elevation at the south pole, and never below thirty degrees in Taiwan. Additionally, events in Taiwan seem to cut out in the two hour period surrounding local noon, suggesting that these might be highly local. They're also somewhat infrequent - we might only find two hundred such events over an entire year. We could go weeks without seeing one, and then get five events within a few hours. This might also suggest that it has something to do with solar heating of the ionosphere, given the timing of events. Note that all of this stuff is new and isn't discussed in any published literature.
As far as what's been published goes, I think there have been maybe half a dozen papers written since this phenomenon was discovered in the seventies. Here are the ones I've been able to quickly grab:
http://www.youtube.com/watch?v=0Hv--BR0ddE
also briefly addressed here:
http://www.youtube.com/watch?v=ZuRcaxpbYCw
edit: not on topic, should have refrained from posting as it's a VLF radio phenomenon and not a sound.