What are Whistlers?. A whistler is an electromagnetic radio wave that when converted to sound is usually experienced as a descending tone.
What are Whistlers?
Although a whistler is an EM wave, it is a very low frequency wave, much lower than what we normally think of for a radio wave. It is so low that little more than a long wire and audio amplifier is needed to pick it up.
World War I
L. R. Owen Storey first worked out the origin of whistlers in 1952 for his PhD thesis at Cavendish Laboratory University of Cambridge
Owen’s Quite Unpopular Answer:
Ionosphere can typically only produce D ~ 2 s1/2
while dispersions of D ~ 60 s1/2 are measured.
WORSE: H. J. A. Ratcliffe, Owen’s thesis adviser, presents his results at the 1952 URSI meeting in Sydney, but notes Owen’s theoretical interpretation is probably wrong.
“The Father of Whistler Research”
mid – 1950’s
Donald Carpenter, 1963
Discovers the Plasmapause from ground whistlers
Konstantin Gringauz, 1959
Makes the first in-space measurements of the plasmasphere from Luna 2
Since EM whistlers waves travel very great distances, you would expect their strength to fall rapidly as the waves spread out.
That does not happen, because small irregularities in the density of ionized gas (plasma) guides and confines the energy to narrow channels that maintain the strength of the wave until some is passed into the opposite hemisphere and some is reflected back.
You may know that the speed sound travels is different for different types of medium and state of matter or phase.
vsolids > vliquids > vgases
In Air: v = 331 m/s + (0.6 m/s/C)*T
(at 20oC it is 343 m/s)
In Water at 20oC it is 1483 m/s,
while in structural steel is it 4512 m/s
In a dispersive medium waves of different frequency have different wavelengths and consequently move with different speeds.
For Whistler Electromagnetic Waves the dispersion equation is:
(Or just check out the slinky)
Dispersion effect weighted toward equatorial densities
Carpenter & Smith 1964
Well… Really just to make one point. A whistler sounds the way it does, because of how things are in space near the Earth. Anyone can pick up whistlers with a low frequency radio receiver. You know how a sound is different when you hear it outside as compared to in a small room or in a large auditorium. To hear the sound is to also experience the environment where you hear it. Hearing whistlers allows us to experience a small part of space near Earth that is also like that near Jupiter, Saturn, and elsewhere in the universe. It is beautiful to hear and a tool for exploring the universe.
Space is Accessible!
Burton and Boardman Introduced the Sound Spectrograph in 1930