celeste wrote:Researchers expected to find that thunder propagates spherically but found that thunder propagates cylindrically from a lightning bolt.
Was that because of the electric field, or was it just because the sound came from an axial source? In other words, sound from a point source will propagate spherically, but sound from a linear source is going to be cylindrical. Or course, a lightning bolt is highly irregular. But the irregularities cancel out, and what propagates is the superposition.
celeste wrote:There is lots of evidence that lightning is a Birkeland current, with a strong radial electric field.
The field that causes the strike is vertical (typically from a negative charge in the cloud to an induced positive charge in the ground). Then, during the strike, there is an EMP that propagates radially (i.e., cylindrically). But by the time the thunder starts, the discharge is long gone. The local electric field has been neutralized, though a couple of kilometers away there might still be a vertical field, like the one that initiated the strike.
I'd have to read the references, to find out exactly what they're talking about. Otherwise, we'd have to go back and forth a few times, to get in sync on all of the terms, observations, conclusions, etc.
As concerns the refraction of seismic waves inside the Earth, I had an idea. Sound waves are refracted toward the greater density -- this is (sorta) true. But it would be more true to say that they are refracted toward the medium with the lower speed of sound. In the atmosphere, this tends to be the cooler, denser air. But density isn't the issue -- temperature is the critical issue in the speed of sound. So if the Moho is hotter than the overlying rock (because there is a heat source there, and because the surface is cooled by radiative heat loss), the waves will travel faster near the Moho, and slower in the cooler rock above, differences in density under pressure notwithstanding. I don't know why it isn't explained that way, but it sure sounds correct.
I'll see if I can find literature to confirm/deny that such is the case.