OFFICIAL43 Why are sunrise and sunset the best times to listen to VLF signals?

[00:00.00]NARRATOR: Listen to part of a lecture in a physics class. [00:04.35]The professor has been discussing electromagnetic waves.
[00:10.15]MALE PROFESSOR: So are there are any questions before we continue our discussion of different types of electromagnetic waves? [00:16.31]Um, today we'll focus on radio waves, and specifically, very low frequency radio waves. [00:23.60]Ah Yes, Tim?
[00:25.50]MALE STUDENT: Are you going to talk at all about the difference between radio waves and sound waves?
[00:29.35]MALE PROFESSOR: Um, OK. That might be a good place to start, actually. [00:33.08]Sound waves are mechanical in nature, right? [00:36.35] Ah, they can only originate and spread in places where there is some dense physical medium, like atmosphere, or water. [00:44.25]Ah, they result from changes in pressure in that medium, like changes in air pressure. [00:50.26]So they can't travel through a vacuum, where there's no dense physical medium. Which is why they can't travel through interplanetary space.
[00:59.33]Radio waves, on the other hand, are fundamentally different from sound waves. [01:04.21]They are electromagnetic: [01:06.80]they result from oscillations of the electromagnetic field and don't need a physical medium. So they, like other types of electromagnetic wave, can travel basically anywhere—through a vacuum or through atmosphere or water.
[01:24.01]Now, radio waves can be detected. [01:26.52]For example, very low frequency radio waves can be detected with a special type of radio receiver called a very low frequency, radio, or VLF radio. Which can pick up radio waves with very low frequencies , from 3 to 30 kilohertz, which aren't really picked up by a regular household or car radio. [01:49.04]So VLF radios pick up VLF radio waves and convert them to sounds we can hear.
[01:56.09]Um, on Earth the main source of naturally occurring VLF emissions is lightning, which generates a pulse of radio waves every time it flashes.  Ah yes, Laura?
[02:11.42]FEMALE STUDENT: Since you almost always get lightning with thunderstorms, we can pick up VLF waves pretty often, right? [02:18.11]You just have to wait until there's a thunderstorm…
[02:20.37]MALE PROFESSOR: Ah, do you? Have to wait? [02:23.00]VLF receivers are very sensitive and VLF mwaves travel very far. [02:28.88]So we can pick up emissions from lightning that's far away. [02:33.81]So, actually, you can pretty much listen to them all the time, because lightning strikes Earth constantly, about a hundred times per second. [02:43.11]Even if there's no lightning where you are, with a VLF radio you can hear the crackling from storms that are thousands of kilometers away.
[02:52.75]However, some times of day are better than others for picking up VLF waves.[02:58.53] Daytime isn't as good as nighttime, for example. [03:02.33]And what’s more, my colleague Dennis Gallagher says—and in my opinion, he's right—[03:08.59]he says the best time to listen for them is around sunset or sunrise. [03:14.22]That's when there are natural waveguides in the local atmosphere.
[03:18.14]FEMALE STUDENT: Did you say waveguide?
[03:20.13]MALE PROFESSOR: Yes. A waveguide, [03:22.33]usually it refers to a device, like a metal conductor, that's used to guide and direct waves. [03:28.20]But waveguides also occur naturally—[03:32.00]they make a path for radio waves to follow in our atmosphere. [03:35.77]These natural waveguides occur when the Sun is rising or setting, which makes sunrise and sunset good times to pick up VLF emissions.
[03:46.02]Now, there are a few different sounds that you can hear on a VLF receiver, because when lightning strikes, the radio waves travel different distances and in different ways before they reach the receiver.
[04:00.59]Some really interesting ones are called “whistlers.”[04:04.72]Whistlers come from lightning-generated radio waves that leave Earth's atmosphere and travel into Earth's magnetosphere before bouncing back down. [04:15.20]Not all radio waves do this. And the sound they make… well we call them “whistlers” because they sound like slowly descending tone…[04:25.52]and no two whistlers are alike—[04:28.38]uh, to me they're the most intriguing.
[04:32.33]Ah, another interesting sound is the “tweek.”[04:36.55]Tweeks are the result of VLF waves that have traveled a long distance through the waveguides. [04:42.38]They produce a chirpy sound because the higher–frequency parts of the wave reach the radio receiver before the lower–frequency parts. [04:51.76]The entire wave is still considered very low frequency—[04:55.27]it's just that some parts of the wave have lower frequencies than others. OK?