(Note to working scientists and those with scientific credentials: I realize that I have glossed over and simplified some of this subject matter. This is not a scientific journal article, but rather an attempt to bring everyday people to the point where they can follow what I am proposing. If anybody would like to pursue the details of this with me, by all means contact me).
After I first noticed the Hum, a web search quickly led me to David Deming’s 2004 paper, “The Hum: An Anomalous Sound Heard Around the World”. My first reaction – fascination and relief – was similar to many people around the world who write to me when they find the Hum Map (www.thehum.info). In his paper, Deming lays out the logic and evidence that suggest the Hum could be caused by Very Low Frequency (VLF) radio waves used for naval communication. Building on that work, I propose a mechanism that explains the sometimes elusive behaviour of the Hum Phenomenon.
But many of us do not have strong backgrounds in physics or biology, and therefore I am presenting a series of brief crash courses on some basic topics so that greater numbers of people – hearers in particular – can understand what I am proposing. In this post, we will look at how VLF radio waves travel.
Read the primer on EM radiation if you have not done so. We are not discussing low frequency sounds here; that is a completely different type of energy.
FM radio stations fade out quickly as we move away from the transmitter. VLF energy, however, can travel tremendous distances – right around the planet, in fact. That is why the world’s major powers use VLF radio for communication. But VLF radio has another attractive property: an extremely large “skin depth”, which means that VLF radio waves can penetrate materials, such as ocean water, to a much greater distance than radio waves with higher frequencies, such as AM radio, microwaves, and so on. This is why VLF is used to communicate with submerged submarines. VLF radio waves can also penetrate earth as deeply as 150 m. I’ve written previously about the irony of the tinfoil hat being the icon for clinical paranoia, because microwaves have a very small skin depth and a thin layer of foil can, indeed, block and reflect microwave energy (which is why we don’t put aluminum foil in the microwave oven).
VLF waves travel (propagate) in three ways. First, and less important here, is the direct wave that travels line-of-sight distances directly to the receiver. Second is the “ground wave”, which runs between the surface of the Earth and the bottom layer of the ionosphere (an electrically charged layer in the atmosphere). The third is the “skywave”, in which the VLF wave bounces from the ground to the ionosphere and back to the ground, sometimes making multiple “hops” around the globe.
The Ground Wave. Within 100 to 300 km (60 to 180 miles) of the transmitter, the ground wave provides most of the energy of a received VLF signal. The strength of the signal can be sharply affected by mountain ranges, mineral deposits in the ground, ice fields, etc. (http://www.navy-radio.com/manuals/0101-1xx/0101_113-02.pdf). Because the signal spreads out in all directions, the ground wave signal becomes weaker as you move away from the transmitter. But something interesting happens beyond a certain distance from the transmitter: the waves start converging (coming together) and refocus on the opposite side of the planet, at a place called the “antipodal point”. This was first tested in 1925 by Tremellen, Eckersley, and Lunnon (http://ia801505.us.archive.org/5/items/noteonantipodalf182wait/noteonantipodalf182wait.pdf). Let’s take a specific example. The VLF transmitter at Cutler, Maine was, and could still be, one of the most powerful transmitters on the planet, on any frequency. The antipodal point for Cutler Maine is located in the Southern Ocean between Australia and Antarctica. The signal strength at that location coming from VLF Cutler will be much stronger than in, say, South America, which is thousands of kilometres closer to the transmitter. Contact me if you would like further references in this area. To find your antipodal point, click here.
The Skywave. First some basics about the Earth’s atmosphere – specifically, the layers that comprise the ionosphere. The ionosphere is a layer of electrically charged particles, created mainly by the impact of Ultraviolet (UV) radiation from the sun that strikes the atmosphere. Because the particles have an electric charge, they affect the propagation of radio waves. The ionosphere changes at night because the night side of the Earth is not being bombarded with the solar “wind” of UV and x-ray radiation. During a typical 24 hour cycle, layers will predictably form, disappear, and change in height. The ionosphere also changes with the seasons, as regions of the planet experience changing levels of solar radiation. The daytime and nighttime layers of the ionosphere are shown in the diagram below.
You will notice from the diagram above that the skywave will take a longer path to get to a receiver than the ground wave. That means that if you are in a location where you can receive both the skywave and the ground wave, you will receive two identical signals, slightly out of synch with each other. As explained in the primer on EM radiation, this creates what is called an “interference pattern”. I am simplifying this (and what is above) considerably, but in an interference pattern, there can be destructive interference (“dead zones”) and constructive interference (“hot spots).
But it gets more complicated. Suppose you are in, say, Portland, Oregon, relatively close to the massive Jim Creek VLF transmitter in Washington. The ground wave rushes toward you, but there is another ground wave that takes the long way around the planet in the opposite direction (great circle route) and strikes your location from the opposite direction. This can create what is called a “standing wave”, which oscillates in a particular location. http://nvlpubs.nist.gov/nistpubs/jres/68D/jresv68Dn1p27_A1b.pdf
Even professional physicists write about the massive complexity of this topic due to the fact that the models used to discuss this topic are just that – models, and the real world doesn’t always behave in the way that our equations might predict. The main idea from the above is that you could be located in a place that has very high levels of VLF energy, and yet be hundreds or even thousands of kilometers away from any transmitter. Also, local geography and geology cause variations in the strength of a received VLF signal. Changing your location by a little as a few dozen kilometers will change the signal strength, especially where oceans meet mountains and valleys/fjords. VLF travels differently at night from during the day, and differently during summer from in winter.