Warning: Extreme Ham Radio Geekery Post

In my 42 years as a licensed ham, one of my constant interests has been the subject of propagation: how the signal gets from point to point.  Most of that interest has centered on the ionosphere and the HF through VHF spectrum.   I can't say I study it at any sort of academic level, just the practicalities of the topic.

The ionosphere is the part of the Earth’s atmosphere that has been ionised by radiation from the sun.  It stretches from about 27 to over 540 nautical miles above the Earth’s surface (commercial aircraft typically fly under a fifth of the lowest limit - 5-1/2 nautical miles) and is generally divided into layers which differ in their behavior.  At the altitude it exists, the density of the air has dropped so much that atmospheric molecules are far apart, and when hit by solar (mostly UV) radiation the gas molecules can lose one or more electrons, turning air into plasma.  Most importantly, it refracts radio waves, affecting radio communication around the world.  Ionospheric propagation varies day by day and season by season, but the High Frequency radio spectrum, from 3 to 30 MHz, is the only place in the entire electromagnetic spectrum where worldwide communication is routine without man-made infrastructure such as towers or radio relays.

I subscribe to the ARRL Propagation Bulletin, which is mailed out at the end of the work week by the American Radio Relay League and available on their website for non-members.  This week's bulletin had a link to a really interesting paper about a new discovery out of Sydney, Australia.  The research comes from a new Australian radio telescope,the Murchison Widefield Array (MWA).   The article, by senior research and lecturer Tara Murphy, describes how an undergraduate researcher, Cleo Loi, has confirmed the existence of something previously theorized: there are tubes of plasma drifting above the earth in the ionosphere. 

The story is worth reading.  In these days of rampant junk science and papers that can't be independently verified, it starts with a "that's funny" type of observation; a postdoctoral researcher from Curtin University, Natasha Hurley-Walker, was examining data from the MWA and came across a night that looked "funny".  Celestial objects were dancing around wildly, distorting strongly in shape and flickering in brightness. She flagged this night as one that the ionosphere had rendered unusable for our astronomy research.  Turning to Dr. Murphy:

Cleo then developed a way of visualising the distortions caused by the ionosphere on the images of distant background galaxies. She took the data Natasha had identified and applied her analysis to it.

When she showed me and other researchers the distortion maps she was generating, we were surprised to see huge waves of correlated motion rippling through the image. They looked like spokes radiating from a point outside the image.

Here we find a key contribution from the undergrad, Cleo Loi.  She transformed the reference frame of the coordinates of everything the MWA recorded from a celestial to an earth-based frame.  This immediately showed that the bands were hanging almost stationary in the Earth’s sky.

Some time later, after working through hundreds of emails of suggestions, Loi had another moment of great insight:

As she was preparing her honours thesis, Cleo had a geometrical insight into explaining the radial spoke-like pattern. She realised that a set of parallel lines viewed at an angle would appear to converge due to perspective distortion, like train tracks going into the distance.

However, without much knowledge of geophysics, it was several weeks until she made a second critical link: the layout of the spokes matched the Earth’s magnetic field. These strange tubular structures were tracing the magnetic field lines, which are parallel to one another but at an angle to the ground. The agreement was perfect.

(A visualization of the plasma tubes - very worthwhile video here).

Consultations with other, more experienced scientists ensued.

Finally, Fred Menk from the University of Newcastle suggested they might be “whistler ducts”. These are cylindrical structures aligned to a magnetic field, where the electron content is higher inside than outside. They are thought to guide the propagation of electromagnetic waves called “whistlers” in the same way that optic fibres guide light.

Whistler ducts had never been seen before, but all their properties deduced by scientists over the years matched what we were seeing with the MWA. Except for one thing: we didn’t know how high they were.

Again, a major contribution by Cleo Loi.

She realised that the MWA could be used stereoscopically to achieve 3D vision, like a giant pair of eyes. By splitting the data from the eastern and western receivers of the MWA, she revealed a slight parallax shift in the distortion pattern that let us triangulate the altitude: around 600km above the ground.

We were all astounded that this idea had worked, confirming that these were likely to be whistler ducts.

The only phenomenon that I'm familiar with which uses the term whistler is a type of VLF signal, (Very Low Frequency - 1 to 30 kHz) explained as lightning strike pulses being ducted to a different hemisphere. 

The visualization image (above) led me to think of transequatorial propagation at VHF.  It has been known among hams since the end of WWII that 10 and 6 meters (primarily) are more likely to open (allow contacts) on north/south transequatorial paths than paths oriented more east/west.  Is this how the transequatorial signals propagate, ducted by the plasma tubes, or is it totally unrelated?  I don't know, but it bears watching.