Streetlight via the ionosphere

Pieter-Tjerk de Boer, PA3FWM pa3fwm@amsat.org

(This is an adapted version of part of an article I wrote for the Dutch amateur radio magazine Electron, March 2020.)

In the so-called Luxembourg effect, the modulation of a strong AM transmitter is transfered to another station's signal in the ionosphere. This happens as the electric field of the strong transmitter accelerates the free electrons in the ionosphere, causing variations in the damping of the other transmitter's signal. When the physicist V.A. Bailey was doing calculations about this, he realised that the effect should be much stronger around 1.4 MHz, and he thought of a remarkable application of this phenomenon.

When electrons are moving in a magnetic field, they are inclined to move in circles. As it turns out, how long it takes to make one revolution only depends on the strength of the magnetic field. The earth's magnetic field at our latitudes causes free electrons to make about 1.4 million rounds per second. This 1.4 MHz is called the "gyro-frequency". Bailey realised that if one would send an electromagnetic wave, preferably circularly polarized, of this frequency into the ionosphere, the electrons are accelerated much more than at other frequencies. At other frequencies the electrons are dragged back and forth by the radio wave's electric field, and thus acquire and lose speed in every cycle. But at the gyro-frequency they move around precisely at the rhythm of the radiowave and are thus accelerated more and more.

Bailey realised that this acceleration could be so much that light would be produced in the ionosphere, similarly to how polar light is caused by charged particles hitting the atmosphere from outer space. According to his calculation [5] a 500 kW transmitter feeding an antenna array of 500 dipoles should be enough to give a clearly visible glow. With a gigawatt(!) and a more modest antenna (wider beam) a light intensity should be produced similar to that of the full moon over an area of some 10000 km² (about a quarter of the Netherlands). He even filed a patent on this [6].

In 1961 an experiment has been done to demonstrate this effect, using a transmitter near Moscow of about 500 kW and 20 dB antenna gain. But no glow was observed. Much later, in 1978, a very faint glow from this installation has been measured using special detectors, but that glow was far too weak to serve as street lighting [7].

References:

[5] V.A. Bailey: Generation of auroras by means of radio waves. Nature, 1938.
[6] V.A. Bailey: Method of producing and utilising certain electrical conditions in the ionosphere. Australian patent nr. 102635, 1937.
[7] A.V. Gurevich: Nonlinear effects in the ionosphere. Uspekhi Fizicheskikh Nauk, 2007.