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Name: Tor T.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 6/12/2004


Question:
How are radio waves (or other forms of EM radiation) affected by external electric and magnetic fields? For example, passing radio waves through parallel plates or by a magnet (or even the Earth's magnetic field). Are the waves bent, deviated? Is the amplitude or wavelength altered? Will there become any noise and what is this noise, and why does it occur?


Replies:
Strangely, physics has it that in a vacuum, photons do nothing to photons. So radio waves (electromagnetic waves) are nominally unaffected by magnetic and electric fields.

But you're not always in a vacuum. Radio and light waves passing by matter are affected by matter: slowed down, bent, absorbed, maybe even distorted into other frequencies of wave.

Electric and magnetic fields can modify these interaction parameters of a substance, depending a lot on which substance it is..

Out in space is very thin gas, but it's almost all somewhat ionized, a plasma. Earth's ionosphere and radiation belts are a somewhat denser plasmas. Plasmas are affected by magnetic fields: given cyclotron resonance frequencies and a directionality for conducting slow waves along magnetic field lines. Some very low frequency radio waves are strongly bent by the magnetized plasma surrounding Earth. Normal earth-surface lightning radiates audio-frequency radio waves into space, which sometimes are bent to travel 1/3-way around the world following a magnetic field line. On the way, low frequencies are slowed down more than high, and sometimes can even be amplified. With a VLF radio receiver one then hears "whistlers", slowly falling tones reminiscent of old sci-fi movies. When there is less bending available, one hears "chirps" which are the same thing, but from only 1000 miles away, or "pops" which are from lightning even closer. (http://spaceweather.com/glossary/inspire.html)

Certainly the ionosphere makes hissing radio noise, much louder than thermal noise at frequencies below about 20-50 MHz. This noise does depend on how much Earth's magnetospheric plasma "is "pumped up" by space weather such as sunspots. A large slow magnetic wave from a solar flare can pump up the plasma just as effectively as the particle radiation or UV. The noise is louder because the free electrons in the plasma are bouncing around at energies equivalent to 10,000 degrees, even though their gas molecules are only moving the right amount for 300 degrees Kelvin. The "electron temperature" can be so different because the electrons are light and easily driven, and the ions and molecules are heavy and only slowly catch energy from the zinging loose electrons. It's a bit like wind driving sand through a bunch of rubber balls.

For mapping out what plasmas do, you might want to learn what the "plasma cutoff frequency" is about. For DC and low frequency electric fields plasmas are a conductor, a short circuit. Frequencies too high cannot move the particles far enough to matter, and only a little absorption occurs. Higher densities of charges particles mean higher cutoff frequencies. That's why AM radio waves (1MHz) bounce around the world, trapped between our ionosphere and the sea, but cell phones and radars (>>100MHz) sing right out into space. There is an even lower cutoff frequency for the wave-bending that makes whistlers.

In microwave and laser equipment there are a few components which are magnetically controlled. An "circulator" is a slab of ferrite, biased with a permanent magnet, so ~10GHz radio waves travelling through it tend to curve left or right. We put three wave guide ports around this slab, A, B, and C. Waves from A are focused on B but absent at C. Likewise, B->C, and C->A. Can't do that without a magnetically anisotropic material. If we install a perfect wave-aborber at port C (merely the right resistor), waves can go through from A->B, but backwards waves B-> A are blocked, absorbed. That is called an "isolator". This asymmetry is also tough to do with any other technique. There are laser/wave guide isolators made with magnetic fields on clear crystals like YAG (yttrium aluminum garnet, Y3 Al5 O12) and crossed polarizers. There is a microwave oscillator-tuning technique, where the susceptibility of a "YIG" crystal (yttrium iron garnet) is very dependent on an adjustable DC electromagnet's field.

Electric field effects are a little less commonly used because they are weaker. If one puts parallel plates in a long tube filed with a non-conductive fluid and applies hundreds or thousands of volts, for some liquids or gasses the molecules will partially line up with the electric field lines, and the speed of light-waves travelling sideways between the plates will change by a few parts per million. This small change in speed can be leveraged to change the polarization of the light (Kerr cell), or change a resonant cavity from blocking to transmitting, or shift a wave between adjacent wave guides. Lately a tiny solid chip version of this (Mach-Zender modulators) has become important in making light-wave switches for fiber optic communication. These benefit from being very small: they use a few volts over a millionth of a meter, instead of a few thousand volts over a thousandth of a meter. Lithium Niobate is the nonlinear crystal most often used for these.

Most of the time when matter's wave-susceptibility is easily influenced by small constant fields, there is substantial wave-loss or absorption mixed in with the wave-bending or slowing. Usually what you want in components is all bending and no absorbing. So the search for better materials and ways to use them, though it has been tried for most of a century, is still ongoing.

Jim Swenson



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