The Production of Radio Waves
The four fundamental ways in which radio waves are produced are:
- Nonthermal or Synchrotron emission - Synchrotron emission is the most common form of radio emission from outside our solar system and is why the Milky Way appears so bright at radio wavelengths. It is associated with relativistic electrons or cosmic ray electrons, so named because they travel at speeds comparable with that of light. These electrons feel an electromagnetic force from magnetic fields between the stars in interstellar space and inside some cosmic objects. They are caused to spiral in their motion and in doing so emit radiation. This radiation was first noted as static in radio sets near particle accelerators of the Synchrotron type. Within these structures charged particles were being accelerated to near relativistic speeds using magnetic fields, the interaction of the electrons with these fields produced strong enough radio emissions to interfere with nearby receivers.
- Thermal emission - This is emission caused by the collisions of non-relativistic electrons, whose speed is determined purely by the kinetic temperature. This raises an important point in temperature measurement. Often in radio astronomy temperature will be talked about in terms of kinetic temperature a measure of the kinetic motions of the particles in an object and not the same as the temperature a thermometer would read.
- Plasma oscillations - In areas of space where plasma exists, for example in stars, the electrons can oscillate at a rate determined by the density. The motion of the charged particles produces a wave whose wavelength is determined by the oscillation frequency.
- Spectral line (monochromatic) emission - An atom or molecule can absorb or emit radiation by changing its state of motion and subsequently energy level. The change of state can be in the form of an electron changing the level of its orbit or even the spin about its own axis, in the case of molecules also the rotation and relative vibrations. The common factor is that when an electron changes its state of motion, and so energy level, a photon specific in wavelength to that change is either absorbed or emitted.