Swedish Institute of Space Physics
How we discovered Stimulated Electromagnetic Emissions
Far above the biosphere and the ozone layer we have the ionosphere, where, fueled by the sun, atoms and molecules are constantly being split into electrons and ions, which give the ionosphere its properties and its name.
Since the pressure is extremely low (the entire ionosphere weighs less than one tonne!), the electrons and ions are allowed to exist for quite some time before they recombine.
A gas which contains separated charged particles is called a plasma and plasma can be considered a fourth state of matter.
This fourth state is utterly rare on the surface of the earth. Only where the thunder strikes the enormous discharge creates as plasma lasting for a fraction of a second, and it was not until the 20th century that man was able to create a laboratory plasma.
Although rare on earth, plasma is abundant in space. Our sun and many stars are made of plasma, and as much as 99% of the Universe is in the plasma state. Our nearest and most accessible plasma "laboratory" is the ionosphere. By using electromagnetic waves as our messengers through space, systematic studies of this plasma can be conducted.
The Institute of Space Physics, Uppsala Division, (IRFU), has taken part in numerous campaigns over the last two decades with the aim to investigate and understand interactions between electromagnetic waves and the ionospheric space plasma.
In the beginning of the eighties, electromagnetic wave interactions in plasmas were often explained by a simple model, known as stimulated scattering, not very different from how a ball hits a wall. The ball bounces back, but with somewhat less energy, and perhaps in another direction.
The bounce exerts pressure on the wall and this pressure is spread as a sound wave within the wall. Imagine the ball as a photon, a quantised electromagnetic wave, and the wall as a layer of the ionosphere. However, when it comes to radio waves, interactions are electromagnetic instead of mechanical.
When an electromagnetic radio wave is emitted from the ground, it could for example be a high frequency (HF) radio wave from a radio station.
(image left from: apollo.lsc.vsc.edu, click for detail)
At the end of the seventies, huge high power HF transmitting facilities were built, transmitting electromagnetic waves with powers high enough to induce turbulence in the ionosphere strong enough to be separated from the natural background radio noise.
Through radar probing, the existence of enhanced plasma waves was soon discovered, at frequencies offset from the radar frequencies.
Through [more] radar probing, one could also see that in the region into which the radio wave was pumped, a turbulence, reflecting radar beams as turbulent air reflects light, was induced.
It was indeed an important week when the IRFU campaign, referred to as "Heating 1," took place, in the autumn of 1981. Old theories were shattered and the ground was laid for an entirely new way of viewing the interactions between a plasma and electromagnetic waves, in space as well as in the laboratory.
It is fair to say that one can regard this first "Heating" project in 1981 as the seed to a new non-linear area in of plasma physics. It was also the discovery of a new technique to investigate the space plasma called stimulated electromagnetic emissions (SEE). This technique makes it possible to study the space plasma from the ground in a new way and it is nowadays used for studies of plasma processes and can even be used as a diagnostic tool to determine local properties of different space plasma. (image right: The SEE changes character when the pump frequency is swept up and down through the 4th harmonic of the electron cyclotron frequency in the ionosphere.)