physics.irfu.se/SEE/ INSTITUTET FÖR RYMDFYSIKUPPSAL A Swedish Institute of Space Physics How we dicovered Stimulated Electromagnetic Emissions Far above the biosphere and the ozone layer we have the ionosphere, where, fuelled 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 interactions between our terrestrial space plasma and the electromagnetic waves, both from cosmos and, as in the SEE-experiments, from high power HF radio transmitters, are not as simple as one could imagine. In the plasma, charges interact with electric and magnetic fields. Thus the charges get accelerated by the electric field components of the radio waves. When charges in one volume move, a charge imbalance is created inducing new electric fields in the plasma which, in their turn, interact with other charges. The ionospheric plasma consist of several different kinds of molecules and consequently different kinds of ions, mainly atomic oxygen, molecular oxygen and nitric oxide. Some of these ions are slower to accelerate than others because of their different masses. Their original random motion, the plasma temperature, which in its turn can be divided into ion temperature and electron temperature, also influences the wave-plasma interactions. At least one more major complication must be added to the equation: the ionospheric plasma is magnetised by the Earths magnetic field. The moving charges are influenced by the magnetic fields so that they accelerate perpendicularly to the motion and the magnetic field. The accelerating charges induce magnetic fields in their turn
Posted on: Tue, 06 Jan 2015 05:40:43 +0000
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