Observations of Non-Linear Processes in the Ionosphere
The purpose of this paper is to review observations of auroral electron distributions in the light of the physics of beam-plasma interactions. I will do this by relating the discussion to a particular problem in auroral physics which I believe can only be solved with reference to such processes. The question is — how are electrons accelerated to form discrete auroral arcs? It is usually accepted that they are accelerated by an electric field parallel to the magnetic field. Some of the earliest evidence for this view came from measurements of electron spectra which contained sharp peaks at energies in the kiloelectron volt range and pitch-angle distributions that were strongly field-aligned. (Albert 1967, Evans 1968, Westerlund 1969). An example is shown figure 1 (Arnoldy et al 1974). These observations raised a number of problems. The first problem to receive attention was to understand how electric potential differences of kilovolts could be maintained along magnetic field lines. It is now known that several processes might contribute, e.g. the magnetic mirror force, anomalous resistivity, formation of double layers, electrostatic shocks, although it is not known which is the most important.
KeywordsPitch Angle Plasma Sheet Electrostatic Wave Suprathermal Electron Parallel Electric Field
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