AURORAL LINESAURORAL LINES
The currents themselves are created by the 'plasma generator' effect: The solar wind flows past the magnetic field of the earth, generating an electric field. Then electrons are attracted in one direction while protons feel a force in the opposite direction. A charge separation occurs. The particles complete the 'circuit' by spiralling along the earth's magnetic field lines. Electrons flow down to the ionosphere then through it and finally back up field lines on the other side of the earth. This vast generator provides the necessary electron acceleration to collisionally excite oxygen ions in the ionosphere creating powerful auroral storms.
Because of the extremely low densities in these regions, the lifetime of forbidden transitions is shorter than the mean time between collisions. The statistical distribution of the excited states of these ions thus favours an overpopulation of the metastable states and forbidden transition dominate the spectrum. McLennan (1928) and Paschen (1930) have explained the green and red auroral lines as corresponding to forbidden transitions (1S-1D and 1D-3P, respectively) of the neutral O atom.
The auroral lines have also been obtained in the laboratory in suitable light sources (McLennan and Shrum 1925; Paschen 1930); for example, in discharges through argon with a small addition of oxygen. The destruction of the metastable atoms is considerably hindered by the argon. This artificial production of the green auroral line made possible the study of its Zeeman effect. From this it follows definitely that a quadrupole transition is involved (Frerichs and Campbell, 1930). Since it is a singlet transition, the normal Zeeman effect with three components would have been expected for dipole radiation. Actually, two additional components were observed at twice the distance from the middle line - an effect in agreement with the theory for quadrupole transitions.