A Theory of Auroras and Magnetic Storms

Abstract

The physics of the atmosphere of the earth under a quiet sun is discussed in detail. The daytime temperatures above 100 km increase with the height to roughly 1000°K at a 400 km level; new tables of the molecular density of the atmosphere to great heights are given. In the region above 450 km, where the molecular free paths are very long, a portion of the highly absorbed ultra-violet light of the sun is converted into kinetic energy, by processes of atomic excitation and ionic recombination, and produces ${10}^6$ atoms ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ which fly out from the earth with velocities of 10 km ${\sec }^{-1\mathrm{}}$ or more. The atoms attain levels of 30000 to 50000 km in 3 hours and are then ionized by the ultra-violet sunlight. The ion pairs thus formed spiral about the lines of force of the earth's magnetic field and a majority are guided to the polar latitudes. They fall into a zone roughly 25° from the magnetic poles and give rise to the auroras there; this is the observed zone of maximum auroral frequency. It takes the ions 9 hours to travel from the equator to the poles, and therefore the aurora occurs more often in the early hours of the night than in the later hours, as is observed. The fact that short wireless waves traverse polar regions supports the view that the ionization is due to the ion influx from lower latitudes; for the sunlight is too weak to make many ions there. It is assumed that the sun, when active, emits a sudden (½ hour) blast of ultra-violet light. For example, if $\frac{1}{10000}$ part of the solar surface, normally at a temperature of 6000°, were removed and there were exposed the black-body radiations at 30000°, the solar constant would be increased by 1 percent and the ultra-violet energy, $\lambda 500$ to 1000A, by ${10}^5$. This ultra-violet energy, completely absorbed in the high lying (200 km) atmospheric gaseous layers, blasts out these layers to produce ions up to 40000 km. Due to gravity and the earth's magnetic field the first effect of the high flying ions is to produce a sudden current, ${10}^6$ amperes, in planes parallel to the equator, which causes a magnetic field ${10}^{-3\mathrm{}}$ gauss simultaneously over the whole earth, as is observed in the first phase of the world-wide magnetic storms. Numbers of ions descend to the zones 23° from the magnetic poles and form there diamagnetic concentrations of considerable intensity (also give rise to the auroras). On the assumption that the blast of ultra-violet light does not die away abruptly but continues with lessening intensity for a day or so, the diamagnetic concentrations wax with the day and wane with the night. The changes in the earth's magnetic field caused by this diamagnetism are found to agree in nearly every detail with the observed complicated diurnal storm variations in the three magnetic field components at all latitudes.