Alfvén's Critical Velocity Hypothesis

Abstract

IN his theory on the origin of the solar system¹, Alfvén makes an assumption which is central to the entire theory². This assumption is that when there is a relative velocity between a plasma and a neutral gas in the presence of a magnetic field, a strong ionization process will set in when the relative velocity reaches a critical value U_crit such that where m_g and V_i are the mass and ionization potential of the gas atom. Using this assumption, Alfvén has been able to predict fairly accurately the mass distribution of the planets and satellites in the solar system. A study of the ionization mechanism leads to some difficulties, however; first, the threshold energy for ionization in a collision between a gas atom and an ion of the same gas is 2 eV_i (ref. 3) and, second, the ionization cross-section of ions is very small compared with that of electrons, except at very high energies. It has been suggested⁴ that the ionization must be caused by the electrons and that energy is transferred from the ions to the electrons by some kind of space charge mechanism which is not discussed in detail. The purpose of this communication is to point out three simple ionization mechanisms which lend support to the critical velocity hypothesis.