Alfvén waves and birkeland currents

Abstract

Hannes Alfvén developed concepts over a half-century ago that continue to influence and guide modern space plasma physics. He derived the guiding center approximation for determining the motion of charged particles in a magnetic field and introduced the idea of a partial ring current formed from trapped radiation in the Earth's magnetic field. He expanded on the suggestion of electric currents flowing into and away from the auroral regions along geomagnetic field lines that was made at the turn of the century by the Norwegian scientist, Kristian Birkeland. Alfvén introduced the concept of a new type of wave motion that involved the coupling between mechanical and electromagnetic fields. These waves are called magnetohydrodynamic waves and are also commonly referred to as "Alfvén waves." In his theory of the origin of the solar system, Alfvén proposed that when a neutral gas streams through a plasma across magnetic field lines at sufficiently high velocity, a discharge-like process can occur in which the neutral gas begins to ionize rapidly. His "Critical Ionization Velocity" (CIV) effect has been demonstrated in laboratory experiments and in space. He applied Langmuir's theory of a probe in a plasma to explain the acceleration of electrons along geomagnetic field lines by "double layers" which produced aurora. Alfvén's ideas were not immediately accepted. His view of Birkeland currents became a source of controversy and intense debate for decades, which could be resolved only after the dawn of the space age. Alfvén regarded the space age as being a revolution in science, comparable to the introduction of the telescope by Galileo. He pointed out the ability of spacecraft to observe a wide range of physical parameters in comparison to the limited "visual light universe" based on ground based telescopes. An enormous amount of data is collected every day from a fleet of international satellite missions which depend upon the plasma physics principles introduced by Hannes Alfvén for interpretation and analysis. This paper describes a small fraction of the ever-increasing knowledge base derived from these space missions, concerning our understanding of Birkeland currents and Alfvén waves.