Auroral particle acceleration processes: the legacy of Hannes Alfvén

Abstract

Impacting corpuscular radiation was believed to be the main cause for the aurora even before the space age. However, the means by which charged particles become energized, eventually impacting into the upper atmosphere was a matter of controversy - a controversy which died out only very slowly with time. Hannes Alfvén was the lead scientists in the 1950:ies and 1960:ies who dared to challenge the ruling paradigm of space plasma physics - by 1958 suggesting field-aligned electric “discharges” as the cause of the aurora. Field-aligned electric acceleration of charged particles became a matter of debate for nearly three decades with indirect proofs for its existence steadily mounting with time, yet all the time meeting strong objections. In retrospect one may wonder why rebutting was so important. Why clinging to the concept of infinite conductivity along magnetic field lines? The reason was simple, because it challenged a ruling paradigm in space plasma physics, requiring a strong modification of “ideal MHD”.The notion of “frozen-in-field-lines”, invented by Hannes Alfvén, is a useful concept in describing large scale morphologies. However, observations from space plasma experiments shows that the condition of frozen-in-field-lines is violated in most interesting regions where plasma acceleration occurs. Alfvén wanted to remedy what he considered the “misuse of MHD” by emphasizing currents instead of magnetic fields in space plasma physics.In this paper I will review the energy and momentum coupling originally proposed by Alfvén and their consequences for the acceleration of charged particles. Plasma acceleration processes, well known and recognized from studies of planetary magnetospheres in our solar system, are applicable also for more remote space objects in the Universe - such as stars and galaxies. The many astrophysical implications of his theories is perhaps the most important legacy of Hannes Alfvén. In fact, new evidence for his theories are emerging from contemporary deep space data.