Electric Fields and Field-Aligned Currents Associated with Aurora. The Heritage of Hannes AlfvénÂÂ

Abstract:

For the discovery of magneto-hydrodynamic waves, published in Nature 1942, and for other important fundamental contributions to plasma physics, Professor Hannes Alfvén was awarded the Nobel Prize in Physics 1970. He was first to propose that electric fields with a component aligned with the Earth’s magnetic field (1958) produced aurora. The first in situobservations of accelerated particles producing aurora were presented by McIlwain (1960). Alfvén argued strongly for the importance of measuring electric fields in space plasmas, which initiated the development of the double probe technique, first presented by his graduate student Ulf Fahleson in a PhD dissertation 1967 at KTH. The technique was subject to much further development in close collaboration with Professor Forrest Mozer, University of California, Berkeley. It has been used to measure electric fields on numerous rocket and satellite missions, such as the NASA missions S3-3, Dynamics Explorer, Polar, FAST, and Themis, the Viking and Freja satellites, and the ESA Cluster mission. From these and other missions, there are today overwhelming experimental evidence that quasi-static parallel electric fields play a key role to accelerate auroral particles. In the mid 90’s, diverging electric field structures and downward electric fields were discovered in the downward current region by the Freja satellite. Another, closely related, and very efficient acceleration process is by time-varying parallel electric fields of kinetic and inertial Alfvén waves, responsible for producing polar boundary intensifications and other types of dynamic aurora. With the Cluster satellite launches 2000, multi-point observations on auroral field lines became a reality and later direct crossings of the acceleration region allowed multi-probing of this key region, the first of its kind. Results are presented from event and statistical studies of Cluster data, addressing various open issues such as: the relative role by quasi-static and time-varying electric fields for producing aurora; the altitude distribution of the parallel electric field and potential; their temporal evolution; the structure and stability of the acceleration region; the auroral density cavity and its distribution in altitude with respect to the AAR; the field-aligned current closure in surges.