The theory of the acceleration of auroral particles is reviewed, focusing on developments in the last 15 years. We discuss elementary plasma physics processes leading to acceleration of electrons to energies compatible with emission observed for quiet, discrete auroral arcs, defined as arcs that have time scales of minutes or more and spatial scales ranging from less than 1 km to tens of kilometers. For context, earlier observations are first described briefly. The theoretical fundamentals of auroral particle acceleration are based on the kinetic theory of plasmas, in particular the development of parallel electric fields. These parallel electric fields can either be distributed along the magnetic field lines, often associated with the mirror geometry of the geomagnetic field, or concentrated into narrow regions of charge separation known as double layers. Observations have indicated that the acceleration process depends on whether the field-aligned currents are directed away from the Earth, toward the Earth, or in mixed regions of currents often associated with the propagation of Alfvén waves. Recent observations from the NASA Fast Auroral SnapshoT (FAST) satellite, the ESA satellite constellation Cluster, and the Japanese Reimei satellite have provided new insights into the auroral acceleration process and have led to further refinements to the theory of auroral particle acceleration.
Bibliographical noteFunding Information:
The authors wish to thank the International Space Science Institute (ISSI), Bern, for organization of the reviews and for support of the team meeting. RLL and YS have been supported by NSF grant AGS 1558134. MME acknowledges support from the Romanian Ministry of Research (PCCDI Grant VESS), the Romanian Space Agency (STAR project 182-OANA), the Belgian Solar Terrestrial Center of Excellence (STCE), the BRAIN-BE project MOMA BR/175/A2/MOMA. OM acknowledges support by SIFACIT contract 4000118383/16/I–EF with ESA and STAR EXPRESS contract 119/2017 with Romanian Space Agency. THW is supported by JSPS KAKENHI Grant Number JP16H04086 and JP17H01177.
- Particle acceleration