Alfvén waves and aurora at Earth and Jupiter: Comparative analysis

The formation of discrete auroral arcs is directly connected with the dynamics of Alfvén waves propagating along auroral field lines. The auroral acceleration process has been most directly studied at Earth and at Jupiter, due to multiple satellites such as S3-3, Polar, Viking, and FAST in polar orbit around Earth and the Juno satellite in polar orbit around Jupiter. These observations have indicated two primary mechanisms for the acceleration of auroral particles: a quasi-static acceleration due to parallel electric fields that produces a nearly monoenergetic distribution of electrons, and a broadband acceleration mainly due to acceleration in the parallel electric fields of kinetic Alfvén waves. Monoenergetic acceleration in upward current regions is more common at Earth, while at Jupiter, broadband acceleration appears to dominate and occurs in both upward and downward current regions. Parallel electric fields are favored when the ratio of the plasma density to magnetic-field strength is lowest due to the lack of available charge to carry the field-aligned current. Broadband acceleration is dominant when the parallel electric fields are fluctuating on rapid timescales and when the current systems are filamented, which can occur due to phase mixing or a turbulent cascade to smaller scales.