Equatorial auroral electron beams at Saturn, Jupiter, and Earth; A comparative examination

The Cassini Magnetospheric Imaging Instrument (MIMI) has now discovered bi-directional electron beams with broad energy distributions near the magnetic equator of Saturn's magnetosphere (J. Saur et al., 2005). The beams are narrowly collimated along the local magnetic field vector, and occur at positions that map magnetically to the vicinity of Saturn's aurora as measured by the Hubble Space Telescope. Similar electron beams have been observed at both Earth and Jupiter. It is thus becoming likely that the upward (anti-planetary), low altitude, stochastic acceleration of electron beams, as distinct from the downward, often coherent, acceleration that directly causes bright auroral emissions, is a universal characteristic of strong auroral phenomena. Also, these beams now serve an important function of diagnosing auroral processes and mapping at high altitudes, since the signatures of downward acceleration that generate strong auroral emissions have not been identified there. At Earth the upward acceleration is known to occur in the regions of the downward magnetic field-aligned electric current that constitutes the closure currents of the global auroral electric circuit. Here we examine in greater detail the context and relative characteristics of the electron beams at Saturn, Jupiter, and Earth; and we use the beams to address the importance of spatial structuring and turbulence in the generation of global auroral displays.