FAST observations of downward current regions: Effect of ionospheric constraints on parallel signatures

Abstract

[1] Downward current region auroral crossings by the FAST (Fast Auroral Snapshot) spacecraft show that divergent electric fields which are perpendicular to the geomagnetic field () have two types of structures: those whose potential contours close below the spacecraft but above the ionosphere (U-shaped), and those that are not completely closed but partially couple to the low-altitude ionosphere (composite). Using FAST data from above 3000 km altitude, we investigate parallel signatures of particle motions in these downward current regions, focusing on the distinctions between U-shaped and composite potential structures. We analyze probability density functions of electron velocity moments and ion energies, and power spectral density scaling laws of wave turbulence above the potential drop. Results indicate that U-shaped potential structures show aspects of fully developed turbulence. Composite structures are often characterized by intermittent signatures, possibly because of lower ionospheric constraints. These results support a picture of an evolutionary process from composite to U-shaped potential structures in downward current regions.