Auroral Small-and Meso-Scale Structures, Origin and Function

International audience; Discrete auroral arcs regularly display small- and meso-scale distortions that can appear suddenly and move with speeds that are not related to plasma speeds in the ionosphere but rather represent properties of the acceleration processes in the magnetosphere. The temporal and spatial structure of each small-scale structure is thus evidence for its distinct dynamic role in the interaction between the hot magnetospheric and the cold, dense ionospheric plasmas. We analyzed passes of Reimei and FAST over the network of THEMIS Ground-Based Observatories (GBO) with all-sky cameras to combine small- and medium-scale auroral imaging with in-situ measurements of the precipitating particles in order to determine the properties and characteristics of auroral arcs and embedded small-scale structures. The THEMIS or Cluster spacecraft provided additional measurements of magnetospheric plasma and the GBO magnetometers allowed for the determination of ionospheric currents and their dynamic changes. The combination of in-situ and remotely determined auroral arc properties allowed for the complete characterization of a substorm breakup arc and embedded small-scale structures. We find consistency between theoretical expectations and observed values for arc and fold speeds and dimensions. The leading edge of the breakup arc with Alfvenic accelerated electrons exhibits the fastest fold speeds while the wider inverted-V arc shows less structure and internal speeds. Twin vortex shear flows observed by two of the THEMIS spacecraft are related to the poleward expansion of the breakup arc, the development of the substorm current wedge, and the appearance of a strong upward/downward current system. The scale size and motion of the small-scale structures signifies the dynamics of the magnetospheric plasma and the acceleration processes responsible for their generation.