Small and meso‐scale properties of a substorm onset auroral arc

Abstract

[1] We present small and meso-scale properties of a substorm onset arc observed simultaneously by the Reimei and THEMIS satellites together with ground-based observations by the THEMIS GBO system. The optical observations revealed the slow equatorward motion of the growth-phase arc and the development of a much brighter onset arc poleward of it. Both arcs showed the typical particle signature of electrostatic acceleration in an inverted-V structure together with a strong Alfvén wave acceleration signature at the poleward edge of the onset arc. Two THEMIS spacecraft encountered earthward flow bursts around the times the expanding optical aurora reached their magnetic footprints in the ionosphere. The particle and field measurements allowed for the reconstruction of the field-aligned current system and the determination of plasma properties in the auroral source region. Auroral arc properties were extracted from the optical and particle measurements and were used to compare measured values to theoretical predictions of the electrodynamic model for the generation of auroral arcs. Good agreement could be reached for the meso-scale arc properties. A qualitative analysis of the internal structuring of the bright onset arc suggests the operation of the tearing instability which provides a 'ropelike' appearance due to advection of the current in the sheared flow across the arc. We also note that for the observed parameters ionospheric conductivity gradients due to electron precipitation will be unstable to the feedback instability in the ionospheric Alfvén resonator that can drive structuring in luminosity over the range of scales observed.

Non-propagating electric and density structures formed through non-linear interaction of Alfvén waves

“…The perpendicular scale size of most of these waves was on the order of a kilometer or less, as expected for acceleration by inertial Alfvén waves. The role of the Alfvénic acceleration in the poleward side of arcs was confirmed by conjugate observations from space and from the ground (Frey et al, 2010). It was proposed by Klatt et al (2005) and others (Asamura et al, 2009;Chen et al, 2005), that the Alfvén waves might be triggered by the electron beam caused by the quasi-static electric fields.…”

Birth and life of auroral arcs embedded in the evening auroral oval convection: A critical comparison of observations with theory

“…The alternative of installing imagers on the spacecraft usually does not allow detailed studies of individual arcs because of the limited spatial resolution, the poor cadence of imaging, and the fast motion of the spacecraft. The Japanese satellite, Reimei, overcame much of these drawbacks by its low orbit and active pointing of the camera toward the magnetic foot points [e.g., Sakanoi et al , 2003; Frey et al , 2010]. On the other hand, the low orbit meant to forego the possibility of intersecting the auroral acceleration regions, not to speak of the absence of an onboard magnetometer and electric field instrument.…”

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

“…Fine scale structures of the aurora in both the temporal and the spatial scale have recently been investigated. Many cases of fine‐scale structures in discrete auroras were demonstrated by both spacecraft investigations and ground‐based observations, such as Alfvenic auroras [ Semeter et al , 2008; Frey et al , 2010] and flickering auroras [ Gustavsson et al , 2008; Whiter et al , 2008, 2010; Yaegashi et al , 2011; Kataoka et al , 2011]. While diffuse auroras have been generally regarded as uniform and not discrete structures, fine scale structures were also reported [e.g., Wallis et al , 1979; Pedersen et al , 2007].…”