What high altitude observations tell us about the auroral acceleration: A Cluster/DMSP conjunction

Abstract

Magnetic conjugate observations by Cluster and DMSP F14 satellites are used to study the field lines of auroral arc. Cluster is well above the acceleration region and observes upward keV ion beams and bipolar electric structures. The integrated potential at Cluster altitudes shows a dip that is consistent with the keV electron acceleration energy at low altitude. The earthward Poynting flux at Cluster altitudes is comparable to the electron energy flux at low altitudes. Thus, for this event the auroral acceleration can be described as a quasi‐stationary potential structure with equipotential lines reaching the Cluster altitudes. The arc forms at the outer edge of the plasma sheet at a density gradient. Multiple Cluster satellite measurements allow us to study the density increase associated with the development of the arc, and to estimate the velocity of the structure. The quasi‐potential structure itself may be part of an Alfvén wave.

Mesoscale PIC simulation of double layers and electron holes affecting parallel and transverse accelerations of electrons and ions

“…We see that the density is enhanced in the region of the low potentials. Using magnetic conjunction between the Cluster satellites and DMSP, Vaivads et al [2003] reported that as the Cluster satellites 1, 2 and 3 crossed the auroral arc a density enhancement was found at 4.7R e geocentric altitude while at the low altitude of DMSP (∼800 km) an extended density cavity was found. While the formation of the lowaltitude density cavity is well known from satellites like FAST [McFadden et al, 1999], the density enhancement measurements at high altitude, perhaps in the source region of the auroral arcs, is an unexplained new finding.…”

“…We note the density enhancement in the central part of the V-shaped potential structure. Later in section 7 we discuss that the density structure like in Figures 4c and 5b is inferred from observations during magnetic conjunctions between the low-altitude DMSP and high-altitude Cluster satellites [Vaivads et al, 2003].…”

“…[6] We report here that auroral arc-associated fieldaligned density structures, consisting of density cavity at the bottom (topside ionosphere) and density enhancement in the source region at high altitudes [Vaivads et al, 2003] in the magnetosphere are inherent features of a V-shaped auroral potential structure in the AUCR. The cavity at the bottom is a consequence of the acceleration of charged particles by the double layer and processes driven by it.…”

Auroral and sub-auroral phenomena: an electrostatic picture

“…The association of magnetospheric interfaces with these strong electric fields has been demonstrated in various case studies (e.g. Vaivads et al, 2003;Johansson et al, 2006;Kullen et al, 2008;Liléo et al, 2008). Note that the electrostatic model presented here predicts that transverse ionospheric electric fields can exist on both sides of an arc, which must give rise to oppositely directed plasma convection along the arc (Kullen et al, 2008).…”

“…Stronger converging fields lead to higher parallel potential differences, stronger upward field-aligned currents, even higher ionospheric conductivity, and accelerated upgoing ions, all typical of discrete auroral arcs (e.g. Lyons, 1981;Carlson et al, 1998;Vaivads et al, 2003;Figueiredo et al, 2005;Liléo et al, 2008;Echim et al, 2009). A statistical study by Johansson et al (2007) points out that the scale sizes of intense electric field signatures, of the field-aligned currents, and of the density gradients observed above the auroral zone at 5-7 R E geocentric distance were compatible, supporting the overall picture of a magnetospheric interface associated with the arc structure, and determining its thickness.…”