Magnetosphere-ionosphere coupling during periods of extended high auroral activity: a case study

Abstract

Abstract.Results are presented from a case study of a plasma boundary crossing by the Cluster spacecraft during an extended period of high auroral activity. The boundary between the magnetotail lobe region of the Southern Hemisphere and the plasma sheet boundary layer, was characterized by intense electric and magnetic field variations, structured upward accelerated ion beams, narrow-scale large field-aligned Poynting fluxes directed upward away from the ionosphere, and a relatively sharp plasma density gradient.The observations are shown to be consistent with the concept of a multi-layered boundary with temporal and/or spatial variations in the different layers. H + and O + ion beams are seen to be accelerated upwards both by means of a fieldaligned electric field and by magnetic pumping caused by large-amplitude and low-frequency electric field fluctuations. The peak energy of the ion beams may here be used as a diagnostic tool for the temporal evolution of the spatial structures, since the temporal changes occur on a time-scale shorter than the times-of-flight of the detected ion species.The case study also shows the boundary region to be mainly characterized by a coupling of the detected potential structures to the low ionosphere during the extended period of high auroral activity, as indicated by the intense fieldaligned Poynting fluxes directed upward away from the ionosphere.

Observation of an inner magnetosphere electric field associated with a BBF-like flow and PBIs

“…The quasi-static electric field structures observed by Cluster were also associated with Earthward Poynting fluxes (e.g., Johansson et al, 2004), although one event with Poynting flux away from Earth has been reported (Liléo et al, 2008). The Cluster events were typically located at plasma boundaries Johansson et al, 2006).…”

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.…”

“…Since the magnetospheric convection is driven by the solar wind-magnetosphere interaction, the solar wind is the source of the energy dissipated in the ionosphere in auroral phenomena. Magnetospheric flow shears are expected, for instance, on closed field lines near the plasmasphere and near the edges of plasmaspheric plumes, especially when hot plasmasheet plasma is injected in the inner magnetosphere during a substorm (McIlwain, 1974;Newell and Meng, 1987;Baker and McPherron, 1990), and in the low latitude boundary layer where the antisunward flow of magnetosheath plasma interfaces with the magnetospheric plasma (Lundin and Evans, 1985;Feldstein et al, 2001;Echim et al, 2008), but also in the plasmasheet (Galperin and Feldshtein, 1989;Baumjohann et al, 1990;Angelopoulos et al, 1992;Chen et al, 2000;Figueiredo et al, 2005;Hamrin et al, 2006;Marghitu et al, 2006;Liléo et al, 2008;Johansson et al, 2009). A second type of electric fields are charge separation electric fields, possibly strengthened by the presence of shear flows, especially at interfaces between cold (plasmasphere/plasmatrough or lobe) and hot (plasmasheet) plasmas, as invoked for discrete arc and subauroral ion drift generators (Ejiri et al, 1980;Feldstein and Galperin, 1985;Yeh et al, 1991;Roth et al, 1993;Lemaire et al, 1998;De Keyser et al, 1998;De Keyser, 1999Johansson et al, 2006;Echim et al, 2007); such fields are capable of creating fine scale structure.…”