Multi-spacecraft observation of plasma dipolarization/injection in the inner magnetosphere

University of California, Berkeley, Berkeley, California, United States
Annales Geophysicae (Impact Factor: 1.71). 03/2007; 25(3). DOI: 10.5194/angeo-25-801-2007
Source: OAI


Addressing the origin of the energetic particle injections into the inner magnetosphere, we investigate the 23 February 2004 substorm using a favorable constellation of four Cluster (near perigee), LANL and Geotail spacecraft. Both an energy-dispersed and a dispersionless injection were observed by Cluster crossing the plasma sheet horn, which mapped to 9–12 RE in the equatorial plane close to the midnight meridian. Two associated narrow equatorward auroral tongues/streamers propagating from the oval poleward boundary could be discerned in the global images obtained by IMAGE/WIC. As compared to the energy-dispersed event, the dispersionless injection front has important distinctions consequently repeated at 4 spacecraft: a simultaneous increase in electron fluxes at energies ~1..300 keV, ~25 nT increase in BZ and a local increase by a factor 1.5–1.7 in plasma pressure. The injected plasma was primarily of solar wind origin. We evaluated the change in the injected flux tube configuration during the dipolarization by fitting flux increases observed by the PEACE and RAPID instruments, assuming adiabatic heating and the Liouville theorem. Mapping the locations of the injection front detected by the four spacecraft to the equatorial plane, we estimated the injection front thickness to be ~1 RE and the earthward propagation speed to be ~200–400 km/s (at 9–12 RE ). Based on observed injection properties, we suggest that it is the underpopulated flux tubes (bubbles with enhanced magnetic field and sharp inner front propagating earthward), which accelerate and transport particles into the strong-field dipolar region.

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    • "To understand the behavior of the electric fields associated with the DFs and acceleration of particles in DFs, we have studied the various electric field terms in the generalized Ohm's law at subproton scales [e.g., Baumjohann and Treumann, 1996; Fu et al., 2012]: the Hall (J × B∕n e e), convection (−V i × B) and electron pressure gradient (−∇p e ∕n e e). Previous studies revealed [Apatenkov et al., 2007; Sergeev et al., 2009] that DF could possess large electric field (up to 40 mV/m) and had strong dawn-dusk component in the spacecraft frame. Recent Cluster observation showed that the electric field in the DF layer was balanced by the three terms, and in the plasma flow frame, electric field has only a normal component [Fu et al., 2012], suggesting that the frozen-in condition for ions was violated. "
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    ABSTRACT: Electric fields associated with dipolarization fronts (DFs) have been investigated in the magnetotail plasma sheet using Cluster observations. We have studied each term in the generalized Ohm's law using data obtained from the multi-spacecraft Cluster. Our results show that in the plasma flow frame electric fields are directed normal to the DF in the magnetic dip region ahead of the DF as well as in the DF layer, but in opposite directions. Case and statistical studies show that the Hall electric field is important while the electron pressure gradient term is much smaller. The ions decouple from the magnetic field in the DF layer and dip region (E + Vi × B ≠ 0), whereas electrons remain frozen-in (E + Ve × B = ∇pe/nee).
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    • "The slab problem was discussed here in the context of hot plasma sheet plasma injection into the plasmatrough (e.g. Apatenkov et al., 2007; Zhang et al., 2008). Similar structures may be found elsewhere in planetary magnetospheres, for example – plasma penetrating into the magnetosphere at the dayside (Marchaudon et al., 2009; Lundin et al., 2003), where the plasma velocity and the associated momentum are of prime importance; in the context of the socalled impulsive penetration mechanism, the electric field inside the slab is called the " polarisation electric field " and it corresponds to charges of opposite sign accumulating in the interfaces on either side (Lemaire, 1977; Lemaire and Roth, 1978; Echim and Lemaire, 2000; Lundin et al., 2003); – bipolar auroral structures, in particular structures associated with polar cap arcs, often characterised by an electric potential well; in that case the external electric potential differences strongly affect the configuration , while the cross-field flow might be relatively unimportant (Maggiolo et al., 2006, 2011, 2012; De Keyser et al., 2010); and – plasma fingers, resulting from interchange motion, that penetrate into the plasmasphere (Burch et al., 2005; Mitchell et al., 2009). "
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    ABSTRACT: We consider cross-field plasma flow inside a field-aligned plasma slab embedded in a uniform background in a 1-dimensional geometry. This situation may arise, for instance, when long-lasting reconnection pulses inject plasma into the inner magnetosphere. The present paper presents a detailed analysis of the structure of the interfaces that separate the slab from the background plasma on either side; a fully kinetic model is used to do so. Since the velocity shear across both interfaces has opposite signs, and given the typical gyroradius differences between injected and background ions and electrons, the structure of both interfaces can be very different. The behaviour of the slab and its interfaces depends critically on the flow of the plasma transverse to the magnetic field; in particular, it is shown that there are bounds to the flow speed that can be supported by the magnetised plasma. Further complicating the picture is the effect of the potential difference between the slab and its environment.
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    • "Dipolarization frequently occurs beyond geosynchronous orbit [e.g., Lopez et al., 1988] but has also been observed deep in the inner magnetosphere [e.g., Sergeev et al., 1998; Fu et al., 2002; Apatenkov et al., 2007; Ohtani et al., 2007; Nosé et al., 2010]. [14] Lui et al. [1986] and Lui [1993] investigated radial transport of energetic ions with data from the AMPTE/CCE/ MEPA instrument which measures >56 keV protons, >72 keV helium ions, and the carbon-nitrogen-oxygen group mostly dominated by >137 keV oxygen ions. "
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