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

Annales Geophysicae (Impact Factor: 1.52). 01/2007; DOI: 10.5194/angeo-25-801-2007
Source: OAI

ABSTRACT 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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    ABSTRACT: In situ observations and modeling work have confirmed that singly charged oxygen ions, O+, which are of Earth's ionospheric origin, are heated/accelerated up to >100 keV in the magnetosphere. The energetic O+ population makes a significant contribution to the plasma pressure in the Earth's inner magnetosphere during magnetic storms, although under quiet conditions, H+ dominates the plasma pressure. The pressure enhancements, which we term energization, are caused by adiabatic heating through earthward transport of source population in the plasma sheet, local acceleration in the inner magnetosphere and near-Earth plasma sheet, and enhanced ion supply from the topside ionosphere. The key issues regarding stronger O+ energization than H+ are nonadiabatic local acceleration, responsible for increase in O+ temperature, and more significant O+ supply than H+, responsible for the increase in O+ density. Although several acceleration mechanisms and O+ supply processes have been proposed, it remains an open question what mechanism(s)/process(es) play the dominant role in stronger O+ energization. This review paper summarizes important previous spacecraft observations, introduces the proposed mechanisms/processes that generate O+-rich energetic plasma population, and outlines possible scenarios of O+ pressure abundance in the Earth's inner magnetosphere.
    Journal of Geophysical Research Atmospheres 07/2013; 118(7):4441-4464. · 3.44 Impact Factor
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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.
    Annales Geophysicae 08/2013; 31(8):1297-1314. · 1.52 Impact Factor
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    ABSTRACT: [1] We present the evolution of dipolarizations in the near-Earth tail during a substorm on 15 March 2009, based on the two-point measurements in the nightside plasma sheet at X ~ −8.0 RE. The earthward-moving dipolarization, the magnetic flux pileup, and the tailward-moving dipolarization were observed. For the 30–200 keV electrons, betatron acceleration was the dominant process, which was caused by the much larger gradient of the magnetic field there during the earthward-moving dipolarization or by a local compression of the magnetic field during the magnetic flux pileup and tailward-moving dipolarization. These near-perpendicular distributions for the 30–200 keV electrons are interpreted as produced by a two-step acceleration: Electrons were first accelerated in the dipolarization fronts in the midtail or the near-Earth tail and then were further accelerated near the tail current disruption region. For the more than 200 keV electrons, Fermi acceleration was the dominant process, which was caused by the shrinking length of magnetic field line during the tailward-moving dipolarization. The source region of the more than 200 keV electrons may be near the tail current disruption region, but these electrons were accelerated locally. These field-aligned electrons can precipitate into the ionosphere and form the discrete auroral arcs. Two parallel arcs were clearly observed around the substorm onset: one propagated equatorward, another propagated poleward. We suggest that the earthward-moving dipolarization, the magnetic flux pileup, and the tailward-moving dipolarization near the tail current disruption region can well explain the auroral evolution around the substorm onset.
    Journal of Geophysical Research: Space Physics 07/2013; 118(7). · 3.44 Impact Factor

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