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| Titel |
Cluster view of the plasma sheet boundary layer and bursty bulk flow connection |
| VerfasserIn |
O. W. Lennartsson, L. M. Kistler, H. Rème |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 27, no. 4 ; Nr. 27, no. 4 (2009-04-08), S.1729-1741 |
| Datensatznummer |
250016495
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| Publikation (Nr.) |
 copernicus.org/angeo-27-1729-2009.pdf |
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| Zusammenfassung |
| The high-latitude boundaries of the plasma sheet (PSBL)
are dynamic latitude zones of recurring and transient (minutes to tens of
minutes) earthward and magnetic field-aligned bursts of plasma, each being
more or less confined in longitude as well, whose ionic component is
dominated by protons with flux, energies and density that are consistent
with a central plasma sheet (CPS) source at varying distance (varying rates
of energy time dispersion), sometimes as close as the ~19 RE
Cluster apogees, or closer still. The arguably most plausible source
consists of so called "bursty bulk flows" (BBFs), i.e. proton bulk flow
events with large, positive and bursty GSE vx. Known mainly from CPS
observations made at GSE x>−30 RE, the BBF type events probably
take place much further downtail as well. What makes the BBFs an especially
plausible source are (1) their earthward bulk flow, which helps explain the
lack of distinctive latitudinal PSBL energy dispersion, and (2) their
association with a transient strong increase of the local tail Bz
component ("local dipolarization"). The enhanced Bz provides
intermittent access to higher latitudes for the CPS plasma, resulting in
local density reductions in the tail midplane, as illustrated here by proton
data from the Cluster CIS CODIF instruments. Another sign of kinship between
the PSBL bursts and the BBFs is their similar spatial fine structure. The
PSBL bursts have prominent filaments aligned along the magnetic field with
transverse flux gradients that are often characterized by local ~10 keV
proton gyroradii scale size (or even smaller), as evidenced by Cluster
measurements. The same kind of fine structure is also found during Cluster
near-apogee traversals of the tail midplane, as illustrated here and implied
by recently published statistics on BBFs obtained with Cluster multipoint
observations at varying satellite separations. Altogether, the Cluster
observations described here mesh rather well with theories about so called
plasma sheet "bubbles," i.e. earthward drifting closed magnetic flux tubes
with reduced particle pressure and enhanced magnetic field strength at their
apex. It is argued that such bubbles may be initiated by localized
diamagnetic instabilities. |
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