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| Titel |
Dynamics of thin current sheets: Cluster observations |
| VerfasserIn |
W. Baumjohann, A. Roux, O. Contel, R. Nakamura, J. Birn, M. Hoshino, A. T. Y. Lui, C. J. Owen, J.-A. Sauvaud, A. Vaivads, D. Fontaine, A. Runov |
| 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 ; 25, no. 6 ; Nr. 25, no. 6 (2007-06-29), S.1365-1389 |
| Datensatznummer |
250015865
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| Publikation (Nr.) |
 copernicus.org/angeo-25-1365-2007.pdf |
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| Zusammenfassung |
| The paper tries to sort out the specific signatures of the
Near Earth Neutral Line (NENL) and the Current Disruption
(CD) models, and looks for these signatures in Cluster data
from two events. For both events transient magnetic
signatures are observed, together with fast ion flows. In the
simplest form of NENL scenario, with a large-scale two-dimensional
reconnection site, quasi-invariance along Y is
expected. Thus the magnetic signatures in the S/C frame are
interpreted as relative motions, along the X or Z direction,
of a quasi-steady X-line, with respect to the S/C. In the
simplest form of CD scenario an azimuthal modulation is
expected. Hence the signatures in the S/C frame are
interpreted as signatures of azimuthally (along Y) moving
current system associated with low frequency fluctuations of
Jy and the corresponding field-aligned currents (Jx). Event 1
covers a pseudo-breakup, developing only at high latitudes.
First, a thin (H≈2000 km≈2ρi, with ρi
the ion gyroradius)
Current Sheet (CS) is found to be quiet. A slightly thinner
CS (H≈1000–2000 km≈1–2ρi), crossed about 30 min later,
is found to be active, with fast earthward ion flow bursts
(300–600 km/s) and simultaneous large amplitude fluctuations
(δB/B~1). In the quiet CS the current density Jy is carried
by ions. Conversely, in the active CS ions are moving
eastward; the westward current is carried by electrons that
move eastward, faster than ions. Similarly, the velocity of
earthward flows (300–600 km/s), observed during the active
period, maximizes near or at the CS center. During the active
phase of Event 1 no signature of the crossing of an X-line is
identified, but an X-line located beyond Cluster could
account for the observed ion flows, provided that it is
active for at least 20 min. Ion flow bursts can also be due
to CD and to the corresponding dipolarizations which are
associated with changes in the current density. Yet their
durations are shorter than the duration of the active period.
While the overall ∂Bz∂t is too weak to accelerate ions up to
the observed velocities, short duration ∂Bz∂t can produce
the azimuthal electric field requested to account for the
observed ion flow bursts. The corresponding large amplitude
perturbations are shown to move eastward, which suggests that
the reduction in the tail current could be achieved via a
series of eastward traveling partial dipolarisations/CD. The
second event is much more active than the first one. The
observed flapping of the CS corresponds to an azimuthally
propagating wave. A reversal in the proton flow velocity,
from −1000 to +1000 km/s, is measured by CODIF. The overall
flow reversal, the associated change in the sign of Bz and
the relationship between Bx and By suggest that the
spacecraft are moving with respect to an X-line and its
associated Hall-structure. Yet, a simple tailward retreat of
a large-scale X-line cannot account for all the observations,
since several flow reversals are observed. These quasi-periodic
flow reversals can also be associated with an
azimuthal motion of the low frequency oscillations. Indeed,
at the beginning of the interval By varies rapidly along the
Y direction; the magnetic signature is three-dimensional and
essentially corresponds to a structure of filamentary field-aligned
current, moving eastward at ~200 km/s. The transverse
size of the structure is ~1000 km. Similar structures are
observed before and after. These filamentary structures are
consistent with an eastward propagation of an azimuthal
modulation associated with a current system Jy, Jx. During
Event 1, signatures of filamentary field-aligned current
structures are also observed, in association with modulations
of Jy. Hence, for both events the structure of the magnetic
fields and currents is three-dimensional. |
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