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Titel |
EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows and their signatures in the ionosphere |
VerfasserIn |
T. Pitkänen, A. T. Aikio, O. Amm, K. Kauristie, H. Nilsson, K. U. Kaila |
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 ; 29, no. 2 ; Nr. 29, no. 2 (2011-02-10), S.299-319 |
Datensatznummer |
250016969
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Publikation (Nr.) |
copernicus.org/angeo-29-299-2011.pdf |
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Zusammenfassung |
We report observations of a sequence of quiet-time Earthward bursty bulk
flows (BBFs) measured by the Cluster spacecraft in the near-tail plasma sheet
(XGSM ~ −12 to −14 RE) in the evening
sector, and by simultaneous high-resolution measurements in the northern
conjugate ionosphere by the EISCAT radars, a MIRACLE all-sky camera and
magnetometers, as well as a meridian-scanning photometer (MSP) in the
Scandinavian sector on 17 October 2005.
The BBFs at Cluster show signatures that are consistent with the plasma
"bubble" model (Chen and Wolf, 1993,
1999), e.g. deflection and compression of the
ambient plasma in front of the Earthward moving bubble, magnetic signatures
of a flow shear region, and the proper flows inside the bubble. In addition,
clear signatures of tailward return flows around the edges of the bubble can
be identified. The duskside return flows are associated with significant
decrease in plasma density, giving support to the recent suggestion by
Walsh et al.
(2009) of formation of a depleted wake. However, the same feature is
not seen for the dawnside return flows, but rather an increase in density.
In the ionosphere, EISCAT and optical measurements show that each of the
studied BBFs is associated with an auroral streamer that starts from the
vicinity of the polar cap boundary, intrudes equatorward, brakes at
68–70° aacgm MLAT and drifts westward along the proton oval. Within
the streamer itself and poleward of it, the ionospheric plasma flow has an
equatorward component, which is the ionospheric manifestation of the
Earthward BBF channel. A sharp velocity shear appears at the equatorward edge
of a streamer. We suggest that each BBF creates a local velocity shear in the
ionosphere, in which the plasma flow poleward of and inside the streamer is
in the direction of the streamer and southeastward. A northwestward return
flow is located on the equatorward side. The return flow is associated with
decreased plasma densities both in the ionosphere and in the magnetosphere as
measured by EISCAT and Cluster, respectively. In summary, we present the
first simultaneous high-resolution observations of BBF return flows both in
the plasma sheet and in the ionosphere, and those are in accordance with the
bubble model. The results apply for the duskside return flows, but the
manifestation of dawnside return flows in the ionosphere requires further
studies.
Finally, EISCAT measurements indicate increased nightside reconnection rate
during the ~35-min period of BBFs. We suggest that the observed
temporal event of IMF rotation to a more southward direction produces
enhanced open flux transport to the nightside magnetotail, and consequently,
the nightside reconnection rate is increased. |
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