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| Titel |
Motion of the dayside polar cap boundary during substorm cycles: I. Observations of pulses in the magnetopause reconnection rate |
| VerfasserIn |
M. Lockwood, J. Moen, A. P. Eyken, J. A. Davies, K. Oksavik, I. W. McCrea |
| 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 ; 23, no. 11 ; Nr. 23, no. 11 (2005-12-21), S.3495-3511 |
| Datensatznummer |
250015419
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| Publikation (Nr.) |
 copernicus.org/angeo-23-3495-2005.pdf |
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| Zusammenfassung |
| Using data from the EISCAT (European Incoherent Scatter)
VHF radar and DMSP (Defense Meteorological Satellite Program) spacecraft
passes, we study the motion of the dayside open-closed field line boundary
during two substorm cycles. The satellite data show that the motions of ion
and electron temperature boundaries in EISCAT data, as reported by Moen et al. (2004),
are not localised around the radar; rather, they reflect motions of the
open-closed field line boundary at all MLT throughout the dayside auroral
ionosphere. The boundary is shown to erode equatorward when the IMF points
southward, consistent with the effect of magnetopause reconnection. During
the substorm expansion and recovery phases, the dayside boundary returns
poleward, whether the IMF points northward or southward. However, the
poleward retreat was much faster during the substorm for which the IMF had
returned to northward than for the substorm for which the IMF remained
southward - even though the former substorm is much the weaker of the two.
These poleward retreats are consistent with the destruction of open flux at
the tail current sheet. Application of a new analysis of the peak ion
energies at the equatorward edge of the cleft/cusp/mantle dispersion seen by
the DMSP satellites identifies the dayside reconnection merging gap to
extend in MLT from about 9.5 to 15.5 h for most of the interval. Analysis
of the boundary motion, and of the convection velocities seen near the
boundary by EISCAT, allows calculation of the reconnection rate (mapped down
to the ionosphere) from the flow component normal to the boundary in its own
rest frame. This reconnection rate is not, in general, significantly
different from zero before 06:45 UT (MLT<9.5 h) - indicating that the X
line footprint expands over the EISCAT field-of-view to earlier MLT only
occasionally and briefly. Between 06:45 UT and 12:45 UT (9.5<MLT<15.5 h)
reconnection is continuously observed by EISCAT, confirming the (large)
MLT extent of the reconnection footprint deduced from the DMSP passes. As
well as direct control by the IMF on longer timescales, the derived
reconnection rate variation shows considerable pulsing on timescales of
2-20 min during periods of steady southward IMF. |
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