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Ann. Geophys., 22, 2355-2367, 2004
www.ann-geophys.net/22/2355/2004/
© European Geosciences Union 2004
Cluster observations of continuous reconnection at the magnetopause under steady interplanetary magnetic field conditions
T. D. Phan1, M. W. Dunlop2, G. Paschmann3, B. Klecker3, J. M. Bosqued4, H. Rème4, A. Balogh5, C. Twitty1, F. S. Mozer1, C. W. Carlson1, C. Mouikis6, and L. M. Kistler6
1Space Science Laboratory, Berkeley, USA
2Rutherford Laboratory, Oxford, UK
3Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
4Centre d’Etude Spatiale des Rayonnements, Toulouse, France
5Imperial College, London, UK
6University of New Hampshire, Durham, NH, USA
Abstract. On 26 January 2001, the Cluster spacecraft detected high-speed plasma jets
at multiple crossings of the high-latitude duskside magnetopause (MP) and
boundary layer (BL) over a period of more than 2h. The 4 spacecraft
combined spent more than half of this time in the MP/BL and jets were
observed whenever a spacecraft was in the MP. These observations were made
under steady southward and dawnward interplanetary magnetic field (IMF)
conditions. The magnetic shear across the local MP was ~100° and
β~1 in the adjacent magnetosheath. The jet velocity is in
remarkable agreement with reconnection prediction throughout the entire
interval, except for one crossing that had no ion measurements inside the
current layer. The flow speed measured in the deHoffmann Teller frame is
90% of the Alfvén speed on average for the 10 complete MP current
layer crossings that are resolved by the ion measurements. These findings
strongly suggest that reconnection was continuously active for more than two
hours. The jets were directed persistently in the same northward and
anti-sunward direction, implying that the X-line was always below the
spacecraft. This feature is inconsistent with patchy and random reconnection
or convecting multiple X-lines. The majority of MP/BL crossings in this
two-hour interval were partial crossings, implying that they are caused by
bulges sliding along the MP, not by inward-outward motion of a uniformly
thin MP/BL. The presence of the bulges suggests that, although reconnection
is continuously active under steady IMF conditions, its rate may be
modulated. The present investigation also reveals that (1) the predicted ion
D-shaped distributions are absent in all reconnection jets on this day, (2)
the electric field fluctuations are larger in the reconnecting MP than in
the magnetosheath proper, but their amplitudes never exceed 20mV/m, (3) the
ion-electron differential motion is ~20km/s for the observed MP
current density of ~50nA/m2 (∇× B), thus inconsequential for the deHoffmann-Teller and
Walén analyses, (4) flows in an isolated flux transfer event (FTE) are
directed in the same direction as the MP jets and satisfy the Walén
relation, suggesting that this FTE is also generated by reconnection.
Finally, the present event cannot be used to evaluate the validity of
component or anti-parallel merging models because, although the magnetic
shear at the local MP was ~100°(≪180°), the X-line
may be located more than 9RE away (in the opposite hemisphere), where
the shear could be substantially different.
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