Cluster observations of continuous reconnection at the magnetopause under steady interplanetary magnetic field conditions

摘要

On 26 January 2001, the Cluster spacecraft detected high-speed plasma jetsat multiple crossings of the high-latitude duskside magnetopause (MP) andboundary layer (BL) over a period of more than 2h. The 4 spacecraftcombined spent more than half of this time in the MP/BL and jets wereobserved whenever a spacecraft was in the MP. These observations were madeunder steady southward and dawnward interplanetary magnetic field (IMF)conditions. The magnetic shear across the local MP was ~100° andβ~1 in the adjacent magnetosheath. The jet velocity is inremarkable agreement with reconnection prediction throughout the entireinterval, except for one crossing that had no ion measurements inside thecurrent layer. The flow speed measured in the deHoffmann Teller frame is90% of the Alfvén speed on average for the 10 complete MP currentlayer crossings that are resolved by the ion measurements. These findingsstrongly suggest that reconnection was continuously active for more than twohours. The jets were directed persistently in the same northward andanti-sunward direction, implying that the X-line was always below thespacecraft. This feature is inconsistent with patchy and random reconnectionor convecting multiple X-lines. The majority of MP/BL crossings in thistwo-hour interval were partial crossings, implying that they are caused bybulges sliding along the MP, not by inward-outward motion of a uniformlythin MP/BL. The presence of the bulges suggests that, although reconnectionis continuously active under steady IMF conditions, its rate may bemodulated. The present investigation also reveals that (1) the predicted ionD-shaped distributions are absent in all reconnection jets on this day, (2)the electric field fluctuations are larger in the reconnecting MP than inthe magnetosheath proper, but their amplitudes never exceed 20mV/m, (3) theion-electron differential motion is ~20km/s for the observed MPcurrent density of ~50nA/m2 (∇× B), thus inconsequential for the deHoffmann-Teller andWalén analyses, (4) flows in an isolated flux transfer event (FTE) aredirected in the same direction as the MP jets and satisfy the Walénrelation, suggesting that this FTE is also generated by reconnection.Finally, the present event cannot be used to evaluate the validity ofcomponent or anti-parallel merging models because, although the magneticshear at the local MP was ~100°(≪180°), the X-linemay be located more than 9R_E away (in the opposite hemisphere), wherethe shear could be substantially different.