Characteristics of the near-tail dawn magnetopause and boundary layer

The paper discusses properties of the near-tail dawnside magnetopause and boundary layer, as obtained from Cluster plasma and magnetic field measurements dur- ing a single skimming orbit on 4 and 5 July 2001 that in- cluded 24 well-defined magnetopause crossings by all four spacecraft. As a result of variations of the interplanetary magnetic field, the magnetic shear across the local magne- topause varied between 0 and 180 . Using an improved method, which takes into account magnetopause accelera- tion and thickness variation, we have determined the magne- topause orientation, speed, thickness and current for the 96 individual magnetopause crossings. The orientations show clear evidence of surface waves. Magnetopause thicknesses range from 100 to 2500 km, with an average of 753 km. The magnetopause speeds range from less than 10 km s 1 up to more than 300 km s 1 , with an average of 48 km s 1 . Both results are consistent with earlier ISEE and AMPTE results obtained for the dayside magnetopause. Importantly, scal- ing the thicknesses to the ion gyro radius or the ion inertial length did not reduce the large dynamic range. There is also no significant dependence of thickness on magnetic shear. Current densities range from 0.01µA m 2 up to 0.3 uA, with an average value of 0.05 µA m 2 . By including some extra crossings that did not involve all four spacecraft, we were able to apply the Wal´ en test to a total of 60 crossings by Cluster 1 and 3, and have classified 19 cases as rotational discontinuities (RDs), of which 12 and 7 were crossings sun- ward and tailward of an X-line, respectively. Of these 60 crossings, 26 show no trace of a boundary layer. The only crossings with substantial boundary layers are crossings into the plasma mantle. Of the 26 crossings without a bound- ary layer, 8 were identified as RDs. Since reconnection pro- duces wedge-shaped boundary layers emanating from the X- line, RDs without boundary layer may be considered cross- ings close to the X-line, in which case the observed magnetic shear and Alfv´ en Mach number should be representative of the conditions at the X-line itself. It is therefore important that four of the eight cases had shear angles 100 , i.e. the reconnecting fields were far from being anti-parallel, and that all eight cases had Alfv´ en Mach numbers MA>1 in the ad- joining magnetosheath. Another important conclusion can be drawn from the crossings without a boundary layer that were tangential discontinuities (TDs). To observe TDs with no boundary layer at such large distances from the subso- lar point appears to rule out diffusion over large portions of the magnetopause as an effective means for plasma transport across the magnetopause.