Reconstruction of two-dimensional magnetopause structures from Cluster observations: verification of method

Abstract

Abstract.A recently developed technique for reconstructing approximately two-dimensional (∂/∂z≈0), timestationary magnetic field structures in space is applied to two magnetopause traversals on the dawnside flank by the four Cluster spacecraft, when the spacecraft separation was about 2000 km. The method consists of solving the GradShafranov equation for magnetohydrostatic structures, using plasma and magnetic field data measured along a single spacecraft trajectory as spatial initial values. We assess the usefulness of this single-spacecraft-based technique by comparing the magnetic field maps produced from one spacecraft with the field vectors that other spacecraft actually observed. For an optimally selected invariant (z)-axis, the correlation between the field components predicted from the reconstructed map and the corresponding measured components reaches more than 0.97. This result indicates that the reconstruction technique predicts conditions at the other spacecraft locations quite well.The optimal invariant axis is relatively close to the intermediate variance direction, computed from minimum variance analysis of the measured magnetic field, and is generally well determined with respect to rotations about the maximum variance direction but less well with respect to rotations about the minimum variance direction. In one of the events, field maps recovered individually for two of the spacecraft, which crossed the magnetopause with an interval of a few tens of seconds, show substantial differences in configuration. By comparing these field maps, time evolution of the magnetopause structures, such as the formation of magnetic islands, motion of the structures, and thickening of the magnetopause current layer, is discussed.

Triple crossings of a string of magnetic islands at duskside magnetopause encountered by AMPTE/IRM satellite on 8 August 1985

“…The Walén analysis shows that the slope is increased for crossing 9 as compared to crossing 4. Similar results have been found for Cluster event at different locations [Hasegawa et al, 2004] and attributed to different reconnection rate. The event shown here may also bear similar effects.…”

“…In the Cluster case the differences between the reconstructed and predicted maps by other spacecraft are attributed to the time evolution of the reconnection process [Hasegawa et al, 2004]. In the present case, the differences among the three field maps however are likely due to the fact that different portions of a large coherent structure are seen by the same spacecraft; in particular, crossing 3 and 9 are 1 hour apart to involve the same structures.…”

“…It is also possible to fit the function P t (A) with two branches of curve representing the difference on both sides of the current layer, as was discussed by Hu and Sonnerup [2003]. The reconstruction method has also been applied to the Cluster spacecraft data and the 2-D magnetopause structure is obtained by optimizing the correlated coefficients between the field components predicted from the reconstructed maps and the corresponding measured components [Hasegawa et al, 2004]. Recently, the GS reconstruction method is further extended to incorporate significant field-aligned flow or perpendicular flow in the original static MHD case to represent more general situation at the magnetopause and boundary layer, respectively.…”

“…Understanding of its structure and dynamics is thus essential to the study of magnetospheric physics. Studies of two-dimensional magnetopause structure utilizing the reconstruction method have been documented in a number of papers that solve the Grad-Shafranov (GS) equation of two-dimensional (2-D) MHD equilibrium as a spatial initial value problem from the data sampled by the single spacecraft while traversing the magnetopause current layer [Sonnerup and Guo, 1996;Hau and Sonnerup, 1999;Sonnerup, 2000, 2003;Teh and Hau, 2004;Hasegawa et al, 2004]. The detailed procedures of the reconstruction method can be referred to the original paper by Hau and Sonnerup [1999]; here we only briefly summarize the basic concepts.…”

ARTEMIS Science Objectives

“…Radial separations will provide the observations needed to determine how rapidly plasmoid diameters change with distance downstream (e.g., Slavin et al 1999). Grad-Shafranov reconstruction techniques (Hasegawa et al 2004) will be used to determine the plasma and magnetic field structure within the plasmoids from time series measurements and an assumption that they are in near pressure balance with their immediate environment. Fig.…”