Four-spacecraft determination of magnetopause orientation, motion and thickness: comparison with results from single-spacecraft methods

Abstract

Abstract. In this paper, we use Cluster data from one magnetopause event on 5 July 2001 to compare predictions from various methods for determination of the velocity, orientation, and thickness of the magnetopause current layer. We employ established as well as new multi-spacecraft techniques, in which time differences between the crossings by the four spacecraft, along with the duration of each crossing, are used to calculate magnetopause speed, normal vector, and width. The timing is based on data from either the Cluster Magnetic Field Experiment (FGM) or the Electric Field Experiment (EFW) instruments. The multi-spacecraft results are compared with those derived from various singlespacecraft techniques, including minimum-variance analysis of the magnetic field and deHoffmann-Teller, as well as Minimum-Faraday-Residue analysis of plasma velocities and magnetic fields measured during the crossings. In order to improve the overall consistency between multi-and single-spacecraft results, we have also explored the use of hybrid techniques, in which timing information from the four spacecraft is combined with certain limited results from single-spacecraft methods, the remaining results being left for consistency checks. The results show good agreement between magnetopause orientations derived from appropriately chosen single-spacecraft techniques and those obtained from multi-spacecraft timing. The agreement between magnetopause speeds derived from single-and multi-spacecraft methods is quantitatively somewhat less good but it is evident that the speed can change substantially from one crossing to the next within an event. The magnetopause thickness varied Correspondence to: S. E. Haaland stein.haaland@issi.unibe.ch substantially from one crossing to the next, within an event. It ranged from 5 to 10 ion gyroradii. The density profile was sharper than the magnetic profile: most of the density change occured in the earthward half of the magnetopause.

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

“…The MVABC and HT analysis for the interval 06:23:03-06:23:44 UT yield (all vectors are in GSE): the magnetopause normal vector, n=(0.6098, −0.7862, 0.0999); the constant HT frame velocity, V H T =(−248.6, −102.5, 68.6) km/s with the correlation coefficient, cc H T =0.9660 (These two vectors are very close to, but not identical to those reported in Haaland et al (2004)). The usage of the constraint B n =0 might be questionable for this event, since, as shown later, the Walén relation is relatively well satisfied, i.e.…”

“…the boundary may be of the rotational discontinuity-type. Nonetheless, we use the constraint because the orientation of the normal with, rather than without, the constraint is more consistent with those from various other methods (Haaland et al, 2004). Also the result without the constraint leads to an unlikely large B n value.…”

“…8). The crossing by C2 also had a long duration, which may, however, have been the result, at least in part, of a smaller magnetopause speed (Haaland et al, 2004). Such changes in speed are not accommodated by the map, which is based on a constant HT velocity.…”

“…This event has also been investigated in detail by Haaland et al (2004), with the objective of comparing singleand multi-spacecraft determinations of magnetopause orientation, speed, and thickness. Time plots of number density, temperature, magnitude and three GSE components of the magnetic field, and bulk flow speed are shown in Fig.…”

“…The normal motion of the magnetopause is negative, i.e. earthward, as required, although Haaland et al (2004) Correlation from Cluster−3 Reconstruction The difference between this number and 56.2 km/s indicates the presence of an inward flow of plasma across the magnetopause. The magnetic field map reconstructed for this C3 magnetopause crossing, using the data from 06:22:00 to 06:25:21 UT, is shown in Fig.…”