Four‐point Cluster application of magnetic field analysis tools: The discontinuity analyzer

Abstract

[1] The Cluster spacecraft have collected 3-D information on magnetic field structures at small to medium scales (100-1000 km) in the Earth's dayside magnetosphere, magnetosheath, and near solar wind. We focus here on the first application of the discontinuity analyzer analysis technique (determination of the geometry and motion of magnetic discontinuities, using interspacecraft timing and variance information) for the case of boundaries that are planar over the four-spacecraft array. This multipoint technique uses minimum variance of the magnetic field to determine the boundary normals at each spacecraft. Identification of the four spacecraft crossings requires accurate timing of encounters at each spacecraft, which depends on key features in each time series being stationary in the frame of the boundary. The technique is tested for selected boundary crossing events (magnetopause and bow shock, and one TD in the magnetosheath), for which the independently determined normals are closely ($1-2°) colinear. Closely aligned normals, on spatial scales of $600-1000 km, were found for most crossings studied. For such a planar geometry, the motion can be determined unambiguously and shows that significant acceleration (up to $10 km s À2 ) of the boundary over the spacecraft is nearly always present. This has the consequence that previous estimates of boundary scales, based on constant motion between two spacecraft, may be in error by factors of two or three and that implied variations in boundary thickness could therefore be predominantly due to variations in speed. We comment on the error introduced through assumptions of constant motion.

Separating spatial and temporal variations in auroral electric and magnetic fields by Cluster multipoint measurements

“…This means that none of them are pure spatial structures, stationary in the geomagnetic coordinate system used here. However, the fact that there are consistent time shifts enables us to interpret the observations in intervals (1) and (3) as quasi-stationary structures moving over the satellite, similar to Cluster observations of magnetopause crossings Pedersen et al, 2001;Dunlop et al, 2002). By assuming that the structures are elongated in a plane with no variations along the plane, at least over a length comparable to the satellite separations, we can determine the angle α of the plane and its velocity component v ⊥ perpendicular both to B and to the plane itself, which best reproduces the observed time shifts.…”

Observations of concentrated generator regions in the nightside magnetosphere by Cluster/FAST conjunctions

“…As a quality indicator for the estimated current, the quantity |∇·B|/|∇×B| can be used. In theory, this quantity should be identically zero, but in practice it can vary substantially (Paschmann and Daly, 1998;Dunlop et al, 2002) due to, for example, small-scale variations in the magnetic field and measurement errors. It should be noted that a oneto-one correspondence between the value of |∇·B|/|∇×B| and the actual error in the estimated current density is not expected.…”

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

“…In two recent papers, Dunlop et al (2001Dunlop et al ( , 2002 have concluded from studies of Cluster magnetopause events that the magnetopause speed was usually not constant during an event but could change drastically over times of the order of a minute or less, whereas the thickness showed more modest variations. The method employed in reaching this conclusion makes use of magnetopause normal vectors obtained from minimum variance analysis of the magnetic field data taken during each of the four crossings, in addition to the timing information.…”