Analysis of thick, non-planar boundaries using the discontinuity analyser

Dunlop, M. W.; Woodward, T. I.

doi:10.1007/s00585-999-0984-6

Cited by 12 publications

(6 citation statements)

References 15 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“… Dunlop and Woodward [1999] also treat thick, nonplanar discontinuities using a technique called the discontinuity analyser; their paper is basically as discussion of the technique with one example of its use, the AMPTE barium release experiment. They caution that the use of minimum variance analysis in determining DD surface normals (as done in this study) for thick, curved surfaces may not always provide accurate results.…”

Section: Resultsmentioning

confidence: 99%

Two‐dimensional curvature of large angle interplanetary MHD discontinuity surfaces: IMP‐8 and WIND observations

Lepping

McClernan

2003

J. Geophys. Res.

View full text Add to dashboard Cite

[1] This study examines the degree of two-dimensional curvature of solar wind directional discontinuity (DD) surfaces at 1 AU using magnetic field, density, and velocity data from the WIND and IMP-8 spacecraft for a large number (N = 134) of carefully selected events having large ''discontinuity angles'' of 90°or greater. The discontinuity angle (w) is measured in the DD's current sheet, the normal (n) to which is estimated by field variance analysis. The fundamental analysis depends on estimates of these DD surface normals at the two spacecraft and the DD's center-times and positions. On average, the transit time from one DD sighting to the other was 36 minutes, and the associated distance along the normal direction was 137 R E . The transition-interval lengths across the DDs are translated into thicknesses and examined for the amount of change between the two spacecraft observing points. The average thickness is relatively large, 14 R E .; the most probable thickness is %6 R E . All relevant quantities are examined statistically to establish their distributions, average, and degree of change. A weighted average of the radius of curvature is estimated to be 380 R E , but its most probable value is 290 R E . The average w is 140°with a relatively large spread (s = 28°). The average direction of propagation is: longitude (f n ) = 194°and latitude (q n ) = 7°(but hjq n ji = 27°), where f n = 0°is sunward and q n = 0°is the ecliptic plane. Various parameters are studied with respect to DD type, i.e., rotational or tangential discontinuity (RD or TD), defined in terms of the ''ratio'' (in percent) of speed of propagation to net speed of the DD surface, where the net speed is the sum of the convection velocity (along n) plus the propagation speed. The RD %-ratio is moderately small, but the TD ratio is very small or zero. The results by this definition of type are favorably compared to those from the more conventional method, which depends on the absolute strength of the normal component of the magnetic field. There is little difference in any average parameter value according to type. However, the average hwi appears to depend slightly on type with the hwi for the RDs being smaller. The discontinuity type was shown to change commonly in either direction (TD to RD or RD to TD) between the two observing positions, i.e., %40% of the time. It is not clear if these type-changes are spatial or temporal. Shortcomings of the analysis are (1) the need to impose an upper limit on the angular difference of the DD normals between the two observing positions (which eliminated most surfaces of very small radii of curvature), and (2) the inability to distinguish real curvature from shorterscale surface variations, from only two spacecraft data sets. The results of the study should help to caution us as to the simplistic use of the planar DD surface assumption in projecting, to the distance of Earth's magnetosphere, a distantly observed DD surface (e.g., one near L1), especially for studies that depend on accurately predicting the ...

show abstract

Section: Resultsmentioning

confidence: 99%

Two‐dimensional curvature of large angle interplanetary MHD discontinuity surfaces: IMP‐8 and WIND observations

Lepping

McClernan

2003

J. Geophys. Res.

View full text Add to dashboard Cite

show abstract

“…Curved boundaries (or changes in boundary orientation over the spacecraft array) present a more complicated data analysis problem. This is not only because the number of unknowns to be directly determined rapidly exceeds the available equations [ Dunlop et al , 1997], but because known, independent methods for determination of the normals need some qualification in the presence of curvature [ Dunlop and Woodward , 1999]. The key problem is that the effect of both the surface topology and the motion can have equal effects on the crossing times.…”

Section: Methods Definitionmentioning

confidence: 99%

“…Investigation of a variety of methods, which use only single‐spacecraft data in the context of the multispacecraft data set, is underway, extending the study of Bauer et al [2000]. The four‐spacecraft Cluster array, requires the motion to be limited to constant velocity for even lowest order curvature to be estimated [ Dunlop and Woodward , 1998, 1999]. Below, we concentrate on application of the planar form of the DA.…”

Section: Methods Definitionmentioning

confidence: 99%

See 1 more Smart Citation

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

2002

Self Cite

View full text Add to dashboard Cite

[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.

show abstract

“…Other papers [ Dunlop et al , 2001; Glassmeier et al , 2001] explore other techniques that are relevant in some sense to the other data regimes (defined by the comparative spatial and temporal scales). The first of these deals with the application of the discontinuity analyzer [ Dunlop et al , 1997; Dunlop and Woodward , 1998, 1999]. The second addresses four spacecraft analysis in the Fourier domain, describing the first application of a number of related techniques, named the Wave Telescope [ Glassmeier et al , 1995; Motschmann et al , 1996, 1998; Pincon and Motschmann , 1998].…”

Section: Introductionmentioning

confidence: 99%

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

2002

Self Cite

View full text Add to dashboard Cite

For the first time, the Cluster spacecraft have collected 3‐D information on magnetic field structures at small to medium scales in the Earth's dayside magnetosphere. We focus here on the first application of the Curlometer (direct estimation of the electric current density from curl(B), using measured spatial gradients of the magnetic field) analysis technique. The applicability of this multipoint technique is tested, for selected events within the data set, in the context of various mission constraints (such as position, timing, and experimental accuracy). For the Curlometer, nonconstant spatial gradients over the spacecraft volume, time dependence, and measurement errors can degrade the quality of the estimate. The estimated divergence of the magnetic field can be used to monitor (indirectly) the effect of nonconstant gradients in the case of many magnetic field structures. For others, and at highly distorted spacecraft configurations, this test may not reflect the quality of the Curlometer well. The relative scales and relative geometry between the spacecraft array and the structures present, as well as measurement errors, all are critical to the quality of the calculation. We demonstrate that even when instrumental and other errors are known to contribute to the uncertainty in the estimate of the current, a number of current signatures within the magnetosphere can be plausibly determined in direction, if not absolute size. A number of examples show consistent currents at the magnetopause, both separate from, and nearby or in the cusp region. Field‐aligned currents near the polar cap boundary are also estimated reliably. We also demonstrate one example of an anomalous current arising from the effect of a highly distorted spacecraft configuration.

show abstract

Analysis of thick, non-planar boundaries using the discontinuity analyser

Cited by 12 publications

References 15 publications

Two‐dimensional curvature of large angle interplanetary MHD discontinuity surfaces: IMP‐8 and WIND observations

Two‐dimensional curvature of large angle interplanetary MHD discontinuity surfaces: IMP‐8 and WIND observations

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

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

Contact Info

Product

Resources

About