Almost all the methods used in inferring the magnetopause (or any other space plasma discontinuity) macroscopic parameters assume a planar geometry for this boundary and consider that all physical quantities vary only along the normal direction. They can divide them into two categories:

• single-spacecraft techniques, based on the validity of various conservation laws. For example, the minimum variance analysis of the magnetic field (MVAB) is based on the absence of magnetic poles, which implies the constancy of the magnetic field component along the normal. Consequently, in this method the direction of minimum magnetic variance provides the direction of the magnetopause normal. Similarly, the planar method of minimum Faraday residue (MFR) is based on the magnetic flux conservation across the discontinuity. In this case the algorithm finds a direction in space and a velocity (assumed constant) along this direction so that the variation in the tangential component of the electric field (the so called Faraday residue) is minimized.

• since the Cluster mission provides for the first time correlated measurements taken simultaneously at four points in space, the differences in the position and time of the satellites encounter with the terrestrial magnetopause can be used to infer the magnetopause orientation, thickness and velocity. The discontinuity is either considered to move with a constant velocity across the four spacecraft (constant velocity approach or CVA technique), or it is assumed it has a constant thickness during the same interval (constant thickness approach or CTA technique). This four-point timing method usually assumes a planar discontinuity as well, and offers an independent check for the various single-spacecraft techniques.

The 2D-CTTVA (2 Dimensional &minus Constant Thickness, Timing and Variance Analysis) method, developed by the project leader in his PhD, extends the four-point timing method in order to accommodate the more realistic situations when the magnetopause behaves like a two-dimensional (2-D), non-planar discontinuity on the scale of the inter-spacecraft separation distance. Such non-planarity occurs when the magnetopause has a local bulge or indentation, or when a large amplitude wave travels on the magnetopause surface.

References

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