Studying the property of the boundary layers using new multipoint techniques

We present results from new multi-point techniques, which have been developed to study the structure and motion properties of the boundary structures. One new method [Shi et al, GRL, 2005GL022454] analyzes the dimensional character of observed structures using multi-point magnetic field measurements of four or more spacecraft. This technique can provide three directions along which the magnetic field has the minimum, intermediate, and maximum derivatives if the magnetic gradient tensor G=∇ ěc{B} at every moment, resulting in the determination of both the structure's dimensionality and the variation direction, corresponding to the invariant axis orientation for two-dimensional structures and the normal directions for one-dimensional structures. One advantage of this method is that these directions can be determined instantaneously, point by point in the time series, and so can be tracked through each observed structure. A second new multi-point magnetic field method [Shi et al, GRL, submitted] calculates the velocity vector of observed quasi-stationary structures, at every moment. If the magnetic gradient tensor G=∇ ěc{B} has been estimated, the velocity can then be determined as a function of time. For three-dimensional structures, all the three components of the velocity can be calculated by this method. For two-dimensional (or one-dimensional) structures, we can calculate the velocity components along two (or one) directions. One striking property of this method is that the velocity can also be determined instantaneously using four or more spacecraft, which differs from other existed techniques. Using these two methods, we have studied the dimensionality and motion velocity of the cusp boundaries, for example. We find that the initial results (boundary normal and motion) are consistent with other discontinuity-based methods. Because the new methods can provide instantaneous values on the structure and motion of the boundaries, these results give us some new details on the boundary structure.