Determining L-M-N current sheet coordinate directions using virtual spacecraft data from a simulation of asymmetric magnetic reconnection

The first step in analyzing magnetic reconnection events is to establish a coordinate system. The usual coordinate system has the L direction being the direction of the reconnecting component of the magnetic field that varies most, the N direction being the direction of the normal to the current sheet, and the M direction forming the complete triad. At the dayside magnetopause, defining these directions is complicated by the asymmetry in the magnetic field across the current sheet. Here we test three methods to determine the coordinate directions: magnetic variance alone to determine both the L and N directions, and magnetic variance for the L direction combined with another evaluation of the N direction, either the direction of the maximum gradient of the magnetic field or the direction of the minimum variance of the current density. We generate virtual spacecraft data from two and three dimensional asymmetric simulations of magnetic reconnection, and then use these methods to evaluate the coordinate directions. The normal direction rotates depending on which part of the current sheet is being crossed. Using the data from the two-dimensional simulation, for the purpose of deriving the global normal direction representing the entire reconnection structure it's better to use a time interval shifted toward the magnetosphere side of the current sheet. This is because the magnetic field is less distorted on the magnetosphere side of the current sheet. Results from a three-dimensional simulation will also be presented.