Inferring interplanetary flux rope orientation with the minimum residue method

Abstract

[1] There are many 2-D models that can be used to describe the structure of the interplanetary flux rope (IFR), such as the force-free model, the nonforce-free model, and the inertial model. For each model, one or multiple field line invariants exist. In this study, we introduce a new definition of the quantity, residue, based on all field line invariants of a specified flux rope model to measure the deflection between the assumed axis and the true flux rope axis. Then, a new minimum residue (MR) method is proposed to infer the axial orientation of IFR with the observational data from a single spacecraft. For an arbitrarily assumed flux rope axis, the natural coordinate system can be constructed, then a magnetic flux function, A, and each invariant of the specified flux rope model can also be concurrently calculated under this coordinate system. The direction corresponding to the minimum residue is expected to be the real axial orientation. In previous study, the residue was first defined with A and a single invariant Pt of a static equilibrium flux rope model. Here, the new MR method is tested with simulated magnetic cloud data sets constructed from the analytical model outputs of two different flux rope models with “trend noise” added. It shows that the new MR method is applicable in real case analysis and the inferring results are acceptable for cases with small closest approach distance and proper noise level. Compared with results from traditional methods, accuracy of the inferred axial orientation is improved by the new method. The new MR method is also applied to a typical in situ event observed by Wind spacecraft. The comparison of the inferring results from different models indicate that application of a more accurate flux rope model is useful for inferring techniques.