Exploring the biases of a new method based on minimum variance for interplanetary magnetic clouds

DASSO S.; JANVIER, M.; DEMOULIN P.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2018

0004-6361

Context. Magnetic clouds (MCs) are twisted magnetic structures ejected from the Sun and probed by in situ instruments. They aretypically modeled as flux ropes (FRs).Aims. Magnetic field measurements are only available along the 1D spacecraft trajectory. The determination of the FR global characteristicsrequires the estimation of the FR axis orientation. Among the developed methods, the minimum variance (MV) is the mostflexible, and features only a few assumptions. However, as other methods, MV has biases. We aim to investigate the limits of themethod and extend it to a less biased method.Methods. We first identified the origin of the biases by testing the MV method on cylindrical and elliptical models with a temporalexpansion comparable to the one observed in MCs. Then, we developed an improved MV method to reduce these biases.Results. In contrast with many previous publications we find that the ratio of the MV eigenvalues is not a reliable indicator of theprecision of the derived FR axis direction. Next, we emphasize the importance of the FR boundaries selected since they stronglyaect the deduced axis orientation.We have improved the MV method by imposing that the same amount of azimuthal flux should bepresent before and after the time of closest approach to the FR axis. We emphasize the importance of finding simultaneously the FRaxis direction and the location of the boundaries corresponding to a balanced magnetic flux, so as to minimize the bias on the deducedFR axis orientation. This method can also define an inner flux-balanced sub-FR. We show that the MV results are much less biasedwhen a compromize in size of this sub-FR is achieved.Conclusions. For weakly asymmetric field temporal profiles, the improved MV provides a very good determination of the FR axisorientation. The main remaining bias is moderate (lower than 6) and is present mostly on the angle between the flux rope axis andthe plane perpendicular to the Sun?Earth direction.