Measurements of Magnetic Cloud Expansion through Multiple Spacecraft in Radial Conjunction

Abstract

The aim of this study is to use multispacecraft measurements of interplanetary magnetic clouds (MCs) to better constrain and understand the effect of expansion on their magnetic field properties. We develop a parameter (γ) for comparing magnetic field components measured at multiple spacecraft. We use the minimum variance technique on the magnetic field data to obtain the axial and azimuthal components. The parameter γ acts at the front boundary as a measure of the global difference in the evolution with heliospheric distance of the axial and azimuthal magnetic field components of MCs. Our goal is to determine whether the studied MCs exhibit self-similar expansion and, if so, whether this expansion is predominantly isotropic or radial, based on the estimated γ. Through our analysis of data from multiple spacecraft, we observe a notable consistency in the γ values across the examples examined. We find that the overall expansion of these MCs tends to be isotropic, while the local expansion of MCs, derived from the γ values measured at the rear boundary of MCs, usually shows anisotropic behavior, particularly when the distances between the observations from the two spacecraft are relatively short. This discovery offers insights for refining flux rope models and advancing our comprehension of the expansion processes associated with coronal mass ejections.