A three-dimensional model of spiral null pair to form ion-scale flux ropes in magnetic reconnection region observed by Cluster

Abstract

The magnetic structure and topology of the three-dimensional magnetic reconnection region are significantly dynamic and complex. Small-scale flux ropes and magnetic null points are frequently detected in the reconnection outflow region and diffusion region due to the increased in situ measurements at high temporal cadences. Previous studies have demonstrated that X-line and small-scale flux ropes are both related to null points. In this study, by applying a fitting-reconstruction method with the input of the Cluster dataset, we reveal three types of spiral null pairs that serve as the skeleton of the flux ropes. Two spiral nulls can be connected by a spine, or by a separator, or by both a spine and a separator. A theoretical model is proposed to explain these spiral null pairs. The observational results and the model indicate that the number of magnetic loops of the flux rope is restricted by the linkage pattern of two nulls, while the flux rope is confined by the two nulls and their fan surfaces. The model predicts that the magnetic perturbations in the reconnection region can transform the linkage types of the nulls and eventually lead to the evolution of flux ropes.