Study of small-scale plasmoid structures in the magnetotail using Cluster observations and Hall MHD simulations

The existence of small‐scale plasmoids associated with the Hall effect has been often observed in the magnetotail. They are considered as the signature of multiple X‐line collisionless reconnection. To study these plasmoids structures, we present some Cluster observations and Hall MHD simulations of their features. In this study, the observation survey is divided into two types. The first one is the isolated plasmoid with two typical plasmoid events the flux‐rope‐like plasmoid on 3 August 2001 and the closed‐loop‐like plasmoid on 22 August 2001. The second type contains multiple successive plasmoids, on 12 September 2001 with three neighboring plasmoids structures observed during a substorm. Especially for the second plasmoid, three main features were observed, including a core field in the plasmoid, a quadrupole magnetic field near the X line, and a local plasma convection within the plasmoid. The Grad‐Shafranov reconstruction method was used to recover the two‐dimensional magnetic field maps for this plasmoid. These results may provide evidence that the small‐scale plasmoids frequently observed in the magnetotail may be produced by multiple X‐line collisionless reconnection. To study the impact of crosstail magnetic field on the structures of small‐scale plasmoids, a 2.5‐D Hall MHD simulation was performed. In the case with a guide field By0, the in‐plane plasma inflows carrying Byflux enter into the plasmoid due to magnetic reconnection. However, there is no such By flux transport process for the case without guide field. These results demonstrate that a crosstail magnetic field is an important factor in the formation of flux‐rope‐like plasmoids. Key PointsMultiple plasmoids associated with Hall effect were observed during a substorm.A crosstail magnetic field is an important factor in the formation of plasmoids.