Magnetic Null-Pairs within Magnetic Reconnection Ion Diffusion Region in the Magnetotail: A Case Study

The 3-dimentional structure of magnetic reconnection ion diffusion region has been studied in this paper. Steady magnetic null-pair structure is found among the Cluster tetrahedron within a thin current sheet when magnetic reconnection takes place in the near-Earth magnetotail. Two magnetic null points in the null-pair are well coupled, with an angle of about 3~7° between the spin line of one and the fan surface of the other. The magnetic null-pair detected in the ion diffusion region, is quasi-stable in spatial structure but fast evolved in time, consistent with the fast reconnection scenario. The spatially steady magnetic null-pair within the diffusion region of the collision less fast magnetic reconnection presents an advanced understanding of the magnetic reconnection process.

Structures of magnetic null points in reconnection diffusion region:Cluster observations

Magnetic reconnection is a very important and fundamental plasma process in transferring energy from magnetic field into plasma. Previous theory, numerical simulations and observations mostly concen-trate on 2-dimensional (2D) model; however, magnetic reconnection is a 3-dimensional (3D) nonlinear process in nature. The properties of reconnection in 3D and its associated singular structure have not been resolved completely. Here we investigate the structures and characteristics of null points inside the reconnection diffusion region by introducing the discretized Poincaré index through Gauss integral and using magnetic field data with high resolution from the four satellites of Cluster mission. We esti-mate the velocity and trajectory of null points by calculating its position in different times, and compare and discuss the observations with different reconnection models with null points based on character-istics of electric current around null points.

Recent studies in satellite observations of three-dimensional magnetic reconnection

Magnetic reconnection is a main process converting the magnetic energy into thermal and kinetic energy in plasmas. It is one of the fundamental problems of crucial importance not only to space plasmas physics and space weather studies,such as the solar flare,coronal mass ejections and magnetospheric substorms,but also to the stability analysis in magnetically confined fusion. In general,except for cases with periodical boundary conditions,three-dimensional(3D) magnetic re-connection occurs on magnetic separatrices generated by magnetic nulls. Here we briefly introduce/review the theories and some recent satellite observations of 3D magnetic reconnection. Topics to be further studied are also discussed.

Evolution of clustered magnetic nulls in a turbulent-like reconnection region in the magnetotail

t Magnetic null points and flux ropes play important roles in the three-dimensional process of magnetic reconnection. In this study, a cluster of null points are reconstructed in the reconnection region in the magnetotail by applying a fitting-reconstruction method to measurements from the Cluster mission. The number of recon- structed null points varies rapidly, presenting a turbulentlike evolution of the magnetic structure. The electron density and the flux of the accelerated electrons were enhanced in this turbulent-like region. During this unstable reconnection process, a B-As-B null structure was formed, showing flux rope features and resembling a secondary island in the observation.

Observation of the lower hybrid waves near the three-dimensional null pair

Magnetic reconnection is a fundamental process in plasma,which is thought to play important roles both in laboratory and natural plasmas through affecting magnetic topology,heating and accelerating particles. During an event on Oct. 1st,2001,the Cluster tetrahedron circled around the magnetic reconnection region several times,and Xiao et al. first identified the null pair and found that the spectrum of the null-point oscillation shows the maximum power near the lower-hybrid frequency. In this paper we report the observation of electromagnetic and electrostatic wave enhancements near lower hybrid frequency associated with the reconnection process near the null pair. The lower hybrid waves(LHWs) with quasi-perpendicular propagation were identified and also confirmed by the power law of the spectrum of electric and magnetic fields.