Magnetic Nulls in the Reconnection Driven by Turbulence

For the first time, we use spacecraft measurements to investigate the magnetic nulls in a reconnection event driven by turbulence in the magnetosheath. We particularly focus on the relation between magnetic-null topologies and currents, which can be decomposed into a component perpendicular to spine (J_⊥) and a component parallel to spine (J_∥). Our new observations include: (1) the total current at spiral nulls is much larger than that at radial nulls; (2) J_∥ is large at spiral nulls but small at radial nulls; (3) at radial nulls, J_⊥ is dominant, while at spiral nulls, J_∥ is dominant; (4) the fan-spine angle at both radial and spiral nulls decreases with J_⊥, with a clear upper boundary; (5) with a database of 715 nulls, we statistically resolve the relation between and J_⊥ as _⊥ = 72.6 · tan (-)^-1 with the correlation coefficient of cc = 0.71 (radial null) and _⊥ = 57.4 · tan (-)^-1 with cc = 0.76 (spiral null), where J_⊥ is in the unit of nA m^-2. Our physical interpretations of these observations are: (1) the current parallel to spine J_∥ significantly determines the null topology, with large J_∥ producing spiral nulls and small J_∥ producing radial nulls; (2) the current perpendicular to spine J_⊥ serves to tilt the fan plane to the spine, for both spiral and radial nulls. All of these observations and conclusions significantly improve our understanding of magnetic reconnection.