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(perpendicular to)) and a component parallel to spine (J(parallel to)). Our new observations include: (1) the total current at spiral nulls is much larger than that at radial nulls; (2) J(parallel to) is large at spiral nulls but small at radial nulls; (3) at radial nulls, J(perpendicular to) is dominant, while at spiral nulls, J(parallel to) is dominant; (4) the fan-spine angle theta at both radial and spiral nulls decreases with J(perpendicular to), with a clear upper boundary; (5) with a database of 715 nulls, we statistically resolve the relation between theta and J(perpendicular to) as J(perpendicular to) = q J(perpendicular to) = 72.6 . tan(theta)(-1) with the correlation coefficient of cc = 0.71 (radial null) and J(perpendicular to) = 57.4 . tan(theta)(-1) with cc = 0.76 (spiral null), where J(perpendicular to) is in the unit of nA m(-2). Our physical interpretations of these observations are: (1) the current parallel to spine J(parallel to) significantly determines the null topology, with large J(parallel to) producing spiral nulls and small J(parallel to) producing radial nulls; (2) the current perpendicular to spine J(perpendicular to) 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.