Magnetic Nulls in the Reconnection Driven by Turbulence

Abstract

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