Is Quadrupole Structure of Out-of-Plane Magnetic Field Evidence for Hall Reconnection?

The experimental observation of a quadrupole pattern in the ''out-of-plane'' field By is usually interpreted as a tell-tale sign of Hall MHD mediated magnetic reconnection in the vicinity of a reconnecting X-point. In contrast to these conventional expectations, it is shown By means of analytical equations describing the generation of By that quadrupole structure can be formed even in the absence of Hall effects. In general, By can be generated due to inhomogeneous ion flow, inhomogeneous current (Hall effect), or terms proportional to a pre-existing By. This prediction is validated in a 3D global MHD simulation where it is shown that vortex flows at the dayside magnetopause lead to formation of a finite By even in the absence of a pre-existing IMF By. The generated field is shown to have a quadrupolar pattern in By with peak amplitude ∼0.5 of the total field. Similar vortex flows are also observed in 3D global hybrid simulations but the resulting By structure in this case is a combination of the Hall effects and the inhomogeneous ion flows. Thus special care must be taken in using observations of quadrupole By in satellite and experiments to infer information about its particular generation mechanism or attributing it to the Hall MHD reconnection regime.