Local identity of magnetic and hydrodynamic fans

Determine whether the tangential magnetopause, represented by the magnetic fan, is identical to the tangential astropause, represented by the hydrodynamic fan, when the magnetic and velocity null points are co-located.

Background

The paper establishes conditions under which magnetic null points and velocity stagnation points co-locate in ideal, resistive, Hall, and general non-ideal magnetohydrodynamics. Co-location of the nulls, however, does not by itself establish that the associated separatrix structures coincide. The magnetic fan is the tangential structure associated with the magnetopause, whereas the hydrodynamic fan is the corresponding tangential structure associated with the astropause. The authors explicitly leave unresolved whether these two local structures are identical.

References

Even if the null points of flow and magnetic field are co-located, one open issue is whether the tangential magnetopause (magnetic fan) is identical to the tangential astropause (hydrodynamic fan). This requires further investigation.

— The existence of null points of flow and magnetic fields as prerequisites for the presence of astropauses and their co-location  (2610.09933 - Nickeler et al., 7 Oct 2026) in Section 5, Discussion and conclusions (following the discussion of Hall MHD)

Even if the null points of flow and magnetic field are co-located, one open issue is whether the tangential magnetopause (magnetic fan) is identical to the tangential astropause (hydrodynamic fan). This requires further investigation. When the two fans are identical locally, the second issue is whether the two separatrices are also identical globally, that is, whether magnetopause and astropause are identical. Furthermore, the question arises whether only field-aligned flows in the vicinity of the astropause guarantee the existence of an at least local identity of the two separatrices, that is, whether the two separatrix surfaces are tangential.

— The existence of null points of flow and magnetic fields as prerequisites for the presence of astropauses and their co-location  (2610.09933 - Nickeler et al., 7 Oct 2026) in Section 5, Discussion and conclusions (following the discussion of Hall MHD)