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EP: Fachverband Extraterrestrische Physik

EP 13: Astrophysics I

EP 13.4: Vortrag

Freitag, 4. April 2025, 10:00–10:15, ZHG101

On the existence and (non-)uniqueness of null points of flows and magnetic fields as prerequisites for the existence of astropauses — •Dieter Nickeler¹, Kuljeet Singh Saddal^1,2, and Rodrigo Meneses³ — ¹Astronomical Institute AV CR, Ondrejov, Czech Republic — ²Charles University, Prague, Czech Republic — ³Universidad de Valparaiso, Chile

The existence of null points of vector fields is prerequisite for the spanning of separating surfaces. Such surfaces guarantee that topologically disjoint field lines of the corresponding vector fields exist on each side of the separatrix. The existence of separatrices allows to define so-called pauses, e.g. magnetopause (a magnetic separatrix) or astropause (like the heliopause). To analyse the structure of fields with null points, we focus on the stationary approximation. Besides the topological perspective, other physical constraints can require the existence of null points.

We investigate the case of a non-monotonous pressure distribution driving stationary counterstreaming MHD flows such as the interstellar medium flows and the outer stellar wind flows. For a purely ideal hydrodynamical problem, and demanding on the regularity of all involved fields and their derivatives, we demonstrate that the existence of an extremum of the thermal or plasma pressure at a certain point in the generic three-dimensional case automatically implies that this point is also a stagnation point (= null point of the plasma flow). An extended analysis is performed for ideal MHD and further for MHD with additional, general non-ideal terms.

Keywords: Magnetohydrodynamics; Astrospheres; Topological fluid dynamics