Overgangsradius

Overgangsradius, literally the transition radius, is a term used in various scientific fields to denote a radial distance at which the physical regime or governing description of a system changes. It is a modeling concept rather than a fixed physical boundary, and its exact meaning depends on the context. In a radial model, different equations or assumptions may apply inside and outside this radius, with the two regions connected through continuity conditions.

In astrophysics and space physics, overgangsradius is commonly used to describe where one description of a flow or disk transitions to another. For example, in accretion disks around compact objects, there may be a transition radius between an inner hot, radiatively inefficient flow and an outer cooler, thinner disk. In planetary and circumstellar disk studies, a transition radius can mark where the disk’s temperature, opacity, or ionization state changes sufficiently to alter the emitted spectrum or dynamics. In magnetospheric or stellar wind contexts, it can denote where magnetic forces begin to dominate over gas pressure, altering flow behavior.

Determination and notation vary by field. The radius is typically denoted by symbols such as r_tr, R_tr, or r_t and is not a fixed universal constant; it is inferred from models fitted to observations or from theoretical criteria based on timescales, energy balance, or pressure comparisons. Because the transition is often gradual rather than a sharp boundary, the overgangsradius is treated as a characteristic scale rather than a precise discontinuity.

Limitations include model dependence and ambiguity in defining the exact boundary. The overgangsradius serves as a practical tool to simplify complex radial structure and to compare different systems or simulations.