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We investigate and discuss protostellar discs in terms of where the variousnon-ideal magnetohydrodynamics (MHD) processes are important. We find that thetraditional picture of a magnetised disc (where Ohmic resistivity is dominantnear the mid-plane, surrounded by a region dominated by the Hall effect, withthe remainder of the disc dominated by ambipolar diffusion) is a greatoversimplification. In simple parameterised discs, we find that the Hall effectis typically the dominant term throughout the majority of the disc. Moreimportantly, we find that in much of our parameterised discs, at least twonon-ideal processes have coefficients within a factor of 10 of one another,indicating that both are important and that naming a dominant term underplaysthe importance of the other terms. Discs that were self-consistently formed inour previous studies are also dominated by the Hall effect, and the ratio ofambipolar diffusion and Hall coefficients is typically less than 10, suggestingthat both terms are equally important and listing a dominant term ismisleading. These conclusions become more robust once the magnetic fieldgeometry is taken into account. In agreement with the literature we review, weconclude that non-ideal MHD processes are important for the formation andevolution of protostellar discs. Ignoring any of the non-ideal processes,especially ambipolar diffusion and the Hall effect, yields an incorrectdescription of disc evolution.

Do we need non-ideal magnetohydrodynamics to model protostellar discs?