## 4. DiscussionWe have shown in this paper that the numerical simulations by Geertsema & Achterberg (1992) of viscous resistive MHD turbulence in an accretion disc give rise to behaviour characteristic of self-organised criticality in a sandpile. This similarity may help to explain certain observed properties of a range of accreting astrophysical systems, which we have reviewed. Furthermore, the result is of intrinsic scientific interest as an ab initio demonstration of the emergence of SOC from a system of MHD-based equations. Further analysis of the numerical results, to establish how the SOC-sandpile phenomenology arises, would be of great interest both for accretion disc astrophysics and for fundamental plasma physics. At the present stage of development of realistic MHD simulations (for example of the link between smallscale dissipation and the time behaviour of the flux of matter onto the central object), direct numerical proof of the occurence of SOC-sandpile phenomenology is still a remote objective. However, the capacity of the SOC-sandpile paradigm to circumvent complex "full mathematical descriptions" is the basis for its present attraction in many fields of physics. If future work confirms the importance of SOC-sandpile phenomenology in accreting systems that is suggested in the present paper, it would permit a dissociation (at least at zeroth order) of the detailed physics of turbulence in the disc from the global modelling of this class of astrophysical object. © European Southern Observatory (ESO) 1998 Online publication: August 27, 1998 |