Self-organised criticality in astrophysical accretion systems

Seif-organised criticality (SOC) has been proposed as a potentially powerfu l unifying paradigm for interpreting non-diffusive avalanche-type transport in laboratory, space and astrophysical plasmas. After reviewing the most p romising astrophysical sites where SOC might be observable, we consider the theoretical arguments for supposing that SOC can occur in accretion discs. Perhaps the most rigorous evidence is provided by numerical modelling of e nergy dissipation due to magnetohydrodynamic turbulence in accretion discs by G. Geertsema & A. Achterberg (Astron. Astrophys. 255, 427 (1992)); we in vestigate how "sandpile"-type dynamics arise in this model. It is concluded that the potential sites for SOC in accretion systems are numerous and obs ervationally accessible, and that theoretical support for the possible occu rrence of SOC can be derived from first principles.