Self-organised criticality in astrophysical accretion systems

Seif-organised criticality (SOC) has been proposed as a potentially powerful unifying paradigm for interpreting non-diffusive avalanche-type transport in laboratory, space and astrophysical plasmas. After reviewing the most promising 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 energy dissipation due to magnetohydrodynamic turbulence in accretion discs by G. Geertsema & A. Achterberg (Astron. Astrophys. 255, 427 (1992)); we investigate how "sandpile"-type dynamics arise in this model. It is concluded that the potential sites for SOC in accretion systems are numerous and observationally accessible, and that theoretical support for the possible occurrence of SOC can be derived from first principles.