Avalanching and Self-Organised Criticality, a paradigm for geomagnetic activity?

The characterization of global energy storage and release in the coupled solar wind-magnetosphere system remains one of the fundamental problems of space physics. Recently, it has been realised that a new paradigm in physics, that of Self Organised Criticality (SOC) may encapsulate the mixing and merging of flux on many scales in the magnetotail prompting bursty energy release and reconfiguration. SOC is consistent with qualitative measures such as power-law power spectra and bursty bulk flows and with more quantitative tests such as power law burst distributions in auroral indices and auroral optical activity. Here, we present a careful classification of the broad range of systems that fall under the general description of 'SOC'. We argue that some, but not all, of these are consistent with our current understanding of the magnetosphere. We discuss the observed low dimensionality of the dynamic magnetosphere in terms of both SOC model properties, and observables. Observations of burst statistics are highlighted; we show that these are currently suggestive but not sufficient to confirm SOC and in particular we find that auroral indices are not effective at distinguishing the internal dynamics of the magnetosphere from that of the intermittent solar wind driver. This may also elucidate the paradox of predictability and complexity of the coupled solar wind-magnetosphere system.