A SIMPLE AVALANCHE MODEL AS AN ANALOG FOR MAGNETOSPHERIC ACTIVITY

The power law dependence of the power spectrum of auroral indices, and in-situ magnetic field observations in the earth's geotail, may be ev idence that the coupled solar wind-magnetospheric system exhibits scal e free self organised criticality and can to some extent be described by avalanche models. In contrast, the intensity of, and time interval between, substorms both have well defined probability distributions wi th characteristic scales. We present results from a simple cellular au tomaton that models avalanches in a one dimensional ''sandpile''; here we examine the simplest case of constant inflow. This model generates a probability distribution of energy discharges due to internal reorg anization that is a power law implying SOC, whereas systemwide dischar ges (flow of ''sand'' out of the system) form a distinct group which d o not exhibit SOC. The energy dissipated in a systemwide discharge fol lows a probability distribution with a well defined mean, as does the time interval between one systemwide discharge and the next. Internal and external avalanches can therefore in principle be identified with distinct processes in the dynamic geotail. If so, the avalanche model places restrictions on the class of physical process that may be invok ed to explain the observed geomagnetic dynamics.