CRITICAL DYNAMICS OF THE SUBSTORM CYCLE IN EARTH'S MAGNETOTAIL

Alex Klimas, NASA/GSFC

The loading-unloading cycle is a fundamental feature of the substorm 
phenomenon. While clearly influenced by the variable solar wind, the 
cycle has its source in the plasma sheet, through transitions from 
stability to instability and back again. To understand the substorm 
cycle, these transitions must be understood within the context of a 
fast flow-driven eddy turbulent plasma sheet in which reconnection 
takes place in varying degrees throughout the cycle. The transitions 
of the substorm cycle are actually transitions from sporadic 
localized instability to global instability, and then back again. 
Further, the substorm cycle must be hysteretic; the threshold for 
transition into global activity must be higher than the threshold for 
transition back again. Only then, can a loading-unloading cycle 
develop. Within the context of the turbulent plasma sheet, what is 
the mechanism for producing hysteresis in the substorm cycle? Uritsky 
et al. (JGR, 2002; GRL, 2003), through analyses of POLAR UVI image 
data, have developed strong evidence for a reconnection based 
scale-free avalanching process in the plasma sheet. They have 
suggested that an avalanche of interacting local instabilities is 
responsible for the transition into global unloading at substorm 
onset. Klimas et al. (JGR, 2004) have studied a numerical driven 
current-sheet model that can evolve into a loading-unloading cycle 
and does exhibit scale-free avalanching that is similar to that 
implied by the UVI image analyses. It will be shown that the global 
loading-unloading cycle of this model is hysteretic, and that the 
source of this hysteresis is at the smallest scales in the model, 
where it is assumed. These results suggest that the global 
loading-unloading cycle of the magnetosphere is a direct consequence 
of hysteresis in the reconnection producing instability that resides 
in the plasma sheet at spatial and temporal scales less than ~1 RE 
and ~1-3 min, respectively. Alternatives that do not contradict the 
Uritsky et al. results are difficult to imagine.