Lognormal form of the ring current energy content

It is shown that the stormtime ring current energy content, from kinetic simulations, has a lognormal distribution. This type of functional form naturally arises from the superposition of many processes with a common initiation event but with differing growth and decay timescales. For the ring current, such a situation occurs from the disparate timescales of energization and decay for the hot ions at various energies, pitch angles, and spatial locations. The summation of this plethora of small currents results in a single current system (the ring current, both partial and symmetric) that has a loss time scale that decreases and then increases during every storm. The consequence is that the stormtime D-st index, which also has a lognormal shape, can be (and in fact is) dominated by this single current system. (C) 2003 Elsevier Science Ltd. All rights reserved.