Stormtime ring-current formation: A comparison between single-and double-dip model storms with similar transport characteristics

Intense magnetic storms often develop in two stages such that a second ring current enhancement begins before the first ring current enhancement has r ecovered to the prestorm level. Since Dst traces of such storms exhibit two dips, we refer to these as double-dip storms. Here we compare double- and single-dip storms with similar convective and diffusive transport character istics for effectiveness at forming the proton ring current. Our model stor ms consist of superposition of almost randomly occurring impulses in the co nvection electric field. We have synthesized a hypothetical double-dip stor m consisting of a moderate 6-hour storm, followed by a 3-hour quiet interva l and then by a more intense 15-hour storm, for a total duration of 24 hour s. For comparison, we consider a single-dip model storm with an unmodulated 24-hour main phase during which the root-mean-square enhancement of the cr oss-tail potential drop is made equal to the time-weighted rms enhancement for the double-dip model storm. This leads to comparable time-averaged diff usion coefficients for our single- and double-dip model storms. The mean en hancement of the cross-tail potential drop of the two storms are also compa rable. When the stormtime proton plasma sheet distribution, the source of r ing current protons, is left unchanged from its quiet (prestorm) level, we find little difference in proton energy content per unit R (which is geocen tric distance normalized by R-E) between our double- and single-dip model s torms. The proton-energy content of the magnetosphere is roughly increased by a factor of 2.5 by either model storm under this scenario, in which the overall amount of stormtime transport, whether convective or diffusive, is nearly the same for the double- and single-dip model storms. As in our earl ier work, we require here an enhanced stormtime plasma sheet population (in addition to enhanced particle transport) in order to achieve (for example) the 20-fold increase in /Dst/ characteristic of a large storm. Only when w e invoke a two-stage stormtime enhancement of the boundary (plasma sheet) p hase space density in combination with the two-stage enhancement in particl e transport, our double-dip model storm does show a much larger total energ y content than our single-dip model storm with a one-stage enhancement of t he boundary spectrum. This suggests that plasma sheet preconditioning may b e important for the development of especially intense storms.