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

Abstract

Abstract. Intense magnetic storms often develop in two stages such that a second ring current enhancement begins before the first ring current enhancement has recovered 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 characteristics for effectiveness at forming the proton ring current. Our model storms consist of superposition of almost randomly occurring impulses in the convection electric field. We have synthesized a hypothetical double-dip storm consisting of a moderate 6-hour storm, followed by a 3-hour quiet interval and then by a more intense 15-hour storm, for a total duration of 24 hours. 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 cross-tail potential drop is made equal to the timeweighted rms enhancement for the double-dip model storm. This leads to comparable timeaveraged diffusion coefficients for our single-and double-dip model storms. The mean enhancement of the cross-tail potential drop of the two storms are also comparable. When the stormtime proton plasma sheet distribution, the source of ring current protons, is left unchanged from its quiet (prestorm) level, we find little difference in proton energy content per unit R (which is geocentric distance normalized by RE) between our double-and single-dip model storms. The protonenergy 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 earlier 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 IDstl characteristic of a large storm. Only when we invoke a two-stage stormtime enhancement of the boundary (plasma sheet) phase space density in combination with the two-stage enhancement in particle transport, our double-dip model storm does show a much larger total energy content than our single-dip model storm with a one-stage enhancement of the boundary spectrum. This suggests that plasma sheet preconditioning may be important for the development of especially intense storms.

Ring Current Energy Input and Decay

“…This preconditioning of the ring current development for this storm was discussed by Kozyra et al (2002), finding that the strong convection of each new stage cleared out most of the ions from the previous injections. This is consistent with the Chen et al (2000) study, which found that single-and double-dip storms with comparable energy inputs produce similarly sized ring currents. The storm in July 2000 also has an unusual time sequence, with a small storm the day before the ICME reached Earth (Figure 8).…”

Relative contribution of electrons to the stormtime total ring current energy content

“…In this study the width of DK for protons is estimated from a simulated stormtime pitch angle distribution of 48 keV protons at L = 3 [Chen et al, 2000] that agree well with CRRES observations. For electrons it is estimated from pitch angle distribution of 35-70 keV electrons at L = 3 observed by Explorer 45 (orbit 102 outbound) [Lyons and Williams, 1975b].…”

Storm-time ring current: model-dependent results

“…From the other side, BC1 and BC2 are different in terms of the shape of the distribution (BC1 has Maxwellian and BC2 kappa distribution) and also their average parameters are not the same (n = 0.5 cm −3 and T = 5 keV for BC1 and (< T >= 7.8 keV and < n >= 0.97 cm −3 for BC2). Not only temporal variations but also the magnitudes of number density are important for the ring current energy increase (Kozyra et al, 1998;Chen et al, 2000;Ebihara et al, 2005). With the temperature it is not straightforward, since a cold dense plasma sheet can E field BC1: Maxwell at 6.6 Re, n=0.5 cm -3 , T=5 keV BC2: kappa at 6.6 Re, n, T || and T ⊥ from LANL BC3: Maxwell at 10 Re, T= 5 keV, n ps =0.025n sw +0.395 BC4: Maxwell at 10 Re, T, n from Tsyganenko and Mukai (2003) Boundary conditions lead to an enhanced ring current (Chen et al, 2007;Lavraud and Jordanova, 2007).…”

“…There have been a number of investigations to study that. For example, Kozyra et al (1998);Chen et al (2000) and Ebihara et al (2005) have shown that plasma sheet density is of key importance to a strong ring current. Ebihara et al (2005) investigated the nonlinear impact of the plasma sheet density on the total energy of the storm-time ring current.…”