Revisiting the Role of Plasma Sheet Bubbles in Stormtime Energy Transport Using RCM-I
Plasma sheet bubbles, defined as entropy-depleted flux tubes, are widely regarded as an efficient mechanism for transporting plasma into the inner magnetosphere during geomagnetic storms. Equilibrium simulations using the Rice Convection Model (RCM-E) predict that bubbles can account for up to 60% of storm-time ring current energy during intense storms. However, global simulations and observations suggest a more moderate net contribution. In this study, we quantify the contribution of plasma sheet bubbles to ring current buildup using a Lagrangian particle backtracking technique applied to three idealized storm simulations conducted with the inertialized Rice Convection Model (RCM-I). A stratified ensemble of about 100,000 test particles, weighted by local plasma pressure and entropy, was traced backward in time to determine whether their energy originated from bubble injections, non-bubble plasma sheet transport, or pre-existing trapped populations. Our results show that bubble contributions increase with storm intensity but saturate near 40% of the total ring current energy inside R<6.6Re, even for strong storms (Dst about -180nT). The trapped population remains comparably important (about 40%), while non-bubble transport contributes about 15%. This saturation is notably lower than the 61% predicted by RCM-E and is attributed to inertial braking, which generates oscillatory flows and tailward return streams that remove approximately 40% of the inward bubble energy flux. When only newly transported plasma is considered, bubbles account for about 73% of the inward transport, consistent with global MHD and flux-based studies. These results reconcile equilibrium modeling, global simulations, and spacecraft observations by demonstrating that bubbles dominate inward transport but do not fully replace the resident ring current population due to inertial limitations.