On the contribution of plasma sheet bubbles to the storm time ring current

Abstract

Particle injections occur frequently inside 10 Re during geomagnetic storms. They are commonly associated with bursty bulk flows or plasma sheet bubbles transported from the tail to the inner magnetosphere. Although observations and theoretical arguments have suggested that they may have an important role in storm time dynamics, this assertion has not been addressed quantitatively. In this paper, we investigate which process is dominant for the storm time ring current buildup: large‐scale enhanced convection or localized bubble injections. We use the Rice Convection Model‐Equilibrium (RCM‐E) to model a series of idealized storm main phases. The boundary conditions at 14–15 Re on the nightside are adjusted to randomly inject bubbles to a degree roughly consistent with observed statistical properties. A test particle tracing technique is then used to identify the source of the ring current plasma. We find that the contribution of plasma sheet bubbles to the ring current energy increases from ~20% for weak storms to ~50% for moderate storms and levels off at ~61% for intense storms, while the contribution of trapped particles decreases from ~60% for weak storms to ~30% for moderate and ~21% for intense storms. The contribution of nonbubble plasma sheet flux tubes remains ~20% on average regardless of the storm intensity. Consistent with previous RCM and RCM‐E simulations, our results show that the mechanisms for plasma sheet bubbles enhancing the ring current energy are (1) the deep penetration of bubbles and (2) the bulk plasma pushed ahead of bubbles. Both the bubbles and the plasma pushed ahead typically contain larger distribution functions than those in the inner magnetosphere at quiet times. An integrated effect of those individual bubble injections is the gradual enhancement of the storm time ring current. We also make two predictions testable against observations. First, fluctuations over a time scale of 5–20 min in the plasma distributions and electric field can be seen in the central ring current region for the storm main phase. We find that the plasma pressure and the electric field EY there vary over about 10%–30% and 50%–300% of the background values, respectively. Second, the maximum plasma pressure and magnetic field depression in the central ring current region during the main phase are well correlated with the Dst index.

Physical Processes of Meso-Scale, Dynamic Auroral Forms

“…reported similar results but describes the auroral activity as equatorward extending, which may also be described as streamers. These findings are consistent with the results of global RCM simulations by Yang et al (2015) that show that flow bursts play an important role in the populating the ring current, providing as much as 20 to 60% of ion injections into the ring current during magnetic storms. While recent studies have shown links between injections in the inner magnetosphere and flow bursts further out (Runov et al 2009;Gabrielse et al 2012), other studies have shown that fast plasma flows are typically deflected around the Earth outside of geosynchronous orbit (Juusola et al 2011;McPherron et al 2011).…”

Large‐amplitude electric fields in the inner magnetosphere: Van Allen Probes observations of subauroral polarization streams

“…The data are consistent with a picture of multiple, narrow in MLT injections occurring on timescales of minutes, and the integrated effect producing a buildup of pressure, field‐aligned currents, and the broad region of strong electric fields in the equatorial region. Recent studies have suggested that small‐scale injections may be of equal or greater importance than large‐scale convection in transporting plasma sheet particles into the inner magnetosphere‐based Van Allen Probes observations [ Gkioulidou et al , ] as well as RCM‐E simulations [ Yang et al , ], and our observations support this view.…”

Current Systems in the Earth's Magnetosphere

“…The storm time evolution of currents can be revealed using magnetospheric modeling since simultaneous in situ measurements of all current systems are not possible. It was found that, initially, the tail current dominates the total current magnitude (Alexeev et al, ; Asikainen et al, ; Liemohn, Ganushkina et al, ; Liemohn et al, ; Patra et al, ; Shi et al, ; Sitnov et al, ; Stephens et al, ; Yang et al, ) as the geospace‐driving conditions start to ramp up and the hot particles in the plasma sheet begin to move sunward. The banana current then rises as the peak of the plasma pressure moves inward.…”