14 research outputs found

    Comparison study of ring current simulations with and without bubble injections

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    For many years, stand-alone ring current models have been successfully producing storm time ring current enhancements without specifying explicit localized transient injections along their outer boundaries. However, both observations and simulations have suggested that the frequent burst flows or bubble injections can contribute substantially to the storm time ring current energy. In this paper, we investigate the difference in the spatial and temporal development of the ring current distribution with and without bubble injections using the Rice Convection Model-Equilibrium. The comparison study indicates that the simulation with bubble effects smoothed out along geosynchronous orbit can predict approximately the same large-scale ring current pressure distribution and electric potential pattern as the simulation with bubble effects included. Our results suggest that the increase of the hot plasma population along geosynchronous orbit can be envisaged as an integrated effect of bubble injections from the near-Earth plasma sheet. However, the observed fluctuations in the plasma population and electric field can only be captured when the mesoscale injections are included in the simulation. We also confirmed again that adiabatic convection of fully populated flux tubes cannot inject the ring current from the middle plasma sheet. The paper provides a justification for using stand-alone ring current models in the inner magnetosphere to simulate magnetic storms, without explicit consideration of bubbles and magnetic buoyancy effects inside geosynchronous orbit

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

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    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

    RCM-E simulation of a thin arc preceded by a north-south-aligned auroral streamer

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    The Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager data have recently revealed a repeatable sequence that occurs during many auroral substorms, in which a newly formed thin arc is preceded by an equatorward propagating streamer. The paper aims at modeling this sequence using the Rice Convection Model–Equilibrium. The simulation shows a thin arc arising when a plasma sheet bubble with its PV5/3 reduced to the transition region value arrives at the magnetic transition region. The modeled thin arc consists of two parts: the one east of the streamer is the result of the bubble pushing high PV5/3 flux tubes ahead of it, strengthening the upward region 2 current, and the one west of the streamer is associated with westward drifting bubble particles, sliding along the transition region. The model predicts that (1) the westward and eastward leading edges of the thin arc propagate azimuthally at a speed of ~0.5–2.7 km/s and (2) the streamer-induced thin arc is accompanied by classic signatures of bubble injections

    RCM-E simulation of bimodal transport in the plasma sheet

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    Plasma sheet transport is bimodal, consisting of both large-scale adiabatic convection and intermittent bursty flows in both earthward and tailward directions. We present two comparison simulations with the Rice Convection Model-Equilibrium (RCM-E) to investigate how those high-speed flows affect the average configuration of the magnetosphere and its coupling to the ionosphere. One simulation represents pure large-scale slow-flow convection with time-independent boundary conditions; in addition to the background convection, the other simulation randomly imposes bubbles and blobs through the tailward boundary to a degree consistent with observed statistical properties of flows. Our results show that the bursty flows can significantly alter the magnetic and entropy profiles in the plasma sheet as well as the field-aligned current distributions in the ionosphere, bringing them into much better agreement with average observations

    RCM-E simulation of substorm growth phase arc associated with large-scale adiabatic convection

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    [1] Substorm auroral breakup often occurs on a longitudinally elongated arc at the end of a growth phase. We present an idealized high-resolution simulation with the Rice Convection Model-Equilibrium (RCM-E) to investigate how large-scale adiabatic convection under equilibrium conditions can give rise to an auroral arc. We find that a thin arc that maps to the magnetic transition region at r ~ 8 RE emerges in the late growth phase. The simulation implies that the arc in the premidnight sector is associated with a sheet of additional region 1 sense field-aligned current (FAC) just poleward of the main region 2 FAC, while the arc in the postmidnight sector is associated with the poleward portion of the main upward region 2 FAC. Explanations for the location and the thickness of the arc are proposed, based on the simulation

    Three Dimensional Finite Element Modeling of the Earth's Magnetosphere

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    We demonstrate the feasibility of using a nonconforming finite element method on an unstructured grid in solving a magnetospheric physics problem. We use this approach to construct a global discrete model of the magnetic field of the magnetosphere that includes the effects of shielding currents at the outer boundary (the magnetopause). As in the approach of [17] the internal magnetospheric field model is that of Hilmer and Voigt [3] while the magnetopause shape is based on an empirically-determined approximation [12]. The result is a magnetic field model whose field lines are completely confined within the magnetosphere. The numerical results indicate that the nonconforming discrete model is robust and efficient. Keywords Magnetopause, magnetosphere, Chapman-Ferraro Currents, Nonconforming finite elements, Laplace's equation, Neumann boundary value problem 1991 Mathematical Subject Classification 65M60, 65N50, 65J10, 85A20, 85-08 1 Introduction The Earth's magnetosphere is formed by th..