Intensity of geomagnetic storms depends on the X‐line location in the plasma sheet

Kan, J. R.; Xing, X.; Wolf, R. A.

doi:10.1029/2006ja011945

Cited by 3 publications

(7 citation statements)

References 45 publications

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“…Another way to look at this is to calculate the total PV 5/3 contained in the ring current region. Kan et al [2007] proposed to estimate the flux integral of the entropy function that is required to support a storm time ring current. They define…”

Section: Results From 20 Idealized Stormsmentioning

confidence: 99%

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

et al. 2015

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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.

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Section: Results From 20 Idealized Stormsmentioning

confidence: 99%

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

et al. 2015

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“…Adiabatic acceleration of the particles in the flux tube provides the population for the ring current. Kan et al [2007] discussed this issue and suggested that magnetic storm intensity might be related to the position of the neutral line in the tail during storm time substorms. Storm time substorms whose neutral line form closer to the Earth would produce a greater enhancement of the ring current as compared to storm time substorms whose neutral line forms further from the Earth.…”

Section: Generation Of the Ring Current And Resolution Of The Paradoxmentioning

confidence: 99%

“…Even though some of these increases in magnetospheric activity might involve some changes in the amount of open flux in the tail [ Boudouridis et al , 2005], the loading/unloading pattern of substorms is not evident. Thus we consider the supposition of Kan et al [2007] that the position of “substorm” neutral line is important to be only partially correct, since simulations of the storm time magnetotail do not show the loading‐unloading behavior of substorms, and in fact the term “storm time substorm” may be a misnomer. Also, it is relevant to note that Bargatze et al [1985] found that when was VB s large, the loading‐unloading response in the AE data disappeared, leaving only the directly driven response.…”

Section: Generation Of the Ring Current And Resolution Of The Paradoxmentioning

confidence: 99%

“…By forming relatively close to the Earth, the newly merged flux tubes could have small enough PV γ that quasi‐steady convection can occur without the pressure‐balance inconsistency [ Erickson and Wolf , 1980]. So to modify the argument of Kan et al [2007], the question is now, where does the quasi‐stationary storm time reconnection region form, and how is it related to VB s ?…”

Section: Generation Of the Ring Current And Resolution Of The Paradoxmentioning

confidence: 99%

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Why doesn't the ring current injection rate saturate?

et al. 2009

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[1] For low values of the solar wind electric field, the response of the polar cap potential is essentially linear, but at high values of VB s , the polar cap potential saturates and does not increase further with increasing VB s . On the other hand, the ring current injection rate does increase linearly with VB s and shows no evidence of saturating. If enhanced convection is the origin of the ring current, this poses a paradox. How can the polar cap potential, and thus convection, saturate when the ring current does not? We examine a possible explanation based on the reexamination of the Burton equation by Vasyliunas (2006). We show that this explanation is not a viable solution to the paradox since it would require a changing polar cap flux, and we demonstrate that the polar cap flux saturates (at around 1 GWb) as the polar cap potential saturates. Instead, we argue that during storms a quasi-steady reconnection region forms in the tail near the Earth. This reconnection region moves closer to the Earth for higher values of solar wind B s , although the polar cap potential, the dayside merging and nightside reconnection rates, and the amount of open flux do not change much as a function of B s once the polar cap potential has become saturated. As the neutral line moves closer, the volume per unit magnetic flux in the closed field line region is less. Flux tubes leaving the reconnection region in general have lower PV g as B s increases, and lower PV g flux tubes can penetrate deeper into the inner magnetosphere, leading to a corresponding greater injection of particles into the inner magnetosphere. Thus a reconnection region that is closer to Earth is more effective in creating a strong ring current. This leads to a continued dependence of the ring current injection rate on VB s , although the polar cap potential has saturated.

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“…During substorm expansion phase, sunward convection in the plasma sheet is enhanced primarily by the nightside reconnection driven by the dipolarization‐induced NEXL (near‐Earth X line) as proposed by Kan [2007]. During the storm main phase, sunward convection penetrating deeper into the inner magnetosphere is also driven by dipolarization‐induced NEXL as proposed by Kan et al [2007]. Noted that the dipolarization‐induced NEXL, formed within a few minutes after dipolarization, is proposed to drive the expanding auroral bulge during substorm expansion phase [ Kan , 2007].…”

Section: Formulation Of Sw‐m‐i Coupling Model For Polar Cap Potentialmentioning

confidence: 95%

Saturation of polar cap potential: Nonlinearity in quasi‐steady solar wind‐magnetosphere‐ionosphere coupling

Kan

Wang

et al. 2010

J. Geophys. Res.

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[1] We propose a quasi-steady nonlinear circuit model for the solar wind-magnetosphereionosphere (SW-M-I) coupling to study the observed saturation of polar cap potential. The oval conductance is shown to be a nonlinear circuit element since it increases with increasing dayside reconnection E field driving the proposed circuit. Oval conductance is produced by precipitating particles energized by enhanced sunward convection in the plasma sheet driven by reconnection at the dayside magnetopause and in the plasma sheet. The asymptotic saturation potential is shown to increase with (1) decreasing internal resistance of the dynamo region, (2) increasing length of dayside reconnection line, (3) increasing ratio of nightside to dayside reconnection potentials, and (4) increasing ratio of nightside to dayside internal resistances.

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Intensity of geomagnetic storms depends on the X‐line location in the plasma sheet

Cited by 3 publications

References 45 publications

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

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

Why doesn't the ring current injection rate saturate?

Saturation of polar cap potential: Nonlinearity in quasi‐steady solar wind‐magnetosphere‐ionosphere coupling

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