Coupling of a global MHD code and an inner magnetospheric model: Initial results

Abstract

[1] This paper describes the coupling of BATS-R-US (Block Adaptive Tree Solar-wind Roe-type Upwind Scheme), a magnetohydrodynamics (MHD) code representing the Earth's global magnetosphere and its coupling to the ionosphere and solar wind, and the Rice Convection Model (RCM), which represents the inner magnetosphere and its coupling to the ionosphere. The MHD code provides a time-evolving magnetic field model for the RCM as well as continuously updated boundary conditions for the electric potential and plasma. The RCM computes the distribution functions of inner magnetospheric particles, including transport of inner plasmasheet and ring current particles by gradient/curvature drifts; it thus calculates more accurate inner magnetospheric pressures, which are frequently passed to the MHD model and used to nudge the MHD values. Results are presented for an initial run with the coupled code for the case of uniform ionospheric conductance with steady solar wind and southward interplanetary magnetic field (IMF). The results are compared with those for a run of the MHD code alone. The coupled-code run shows significantly higher inner magnetospheric particle pressures. It also exhibits several well-established characteristics of inner magnetospheric electrodynamics, including strong region-2 Birkeland currents and partial shielding of the inner magnetosphere from the main force of the convection electric field. A sudden northward turning of the IMF causes the ring current to become more nearly symmetric. The inner magnetosphere exhibits an overshielding (dusk-to-dawn) electric field that begins about 10 min after the northward turning reaches the magnetopause and lasts just over an hour.

Simulation of the interchange instability in a magnetospheric substorm site

“…Then, it is very reasonable to suggest that the relaxation of these pressure gradients after the northward turning of B z IMF and the reduction in the sunward convection, occurs on the same time scale, inferring the persistence of the shielding (dusk-to-dawn) electric field for about an hour. The validity of this suggestion has been proved in the computer simulation of De Zeeuw et al (2004). Using the RCM MHD code, the authors showed that about 10 min after the northward turning of the IMF reaches the magnetopause the inner magnetosphere exhibits an overshielding electric field which lasts just over an hour.…”

“…On the other hand, the Region 2-type FACs, which arise in the nighttime near-Earth tail during the growth phase (Fig. 2), are related to the pressure gradients developing here and thus have much a larger relaxation time of ∼1 h (Rezhenov et al, 1980;De Zeeuw et al, 2004). During this period they solely control the convection in the inner magnetosphere.…”

“…We complete this section with the reference to the study of De Zeeuw et al (2004), in which the physics of duskto-dawn electric field formation in the inner magnetosphere in response to B z IMF turning northward (a so-called overshielding effect) is considered in detail.…”

“…As mentioned above, this is the potential of the remnant polarization electric field, or the "overshielding" field, as termed by De Zeeuw et al (2004), persisting in the inner magnetosphere after a northward swing of B z IMF and a reduction in the earthward convection. The contours of ϕ(x, y) shown in Fig.…”

The transition to overshielding after sharp and gradual interplanetary magnetic field northward turning

“…Fejer et al [1990] proposed that the magnetic field reconfiguration of the magnetosphere due to a northward turning and the associated shrinkage of the polar cap may effectively prolong the overshielding time period. This effect has been confirmed by the Rice Convection Model (RCM) with a time-dependent magnetic field model [Sazykin, 2000;De Zeeuw et al, 2004;Maruyama et al, 2007;Zhang et al, 2009] and has been discussed with observations [Wei et al, 2008b[Wei et al, , 2009. On the other hand, the IMF northward turning is preferable to trigger substorms [Lyons et al, 1997], but the effect of substorm on overshielding, reduction [e.g., Huang et al, 2004;Huang, 2009] or enhancement [Wei et al, 2009;Zhang et al, 2009] is still a controversial issue.…”