Simulations of the magnetosphere for zero interplanetary magnetic field: The ground state

Abstract

A global MHD simulation code, the Integrated Space Weather Prediction Model, is used to examine the steady state properties of the magnetosphere for zero interplanetary magnetic field. In this “ground state” of the system, reconnection at the magnetopause is absent. Topics reported here include (1) qualitative description of global magnetic field, plasma flow, and current systems (Chapman‐Ferraro, geotail, Region 1 and Region 2 currents) (2) quantitative parametric studies of shock jump conditions, magnetopause and shock standoff distance, polar cap voltage, and total Region 1 current for different solar wind speeds and ionospheric Pedersen conductances; and (3) quantitative analysis of the low‐latitude boundary layer (LLBL) and its coupling to the ionosphere. The central part of the geomagnetic tail is found to be very long, extending beyond the downstream end of the simulation box at X = −300 R E . Along each flank a “wing‐like” region containing closed, albeit strongly stretched, field lines is present. Each such region contains a narrow convection cell, consisting of the tailward flowing LLBL and an adjoining narrow channel of sunward return flow. These cells are the result of viscous‐like interaction along the magnetospheric flanks, with an effective kinematic viscosity, entirely of numerical origin, estimated to be ν = 1.8 × 10 8 m 2 s −1 . Except in certain regions near the magnetopause, the magnetosheath flow is steady and laminar while the internal motion in the tail displays turbulent vortical motion in the plasma sheet. Plasma transport in the tail occurs as a result of this turbulence, and substantial turbulent plasma entry across the equatorial magnetopause is seen in the region −10 R E < X < 0 R E behind the torus of dipolar field lines. The polar cap potential Δ ϕPC is 29.9 ± 1.4 kV for V SW = 400 km s −1 and ∑ P = 6 mho, which is in reasonable agreement with results inferred from satellite observations. About half of Δ ϕPC can be attributed to the LLBLs with the remainder coming from a dawn‐to‐dusk potential drop along the dayside magnetopause, caused by nonlinearly switched resistivity, added explicitly to the MHD equations, and/or by numerical diffusion. The magnetospheric voltage‐current relation at V SW = 400 km s −1 has a constant negative slope with an open circuit voltage of Δ ϕPC = 38.5kV. The total Region 1 current (into the northern dawn hemisphere) is 0. 66 MA (at V SW = 400 km s −1 and ∑ P = 6 mho). It maximizes at about 2. 83 MA during short‐circuit conditions (∑ P = ∞; Δ ϕPC = 0).