Dipole Tilt Effect on Magnetopause Reconnection and the Steady‐State Magnetosphere‐Ionosphere System: Global MHD Simulations

Abstract

The Earth's dipole tilt angle changes both diurnally and seasonally and introduces numerous variabilities in the coupled magnetosphere‐ionosphere system. By altering the location and intensity of magnetic reconnection, the dipole tilt influences convection on a global scale. However, due to the nonlinear nature of the system, various other effects like dipole rotation, varying interplanetary magnetic field (IMF) orientation, and nonuniform ionospheric conductance can smear tilt effects arising purely from changes in coupling with the solar wind. To elucidate the underlying tilt angle dependence, we perform magnetohydrodynamic (MHD) simulations of the steady‐state magnetosphere‐ionosphere system under purely southward IMF conditions for tilt angles from 0–90°. We identify the location of the magnetic separator in each case and find that an increasing tilt angle shifts the 3‐D X line southward on the magnetopause due to changes in magnetic shear angle. The separator is highly unsteady above 50° tilt angle, characteristic of regular flux transfer event (FTE) generation on the magnetopause. The reconnection rate drops as the tilt angle becomes large, but remains continuous across the dayside such that the magnetosphere is open even for 90°. These trends map down to the ionosphere, with the polar cap contracting as the tilt angle increases, and region I field‐aligned current (FAC) migrating to higher latitudes with changing morphology. The tilt introduces a north‐south asymmetry in magnetospheric convection, thus driving more FAC in the Northern (sunward facing) hemisphere for large tilt angles than in the Southern independent of conductance. These results highlight the strong sensitivity to onset time in the potential impact of a severe space weather event.

Interplanetary Shock‐Induced Magnetopause Motion: Comparison Between Theory and Global Magnetohydrodynamic Simulations

“…Gorgon is a 3‐D simulation code with resistive MHD and hydrodynamic capabilities, originally developed to study high‐energy‐density laboratory plasmas (Chittenden et al., 2004; Ciardi et al., 2007). Gorgon has been adapted and applied to planetary magnetospheres in several contexts, including: the inclined and rotating Neptunian magnetosphere (Mejnertsen et al., 2016), the variable motion of the terrestrial bow shock (Mejnertsen et al., 2018), and the effects of dipole‐tilt on terrestrial magnetopause reconnection and ionospheric current systems (Eggington et al., 2020).…”

Control of Magnetopause Flux Rope Topology by Non-local Reconnection

“…In this work, we simulate the magnetosphere using the Gorgon 3D magnetohydrodynamic code. Gorgon was initially developed for studying high energy, collisional plasma interactions such as Z-pinches (Chittenden et al, 2004;Jennings, 2006;Jennings et al, 2010), laser-plasma interactions (Smith et al, 2007) and magnetic tower jets (Ciardi et al, 2007), but has recently been adapted to simulate planetary magnetospheres and their interaction with the solar wind (Desai et al, 2021;Eggington et al, 2020;Mejnertsen et al, 2016Mejnertsen et al, , 2018.…”

Drift Orbit Bifurcations and Cross-field Transport in the Outer Radiation Belt: Global MHD and Integrated Test-Particle Simulations

“…The resistive MHD simulation code, Gorgon, was developed for the study of high-energy-density laboratory plasmas [Chittenden et al, 2004, Ciardi et al, 2007 and has since been adapted to the study of the solar wind-magnetosphere interaction [Mejnertsen et al, 2016, Eggington et al, 2020, Desai et al, 2021. Gorgon is differentiated from other global MHD codes in that it solves for the vector potential on a staggered grid and automatically conserves the divergence of the magnetic field, ∇ • B = 0, to machine precision without the need for an additional divergence cleaning operation.…”

“…In this study we use the global MHD model, Gorgon , Eggington et al, 2020, Desai et al, 2021, and integrated test-particle simulations to demonstrate that this modelling approach is capable of capturing and further studying DOBs. Section 2 first describes the global MHD simulations and outlines the resolution requirements of ≈1/4 R E as necessary to properly resolve the non-dipolarity in the locally enhanced magnetic fields near the sub-solar point.…”