Abstract
AbstractThe 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.
Highlights
The interaction between the solar wind and the magnetosphere drives a highly complex dynamical system, affecting conditions in the near‐space environment and on the ionosphere
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
One description which attempts to predict the X line location on the magnetopause is antiparallel reconnection, in which reconnection occurs where the shear angle is largest (Crooker, 1979). This angle depends on the relative orientation of the interplanetary magnetic field (IMF) and the planetary dipole axis; the angle that the latter makes with the Z axis of the Geocentric Solar Magnetic (GSM) coordinate system is the “dipole tilt angle” and varies annually between ±34° (Hapgood, 1992)
Summary
The interaction between the solar wind and the magnetosphere drives a highly complex dynamical system, affecting conditions in the near‐space environment and on the ionosphere. Reconnection occurs between the planetary magnetic field and the interplanetary magnetic field (IMF) in the solar wind, opening “closed” magnetospheric field and driving large‐scale convective flows (Dungey, 1961). One description which attempts to predict the X line location on the magnetopause is antiparallel reconnection, in which reconnection occurs where the shear angle is largest (Crooker, 1979). This angle depends on the relative orientation of the IMF and the planetary dipole axis; the angle that the latter makes with the Z axis of the Geocentric Solar Magnetic (GSM) coordinate system is the “dipole tilt angle” (denoted by μ) and varies annually between ±34° (Hapgood, 1992). Introducing an arbitrary and nonzero IMF By component splits the reconnection line at noon, with antiparallel regions in each of the dawn and dusk hemispheres
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