How a realistic magnetosphere alters the polarizations of surface, fast magnetosonic, and Alfv\'en waves

TL;DR

This study uses global MHD simulations to show how realistic Earth's magnetosphere affects the behavior and detection of surface, fast magnetosonic, and Alfvén waves, revealing significant differences from simplified models.

Contribution

It demonstrates the impact of realistic magnetospheric features on wave properties and detection, highlighting the limitations of traditional box models and proposing improved analysis methods.

Findings

Large-scale wave predictions are significantly altered in realistic magnetospheres.

Reversal of radial ordering of wave resonances can occur.

Detection techniques may fail to identify standing waves in complex environments.

Abstract

System-scale magnetohydrodynamic (MHD) waves within Earth's magnetosphere are often understood theoretically using box models. While these have been highly instructive in understanding many fundamental features of the various wave modes present, they neglect the complexities of geospace such as the inhomogeneities and curvilinear geometries present. Here we show global MHD simulations of resonant waves impulsively-excited by a solar wind pressure pulse. Although many aspects of the surface, fast magnetosonic (cavity/waveguide), and Alfv\'en modes present agree with the box and axially symmetric dipole models, we find some predictions for large-scale waves are significantly altered in a realistic magnetosphere. The radial ordering of fast mode turning points and Alfv\'en resonant locations may be reversed even with monotonic wave speeds. Additional nodes along field lines that are not present in the displacement/velocity occur in both the perpendicular and compressional components of the magnetic field. Close to the magnetopause the perpendicular oscillations of the magnetic field have the opposite handedness to the velocity. Finally, widely-used detection techniques for standing waves, both across and along the field, can fail to identify their presence. We explain how all these features arise from the MHD equations when accounting for a non-uniform background field and propose modified methods which might be applied to spacecraft observations.