Magnetopause ripples going against the flow form azimuthally stationary surface waves

M O Archer,M D Hartinger,D J Southwood,F Plaschke,L Rastaetter

doi:10.1038/s41467-021-25923-7

Abstract

Surface waves process the turbulent disturbances which drive dynamics in many space, astrophysical and laboratory plasma systems, with the outer boundary of Earth’s magnetosphere, the magnetopause, providing an accessible environment to study them. Like waves on water, magnetopause surface waves are thought to travel in the direction of the driving solar wind, hence a paradigm in global magnetospheric dynamics of tailward propagation has been well-established. Here we show through multi-spacecraft observations, global simulations, and analytic theory that the lowest-frequency impulsively-excited magnetopause surface waves, with standing structure along the terrestrial magnetic field, propagate against the flow outside the boundary. Across a wide local time range (09–15h) the waves’ Poynting flux exactly balances the flow’s advective effect, leading to no net energy flux and thus stationary structure across the field also. Further down the equatorial flanks, however, advection dominates hence the waves travel downtail, seeding fluctuations at the resonant frequency which subsequently grow in amplitude via the Kelvin-Helmholtz instability and couple to magnetospheric body waves. This global response, contrary to the accepted paradigm, has implications on radiation belt, ionospheric, and auroral dynamics and potential applications to other dynamical systems.

Highlights

Surface waves process the turbulent disturbances which drive dynamics in many space, astrophysical and laboratory plasma systems, with the outer boundary of Earth’s magnetosphere, the magnetopause, providing an accessible environment to study them
The motion of the outer boundary of a magnetosphere, the magnetopause, is of primary importance in dictating global magnetospheric dynamics since it controls the flow of mass, energy and momentum from the solar wind into the terrestrial system having direct and indirect space weather impacts on the radiation belts, auroral oval and ionosphere[7,8,9]
We use Time History of Events and Macroscale Interactions during Substorms (THEMIS)[36] spacecraft observations from satellites A–E (THA–THE) during the previously reported event of magnetopause surface eigenmode (MSE) triggered by a magnetosheath jet on August 7, 2007

Summary

Introduction

Surface waves process the turbulent disturbances which drive dynamics in many space, astrophysical and laboratory plasma systems, with the outer boundary of Earth’s magnetosphere, the magnetopause, providing an accessible environment to study them. Magnetopause surface waves are thought to travel in the direction of the driving solar wind, a paradigm in global magnetospheric dynamics of tailward propagation has been well-established. Further down the equatorial flanks, advection dominates the waves travel downtail, seeding fluctuations at the resonant frequency which subsequently grow in amplitude via the Kelvin-Helmholtz instability and couple to magnetospheric body waves This global response, contrary to the accepted paradigm, has implications on radiation belt, ionospheric, and auroral dynamics and potential applications to other dynamical systems. Surface waves have been observed and modelled within tokamak experiments[1], plasma tori surrounding planets[2], the solar atmosphere (e.g. in coronal loops3), the heliopause[4], accretion disks[5] and astrophysical/relativistic jets[6] to name a few This makes understanding surface waves of universal importance. The models predict an exception, in agreement with several observations, only at the early post-noon magnetopause, since pressure fronts aligned with the Parker spiral interplanetary magnetic field (IMF)

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