The Effect of Fast Normal Mode Structure and Magnetopause Forcing on FLRs in a 3‐D Waveguide. Issue 1 (17th January 2019)

• Record Type: Journal Article
• Authors: Elsden, T.; Wright, A. N.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 1(2019)
• Page Start: 178
• Page End: 196
• Publication Date: 2019-01-17
• Subjects: ULF waves -- field line resonance -- waveguide modes -- magnetosphere -- MHD waves; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2018JA026222
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: This paper investigates the excitation of waveguide modes in a nonuniform dipole equilibrium and, further, their coupling to field line resonances (FLRs). Waveguide modes are fast compressional ultralow frequency (ULF) waves, whose structure depends upon the magnetospheric equilibrium and the solar wind driving conditions. Using magnetohydrodynamic simulations, we consider how the structure of the excited waveguide mode is affected by various forms of magnetopause driving. We find that the waveguide supports a set of normal modes that are determined by the equilibrium. However, the particular normal modes that are excited are determined by the structure of the magnetopause driver. A full understanding of the spatial structure of the normal modes is required in order to predict where coupling to FLRs will occur. We show that symmetric pressure driving about the noon meridian can excite normal modes which remain around to drive resonances for longer than antisymmetric driving. Further, the critical quantity in terms of efficient coupling is the magnetic pressure gradient aligned with the resonance. Plain Language Summary: Earth's magnetic field is constantly being disturbed by the outflow from the Sun (the solar wind). Such disturbances generate periodic oscillations of the Earth's magnetic field, which can be measured by instruments on the ground and in space. These oscillations (waves) have been shown to have an impact on the aurora and the motion of energetic particles within the radiation belts (regions of trapped energetic particles around the Earth). In this study, we use a computational model of the Earth's magnetic field to investigate how these waves propagate. We consider various ways of driving the system, together with different density distributions and measure the resulting wave response. We identify that the system supports what are called "fast normal modes, " which are the natural waves of the system. These can be thought of in the same way as blowing across an open bottle, which will produce a specific frequency of sound. We discuss how the structure of these modes affects how energy propagates and accumulates throughout the Earth's magnetic environment. Key Points: Waveguide normal mode structure is determined by the equilibrium Structure of waveguide normal modes is key in determining coupling efficiency and resonance location Magnetopause driver determines which normal modes are excited and hence the FLR locations