Properties of Magnetohydrodynamic Normal Modes in the Earth's Magnetosphere.

The Earth's magnetosphere supports a variety of Magnetohydrodynamic (MHD) normal modes with Ultra Low Frequencies (ULF) including standing Alfvén waves and cavity/waveguide modes. Their amplitudes and frequencies depend in part on the properties of the magnetosphere (size of cavity, wave speed distribution). In this work, we use ∼13 years of Time History of Events and Macroscale Interactions during Substorms satellite magnetic field observations, combined with linearized MHD numerical simulations, to examine the properties of MHD normal modes in the region L > 5 and for frequencies <80 mHz. We identify persistent normal mode structure in observed dawn sector power spectra with frequency‐dependent wave power peaks like those obtained from simulation ensemble averages, where the simulations assume different radial Alfvén speed profiles and magnetopause locations. We further show with both observations and simulations how frequency‐dependent wave power peaks at L > 5 depend on both the magnetopause location and the location of peaks in the radial Alfvén speed profile. Finally, we discuss how these results might be used to better model radiation belt electron dynamics related to ULF waves. Plain Language Summary: The solar wind constantly disturbs plasma in the near‐Earth space environment on a broad range of frequencies. However, plasma waves in the Earth's magnetosphere, a region of space where the Earth's magnetic field plays a dominant role in shaping plasma dynamics, often exhibit standing wave structure with a narrow range of frequencies. In other words, the magnetosphere selects standing waves with discrete frequencies from drivers with a broadband frequency spectrum. These standing waves have properties that depend on the size of the magnetosphere and plasma wave speeds. In this study, we use a database of magnetic field measurements from the Time History of Events and Macroscale Interactions during Substorms satellites along with numerical simulations to isolate natural frequencies from noisy and variable driving conditions and extract standing wave spatial structure. We show how the standing wave properties change as the outer boundary of the magnetosphere and internal wave speeds change. We finally discuss how the properties of these standing waves might be used to improve space weather models. Key Points: Magnetohydrodynamic normal modes are identified using ∼13 years of observations and comparisons with numerical simulationsRadial Alfvén speed profile peaks outside 8 Earth radii significantly alter frequencies and spatial structure of normal modesFrequencies and nodal structure of cavity/waveguide modes vary with magnetopause location, with power peaks well inside the magnetopause [ABSTRACT FROM AUTHOR]