Buoyancy Waves in Earth's Magnetosphere: Calculations for a 2‐D Wedge Magnetosphere

• Published in: Journal of geophysical research. Space physics 2018-05, Vol.123 (5), p.3548-3564
• Main Authors: Wolf, R. A., Toffoletto, F. R., Schutza, A. M., Yang, J.
• Format: Article
• Language: English
• Subjects: Atmosphere; Brake presses; Braking; braking oscillations; Buoyancy; Bursting strength; bursty bulk flows; Computational fluid dynamics; Configurations; Differential equations; Earth; Earth magnetosphere; Extremely low frequencies; Fluid flow; Gravitation; magnetic buoyancy; Magnetic fields; Magnetic properties; Magnetohydrodynamic waves; Magnetohydrodynamics; Magnetosphere; Magnetotails; Mathematical models; Oscillations; Plasmasphere; Propagation; Rapid flow; Refraction; Space plasmas; ULF waves; Wave equations; Wave propagation; Wedges
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2017JA025006

To improve theoretical understanding of the braking oscillations observed in Earth's inner plasma sheet, we have derived a theoretical model that describes k∥ = 0 magnetohydrodynamic waves in an idealized magnetospheric configuration that consists of a 2‐D wedge with circular‐arc field lines. The low‐frequency, short‐perpendicular‐wavelength mode obeys a differential equation that is often used to describe buoyancy oscillations in a neutral atmosphere, so we call those waves “buoyancy waves,” though the magnetospheric buoyancy force results from magnetic tension rather than gravity. Propagation of the wave is governed mainly by a position‐dependent frequency ωb, the “buoyancy frequency,” which is a fundamental property of the magnetosphere. The waves propagate if ωb > ω but otherwise evanesce. In the wedge magnetosphere, ωb turns out to be exactly the fundamental oscillation frequency for poloidal oscillations of a thin magnetic filament, and we assume that the same is true for the real magnetosphere. Observable properties of buoyancy oscillations are discussed, but propagation characteristics vary considerably with the state of the magnetosphere. For a given event, the buoyancy frequency and propagation characteristics can be determined from pressure and density profiles and a magnetic field model, and these characteristics have been worked out for one typical configuration. A localized disturbance that initially resembles a dipolarizing flux bundle spreads east‐west and also penetrates into the plasmasphere to some extent. The calculated amplitude near the center of the original wave packet decays in a few oscillation periods, even though our calculation includes no dissipation. Plain Language Summary Plasma in the near‐Earth region of space exhibits many kinds of ultralow‐frequency waves. The present paper deals with a specific class of space plasma ultralow‐frequency waves that have unique properties and have not been much studied. They can be called “buoyancy waves,” because they are mathematically equivalent to buoyancy waves in a neutral atmosphere, but the buoyancy force in near‐Earth space plasmas is due to magnetic tension rather than gravity. This paper develops a theory of near‐Earth space buoyancy waves, by considering a simplified geometry for which the wave equations can be solved analytically. The frequency and propagation characteristics of the waves are determined mainly by a parameter called the “buoyancy frequency,” which can be calcula