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- Title
Nodal Structure of Toroidal Standing Alfvén Waves and Its Implication for Field Line Mass Density Distribution.
- Authors
Takahashi, Kazue; Denton, Richard E.
- Abstract
We have conducted a statistical study of toroidal mode standing Alfvén waves detected by the Van Allen Probes spacecraft in the dayside inner magnetosphere, with an emphasis on the nodal structure of the fundamental through fifth harmonics. We developed a technique to accurately assign harmonic mode numbers to peaks in the power spectra of the electric (Eν) and magnetic (Bϕ) field components of toroidal waves and then determine the spectral intensities of Eν and Bϕ and the coherence and cross‐phase between these field components for each harmonic. The magnetic latitude (MLAT) dependence of these quantities was statistically examined to determine the location of the nodes. In addition to the equatorial nodes located close to the equator (MLAT = 0), we identified several nodes away from the equator within the MLAT range from −20° to +20°. We found that the Eν‐Bϕ cross‐phase is very close to ±90° except near the nodes, indicating that the fixed‐end approximation is appropriate in modeling dayside toroidal waves. Noting that the node latitudes depend on the distribution of the mass density (ρ) along the background magnetic field, we inferred the distribution from the nodes observed at L = 4–6. If we adopt a model field line mass density (ρ) distribution of the form ρ ∝ (1/r)α, where r is geocentric distance to the field line and α is a free parameter, the statistically determined node latitudes indicate that α∼1.5 is appropriate for both the plasmasphere and the plasmatrough. Plain Language Summary: Interaction of the solar wind plasma with the outer boundary of Earth's magnetosphere results in compressional magnetohydrodynamic waves in the 1–100 mHz band that propagate into the magnetosphere. Each geomagnetic field line is stimulated by the waves and executes its own discrete eigenmode oscillations because the field line is tied to the rigid ionosphere. The oscillations contain valuable information on how the plasma mass is distributed along the field line. Unlike the mass distribution on a guitar string, which is uniform, the mass distribution of geomagnetic field lines changes with distance from the ionosphere. The mass distribution affects various magnetospheric phenomena but is difficult to directly determine using particle experiments on spacecraft. Fortunately, we can estimate the mass distribution from the location of the nodes of the field line eigenmode oscillations, which can be determined using spacecraft observations of the electric and magnetic fields of the oscillations. In this study, we use data from the Van Allen Probes spacecraft to demonstrate that the node location can be determined for the fundamental through fifth harmonics and describe how this information is used to estimate the field line mass density distribution. Key Points: We determine the power and phase of the electric and magnetic fields of toroidal waves detected by spacecraftWe determine the node latitudes of five harmonics from the latitude dependence of the power and phaseWe compare the node latitudes with theoretical models to infer the field line mass distribution
- Subjects
PLASMA Alfven waves; PLASMA waves; VAN Allen radiation belts; SPACE vehicles; MAGNETOSPHERE
- Publication
Journal of Geophysical Research. Space Physics, 2021, Vol 126, Issue 5, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2020JA028981