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Nodal Structure of Toroidal Standing Alfvén Waves and Its Implication for Field Line Mass Density Distribution


AuthorTakahashi, Kazue; Denton, Richard;
KeywordsToroidal Alfven waves; inner magnetosphere; Nodal structure; Field line mass density distribution; Van Allen Probes
AbstractAbstract 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.
Year of Publication2021
JournalJournal of Geophysical Research: Space Physics
Volumen/a
Number of Pagese2020JA028981
Section
Date Published03/2021
ISBN
URLhttps://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JA028981
DOIhttps://doi.org/10.1029/2020JA028981