Nodal Structure of Toroidal Standing Alfvén Waves and Its Implication for Field Line Mass Density Distribution
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Abstract 
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 crossphase 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 EνBϕ crossphase is very close to ±90° except near the nodes, indicating that the fixedend 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 Publication 
2021

Journal 
Journal of Geophysical Research: Space Physics

Volume 
n/a

Number of Pages 
e2020JA028981

Date Published 
03/2021

URL 
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JA028981

DOI 
https://doi.org/10.1029/2020JA028981
