7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Excitation of ULF waves and transport of plasma through wave-particle
interaction in the Earth’s magnetosphere

Kazuhiro Yamamoto1, A.V. Rubtsov2, D. V. Kostarev2, P. N. Mager2, D. Yu. Klimushkin2, M. Nosé3,A. Matsuoka4, K. Asamura5, Y. Miyoshi6, S. Yokota7, S. Kasahara1, Y. Kasahara8, A. Kumamoto9, F.
Tsuchiya9, M. Shoji6, S. Nakamura6, and I. Shinohara5
1Graduate School of Science, The University of Tokyo, 2 Institute of Solar-Terrestrial Physics SBRAS, 3 School of Data Science, Nagoya City University, 4 Graduate School of Science, Kyoto
University, 5 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 6Institute for Space-Earth Environmental Research, Nagoya University, 7 Graduate School ofScience, Osaka University, 8 Graduate School of Natural Science and Technology, Kanazawa
University, 9 Graduate School of Science, Tohoku University
e-mail (speaker): yamamoto.kazuhiro@eps.s.u-tokyo.ac.jp
We examined the Arase satellite data on 19th
November 2018. Firstly, electromagnetic fieldoscillations of poloidal Pc 5 (~4.4 mHz) waves weredetected from 02:40 to 03:40 UT at R = 5.7-6.0 RE
around the magnetic equator on the duskside, where R is
a radial distance from the center of the Earth, RE is theEarth’s radius, and UT is the universal time. From thelarge amplitude ratio between the radial magnetic field
and the azimuthal electric field, these waves are thesecond harmonic mode of the standing Alfvén wave,
because the second harmonic wave has its node ofelectric field oscillations around the equator. From thefinite Larmor radius effect [1], we estimated that thepoloidal waves propagate eastward with a high wave
number (m ~ 220).
At the same time, earthward penetration of energetic
protons was observed at ~3-30 keV. The energy andspatial distributions of energetic ions are formed by theions transported from the Earth’s plasmasheet and arecalled “nose structure”. There were two energy bands ofthe nose structure at ~5 keV and ~20 keV. In the pitch
angle (α) distributions of protons, proton fluxes at 5-25
keV oscillated with the frequency same as the poloidalwaves. Proton fluxes of the bouncing population (α ~30/150 degrees) were coherently modulated by thewaves. Proton fluxes at α > 90 degrees show antiphaseoscillations for those at α < 90 degrees. These
characteristics imply that strong interaction of protons
and the second harmonic poloidal standing Alfvén wave.
The destabilization condition of the drift-bounce
resonance is summarized as df/dW =∂f/∂W+(dL/dW)∂f/∂L > 0, where f is the distributionfunction, W is particle kinetic energy, and L is the Lshell, which represents the radial distance of a field line
at the equator. This equation is similar to thedestabilization condition of toroidal Alfvén eigenmodes(TAE) in the laboratory plasma. From this equation, wefound that waves were excited by a positive radial
gradient of the proton distribution function if the waves
propagate eastward.
After the poloidal wave excitation, the Arase satellite
moved into a region where β reached up to ~1. In thisregion, compressional Pc 4-5 waves were observed.
Proton fluxes at 20-30 keV around α = 90 degrees
coherently oscillated, indicating that equatoriallybouncing protons are related to the generation ofcompressional waves. These flux oscillations correspondto the drift resonance with westward propagating waves
with azimuthal wave numbers |m| of 120–160, which isin agreement with observational values.
We calculated the anisotropy parameter Γ for the drift
mirror mode instability. Γ was lower than −0.5,
excluding drift-mirror mode interpretation. Based on therecent theoretical studies [2,3], we calculated theeigenfrequency and pressure perturbations for the drift
compressional mode. The calculated values were close to
the observed compressional waves, suggesting that theobserved compressional wave is the drift compressionalmode. The instability condition for the drift resonance
was satisfied by a negative radial gradient of the proton
phase space density. In addition, an outward proton
temperature gradient is sometimes positive. This result
suggests that both the drift resonance and the ion
temperature gradient instability can provide free energy
to the wave. Similar drift waves can be seen in thelaboratory plasma and it is considered to cause a radial
diffusion of the plasma. In the magnetospheric plasma, aradial diffusion leads to the decay of the ring current,
which is one of the global current systems in themagnetosphere and changes the magnetic field strength
on the surface of the Earth.
References[1] Su, S.-Y., Konradi, A., & Fritz, T. A. (1977). On
propagation direction of ring current proton ULF waves
observed by ATS 6 at 6.6 RE. Journal of GeophysicalResearch, 82 (13), 1859–1868. doi:
10.1029/JA082i013p01859[2] Mager, P. N., Klimushkin, D. Yu., & Kostarev, D. V.
(2013). Drift-compressional modes generated by inverted
plasma distributions in the magnetosphere. Journal ofGeophysical Research: Space Physics, 118, 4915–4923.
doi: 10.1002/jgra .50471[3] Takahashi, K., Crabtree, C., Ukhorskiy, A. Y., Boyd,A., Denton, R. E., Turner, D., . . . Spence, H. E. (2022).
Van Allen Probes observations of symmetric stormtime
compressional ULF waves. Journal of GeophysicalResearch: Space Physics, 127 (2), e2021JA030115. doi:
10.1029/2021JA030115