7 Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya
thGlobal drift kinetic simulations of internally driven ULF waves in the Earth’s
inner magnetosphere
T. Yamakawa , K. Seki , T. Amano , Y. Miyoshi , N. Takahashi , A. Nakamizo , and K. Yamamoto
School of Science, University of Tokyo
Institute for Space-Earth Environmental Research, Nagoya University
Applied Electromagnetic Research Institute, National Institute of Information andCommunications Technology
e-mail (speaker): yamakawa@eps.s.u-tokyo.ac.jp
11123

3

1

12

3

Ultra-low frequency (ULF) waves are electromagnetic
pulsations that are widely observed in the magnetosphereand the ionosphere. The excitation mechanism andglobal distribution of ULF waves are keys to understandthe variation of the outer radiation belt, since ULF waves
in the Pc5 range (1.6-6.7 mHz) can drive radial transportof radiation belt electrons (MeV).
ULF waves can be generated by ring current ionsassociated with the injection from the plasma sheet
during substorms, which are called as internally driven
ULF waves. Theoretically, Southwood (1976) [1]
proposed that drift-bounce resonance was a candidate
excitation mechanism of ULF waves. Previousspacecraft observations suggested the excitation ofinternally driven ULF waves through the drift-bounce
resonance. Previous studies of ULF waves assumed thesimplified situations such as locally homogeneous
plasmas and where and how ULF waves are generated
by ring current ions in the inner magnetosphere are far
from understood.
In this study, we have investigated the excitation ofULF waves based on the magnetosphere-ionosphere
coupled model. In the magnetosphere, we used
GEMSIS-RC model [2], in which 5-D drift-kinetic
equation for ion phase space density (PSD) and Maxwell
equations are solved self-consistently under theassumption that the first adiabatic invariant is conserved.
GEMSIS-RC model is capable of describing theexcitation and propagation of ULF waves, since themodel can assume the force-imbalanced state. In theionosphere, GEMSIS-POT model [3] was used, which isa global 2-D potential solver. The coupled model enabled
us to simulate ion injection from the plasma sheet by theconvection electric field. In order to investigate theeffects of the dynamics of cold plasma on the excitationof ULF waves, we also implemented the simulation code
to describe the dynamics of cold plasma into
GEMSIS-RC model and compared simulation results
between the case of constant density (Case a) [4] and theinclusion of the dynamics of cold plasma (Case b).
Simulation results showed the excitation of two types ofPc5 ULF waves in both cases. Figure 1 shows the global
distribution of two types of Pc5 waves in Case a. First,
we found the drift resonance excitation of Pc5 waves on
the dayside. The waves were driven by positive energy
gradient of ion PSD with the energy of 50-120 keV.
Second, Pc5 waves were excited by the drift-bounce
resonance on the duskside. The waves were driven by

inward gradient of ion PSD with the energy of 50-80 keV.
In Case b, by including the dynamics of cold plasma, theplasmapause was formed on the nightside. Simulation
results suggested that ULF waves were also generated on
the nightside and dawnside near the density gradient inaddition to two types of Pc5 waves. We will also reporton the effects of the dynamics of cold plasma on theexcitation of ULF waves and future plans about thesimulation of ULF waves of weakly magnetized planets.

Figure 1. Global distribution of poloidal wave power
spectra of two types of Pc5 waves in the plane with
MLAT = 1° in Case a.
References[1] Southwood (1976). A general approach to
low-frequency instability in the ring current plasma. J.
Geophy. Res., 81(19), 3340-3348,
doi:10.1029/JA081i019p03340.
[2] Amano et al. (2011). Self-consistent kinetic
numerical simulation model for ring current particles inthe Earth’s inner magnetosphere. J. Geophy. Res., 116,A02216, doi:10.1029/2010JA015682.
[3] Nakamizo et al. (2012). Effect of R2-FAC
development on the ionospheric electric field patterndeduced by a global ionospheric potential solver. J.
Geophy. Res., 117(A9), A09231,
doi:10.1029/2012JA017669.
[4] Yamakawa et al. (2022). Excitation of two types ofstorm-time Pc5 ULF waves by ring current ions based on
the magnetosphere-ionosphere coupled model. J. Geophy.
Res., 127, e2022JA030486, doi:10.1029/2022JA030486.