[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-12] A drift kinetic simulation of internally driven ULF waves based on multi-point spacecraft observations in the ionosphere and the magnetosphere

*Kazuhiro Yamamoto¹, Kanako Seki¹, Tomotsugu Yamakawa¹, Takanobu Amano¹, Aoi Nakamizo², Yoshizumi Miyoshi³ (1.Graduate School of Science, The University of Tokyo, 2.National Institute of Information and Communications Technology, 3.Institute for Space-Earth Environmental Research, Nagoya University)

Keywords:ULF wave, ring current, simulation, drift bounce resonance

The ultra-low frequency (ULF) waves with wave periods of >10 s are MHD mode waves and are commonly observed in the magnetosphere of the planets of our solar system. These waves obtain their energy from the solar wind or local plasma instability, and then they are amplified as standing waves along a field line or inside a plasma cavity. Therefore, ULF waves are essentially related to the coupled system of the magnetosphere, plasmasphere, and ionosphere/conductive surface of the planet. Many studies proposed that ULF waves can accelerate charged particles of Earth’s outer radiation belt and ring current, but it is required to reveal the global distribution of ULF waves for the evaluation of the effectiveness of the wave-particle interaction, which may be different among the planets with different coupled systems. To understand the ULF wave excitation, a numerical simulation study using a global ring current model (Amano et al., 2011) and an ionospheric potential solver (Nakamizo et al., 2012) was conducted (Yamakawa et al., 2022). In the simulation, poloidal mode waves were excited by ring current ions initially placed at the outer boundary of the simulation box under a constant ionospheric Region 1 field-aligned current (R1FAC), which drives magnetospheric convection and plasma transport. However, the setting of the simulation should be considered carefully when we compare a simulation result with observation and evaluate the capability of the model because the setting of spatial and velocity distributions of ions and variations of R1FAC change the simulation result.

In this study, we focus on the scenario of the excitation of second harmonic poloidal waves observed by Van Allen Probes in Earth’s inner magnetosphere on 29 October 2013. We used the data obtained from LANL and Iridium satellite measurements of the ring current ions at the geosynchronous orbit and the field-aligned current in the ionosphere to set realistic simulation inputs. Using the coupled model of Amano et al. (2011) and Nakamizo et al. (2012), we conducted a drift kinetic simulation of the ring current and electromagnetic field evolution based on the observational data set. Since higher energy ions drift westward, the velocity distribution of ion phase space density measured by the LANL satellite was not uniform for the magnetic local time (MLT). We set the kappa parameter of the ions at the outer boundary as a function of MLT. There was a moderate substorm ~3 hours before the poloidal wave detection. R1FAC density increased to ~1.7 uA/m² during the substorm expansion phase and then gradually decreased to <0.3 uA/m², according to Iridium satellite observations. We obtained the temporal function of R1FAC by the gaussian fitting of the data. From the inhomogeneous and temporal inputs, we found ULF oscillations near the location and timing of the Van Allen Probes observation in our simulation result. However, the waves were fundamental compressional modes. In this presentation, we will discuss the agreement and disagreement of the simulation results to reveal the importance of temporal and spatial variations of the ionospheric and magnetospheric conditions, which were simplified in the previous simulation studies.