[P-EM11] Dynamics of the Inner Magnetospheric System

[PEM11-P12] Nightside ion mass density estimated by transient toroidal mode waves observed by the Arase satellite

*Mariko Teramoto¹, Masahito Nose², Ayako Matsuoka³, Yoshiya Kasahara⁴, Atsushi Kumamoto⁵, Fuminori Tsuchiya⁵, Yoshizumi Miyoshi², Masafumi Shoji², Satoko Nakamura², Masahiro Kitahara², Shun Imajo³, Iku Shinohara⁶ (1.Graduate school of engineering, Kyushu Institute of Technology, 2.ISEE, Nagoya Univeristy , 3.Graduate school of science, Kyoto University, 4.Graduate school of natural science and technology, Kanazawa University, 5.Graduate school of science, Tohoku University, 6.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)

Keywords:substorm, ultra-low frequency waves, Arase satellite

The mass density of cold plasma is a key parameter in the magnetosphere. The ion mass density in the magnetosphere controls the growth and evaluation of electromagnetic ion waves and the enhanced process of magnetospheric reconnection. Previous studies have evaluated the mass density in the inner magnetosphere with the magnetoseismic technique, using the toroidal mode ultra-low frequency waves observed by the satellite. In this technique, the wave equations are solved, which provides the relationship between the frequency of the toroidal waves and mass density. Although the magnetoseismic technique is commonly used for ion mass estimations, the method is applied for ULF waves observed by the equatorial-orbiting satellite for the dayside magnetosphere. This is because the toroidal mode ULF waves with harmonic mode structures are easily observed on the dayside by the equatorial-orbiting satellite. Few studies have investigated the distribution and evaluation of the ion mass density on the nightside using the magnetoseismeic technique.
Thus, we aimed to estimate the ion mass density and average ion mass on the nightside in the inner magnetosphere using the magnetoseismic technique and data from the several toroidal mode events observed on the nightside by the Arase satellite. Due to its unique inclination of ~31°, the Arase satellite can easily observe fundamental toroidal mode waves on the nightside off equator at substorm onset termed transient toroidal waves (TTWs). The waves can be excited simultaneously with the onset of Pi2 pulsations. TTWs can last a long time, for approximately 10–20 min, whereas Pi2 pulsations are damping poloidal waves. We investigated TTWs from the event list of Pi2 pulsations observed by the Arase satellite [Teramoto et al., 2022] and found that 59 TTWs events occurred on the nightside (06–18 MLT) between May 2017 to September 2019. To monitor the plasma mass density in the nightside magnetosphere during substorms, we applied the magnetoseismic technique for one of the TTW events on September 06, 2018, in which Arase was located at L~6.7–7.1 and 0.1–1.5 MLT outside the plasmasphere and a small substorm was excited. We determined the fundamental toroidal frequency of TTWs from the spectra of the azimuthal component of the magnetic field and derived the mass density from the observed toroidal frequency. We also determined the electron number density from the plasma wave experiment from Arase. We found that the average ion mass, which is derived from the electron and ion number densities, was 6.2–8.5 amu.