[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-P16] Analysis of linear MHD waves in a curvilinear background magnetic field using local frame and its application to ULF waves in the inner magnetosphere

*Ko Isono¹, Akimasa Yoshikawa², Yuto Katoh¹, Yohei Kawazura^3,1, Atsushi Kumamoto¹ (1.Department of Geophysics, Graduate School of Science, Tohoku University, 2.Department of Earth and Planetary Sciences, Kyushu University, 3.Frontier Research Institute for Interdisciplinary Sciences, Tohoku University)

Keywords:ULF waves, Plasma physics

ULF waves stand for ultra-low frequency waves, which are low-frequency oscillations observed in the Earth's inner magnetosphere, specifically, oscillations with periods ranging from a few seconds to a few hundred seconds. The excitation and propagation processes of ULF waves have been extensively studied. Field line resonance (FLR) is a process in which Alfvén waves resonate with fast modes that are excited at the magnetopause and propagate toward the inner magnetosphere. This process excites ULF waves in the inner magnetosphere and explains field line oscillations associated with the Pc4-5 phenomenon (Dungey, 1954; Chen & Hasegawa, 1974; Southwood, 1974). This is one of the most important processes in the excitation of ULF waves whose energy source is outside the magnetosphere; the FLR process has been extensively studied theoretically in cartesian, cylindrical, and dipole coordinate systems (e.g., Mann et al, 1995; Allan et al, 1986; Wright & Elsden, 2016). Previous studies revealed that the regions where fast modes and Alfvén waves resonate are formed by the spatial variation of the Alfvén speed due to the plasma mass density distribution, strength of the magnetic field, and field line length in the inner magnetosphere. One of the factors that have been focused on in the FLR process is the geometry of the background magnetic field. Using the MHD equations in the field-aligned coordinate system, Wright & Elsden (2016) theoretically showed that dipole scaling factors have a significant influence on the conditions of Alfvén wave generation and polarization characteristics produced by FLR.

We apply an analytical method of Yoshikawa (JpGU, 2023) to investigate the role of the geometrical characteristics of the background magnetic field geometry, i.e., the curvature and torsion of the field lines, on generating FLR in an arbitrary background magnetic field geometry. The method uses a local frame with three axes: tangential, normal, and subnormal to the magnetic field. The local frame makes it possible to analyze the effects of geometrical properties of arbitrary background magnetic field geometries. The inner magnetosphere changes its structure drastically due to currents caused by the solar wind-magnetosphere interaction and other factors. By focusing on the geometrical shape of the magnetic field lines, rather than the shape of the magnetosphere, the process of FLR formation and the region where FLR occurs can be studied in a realistic magnetosphere shape.
We derived the MHD equations in the local frame and performed the linearization. We examined the impacts of the curvature and tortuosity of the magnetic field lines on the orientation of plasma oscillation and its evolution. The findings propose that the excitation of Alfvén waves is influenced by magnetospheric compression owing to increase of the dynamic pressure of solar wind and by torsion of the flux tube. Furthermore, it was denoted that the polarization of ULF waves might be altered more considerably than that in homogeneous potential magnetic fields.