Two modes of ion-ion hybrid waves in magnetospheric plasma

Abstract

This paper is concerned with the structure of ion-ion hybrid (IIH) waves in multicomponent magnetospheric plasma in the dipole geomagnetic field model. Like Alfvén waves, in IIH waves the field line can oscillate in different directions: in the azimuthal direction (the toroidal modes) and in the radial direction (the poloidal modes). Contrary to Alfvén waves, in IIH waves only the equatorial part of the field line can oscillate, since due to the heavy ions admixture the IIH waves are trapped in the resonator in the vicinity of the geomagnetic equator. The equatorial localization of the waves allow the calculation of the parallel structure of the toroidal and poloidal IIH modes analytically. It was found that the toroidal and poloidal modes have slightly different structures and that their frequencies are also slightly different. The difference between toroidal and poloidal frequencies (the polarization splitting) is small with respect to the basic harmonics of each mode. It was found that IIH modes have a considerable compressional component of the wave's magnetic field.

Transverse resonator for ion-ion hybrid waves in dipole magnetospheric plasma

“…It was found that the analogy of the poloidal and toroidal oscillations can exist also for the IIH modes, with the difference that in this case, only a near-equatorial part of the field line oscillates [35] (figure 1). Thus, the existence of the transverse resonator for IIH modes can be expected, which is an analogy of the large-scale resonator for the poloidal Alfvén modes.…”

“…Let us consider the azimuthally small scale perturbations. Then the wave's electric field can be expressed as [14,35]…”

“…In a homogeneous plasma and in the ω/Ω c → 0 limit, the "potential" Φ corresponds to the Alfvén wave, and in the general case-to the IIH mode. Then, as was shown in [35], equation (1) can be reduced to the following equation for the IIH mode:…”

“…where m is the azimuthal wave number playing the role of the azimuthal wave vector component k 2 , LT and LP are toroidal and poloidal operators in the near-equatorial approximation (for detail see [35]),…”

Magnetic tilt effect on externally driven electromagnetic ion cyclotron (EMIC) waves

“…External compressions are one of the possible sources of waves in the Pc frequency range in the magnetosphere (Anderson & Hamilton, 1993;Keika et al, 2013;Gamayunov & Engebretson, 2021), and the EMIC waves can be externally generated via mode conversion at the ion-ion hybrid (IIH) resonance (Klimushkin et al, 2006;D.-H. Lee et al, 2008;Mikhailova et al, 2019) in analogy with the standard field-line resonance (e.g., D.-H. Lee & Lysak, 1989). This mode conversion occurs as a compressional wave propagates radially inward into an increasing magnetic field region (or into a heavy-ion density gradient) and encounters the IIH resonance.…”