Electron Inertial Effects on Linearly Polarized Electromagnetic Ion Cyclotron Waves at Earth's Magnetosphere

Abstract

We discuss a role of the electron inertial effect on linearly polarized electromagnetic ion cyclotron (EMIC) waves at Earth. The linearly polarized EMIC waves have been previously suggested to be generated via mode conversion from the fast compressional wave at the ion-ion hybrid (IIH) resonance. When the electron inertial effects are neglected, the wave normal angle of the mode-converted IIH waves is 90 • because the wave vector perpendicular to the magnetic field becomes infinite at the IIH resonance. When the electron inertial effect is considered, the mode-converted IIH waves can propagate across the magnetic field lines, and the wavelength perpendicular to the magnetic field approaches the electron inertial length scale near the Buchsbaum resonance. These waves are referred to as electron inertial waves. Due to the electron inertial effect, the perpendicular wave number to the ambient magnetic field near the IIH resonance remains finite, and the wave normal angle is less than 90 • . The wave normal angle where the maximum absorption occurs in a dipole magnetic field is 30-80 • , which is consistent with the observed values near the magnetic equator. Therefore, the numerical results suggest that the linearly polarized EMIC wave generated via mode conversion near the IIH resonance can be detected in between the Buchsbaum and the IIH resonance frequencies, and these waves can have normal angle less than 90 • . Key Points:• Mode-converted ion-ion hybrid waves can propagate toward the Buchsbaum resonance due to the electron inertial effects • Electron inertial effects allow the mode-converted ion-ion hybrid waves have medium normal angle less than 90 • • Linearly polarized EMIC waves generated via mode conversion can have relatively small normal angle between 30 and 80 •

The First Observation of N+ Electromagnetic Ion Cyclotron Waves

“…The reduced part of Figure 2 showing constant values is given in Figure S1. radial extent of the resonance through dispersive effects is suggestive of a common source region as shown in the recent simulations (Kim et al, 2019). The generation mechanism of this N + EMIC wave event based on the possible sources discussed above needs further analysis and will be investigated in the future.…”

“…The local plasma conditions do not favor a local generation mechanism. However, linearly polarized, small wave normal angles, N + EMIC waves observed at low L-shells can be generated via the mode conversion of compressional waves at the ion-ion hybrid/Buchsbaum resonance (Lee et al, 2008;Kim et al, 2015Kim et al, , 2019, and/or due to the free energy from ring velocity distribution of ring current protons ( However, this first observation of N + EMIC wave provides evidence of the N + ions presence in the near-Earth region, in addition to O + , during this event. This study opens up the new avenues of quantifying the relative concentrations of N + and O + based on EMIC wave observations from current missions (e.g., Van Allen Probes, MMS, and Cluster), which can be used as a tool to infer the N + ion concentration using the observed characteristic frequencies (Kim et al, 2015;Min et al, 2015;Miyoshi et al, 2019).…”

“…The local plasma conditions do not favor a local generation mechanism. However, linearly polarized, small wave normal angles, N + EMIC waves observed at low L-shells can be generated via the mode conversion of compressional waves at the ion-ion hybrid/Buchsbaum resonance (Lee et al, 2008;Kim et al, 2015Kim et al, , 2019, and/or due to the free energy from ring velocity distribution of ring current protons (H + ) and oxygen (O + ) (Gamayunov et al, Allen Probes, MMS, and Cluster), which can be used as a tool to infer the N + ion concentration using the observed characteristic frequencies (Kim et al, 2015;Min et al, 2015;Miyoshi et al, 2019). Inferred N + ion concentration can reveal N + torus and N + warm plasma cloak by quantifying O + torus (Nosé et al, 2018(Nosé et al, , 2020(Nosé et al, , 2015(Nosé et al, , 2011 and O + warm plasma cloak (Chappell et al, 2008).…”

Magnetic Tilt Effect on Externally Driven Electromagnetic Ion Cyclotron (EMIC) Waves

“…The dispersion relation of the magnetosonic wave branch can be written as $$${n}_{\perp }^{2}=\frac{\left(R-{n}_{\Vert }^{2}\right)\left(L-{n}_{\Vert }^{2}\right)}{\left(S-{n}_{\Vert }^{2}\right)},$$$ and the IIH resonance occurs where n ⊥ → ∞ at n || 2 ∼ S . The mode‐converted wave propagates along the magnetic field line with a dispersion relation of n || 2 ∼ S and has a large wave normal angle (Kim, Johnson, & Lee, 2019).…”

MMS Measurements and Modeling of Peculiar Electromagnetic Ion Cyclotron Waves

“…Improved modeling and data agreement suggest future studies should apply methods (e.g., section 2.1) to ensure measurement of cold and hot plasma composition or develop improved instrumentation (Zurbuchen & Gershman, 2016) to accurately characterize magnetospheric composition and assess effects on EMIC waves. Realistic, complex composition data could be used in sophisticated wave modeling applications to also investigate composition effects on wave propagation (e.g., Kim et al, 2019) or the waves' effectiveness in energetic particle scattering (e.g., Denton et al, 2019).…”