Whistler‐Mode Waves in Magnetic Ducts

Abstract

Observations from the NASA MMS satellites show packages of ELF whistler‐mode waves localized inside the small‐scale irregularities of the magnetic field. These irregularities are formed by the narrow field‐aligned channels where the magnitude of the background magnetic field inside the channel is greater or less than outside. By analogy with the classical density ducts, we introduce the high‐B duct (HBD), where the magnitude of the field inside the channel is greater than the outside, and the low‐B duct (LBD), where the magnitude of the field inside the channel is less than the outside. We investigate the guiding of the ELF whistler‐mode waves by high‐B and low‐B ducts. We derive the analytical criteria for the wave ducting by these ducts and confirm them with two‐dimension, time‐dependent simulations of the electron‐MHD model. Also, we model ELF whistler‐mode waves observed inside the high‐B and low‐B ducts by MMS satellites.This article is protected by copyright. All rights reserved.

Switching Between Whistler‐Mode Waves on Inhomogeneities of the Plasma Density and Magnetic Field

“…Now let us consider a situation when the plasma density is homogeneous in the considered domain and the background magnetic field is homogeneous along B 0 but inhomogeneous across it (A. V. Streltsov & Nejad, 2023). demonstrate that relation (2) can be used to express B as a function of ω , k ⊥ , k ‖ , and ω pe in the form $$$B=\omega \frac{{m}_{e}}{e}\frac{k}{{k}_{\Vert }}\left(1+\frac{1}{{k}^{2}{\lambda }_{e}^{2}}\right).$$$ …”

“…If the field‐aligned density channels are formed by the density enhancement, they are called high‐density ducts (HDD), and in the case of being formed by density depletion, they are called low‐density ducts (LDD) (A. Streltsov et al., 2006; Chen et al., 2021; Zudin et al., 2019; Hanzelka & Santolík, 2019; Ke et al., 2021; Yearby et al., 2011). Similarly, magnetic‐filed aligned enhancement channels form high‐magnetic ducts (HBD), and their depletion forms low‐magnetic ducts (LBD) (A. V. Streltsov & Nejad, 2023; Nejad & Streltsov, 2023). All these ducts can trap and propagate whistler‐mode waves over considerable distances in the Earth's magnetosphere with minimal attenuation, including from one hemisphere to the other.…”

On the Propagation of Whistler‐Mode Waves in the Magnetic Ducts

“…By intentionally introducing controlled injections of whistler-mode waves from Earth or space into the magnetosphere, the presence of energetic particles in the Earth's radiation belt can be reduced, thereby creating a safer environment for electronics and crews on space platforms (Inan et al, 1985(Inan et al, , 2003.As a notable characteristic, whistler-mode waves in the magnetosphere propagate along the ambient magnetic field, guided by field-aligned plasma density inhomogeneities referred to as the density ducts, which can be formed as the low-density ducts and the high-density ducts (HDD). The presence of these ducts enables the propagation of whistler-mode waves over considerable distances in the Earth's magnetosphere, including from one hemisphere to another, with minimal attenuation.Recently (Streltsov & Nejad, 2023) have pointed out the occurrence of small-scale, localized packages of extremely low frequency (ELF) waves within the inhomogeneities of the ambient magnetic field in the equatorial magnetosphere in the data gathered by the NASA Magnetospheric Multiscale Mission (MMS) satellites, which are interpreted as another mechanism of ELF guiding that happens through the magnetic structures, referred to as high-magnetic duct (high-B duct or HBD) and low-magnetic duct (low-B duct or LBD).For almost 70 years, an extensive study has been done on different density structures, guiding whistler-mode waves in space and laboratory plasmas (…”

“…Recently (Streltsov & Nejad, 2023) have pointed out the occurrence of small-scale, localized packages of extremely low frequency (ELF) waves within the inhomogeneities of the ambient magnetic field in the equatorial magnetosphere in the data gathered by the NASA Magnetospheric Multiscale Mission (MMS) satellites, which are interpreted as another mechanism of ELF guiding that happens through the magnetic structures, referred to as high-magnetic duct (high-B duct or HBD) and low-magnetic duct (low-B duct or LBD).…”

“…(Yu & Yuan, 2022) investigated the ducting of whistler-mode waves by magnetic dips and peaks by analyzing the wave refractive index. In a recent study (Streltsov & Nejad, 2023) utilized the whistler-mode dispersion relation to derive analytical criteria for wave trapping in HBD and LBD and to identify thresholds in the magnitude of the ambient magnetic field that facilitates the trapping of waves with specific frequencies. They also determined the ranges of parallel and perpendicular wavelengths of the waves trapped in LBDs and HBDs.…”