Whistler Waves' Propagation in Plasmas With Systems of Small‐Scale Density Irregularities: Numerical Simulations and Theory

Abstract

The propagation of whistler waves in a magnetized plasma containing multiple small-scale (100 m to 1 km) field-aligned irregularities of enhanced electron density is considered analytically and by means of numerical simulations. Such systems of irregularities can develop in the upper ionosphere during the generation of density ducts by high-frequency heating facilities and other types of active experiments. The simulation parameters are close to those of an active experiment where a whistler wave of 18 kHz emitted by a ground-based very low frequency (VLF) transmitter was received onboard the DEMETER satellite at 700 km above the SURA heater. The study reveals a number of remarkable properties of the VLF waves' propagation, including the existence of specific waveguide modes of the small-scale density structures and of a characteristic transverse size d 0 of the irregularities. Irregularities with small density enhancements around 10-20% and transverse sizes larger than d 0 ∼ 1 km can serve as separate waveguides for VLF waves. In their turn, single irregularities narrower than d 0 cannot be considered as individual ducting structures. Numerical simulations show that, for the analysis of the electromagnetic whistlers' propagation, a system of closely spaced irregularities with scales narrower than d 0 can be modeled by an equivalent ducting structure with a smoothed density profile. Such equivalent structure has the same ducting properties for whistlers and can be produced by averaging with a sliding window of a scale about d 0 the original density distribution.

Kinetic Plasma Turbulence Generated in a 3D Current Sheet With Magnetic Islands

“…As shown in the b and c plots of Figure 7, the particle densities in these points A and B have well recorded inhomogeneities of particle densities clearly seen in Figures 7B,C, which could attribute to the generation of whistler waves in the region near these points as suggested by Zudin et al (2019). This suggestion is also confirmed by studies of McMillan & Cairns (2007) showing that in plasmas with low beta (as we use in our model) the most unstable mode is not occurring at parallel propagation, but may be at intermediate and very oblique angles.…”

“…As shown in the lower plots of Figure 7, the particle densities in these points A and B have well-recorded inhomogeneities of particle densities clearly seen in Figures 7B,C. The lower-hybrid waves can be generated by two-stream instabilities as shown in the energy distribution of Figure 7B (Papadopoulos & Palmadesso, 1976;Fujimoto & Sydora, 2008;Zhou et al, 2014;, or due to the strong density gradient near the separatrices and in the outflow (Drake et al, 2003;Scholer et al, 2003;Divin et al, 2015;Zudin et al, 2019).…”

Whistler‐Mode Waves in the Magnetospheric Ducts: Statistical Study

“…The theory of VLF waves trapping in the density duct has been developed in a number of papers summarized in the monographs by Helliwell (1965), Sazhin (1993), Kondrat'ev et al (1999, Katoh (2014), Zudin et al (2019), Hosseini et al (2021), andXu et al (2020). We follow the formalism developed by Streltsov et al (2006).…”

Direct Evidence Reveals Transmitter Signal Propagation in the Magnetosphere

“…Ducted waves propagate inside a density enhancement or depletion known as a duct, with wave energy and wave normal direction confined nearly along the ambient field lines (Cerisier, 1974; Helliwell, 1965; Sonwalkar & Inan, 1986). Both observations (Moullard et al., 2002; Scarf & Chappell, 1973; Sonwalkar et al., 1994) and modeling (Katoh, 2014; Ke et al., 2021) reveal that wave power can be modulated and focused by density ducts with various spatial scales, even with spatial scales comparable or smaller than signal wavelength (Hosseini et al., 2021; Streltsov et al., 2006; Zudin et al., 2019). Ducted propagation mode has been assumed for numerous theoretical studies on wave‐particle interaction (Gołkowski et al., 2019).…”