Whistler interaction with field‐aligned density irregularities in the ionosphere: Refraction, diffraction, and interference

Abstract

Field-aligned density irregularities (FAI) with kilometer-scale sizes transverse to the background magnetic field are a common feature in the ionosphere at all latitudes and local times. In this paper, we investigate the effect of these irregularities on the transionospheric propagation of very low frequency whistler waves and develop a quantitative description of FAI-related effects on whistler propagation through the lower ionosphere. Using an electron magnetohydrodynamics simulation, we provide two applications of our model. First, we show that the presence of kilometer-scale FAI in the ionosphere can reduce the power observed in the equatorial magnetosphere by more than 10 dB in some cases. Second, we demonstrate that multiple FAIs can act as a discrete lens for whistlers, providing a possible means for increasing wave power in artificial whistler ducting experiments.

Van Allen Probes observations of structured whistler mode activity and coincident electron Landau acceleration inside a remnant plasmaspheric plume

“…It is ultimately possible for any wave normal angle in the range − θ G < θ < θ G (where ) to result from this mechanism, with rays originating at greater distances having larger θ at the center of the enhancement. The evolution of a ducted whistler is illustrated in Figure , which shows results from an electron magnetohydrodynamics simulation of whistler propagation [e.g., Woodroffe and Streltsov , ]. These results demonstrate how an initially field‐aligned whistler undergoes periodic “focusing” as it propagates within a field‐aligned density enhancement, resulting in the creation of a broad spectrum of different wave normal angles despite an initially field‐aligned signal.…”

“…The propagation of a whistler inside a density enhancement has been described in detail by Woodroffe and Streltsov []. The key idea is that even if the wave is initially field aligned, the transverse density structure causes the wave vector to develop a perpendicular component in the direction of the density variation.…”

“…The azimuthal propagation angle (Figure e) also exhibits a characteristic variation, going from ∼90° before encountering the enhancement to ∼0 ∘ within the enhancement, to ∼−90° on the other side. This is what would occur if the waves were being locally refracted toward density enhancement from outside the point where trapping is possible [ Woodroffe and Streltsov , ]; the reversal in directions arises because the density gradient would be oppositely directed to the other side of the enhancement. The ∼0° azimuthal angle within the enhancement suggests that waves are either propagating in the meridional plane or that there are multiple waves whose out‐of‐meridian components are oppositely directed, giving a net appearance of meridional propagation.…”

Observations Directly Linking Relativistic Electron Microbursts to Whistler Mode Chorus: Van Allen Probes and FIREBIRD II

“…Probe A spectral measurements indicate that chorus lower band magnetic field power from 1900 to 2020 UT tends to increase during 5–10 min period enhancements (∼10%) of the plasma density, as measured from EFW antenna potential. It is unclear whether these enhancements help guide the propagation of the chorus to higher magnetic latitudes (e.g., Woodroffe and Streltsov, ) or just enhance their growth rate (Li et al, ).…”

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

“…The electron magnetohydrodynamic model was used to calculate the natural VLF waves' distributions in several ducts with transverse sizes around 10 km produced above the HAARP heater. The interaction of VLF whistlers with a periodic system of field‐aligned density irregularities in the ionospheric E layer was also investigated (Woodroffe & Streltsov, ). In both latter cases, irregularities with transverse sizes larger than a few kilometers were considered.…”