Whistler propagation in nonsymmetrical density channels

Abstract

[1] We present results from a study of whistler propagation in plasma density channels extended along Earth's geomagnetic field and localized across it. Our study is focused on whistler guiding by channels with a nonsymmetrical density profile across the channel. Such profiles are typical for density inhomogeneities observed on/near the plasmapause formed in the Earth's magnetosphere during strong geomagnetic storms/substorms. Considering these density channels as a combination of high-density and low-density ducts, we analyze leakage of the electromagnetic energy from these ducts depending on various parameters of the system. Our results indicate that a whistler with a frequency less than half of the local electron gyrofrequency can propagate along a nonsymmetrical duct with a homogeneous central part, without any leakage, when the width of the duct is equal to an integral multiple of each of the two internal perpendicular wavelengths. We also show that a homogeneous ducting region is not necessary; a low-frequency whistler can be guided along a region of appropriate density within a monotonic density gradient. This finding makes our results particularly relevant for the interpretation of intense whistler dynamics recorded in a number of satellite observations near/on the plasmapause where strong transverse density gradients are observed.

Effect of frequency modulation on whistler mode waves in the magnetosphere

“…The main conclusion from Figure 3 is that the amplitude of the whistler waves in the equatorial region is significantly less than the wave amplitude near the ionosphere due to the divergence of the background magnetic field. This decrease in the amplitude is significant ( E y = 0.2 mV/m at the equator and E y = 1.0 mV/m at the ionosphere), even if we assume that the whistler propagates exactly along B 0 , which may happen if the wave is perfectly trapped inside the density duct [ Inan and Bell , 1977; Streltsov et al , 2007]. Since in the real situation this trapping is always imperfect (transverse gradients in the background plasma and magnetic field generally tend to “scatter” the wave across the ambient magnetic field) and other dissipative mechanisms (for example, wave‐particle interactions, not considered in this model) take place, it is reasonable to expect that in the real magnetosphere, the wave amplitude will decrease from the ionosphere to the equatorial plane even more than is shown in Figure 3.…”

Artificial ducts caused by HF heating of the ionosphere by HAARP

“…Field‐aligned density enhancements (ducts) with km or longer length‐scale can guide whistler waves along the geomagnetic field B 0 [ Streltsov et al , 2007]. Whistler waves generated by lighting discharges or very low frequency (VLF) transmitters, guided by such ducts into the magnetosphere, are known to play an important role on the radiation belts' dynamics [e.g., Koons , 1989; Carpenter et al , 2002].…”

Waves guided by density ducts in magnetoplasma in the nonresonant region of the whistler frequency range

“…It should be noted that expressions (10)- (15) corresponding to the magnetoplasma whose dielectric permittivity tensor has general form (2) are valid for uniform ducts with both enhanced and depleted plasma density and are not limited to the whistler frequency range.…”

“…Whistler waves supported by such structures have been intensely studied because of the important role that they play in many fundamental physical processes in the Earth's ionosphere and magnetosphere [1,2], as well as due to various applications, including VLF diagnostics of the near-Earth space [6], the use of the guiding properties of density ducts to control the characteristics of electromagnetic sources in magnetoplasma [3,[7][8][9], etc. An overwhelming majority of theoretical studies devoted to specific features of excitation and propagation of whistler waves in the presence of density ducts in a magnetized plasma deal with waves in the resonant region of the whistler range, which lies above the lower hybrid resonance frequency and allows the existence of quasi-electrostatic waves [10][11][12][13][14][15][16]. It was shown in those works that, in this frequency range, the ducts whose density is lower than the background level can support eigenmodes of different kinds, whereas ducts with enhanced density can support no more than one eigenmode with a volume-surface field structure (for each fixed value of the azimuthal index) and improper leaky modes.…”