Properties of Whistler Waves' Ducting in Plasmas With Systems of Small-Scale Density Depletions

The ducting of whistler waves by systems of small-scale field-aligned plasma density depletions is studied. Similarly to our previous paper (Zudin et al., 2019, ), we carry out analytical calculations and numerical simulations for the parameters of an active experiment in which very low frequency whistler waves emitted by a ground-based transmitter at a frequency of 18 kHz were received onboard the DEMETER satellite at 700 km above the SURA heating facility. Random-sized density depletions with a level around 10%-20% and perpendicular sizes ranging from 10 m up to about 300 m are considered. The properties of ducted waves are determined by the perpendicular size of individual depletions. Particularly, depletions with a width of more then d0 & SIM; 100 m form separate ducting structures, that is, coupled waveguides capable of exchanging energy by means of mode overlap. Depletions with a width of less than d0 & SIM; 100 m form a common waveguide structure, whose properties are equivalent to those of a wider irregularity with a smoothed density profile. Two important differences are revealed in ducting properties of density depletions compared to density enhancements considered in Zudin et al. (2019, ). First, depletions support highly oblique Gendrin mode waves, rather than quasi-longitudinal whistlers as in the case of density enhancements. Second, the characteristic perpendicular size d0 & SIM; 100 m of density depletions separating the regimes of "coupled waveguides" and of "equivalent ducting structure" with smoothed density profile is by an order of magnitude smaller than for density enhancements of the same 10-20% relative level. Small-scale field-aligned ionospheric plasma density depletions can serve as specific ducting structures for very low frequency whistlersThe ducting properties of systems from multiple small-scale density depletions are studied both analytically and numericallyThe ducting properties of density depletions and enhancements are compared