Ducting of whistler-mode waves by ultra-low frequency electromagnetic waves: a case study

Whistler-mode waves can be guided along the ambient magnetic field by field-aligned irregularities in plasma density, magnetic-field strength, or both. We examine localized whistler-mode wave packets observed by MMS1 within a structured plasma region containing an ultralow-frequency (ULF) electromagnetic disturbance. The packets coincide with variations in electron density and magnetic-field magnitude, consistent with propagation within a compound density–magnetic duct. Two-dimensional electron-magnetohydrodynamic simulations using the measured profiles reproduce the primary trapping regions and observed frequency dependence. Magnetic-field variations associated with the ULF disturbance exert the strongest control over the spatial structure of the wave packets, whereas density variations modify trapping locations and wave-power distribution. These results provide observationally constrained evidence that magnetic structures can play an important role in whistler-mode wave ducting.