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Whistler‐Mode Waves in the Magnetospheric Ducts: Statistical Study

The paper presents results from the statistical study of very low frequency (VLF) whistler‐mode waves observed inside field‐aligned density inhomogeneities by the NASA Van Allen Probe satellites. These observations were conducted in the night‐side equatorial magnetosphere during April 2013 and in the dusk‐side magnetosphere during August 2015. The total number of analyzed events is 225. The events include wave ducting in the so‐called “narrow” high‐density (HDD) ducts, where the variation of the background magnetic field over the duct width is insignificant compared to the change in the density, and “wide” ducts; where the variation of the magnetic field needs to be taken into account. Of the 225 total events, 176 of them were “narrow” events. The analysis revealed that the most common size of narrow, HDD ducts are 100–200 km. These ducts are characterized by the increase in the amplitude of the density of 161%. The background magnetic field in these ducts is in the range of 100–300 nT. The wide ducts are formed by the broader regions, consisting of large‐scale gradients in the density and the magnetic field. We analyzed 49 wide ducts observed in the dusk magnetosphere during August 2015. Frequency of the waves trapped in these ducts is commonly in the range of 400–500 Hz. Our results demonstrate that ducting of the VLF waves by the HDD narrow and wide ducts is a common phenomenon and it may significantly contribute to the VLF energy propagation and distribution in the magnetosphere.