VLF spectrograms registered at Kannuslehto ground station, after cleaning them from strong sferics, reveal VLF noise suppression by whistlers and whistler echo trains, which consists in significant reduction in the noise spectral power after a strong whistler event. We have found similar effect in the VLF data from Van Allen Probe B taken in the equatorial region on L-shell ~ 3. Detailed analysis of the data shows that the whistler echo train as well as the VLF noise have small wave normal angles. Based on this observation, we limit our analysis to parallel (ducted) whistler wave propagation. The persistence of whistler echo train as well as the VLF noise suggests that in the events under discussion, plasma is unstable in the frequency range corresponding to the observed VLF noise band. In an attempt to explain the effect of VLF noise suppression, we follow up the long-standing idea that relates this effect to the reduction of free energy in the unstable plasma distribution by whistler echo train. To develop this idea into qualitative model, we have studied the motion of energetic electrons, responsible for the noise generation, in the field of ducted whistler echo train. We show that energetic electrons that make the main contribution to the growth rate of VLF noise, during their bounce oscillations in the magnetosphere are subject to multiple resonant impacts from the whistler echo train. These lead to energetic electron diffusion in the phase space, and the corresponding reduction in free energy of the unstable distribution.

We study quasi-periodic VLF emissions observed simultaneously by Van Allen Probes spacecraft and Kannuslehto and Lovozero ground-based stations on 25 December 2015. Both Van Allen Probes A and B detected quasi-periodic emissions, probably originated from a common source, and observed on the ground. In order to locate possible regions of wave generation, we analyze wave normal angles with respect to the geomagnetic field, Poynting flux direction, and cyclotron instability growth rate calculated by using the measured phase space density of energetic electrons. We demonstrate that even parallel wave propagation and proper (downward) Poynting flux direction are not sufficient for claiming observations to be in the source region. Agreement between the growth rate and emission bands was obtained for a restricted part of Van Allen Probe A trajectory corresponding to localized enhancement of plasma density with scale of 700~km. We employ spacecraft density data to build a model plasma profile and to calculate ray trajectories from the point of wave detection in space to the ionosphere, and examine the possibility of their exit to the ground. For the considered event, the wave could exit to the ground in the geomagnetic flux tube with enhanced plasma density, that ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point.

VLF auroral hiss at Kannuslehto (KAN), Finland has been analyzed with a reference to progress of 98 isolated magnetic substorms. During 11 winter months of 2015-2018, 91 of them were accompanied by the auroral hiss during the substorm growth phase. No auroral hiss was recorded during the expansion and recovery phases. We found auroral hiss was observed under rising polar cap PC index showing an increasing input of the solar wind energy into the magnetosphere during the substorm growth phase. We also found that KAN was mapped to the vicinity of enhanced Field Aligned Currents (FACs) during the auroral hiss occurrence. For the first time, it was established that the auroral VLF hiss generation in the equatorial part of the auroral oval (KAN location) is a typical signature of a substorm phase.