We study quasiperiodic very low frequency (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 quasiperiodic 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 propagation toward the ground. For the considered event, the wave could propagate toward the ground in the geomagnetic flux tube with enhanced plasma density, which ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point.

The main feature of the magnetosphere consists in its filling with a large variety of waves at different frequency scales with different characteristics and of different origins. The natural electromagnetic waves at the audio range of frequencies (3-30 kHz) termed very low frequency (VLF) emissions are typical for the magnetospheric plasma as well. They are whistler mode waves of magnetospheric origin at the frequencies between the ion and electron gyrofrequency, that have propagated through the ionosphere to the ground.The natural VLF waves known as chorus, hiss, and quasiperiodic (QP) emissions have been widely studied more than 50 years since the classical monograph by Helliwell (1965). The majority of these emissions are usually generated at or near the geomagnetic equator in the magnetosphere through resonant cyclotron interactions with energetic (∼hundreds of keV) electrons of the Earth's radiation belts (e.g.

We study quasiperiodic very low frequency (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 quasiperiodic 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 propagation toward the ground. For the considered event, the wave could propagate toward the ground in the geomagnetic flux tube with enhanced plasma density, which ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point.

Very low frequency (VLF) emissions in the magnetosphere are often observed in the form of periodic or quasiperiodic sequence of bursts. Helliwell (1965) termed periodic emissions (PE) as the bursts having repetition periods of 3-10 s consistent with individual wave packet propagation between conjugate ionospheres. This period can be very stable in time for long time intervals. Quasi-periodic (QP) emissions have longer periods (usually from 20-30 to 300 s), and the inter-element interval can vary smoothly due to several factors (Manninen et al., 2014).Such events can last tens of minutes and even several hours (Manninen et al., 2014), which require the amplification of whistler mode waves in the magnetosphere to compensate for the losses due to nonideal ionospheric reflection and refractive spreading of wave energy. The commonly accepted amplification mechanism of whistler mode waves in the inner magnetosphere is cyclotron resonant interaction with energetic electrons having anisotropic velocity distribution (Trakhtengerts & Rycroft, 2008).Periodic emissions are divided into two subtypes, which Helliwell (1965) termed as dispersive and nondispersive. The period of dispersive PE has a systematic frequency-time dispersion typical of multi-hop

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