ИСТИНА

We present the results of analysis of simultaneous spacecraft and ground-based observations of quasiperiodic VLF emissions. The event was observed on 19 September 2017 by the Cluster, Van Allen Probes, and ERG spacecraft and by magnetic antennas at four ground-based sites including Kannuslehto in Northern Finland, Lovozero at Kola Peninsula, Barentsburg in Svalbard, and Maimaga in Siberia. Emissions were detected at the ground-based stations from 05:40 to 10:00 UT, by Cluster spacecraft from 06:10 to 09:40 UT, and by Van Allen Probes from 06:10 to 10:00 UT. Spacecraft observations correspond to a wide range of L-shells from approximately 4 for Van Allen Probes up to 11.3 for Cluster. Geomagnetic conditions were very quiet. In both spacecraft and ground-based measurements two different modulation periods are clearly seen: the smaller one is about 50 seconds (corresponding to QP-elements generation) and the larger one is about 12 minutes (corresponding to gradual QP-emission amplitude variation). Simultaneously detected QP emissions exhibit clear one-to-one correlation between different observation sites, which indicates the temporal nature of the observed quasiperiodic modulation. Wave propagation analysis indicates that the quasiperiodic emissions have low (below 40°) wave vector angles with respect to the magnetic field. The variations of VLF wave amplitude are shown to correlate with geomagnetic field pulsations, the highest correlation being observed for the compressional component. Correlated long-period magnetic pulsations were also observed on the ground. An analysis of ground-based signal polarisation allowed us to estimate possible locations of the region where the VLF waves propagated to the ground, thus restricting probable position of the source region of QP emissions.