Localization of the sources of narrow-band noise VLF emissions in the range 4-10 kHz from simultaneous ground-based and Van Allen Probes satellite observations

This paper presents the results of simultaneous observations of narrow-band noise VLF emissions in the frequency range 4-10 kHz at Kannuslehto ground station in Northern Finland and by Van Allen Probes (previously RBSP) in the equatorial part of the magnetosphere. The event of December 25, 2015, is considered. During the event, narrow-band noise VLF emissions were detected on the Earth in two frequency ranges, f = 3.5-6 kHz and f = 8-10 kHz, between 1100 and 1300 UT. Narrow-band VLF emissions in the equatorial zone were also observed during that time by the RBSP-B satellite; their frequency was close to the electron equatorial half-gyrofrequency and gradually increased from 3 to 11 kHz during the satellite motion from L = 5.0 to L = 3.0. Analysis of the fine structure of the emissions on the ground showed that their spectral and temporal characteristics corresponded to emissions by the satellites in localized zones at different L-shells. The ground-based observations at lower frequencies correlated with the satellite observations at larger L-shells. In order to localize the regions of the generation of the VLF emissions observed at Kannuslehto auroral station at different frequencies, we calculated the ray trajectories of waves from the equator for the plasma density distributions detected by Van Allen Probes. The calculations of the trajectories showed that the VLF waves detected at Kannuslehto station could travel to the ground only if they propagated in the large-scale density ducts (700-900 km) observed by Van Allen Probes.