Identification of the source of quasiperiodic VLF emissions using ground-based and Van Allen Probes satellite observations
Abstrakt
We report on simultaneous spacecraft and ground-based observations of quasiperiodic VLF emissions and related energetic-electron dynamics. Quasiperiodic emissions in the frequency range 2-6kHz were observed during a substorm on 25 January 2013 by Van Allen Probe-A and a ground-based station in the Northern Finland.
The spacecraft detected the VLF signals near the geomagnetic equator in the night sector at L=3.0-4.2 when it was inside the plasmasphere. During the satellite motion toward higher latitudes, the time interval between quasiperiodic elements decreased from 6min to 3min.
We find one-to-one correspondence between the quasiperiodic elements detected by Van Allen Probe-A and on the ground, which indicates the temporal nature of the observed variation in the time interval between quasiperiodic elements. Multi?omponent measurements of the wave electric and magnetic fields by the Van Allen Probe-A show that the quasiperiodic emissions were almost circularly right-hand polarized whistler mode waves and had predominantly small (below 30 degrees) wave vector angles with respect to the magnetic field.
In the probable source region of these signals (L about 4), we observed synchronous variations of electron distribution function at energies of 10-20keV and the quasiperiodic elements. In the pause between the quasiperiodic elements pitch angle distribution of these electrons had a maximum near 90 degrees, while they become more isotropic during the development of quasiperiodic elements.
The parallel energies of the electrons for which the data suggest direct evidence of the wave-particle interactions is in a reasonable agreement with the estimated cyclotron resonance energy for the observed waves.