Identification of the source of quasiperiodic VLF emissions using ground‐based and Van Allen Probes satellite observations

Abstract

We report on simultaneous spacecraft and ground‐based observations of quasiperiodic VLF emissions and related energetic‐electron dynamics. Quasiperiodic emissions in the frequency range 2–6 kHz 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 6 min to 3 min. 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. Multiсomponent 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°) 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 10–20 keV and the quasiperiodic elements. In the pause between the quasiperiodic elements pitch angle distribution of these electrons had a maximum near 90°, 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.

Oblique Whistler-Mode Waves in the Earth’s Inner Magnetosphere: Energy Distribution, Origins, and Role in Radiation Belt Dynamics

“…Hot (1-50 keV) electron populations with a temperature anisotropy are injected from the plasmasheet during substorms and are believed to provide the necessary free energy source for chorus wave excitation via linear and non-linear processes (e.g., see Andronov and Trakhtengerts 1964;Kennel and Petschek 1966;Cully et al 2011;Titova et al 2015). Recently, Fu et al (2014b) have demonstrated via numerical simulations using measured electron distributions with large enough temperature anisotropies that a low energy (∼ 300 eV) population could drive quasi-electrostatic very oblique upper-band (with ω/Ω ce0 > 0.5) chorus waves while a hotter (> 2 keV) component was simultaneously generating quasi-parallel lower-band (with ω/Ω ce0 < 0.5) chorus waves.…”

Conjugate Observations of Dayside and Nightside VLF Chorus and QP Emissions Between Arase (ERG) and Kannuslehto, Finland

“…For example, Nėmec et al (2016) show that the time delay between QP simultaneously detected in space and on the ground is related to azimuthally propagating compressional ultra low frequency waves responsible for the generation of the QP. Although by their nature, conjugated events give us significant information on the generation and propagation of waves, up until now they remain rare with only a handful of documented cases (Martinez-Calderon, 2016;Nėmec et al, 2016;Titova et al, 2015). Manninen et al (2013) and Manninen et al (2014) found that the variations of QP periods, particularly their gradual increment, can be explained by the auto-oscillation regime of the cyclotron instability in the magnetosphere.…”

New Type of High-Frequency Discrete VLF Emissions ("BIRDS") at Auroral Latitudes

“…Recently, Titova et al (2015) reported simultaneous observations of the high-frequency QP emissions, showing similar spectral and frequency features between ground-based VLF receiver at KAN and Van Allen Probe spacecraft located above UK to SouthWest from KAN. The authors concluded that this QP event was generated inside the plasmasphere at much lower L-shells than KAN, but they did not discuss the propagation properties of these emissions.…”