Polarization analysis of VLF/ELF waves observed at subauroral latitudes during the VLF-CHAIN campaign

Abstract

Chorus wave emissions are one of the most intense naturally occurring phenomena in the very low (VLF) and extremely low frequency (ELF) ranges. They are believed to be one of the major contributors to acceleration and loss of electrons in the radiation belts. During the VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) from 17 to 25 February 2012, several types of VLF/ELF emissions, including chorus, were observed at subauroral latitudes in Athabasca, Canada. To our knowledge, there has not been any comprehensive study of the physical properties of such emissions at these latitudes. In this study, we calculate spectral and polarization parameters of VLF/ELF waves with high temporal resolution. We found that the polarization angle of several emissions depended on both frequency and time. We suggest that the frequency-dependent events, which usually last several tens of minutes, might be the consequence of the broadening of the ray path that the waves follow from their generation region to the ground. Furthermore, time-dependent events, also lasting tens of minutes, have a polarization angle that changes from negative to positive values (or vice versa) every few minutes. We suggest that this could be due to variations of the wave duct, either near the generation region or along the wave propagation path. Using another ground station in Fort Vermillion, Canada, about 450 km northwest of Athabasca, we tracked the movements of the ionospheric exit point of three chorus emissions observed simultaneously at both stations. Although we found that movement of the ionospheric exit point does not follow a general direction, it is subject to hovering motion, suggesting that the exit point can be affected by small-scale plasma processes.

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

“…Martinez-Calderon et al (2015) studied the ionospheric exit point of simultaneously ground-observed chorus waves using two VLF receivers in Canada at similar latitudes. In both cases presented here, the ionospheric footprint of ERG stays within the same area southwest of KAN (less than 300 km from each other).…”

ELF/VLF wave propagation at subauroral latitudes: Conjugate observation between the ground and Van Allen Probes A

“…These data are recorded continuously at 10 min intervals along with riometer data and GPS time signal with a 1 μs accuracy. More details on this system are explained by Ozaki et al [2008], Shiokawa et al [2014], andMartinez-Calderon et al [2015a].…”

Simultaneous observations of magnetospheric ELF/VLF emissions in Canada, Finland, and Antarctica

“…They found that each QP element is simultaneously observed at the conjugate point with an accuracy of 1 s. This symmetry indicates that the interaction between whistler mode waves and hydromagnetic waves occurs near Journal of Geophysical Research: Space Physics 10.1002/2017JA024211 the equator in the magnetosphere. Manninen et al [2014] and Martinez-Calderon et al [2015a] reported VLF emissions simultaneously observed at two separated stations with a separation of about 400-450 km and identified the ionospheric exit area of the emissions through the polarization analysis of the waves.…”