VLF Observations of Solar and Geophysical Forcing on the Polar Ionosphere

Analysis of very low frequency (VLF) radio waves offers a valuable opportunity to investigate the response of both the lower ionosphere and the magnetosphere to a wide range of transient and long-term physical phenomena originating on Earth (e.g., atmospheric waves) or in space (e.g., coronal mass ejections). In this study, we use broadband VLF measurements recorded at Kannuslehto in northern Finland to characterize and examine their links to different geophysical and solar phenomena. The main findings are: (i) the semiannual oscillation in VLF data is associated with geomagnetic activity, while a 27-day solar rotation signal dominates during the declining phase of the solar cycle; (ii) sunrise-related VLF phase perturbations are primarily caused by the attenuation of short-wavelength solar UV radiation by stratospheric ozone; and (iii) banded VLF emissions were detected in the 16–39 kHz range, a frequency band not typically used to study magnetospheric whistler-mode emissions. We further examine the seasonal dependence of the banded emissions using continuous data from 2022 and discuss their possible origin, including the potential role of magnetospheric plasma instabilities, similar to those responsible for auroral hiss.