Spring‐Fall Asymmetry in VLF Amplitudes Recorded in the North Atlantic Region: The Fall‐Effect

Abstract

Very low frequency (VLF: 3-30 kHz) radio waves propagate inside the Earth-ionosphere waveguide monitoring the electrical conductivity of its boundaries. The upper boundary properties of the waveguide can be represented by Wait parameters (Wait & Spies, 1964), namely, the reference height and conductivity gradient of the D-region. The quiescent ionospheric condition can be disturbed by different types of physical phenomena, originating in space (Clilverd et al., 2010;Macotela et al., 2017) or on Earth (Macotela, Clilverd, Manninen, Thomson, et al., 2019). These disturbances, interpreted as perturbations of the D-region ionization levels, produce changes in the Wait parameters, which show up as phase and/or amplitude variations in the VLF signals.It is well known that the long-term variation of the daytime lower ionosphere exhibits distinct seasonal characteristics with high variability in winter, and lower variability in summer (

Ground-based noontime D-region electron density climatology over northern Norway

“…Though there are several case studies and periodic campaigns carried out to explore the D-region electron density variations and their causative mechanisms, we are still lacking to address the seasonal variation of D-region studies using continuous observations. A recent study by Macotela et al (2021) reported that during the fall equinox, there is an increase in the noontime mean amplitude of VLF radio waves in high and mid-latitudes, which do not resemble the solar zenith angle (see Fig. 3).…”

“…Thus, they postulate that the changes in the collision frequency associated with the background temperature could be a prime reason for the observed increase in the noontime VLF amplitude. It is well known that the VLF radio waves are drastically affected by the D-region electron density, however, Macotela et al (2021) did not consider the electron density role (if any) on the fall effect. Therefore, this study focuses to explore the altitudinal, and seasonal variation of noontime electron number density using partial reflection radar observations from 2014 to 2022 at a high-latitude location.…”

“…their contribution may still be significant as they typically occur near noontime. As we initially highlighted the focus of this study is on the asymmetry between spring and autumn as previous studies indicated such behavior in the detected VLF amplitudes of long-distance transmissions (Macotela et al, 2021, see bottom panel Fig. 3).…”

“…Generally, the period from the end of summer and mid of fall is a rather dynamical time of the year, for which we will raise some presumably related observations. Macotela et al (2021) argued that changes in the temperature and semi-diurnal tidal amplitude (see Conte et al, 2018) could be a cause for the fall effect. However, their study did not include electron density's role in the observed fall effect (if any).…”

Delineating the effect of upward propagating migrating solar tides with the TIEGCM-ICON

“…Other studies have also used differences in Hough modes and vertical wavelengths to understand the changes in the SW2 latitudinal variation with altitude (e.g., Azeem et al, 2016;Stober et al, 2021;. In addition, signals of distinct September transitions are observed in the D-region ionosphere in the propagation of very low frequency (VLF) radio wave signals (Macotela et al, 2021), suggesting an association with the mean temperature variation at 70-80 km and the semidiurnal solar tidal enhancement.…”