D‐Region High‐Latitude Forcing Factors

Abstract

The subionospheric very low frequency (VLF) radio wave technique provides the possibility of investigating the response of the ionospheric D‐region to a diversity of transient and long‐term physical phenomena originating from above (e.g., energetic particle precipitation) and from below (e.g., atmospheric waves). In this study, we identify the periodicities that appear in VLF measurements and investigate how they may be related to changes in space weather and atmospheric activity. The powerful VLF signal transmitted from NAA (24 kHz) on the east coast of the United States, and received at Sodankylä, Finland, was analyzed. Wavelet transform, wavelet power spectrum, wavelet coherence, and cross‐wavelet spectrum were computed for daily averages of selected ionospheric, space weather, and atmospheric parameters from November 2008 until June 2018. Our results show that the significant VLF periods that appear during solar cycle 24 are the annual oscillation, semiannual oscillation, 121‐day, 86‐day, 61‐day, and solar rotation oscillations. We found that the annual oscillation corresponds to variability in mesospheric temperature and solar Lyman‐α (Ly‐α) flux and the semiannual oscillation to variability in space weather‐related parameters. The solar rotation oscillation observed in the VLF variability is mainly related to the Ly‐α flux variation at solar maximum and to geomagnetic activity variation during the declining phase of the solar cycle. Our results are important since they strengthen our understanding of the Earth's D‐region response to solar and atmospheric forcing.

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

“…Certainly, more data is required to be able to confirm whether the fall effect is related to the seasonal transition of the mean zonal wind impact on mean temperature, and whether the effect is also observed in other regions of the Earth or not. In the literature, we found a few graphs suggesting the presence of the fall-effect in VLF data (Clilverd et al, 2010;Correia et al, 2011;Macotela, Clilverd, Manninen, Moffat-Griffin, et al, 2019;Neal et al, 2015;Pal & Hobara, 2016). But the investigation of such effects was not the focus of those studies.…”

“…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 (Bertoni et al., 2013; Correia et al., 2011; Macotela, Clilverd, Manninen, Moffat‐Griffin, et al., 2019). The D‐region, created and maintained by solar radiation (Nicolet & Aikin, 1960), depends on the solar zenith angle (SZA), which is a function of time of day, date, and latitude on Earth.…”

“…Analysis of long‐term VLF amplitude signal propagating in the northern hemisphere showed a comparative asymmetry during spring and fall seasons (Macotela, Clilverd, Manninen, Moffat‐Griffin, et al., 2019) but was not investigated at the time. Recently, reviewing the data once more, we realized that this observation has not been sufficiently studied and the mechanism that generates this asymmetry is yet unknown.…”

Measuring Global Signals in the Potential Gradient at High Latitude Sites

“…Such gaps were filled using the moving average of the original time series with a time window length of 3,072 h for both Sodankyla and Halley. This procedure minimizes the introduction of artifacts in the wavelet analysis (Macotela et al, 2019).…”

“…As there is effectively no radon emission at Halley (as the site location is on a ∼1 km thick ice sheet), this is an unlikely explanation for the observed periodicities. We therefore investigate whether a relationship in the time-period domain exists between the PG and local meteorological measurements (for SOD), as indicated by Macotela et al (2019). This involves employing the wavelet coherence and cross-wavelet transform.…”