Ground-based VLF wave intensity variations investigated by the principal component analysis

Abstract

Very low frequency wave intensity variations measured by the Kannuslehto station, Finland in the frequency range 0–12 kHz between 2016 and 2020 are analyzed by the principal component analysis (PCA). As the analyzed ground-based measurements are basically continuous, the length of individual basis vectors entering into PCA is fundamentally arbitrary. To better characterize both long- and short-period variations, two PCAs with different lengths of the basis vectors are eventually performed. Specifically, either daily frequency–time spectrograms or individual frequency spectra are chosen as the PCA basis vectors. Analysis of the first three principal components shows substantial variations of the wave intensity due to seasonal and local time effects. Intensity variations related to the geomagnetic activity characterized by Kp and AE indices and standard deviation of the magnetic field magnitude are less significant. Moreover, PCA allows one to distinguish between nighttime and daytime Kannuslehto variations and study them independently. Solar and geomagnetic activity effects on the daytime and nighttime measurements are discussed. Wave intensity variations related to substorm occurrence are also analyzed. Graphic Abstract

Comparison of Very Low Frequency Wave Intensities Measured by a Low‐Altitude Spacecraft and on the Ground

“…Although possible solar cycle effects are not explicitly considered, the use of the Kp index for parameterization accounts for varying levels of geomagnetic activity. This has been shown, along with seasonal and diurnal variations, to be the main factor controlling the occurrence and properties of extra low frequency/very low frequency (ELF/VLF) waves measured by ground‐based stations (Bezděková et al., 2022; Golden et al., 2011). Although the solar ionizing flux varies through the solar cycle, which may, in principle, affect ground‐measured wave properties due to the varying ionospheric wave attenuation, long‐term ground‐based measurements do not reveal any convincing solar cycle dependence (Smith et al., 2010).…”

Periodic Oscillation of VLF Transmitter Signals Measured in Low and Middle Latitude Regions

“…In this regard, multiple parameters need to be taken into account while analyzing the correlation. PCA is a method specifically developed for this need, which is capable of simplifying complex high-dimensional data while retaining the main features (Bezděková et al, 2021(Bezděková et al, , 2022Pearson, 1901;Silber et al, 2013).…”