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
Highlights
IntroductionNatural (i.e., non-anthropogenic) whistler mode waves observed on the ground are basically of two origins
Natural whistler mode waves observed on the ground are basically of two origins
Note that while the results are plotted as a function of universal time (UT), local time (LT) at Kannuslehto is larger by about 1.5 h
Summary
Natural (i.e., non-anthropogenic) whistler mode waves observed on the ground are basically of two origins Either they are generated directly near the Earth and are typically related with the lightning occurrence, or they originate in the magnetosphere and propagate down to the Earth (Helliwell 1965). To exit through the bottom of the ionosphere, where the refractive index suddenly drops, the Snell’s law tells us that the incident wave vectors need to be oriented nearly vertically down. For this reason, typically only whistler mode waves ducted for a considerable part of their propagation path are able to make it all the way to the ground (Helliwell 1965). Once the waves exit the ionosphere, they can propagate considerable distances in the Earth–ionosphere waveguide before they are detected by a receiver
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