Fig. 8
From: Ground-based VLF wave intensity variations investigated by the principal component analysis
a Scatter plot of individual principal component coefficients PC1 and PC2 obtained for frequency spectra. b–e Frequency spectra obtained using the first two principal component coefficients. Their coordinates are plotted in panel (a) by green crosses and correspond to b [− 150, 130], c [100, 130], d [− 150, − 50], e [200, − 50]