Table 2 Table listing the major findings of this work. The explored parameters are period P = 24, 12, 8, 6, 4.8, 4 h; variance \(\sigma ^2 = 95 \%\) or \(99\%\) provided by PCA; total length of time series used for PCA (maximum—almost 1 year, minimum—2 weeks); selection of time series (separating them into sections); high latitude filtering by applying an SH filter of degree \(N_{cut}\); segment length along which a Fourier transform is applied (24, 48, 72 h); and latitude of observatory stations used for the estimation of the source coefficient. “N/A” stands for not applicable
Explored parameter | Effect on the number of modes | Effect on the spatial features | \(R^2\) results |
|---|---|---|---|
Period (P) | Increases for decreasing P | Long P show larger scale features Short P show smaller and high latitude features | Increased agreement with increasing P |
Variance | Doubles when going from 95 to 99% | Subsequent modes tend towards smaller scale features | Improved when using higher variance |
Length of time series | Increases for longer time series | Only a weak effect on the dominant modes | Improved for longer time series |
Selection of time series | Increases for equinoctial months | Only a weak effect on the dominant modes | Weak improvement for equinoctial months |
High latitude filtering | Decreases (more prominent for lower \(N_{cut}\)) | Features are blurred, but still present at high latitudes | Worsens compared to unfiltered data |
FT segment length | Decreases for longer segment lengths | Longer segment lengths show expected Sq pattern at short P | Improved for short segment lengths |
Observatory latitude | N/A | N/A | Best agreement using only mid latitudes |