K index: ftp://ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_DATA/INDICES/KP_AP/
https://dx.qsl.net/propagation/
https://www.arrl.org/files/file/Technology/tis/info/pdf/0209038.pdf
https://www.electronics-notes.com/articles/antennas-propagation/ionospheric/solar-indices-flux-a-ap-k-kp.php
https://www.qsl.net/w2vtm/hf_solar.html
http://www.n0hr.com/radio_propagation.htm
https://en.wikipedia.org/wiki/Index_of_radio_propagation_articles
https://en.wikipedia.org/wiki/K-index