1. Allen , M. 2016 U.S. Department of Defense space weather services. Power point presentation http://www.swpc.noaa.gov/sites/default/files/images/u33/Mark%20Allen_2016%20SWW%20DoD%20Space%20Weather%20Briefing%20(2).pdf
2. Correlation of radio transmission and solar activity;Anderson;Proc. Inst. Radio Eng. (IRE),1928