450. On an average yearly basis the received Trans-Atlantic signals increased in strength in proportion to the degree of ____. Made on the basis of the 11 year cycle:

Trans-Atlantic radio signal strength varies with the 11-year solar cycle, directly correlating with sunspot activity. Sunspot cycles influence the ionosphere, which reflects radio waves; increased sunspot activity leads to greater ionization and stronger signals. Aurora borealis, extratropical advection, and perigean tides are unrelated to this cyclical variation.