Solar activity and MF skywave propagation

An examination is made of the effect of solar activity on MF skywave propagation, and quantitative expressions describing the effect are presented. It is shown that these equations are valid for all latitudes and for the two commonly used reference hours, two and six hours after sunset. Until recently, it has been assumed that the effect of solar activity was more or less the same for all hours of the day. However, the studies reported show that the effect is the greatest during the hours from sunrise to two hours after sunset. During these hours, the reduction of field strength is a function of sunspot number, geomagnetic latitude, path length and frequency. Beginning at two hours after sunset, the effect diminishes with time, reaching a minimum at about six hours after sunset. At six hours after sunset, frequency ceases to be a factor