Some practical aspects of auroral propagation

Assoclated with visible auroral activity there is some kind of reflection or scattering mechanism that can result in radio communication in the VHF band over distances up to 1000 km with low power transmitters and probably up to 2000 km with high power transmitters. The transmission-loss experienced over such a circuit is somewhat less than is involved in regular VHF scatter-communication but is much greater than for sporadic E openings at VHF. For auroral propagation the direction of arrival is deviated from the great circle plane towards the direction of auroral activity, whereas for regular VHF scatter-transmission, and for sporadic E transmission, propagation is approximately geodesic on most occasions. For auroral propagation the bandwidth available for communication is almost always severeley limited, frequently to the extent that amplitude-modulated (AM) radio-telephony is impossib1e. Moreover the ratio of the fading rate to the carrier frequency is rather more than a power of ten greater than is customary for ionospheric transmiseion under quiet ionospheric conditions. Auroral propagation occurs most frequently at night and is hardly ever experienced between sunrise and noon. It occurs most frequently at the equinoxes and more often in years of high sunspot activity.