Excitation of Lunar Luminescence by Solar Activity

An anomalous increase in surface brightness of the Moon in an area of over 60,000 km ² around and north of the crater Kepler was observed to occur twice on the night of November 1-2, 1963, on eight photographs secured with the 24-inch refractor of the Observatoire du Pic-du-Midi between 22.35-22.42 U.T. on November 1st, and 00.20-00.35 U.T. on November 2nd, on Kodak 1-F plates exposed through an interference filter of 45A˚ half-width centered on 6725A˚. Control photographs taken through an interference filter of 95A˚ half-width centered on 5450A˚ failed to show any such effect. The enhancement in the red was not only observed to recur twice during the same night, but plates taken between 00.20-00.35 U.T. on November 2nd disclosed that the degree of enhancement (resulting nearly in a temporary doubling of surface brightness) increased appreciably within 15 minutes of observation. This observed enhancement is interpreted to be the result of luminescence of the respective parts of lunar surface excited by solar activity. Two Class-1 flares were indeed observed to occur on an identical place of an otherwise calm Sun earlier that day at the Sacramento Peak and McMath-Hulbert Observatories at 13.58 and again at 15.55 U.T. The time interval between these flares suggests that the recurrence of the observed brightening of the Moon may have been due to surface luminescence stimulated by them after a transit time close to 8.5 hours—of sufficient duration to rule out solar X-rays or UV light as the exciting source, and to direct attention to corpuscular radiation. If so, this transit time would correspond to a particle velocity of 5000 km/sec, and the energy flux to a density of the order of 10³ particles/cm ³.