Ghost images of auroral X ray sources observed by directional balloon-borne detectors

Diffusion characteristics of auroral X rays propagating through the atmosphere have been investigated by means of a Monte Carlo simulation. The results are used to estimate the defocusing effect of multiple photon scatterings on auroral X ray images detected at balloon altitudes. It is predicted that the auroral X ray image at an atmospheric depth of 6-14 g/cm2 is occasionally accompanied by a spurious ghost image in the zenith when an auroral X ray source near the zenith is viewed by a collimated detector inclined with respect to the zenith. The ghost image is enhanced with increasing zenith angle of the collimator axis. The maximum spurious contribution occurs for a collimator with ±20 degree opening angle and 40 degree zenith angle. The magnitude of the spurious contribution also depends on the atmospheric depth and the threshold energy of the detector