Defocusing characteristics of auroral X-ray images observed at balloon altitudes.

Abstract

The purpose of the present paper is to estimate defocusing characteristics of auroral X-ray images observed at balloon altitudes, on the basis of propagation properties of photons from the top to deep in the atmosphere. A Monte Carlo simulation of the X-ray propagation down to a selected atmospheric depth between 6 and 14g/cm2 was carried out to deduce the spatial dispersion of the initial source photons with different functions of energy and incident direction. The simulation results obtained can be summarized as follows: 1) the horizontal dispersion at a depth is amplified with increasing energy and incident zenith angle of source photons; 2) a fractional contribution of the upward scattering photons amounts to about one-third of the downward moving photons; 3) the omni-directional X-ray detector provides the defocusing of ±10-12km at balloon altitudes for any spectral type of isotropic photon; and 4) the collimated detector with a limited field of view reduces the dispersion widths, say ±10° in collimation angle, to about one-third of case 3. An example demonstrates defocused images of a ring-shaped photon source, as functions of incident direction, collimation angle and atmospheric depth.