Light Scattering by Particles in the Upper Atmosphere

Abstract

During two flights of a balloon-borne coronagraph to a maximum altitude of 25 km, observations of the wavelength distribution of skylight from 0.37 μ to 0.79 μ at a scattering angle of 2°4 as well as the angular distribution at γ=0.44 μ from scattering angles 1°7 to 58° were secured. At 25 km the daytime sky radiance at a scattering angle of 2°4 is still nearly twice that expected from the Rayleigh sky. At a scattering angle of 10° the observed radiance exceeds the Rayleigh value by only about 10 per cent. At 20° and beyond the two are nearly indistinguishable. The angular and wavelength distributions of the light scattered by the aerosols are used to determine the parameters of the size distribution and the number of contaminating particles above the apparatus. Comparison of calculated angular distributions with the observations shows that between r=0.3 μ and 3.0 μ the particle size distribution is fairly well determined, varying as the −3.5 power of the radius in the stratosphere. An investigation is also made of how uniquely the observations are able to determine the parameters of the size distribution of the scattering particles. Differentiation with respect to height of the inferred concentrations per cm² column above a particular altitude yields the local particle concentration at any altitude. A broad relative maximum in particle concentration or “dust layer” exists at 20 km. The distribution of particle sizes at this altitude as determined by the scattered skylight compares favorably with the distributions determined by direct sampling. We conclude that many of these particles are of stratospheric origin. The observations also demonstrate the existence of extremely thin laminae of scattering particles in the stratosphere. The data allow determination of the optical depth caused by large particles in the atmosphere. At 25 km the optical depth for the non-molecular component is approximately 2×10⁻⁴. The variation of particle concentration with height above 20 km is interpreted on the assumption that two mechanisms transport particles–eddy diffusion, which carries particles upwards, and sedimentation, which transports meteoric debris downwards. The eddy diffusion coefficient in the altitude range 20 to 25 km is found to he approximately 5000 cm² sec⁻¹. The observations also allow the influx of meteoric particles to be determined. The meteoric influxes inferred in this manner are compared with those determined from rocket and satellite experiments, estimates of meteoric dust settling in the air or deposited on the ground, and from the analysis of the zodiacal light. This comparison leads to the conclusions that: 1) a geocentric condensation of the interplanetary dust of approximately a factor of 10³ exists in the neighborhood of the earth; 2) this geocentric concentration is most effective for particles in the size range 0.3 μ to 10 μ; 3) a majority of the particles in the earth's dust cloud are in geocentric, quasi-closed orbits. It is also demonstrated that meteoric dust represents a negligible fraction of the particulate matter below 20 km in the earth's atmosphere.