Twilight observations from a balloon gondola - Preliminary results

Sunlight extinction and atmospheric radiances data are presented for blue and red light photographs taken during twilight in the stratosphere during the falls of 1980 and 1981 and the spring of 1982. The extinction and excess of radiance are shown to originate from an altitude near 60 km in the mesosphere, and they exhibit a large enhancement when the wavelength changes from 0.65 micron to 0.44 micron. The observed excess extinction leads to a zenithal optical depth equal to 0.066, of which 0.0054 is due to scattering by the dust layer. If it is accepted that the dust is responsible for both extinction and scattering in the 60 km layer, the dust scattering albedo is equal to 0.08. This low value is compatible with very small particles exhibiting an almost symmetric scattering phase function.