Solar radiation (190–230 nm) in the stratosphere: Implications for photoelectric emissions from instrumentation at balloon altitudes

Abstract

Photoelectric contamination of conductivity measurements using the relaxation time method and electric field measurements using double‐probe systems have been observed well below maximum balloon altitudes. The solar wavelengths responsible for the photoemissions are primarily those in the 190–230 nm range. At these wavelengths, the atmosphere is partially transparent, allowing a large fraction of the incident solar photons to penetrate deep into the stratosphere. The effects of photoemissions on the electrical measurements have been observed at altitudes as low as 25 km, even for low solar angles. In the relaxation time measurements the photoelectric effects are manifested by a large perturbation of the positive conductivity measurement as a function of altitude and solar angle, with a ratio of the measured positive and negative conductivities greater than 1 above 25 km. We employ a photocurrent model to investigate the contamination of the relaxation time measurements. The model result shows that the solar flux in the range of 190–230 nm is more than sufficient to account for our conductivity observations. In the double‐probe measurements of electric fields, the photoelectric effects appear as perturbations to the probe potentials. As a result of the large flux of energetic solar photons in the stratosphere, many of the requirements for satellite double‐probe systems to reduce photoelectric contamination of electric field measurements are also applicable to balloon‐borne systems. On the basis of our analysis, we recommend that the surfaces of double‐probe instruments in the stratosphere be coated with a material of highly uniform work function in the optimum range of 4.6–5.4 eV. Our observations demonstrate the presence of a large flux of ultraviolet radiation in the stratosphere which is capable of photoemissions from aluminum surfaces and surfaces coated with high work function materials. As a consequence, many types of electrical or chemical measurements made in the sunlit stratosphere may suffer contamination by photoemissions.