Properties of the satellite photoelectron sheath derived from photoemission laboratory measurements


  • Réjean J. L. Grard


The electrical properties of the medium around a probe in the outer magnetosphere and in interplanetary space are modified by photoemission. Information on the energy distribution of the photoemitted electrons is important for the evaluation of the physical parameters of the surrounding photosheath and for the interpretation of the probe measurements. This energy distribution has been determined for various materials exposed to sunlight, at the earth's orbit, by combining laboratory measurements on photoemission with solar spectrum data collected in space. It is found that the photoelectron saturation current density is a function of the material and can vary from a few microamperes per square meter up to several tens of microamperes per square meter; the photoelectron density is of the order of 10²–10³/cm³ in the vicinity of the probe. The shape of the distribution is approximately Maxwellian, and the mean kinetic energy is of the order of 1.5 ev. Current voltage characteristics and conductance of surfaces emitting photoelectrons are also derived. A number of photosheath parameters, such as shielding distance and surface electric field, are tabulated. Finally, the consequences of photoemission for the interpretation of space measurements are discussed.