Determining the photocurrent of spherical probes from one-sonde-shading electric field data

We propose a method for determining the saturation photoelectron current density of spherical probes, designed for measuring the electric field by the double-probe technique. Shading of only one of the probes causes specific changes in the electric field, and using the potential difference ΔV between the two probes created by the shading we obtain an analytical expression for the photocurrent density. We derive this expression in two different ways: directly, from the potential difference; and after calculating the resistance of the plasma layer around the probe. In both cases the value of the photocurrent is determined from the ion current. Data for the concentration and the plasma temperature is needed to determine the ion current. We considered two limiting forms of its collection: (1) SAL-sheath area limited case (thin layer); (2) OML-orbital motion limited case (thick layer). We validate the method using data from Intercosmos Bulgaria - 1300 satellite. The photocurrent density is calculated for two shading-of-the-probe instances. The values we obtain are significantly larger than the ones from laboratory measurements, but close to the photocurrent values deduced from other space experiments.