Some common errors in measurement of spacecraft potential

Summary form only given. Some common spacecraft charging measurement methods and their possible errors are discussed. One of the most common methods is to measure the potential difference between the tip of a boom and the spacecraft body. The boom has to be sufficiently long so that the tip is far away from the sheath electric field of the spacecraft body. The tip surface has to be made of a material which does not easily charge relative to the space plasma. In severe magnetic storms, natural charging of the tip can occur despite the material chosen. When a spacecraft is rotating in sunlight, photoemission from the booms can modulate the potential of the spacecraft body. The amplitude of potential modulation depends on the sun angle, the photoelectron current and the spacecraft potential. Another common method is to equate the energy shift of the electron distribution function to the spacecraft potential. This method often gives errors if there is a local potential barrier confining low energy electrons. Ionization during electron beam emissions may also affect the shift. Also, since the measured electrons may have significant angular momentum, it may be erroneous to identify the shift to the potential energy only. Another common method is to use Langmuir probes near the spacecraft surface. It may happen that the probe, while repelling electrons from the spacecraft, is attracting electrons from the space plasma, or vice versa. In this condition, neither the attractive regime probe formula nor the repulsive one is properly applicable. Finally, we discuss surface potential measurements of deep dielectric charging caused by energetic electrons and ions in space.