Electrostatic Spacecraft Potential Structure and Wake Formation Effects for Characterization of Cold Ion Beams in the Earth's Magnetosphere

International audienceCold plasma (up to few tens of electron volts) of ionospheric origin is present most of the time, in most of the regions of the Earth's magnetosphere. However, characterizing it using in situ measurements is difficult, owing to spacecraft electrostatic charging, as often this charging is at levels comparable to or even higher than the equivalent energy of the cold plasma. To overcome this difficulty, active potential control devices are usually placed on spacecraft that artificially reduce spacecraft charging. The electrostatic potential structure around the spacecraft is often assumed to be spherically symmetric, and corrections are applied to the measured particle distribution functions. In this work, we show that large deviations from the spherical model are present, owing to the presence of long electric field booms. We show examples using Magnetospheric MultiScale spacecraft measurements of the electrostatic potential structure and its effect on the measurement of cold ion beams. Overall, we find that particle detectors underestimate the cold ion density under certain conditions, even when their bulk kinetic energy exceeds the equivalent spacecraft potential energy and the ion beam reaches the spacecraft. Active potential control helps in reducing this unwanted effect, but we show one event with large cold ion density (∼10 cm-3) where particle detectors provide density estimates a factor of 3-5 below the density estimated from the plasma frequency. Understanding these wake effects indirectly constrains some properties of the magnetospheric cold ion component, such as their drift energy, direction, and temperature