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

Cold 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. Plain Language Summary: The near‐Earth space environment is filled with plasma, that is, ionized gas that interacts with electromagnetic fields. Owing to its relative accessibility, it constitutes an invaluable laboratory for understanding how plasmas behave in nature. Many spacecraft missions have been launched with the purpose of studying space plasmas since the 1960s, when the space era began. They carry in situ instrumentation capable of measuring the properties of electric and magnetic fields, as well as the properties of ions and electrons. One problem these missions encounter is that the spacecraft produce their own electromagnetic fields that locally interact with the plasma and modify their properties. In this work, we quantify the effects of electric field booms mounted on spacecraft, which have length scales much larger than the spacecraft itself. The electromagnetic properties of these spacecraft booms strongly affect the detection and characterization of cold plasma, that is, low‐temperature plasma. Cold plasma in the near‐Earth space environment originates in the ionosphere, populates the whole magnetosphere, and constitutes the most abundant magnetospheric population. Key Points: The modeled MMS spacecraft electrostatic potential distribution deviates from spherical because of the electric field boomsMeasurements by particle detectors in the magnetosphere are strongly affected and biased by the complex potential structureCold ion beam properties are constrained by characterizing the ion wake and using a combination of particle and electric field measurements [ABSTRACT FROM AUTHOR]