Particle-in-cell modeling of spacecraft-plasma interaction effects on double-probe electric field measurements

The double-probe technique, commonly used for electric field measurements in magnetospheric plasmas, is susceptible to environmental perturbations caused by spacecraft-plasma interactions. To better model the interactions, we have extended the existing particle-in-cell simulation technique so that it accepts very small spacecraft structures, such as thin wire booms, by incorporating an accurate potential field solution calculated based on the boundary element method. This immersed boundary element approach is effective for quantifying the impact of geometrically small but electrically large spacecraft elements on the formation of sheaths or wakes. The developed model is applied to the wake environment near a Cluster satellite for three distinctive plasma conditions: the solar wind, the tail lobe, and just outside the plasmapause. The simulations predict the magnitudes and waveforms of wake-derived spurious electric fields, and these are in good agreement with in situ observations. The results also reveal the detailed structure of potential around the double probes. It shows that any probes hardly experience a negative wake potential in their orbit, and instead, they experience an unbalanced drop rate of a large potential hill that is created by the spacecraft and boom bodies. As a by-product of the simulations, we also found a photoelectron short-circuiting effect that is analogous to the well-known short-circuiting effect due to the booms of a double-probe instrument. The effect is sustained by asymmetric photoelectron distributions that cancel out the external electric field.