The Sunward Electron Deficit: A Telltale Sign of the Sun’s Electric Potential

As the Parker Solar Probe explores new regions of inner heliosphere, it travels ever deeper into electric potential Sun. In near-Sun environment, a feature electron distribution emerges, in form deficit sunward suprathermal population. The lower boundary this forms cutoff phase space, at an energy determined by drop between observation point and outer heliosphere. We explore characteristics associated cutoff, as well properties plasma which we observe them. occurs ∼60%–80% observations within ∼0.2 au, even more frequently with low β, collisional age, anisotropic core At greater distances, rapidly disappears, halo grows, these two trends likely related. varies linearly local temperature, confirming direct relationship to ambipolar potential. Meanwhile, width β suggesting that diffusion plays role erasing deficit. nearly ubiquitous occurrence heliosphere suggests may need reconsider functional commonly used represent distributions environment.