Torus-shaped velocity distribution of interstellar pickup He+ observed at unprecedented resolution

The heliosphere can be considered as a plasma bubble formed by the solar wind as it flows outward from the Sun, carving through the local interstellar medium. Due to the relative motion of the heliosphere against the local interstellar medium, a continuous stream of interstellar neutrals (ISNs) enters the heliosphere with a defined speed and direction. Upon entering the heliosphere, ISNs are subjected to ionization processes, which further leads to the creation of interstellar pickup ions (PUIs). PUIs are continuously injected into the solar wind. In the velocity space, their velocity distribution (VDF) initially forms a torus-like shape.

The energies of interstellar pickup helium fall within the measurement range of the SupraThermal Electrons and Protons (STEP) sensor of the Energetic Particle Detector (EPD) onboard Solar Orbiter. By sacrificing mass information, STEP achieves a temporal resolution of up to 1 second. This work presents observations from STEP/EPD. We report several clear torus-shaped velocity distributions of interstellar pickup He⁺ observed at unprecedented temporal resolution.