Observations of stable pickup He$^+$ tori in a magnetic flux rope at 0.85 au

Abstract

Interstellar pickup ions originate from the neutral interstellar medium, are ionized in the heliosphere, and picked up by the solar wind. They initially form a torus-shaped velocity distribution function, which is generally believed to be transformed rapidly into an isotropic shell distribution by pitch-angle scattering. With the SupraThermal Electron Proton onboard Solar Orbiter we observe clear torus-shaped velocity distribution functions at an unprecedented one minute resolution. While these tori are variable on a time scale of one minute, they remain stable for over ten hours without signs of significant scattering. We conclude that they are populated by a huge fraction of the expected total number of pick-up ions injected in the past of the same solar wind stream.