Constraining time scales of interstellar pick-up ion transport processes with Solar Orbiter/EPD/STEP

• Applicant: Dr. Verena Heidrich-Meisner
• Subject Area: Astrophysics and Astronomy
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 567568091

Neutral particles from the local interstellar medium can enter our solar system unimpeded. If they are ionized within the heliosphere, the resulting ions follow the interplanetary magnetic field embedded in the solar wind and are called pick-up ions (PUIs) with a characteristic velocity distribution function (VDF). The properties of VDFs of such interstellar PUIs result from a combination of three factors: the characteristics of their interstellar seed population, the conditions at the time of injection into the solar wind, and subsequent transport effects. Thereby, interstellar PUIs provide a unique insight into the neighbourhood of our solar system and facilitate the investigation of fundamental physical processes under conditions that cannot be studied in Earth-bound laboratories. So far, PUI studies have been frequently limited by statistics, either confined to long-term studies (with very few exceptions), or, in the case of He+ PUIs, mainly focused on a single region in the heliosphere, the PUI focusing cone. Here, we adopt a novel approach and utilize an instrument on Solar Orbiter that was designed to measure suprathermal particles in solar energetic particle events but has since proven effective for detecting PUIs, the SupraThermal Electron and Proton (STEP) sensor which is part of the Energetic Particle Detector (EPD) on Solar Orbiter. The good PUI statistics of STEP together with the unique orbit of Solar Orbiter now allow for the first time to provide data-driven estimates of the time scales of PUI transport effects.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Robert F. Wimmer-Schweingruber