Measurements of Nonthermal Signatures in Solar Wind Heavy Ion Velocity Distributions at 1 AU

Heavy ions (with atomic number Z >2) can be regarded as test particles in the solar wind plasma and have become a sensitive diagnostic tool to study (potentially) mass-per-charge dependent kinetic processes in the solar wind such as (preferential) particle acceleration and heating. For our investigation we utilize proton and heavy ion in-situ measurements from the Charge, ELement, and Isotope Analysis System (CELIAS) onboard the SOlar and Heliospheric Observatory (SOHO), that is located at the Lagrange point L1 at a distance of about one astronomical unit (1 AU) from the Sun. The measurement data is recorded during solar minimum between day of year (DOY) 174 and 220 in 1996. Based on an improved response model of the SOLO/CELIAS/CTOF sensor we present high precision measurements of kinetic properties of more than 20 solar wind heavy ion species covering an elemental range from carbon through iron and a mass-per-charge range from 2.3 to 8 amu/e. We investigate systematically 1) differential speeds between heavy ions and protons, 2) kinetic temperatures of heavy ions, and 3) suprathermal tails of heavy ion velocity distributions. The measurements allow in particular precise estimations of the mass-per-charge dependence of differential speeds in certain solar wind regimes (see attached figure). Together with earlier studies, based on measurements from the Advanced Composition Explorer (ACE), these observations provide a largely consistent picture of nonthermal solar wind ion signatures at 1 AU, that can be now compared with measurements from Solar Orbiter at variable solar distances in the very inner heliosphere.