Interstellar Pickup and Suprathermal Ions from New Horizons and Cassini Data

Interstellar gas that approaches the Sun is ionized and accelerated to solar wind speeds, yielding interstellar pickup ions with a suprathermal tail in their energy distributions. Here we use data from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on the New Horizons spacecraft and the Charge-Energy-Mass Spectrometer (CHEMS) sensor of the Magnetospheric IMaging Instrument (MIMI) on the Cassini spacecraft in the keV to hundreds of keV range acquired outside of 5 astronomical units (AU) from the Sun to study these ions. Although PEPSSI does not directly distinguish species at the lowest energies, we show that ion counts in that range are dominated by pickup-helium. A combination of modeling with new laboratory measurements using the PEPSSI engineering model has allowed us to improve on the intensity calibration. The dominant parameter organizing the distributions of protons and helium in the spacecraft frame is the cone angle relative to the Sun. The instruments on Cassini and New Horizons can measure a large range of these angles since those spacecraft are 3-axis stabilized (New Horizons for various intervals of time and otherwise spin stabilized). Transformation of the distribution functions into the co-moving frame of reference reveals that the distributions at these relatively large distances to the Sun are, to first order, isotropic. "Typical" distributions show a sharp drop at about twice the solar wind speed, followed by a suprathermal population that can be described by a power law. It has been known that the overall distribution can exhibit less structure near shocks in the solar wind, when the population of suprathermal particles becomes more intense. Such "shallow" distributions are commonly measured by CHEMS and PEPSSI. We study the relative amplitude and power-law exponent of the suprathermal particles over the solar cycle while CHEMS stays around 10 AU and PEPSSI moves towards 30 AU and beyond. We find that these quantities scale to first order with time, over the solar cycle. The flattest spectra are found after solar maximum and for low overall intensities.