In Situ Observations of Interstellar Pickup Ions from 1 au to the Outer Heliosphere

Interstellar pickup ions are an ubiquitous and thermodynamically important component of the solar wind plasma in the heliosphere. These PUIs are born from the ionization of the interstellar neutral gas, consisting of hydrogen, helium, and trace amounts of heavier elements, in the solar wind as the heliosphere moves through the local interstellar medium. As cold interstellar neutral atoms become ionized, they form an energetic ring beam distribution comoving with the solar wind. Subsequent scattering in pitch angle by intrinsic and self-generated turbulence and their advection with the radially expanding solar wind leads to the formation of a filled-shell PUI distribution, whose density and pressure relative to the thermal solar wind ions grows with distance from the Sun. This paper reviews the history of in situ measurements of interstellar PUIs in the heliosphere. Starting with the first detection in the 1980s, interstellar PUIs were identified by their highly nonthermal distribution with a cutoff at twice the solar wind speed. Measurements of the PUI distribution shell cutoff and the He focusing cone, a downwind region of increased density formed by the solar gravity, have helped characterize the properties of the interstellar gas from near-Earth vantage points. The preferential heating of interstellar PUIs compared to the core solar wind has become evident in the existence of suprathermal PUI tails, the nonadiabatic cooling index of the PUI distribution, and PUIs' mediation of interplanetary shocks. Unlike the Voyager and Pioneer spacecraft, New Horizon's Solar Wind Around Pluto (SWAP) instrument is taking the only direct measurements of interstellar PUIs in the outer heliosphere, currently out to ∼ 47 au from the Sun or halfway to the heliospheric termination shock.

Analysis of Small-scale Magnetic Flux Ropes Covering the Whole Ulysses Mission

Small-scale magnetic flux ropes in the solar wind have been studied for decades via both simulation and observation. Statistical analysis utilizing various in situ spacecraft measurements is the main observational approach, which helps investigate the generation and evolution of these small-scale structures. In this study, we extend the automated detection of small-scale flux ropes based on the Grad-Shafranov reconstruction to the complete data set of in situ measurements of the Ulysses spacecraft. We first discuss the temporal variation of the bulk properties of 22,719 flux ropes found through our approach, namely, the average magnetic field and plasma parameters, etc., as functions of the heliographic latitudes and heliocentric radial distances. We then categorize all identified events into three groups based on event distributions in different latitudes separated by 30°, at different radial distances, and under different solar activities. With the detailed statistical analysis, we conclude the following: (1) the properties of flux ropes, such as the duration, scale size, etc., follow power-law distributions, but with different slope indices, especially for distributions at different radial distances. (2) They are also affected by the solar wind speed, which has different distributions under different solar activities, manifested as a latitudinal effect. (3) The main difference in flux rope properties between the low and high latitudes is attributed to possible Alfvénic structures or waves and to flux ropes with relatively high Alfvénicity. (4) Flux ropes with longer durations and larger scale sizes occur more often at larger radial distances. (5) With a stricter Walén slope threshold, more events are excluded at higher latitudes, which further reduces the latitudinal effects on flux rope properties. The entire database is published online at http://www.fluxrope.info.