Anterograde Collisional Analysis of Solar Wind Ions

Abstract

Owing to its low density and high temperature, the solar wind frequently exhibits strong departures from local thermodynamic equilibrium, which include distinct temperatures for its constituent ions. Prior studies have found that the ratio of the temperatures of the two most abundant ions—protons (ionized hydrogen) and α-particles (ionized helium)—is strongly correlated with the Coulomb collisional age. These previous studies, though, have been largely limited to using observations from single missions. In contrast, this present study utilizes contemporaneous, in situ observations from two different spacecraft at two different distances from the Sun: the Parker Solar Probe (PSP; r = 0.1–0.3 au) and Wind (r = 1.0 au). Collisional analysis, which incorporates the equations of collisional relaxation and large-scale expansion, was applied to each PSP datum to predict the state of the plasma farther from the Sun at r = 1.0 au. The distribution of these predicted α–proton relative temperatures agrees well with that of values observed by Wind. These results strongly suggest that, outside of the corona, relative ion temperatures are principally affected by Coulomb collisions and that the preferential heating of α-particles is largely limited to the corona.

Application of collisional analysis to the differential velocity of solar wind ions

“…Maruca et al (2013) applied this technique to individual θ αp -measurements from the Wind spacecraft (r = 1.0 au) to predict the distribution of θ αp -values at r = 0.1 au, i.e., near-Sun solar wind. This prediction was confirmed by Johnson et al (2023), who further validated the collisional analysis technique by showing that it could be applied to θ αp -measurements from Parker Solar Probe (PSP; r = 0.1-0.27 au) to effectively predict contemporaneous measurements of θ αp from Wind.…”

“…The data set used in this study is the same as the one used by Johnson et al (2023), who provide a detailed discussion of its properties and of the algorithm applied to correct for variations in sampling cadence. The data were derived from ion measurements from the electrostatic analyzer (ESA; Livi et al, 2021) in the SWEAP instrument suite (Kasper et al, 2016) onboard the PSP spacecraft (Fox et al, 2016).…”

“…Collisional analysis was introduced by Maruca et al (2013), combining the effects of expansion and Coulomb collisions on ion temperature ratios in any given parcel of solar wind plasma. A full in-depth review on collisional analysis is presented in Maruca et al (2013), with Johnson et al (2023) applying this technique to PSP data for the first time. This section presents a variation of collisional analysis that, for the first time, addresses ion differential flow rather than differences in ion temperature.…”

“…where μ 0 is the vacuum permeability. These scaling values are used to remain consistent with prior works, such as Maruca et al (2013) and Johnson et al (2023). Additionally, the values are not fixed and can be taken from observations or simulations.…”

Zone of Preferential Heating for Minor Ions in the Solar Wind

“…The high R b and high ε r are likely tied to each other, and, depending on their cause, could also be driving up χ 2 /dof values. Recent work by Johnson et al (2023) has shown, using PSP and Wind data, that the preferential heating of alpha particles persists up to about 0.27 au, beyond which Coulomb collisions provide the dominant thermalizing effect. While this is lower than the alpha-particle result shown here with SWICS data, it is within the range of SWICS R b predictions for ions with a q/m of about 0.5.…”