Coulomb Collisions

Coulomb collisions—interactions between charged particles—gradually erase differences in speed and temperature between ion types. In nearly collision-free solar wind, helium ions and proton beams drift away from the core proton population, but their behaviors diverge: alpha particles slow down more with increased collisions, while proton beams maintain their speed better. This suggests that beams might be less affected by collisions or are sustained by other forces like wave interactions.

Figure showing alpha-proton and proton beam-core differential flow vs. coulomb collision frequency. — Alpha particles and proton beams both differentially flow faster than the bulk or core protons in the solar wind. This differential flow is aligned with the local magnetic field. Over the range of Coulomb Age ( $A_{c}$ , also called Coulomb Number $N_{c}$ ) that the alpha particle differential flow decays, proton beam-core differential flow is constant and independent of Coulomb collisions. From Alterman et al. (2018), The Astrophysical Journal.