Coulomb Collisions as a Candidate for Temperature Anisotropy Constraints in the Solar Wind

• Published in: Astrophysical journal. Letters 2019-01, Vol.871 (1), p.L11
• Main Authors: Vafin, S., Riazantseva, M., Pohl, M.
• Format: Article
• Language: English
• Subjects: Anisotropy; Contours; Coulomb collisions; instabilities; plasmas; Protons; scattering; Shape; Solar wind; Sun: heliosphere; Temperature; Wind observation
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/aafb11

Many solar wind observations at 1 au indicate that the proton (as well as electron) temperature anisotropy is limited. The data distribution in the (Aa, βa, )-plane have a rhombic-shaped form around βa, ∼ 1. The boundaries of the temperature anisotropy at βa, > 1 can be well explained by the threshold conditions of the mirror (whistler) and oblique proton (electron) firehose instabilities in a bi-Maxwellian plasma, whereas the physical mechanism of the similar restriction at βa, < 1 is still under debate. One possible option is Coulomb collisions, which we revisit in the current work. We derive the relaxation rate of the temperature anisotropy in a bi-Maxwellian plasma that we then study analytically and by observed proton data from WIND. We found that increases toward small βp, < 1. We matched the data distribution in the (Ap, βp, )-plane with the constant contour s−1, corresponding to the minimum value for collisions to play a role. This contour fits rather well the left boundary of the rhombic-shaped data distribution in the (Ap, βp, )-plane. Thus, Coulomb collisions are an interesting candidate for explaining the limitations of the temperature anisotropy in the solar wind with small βa, < 1 at 1 au.