Parker Solar Probe observations of collisional effects on thermalizing the young solar wind

Abstract

Solar wind ions exhibit distinct kinetic non-thermal features such as preferential heating and acceleration of alpha particles compared to protons. On the other hand, Coulomb collisions in the solar wind act to eliminate these non-thermal features and gradually lead to thermal equilibrium. Previous observations at 1 au have revealed that even though the local Coulomb collisions in the solar wind plasma are rare, the cumulative effect of the collisions during a transit time of a particle can be important in terms of thermalizing the solar wind plasma populations and reducing the ion non-thermal features. Here, we analyze Parker Solar Probe observations to study the effects of Coulomb collisions on the non-thermal features (alpha-to-proton temperature ratio and differential flow) of young solar wind closer to the Sun than previously possible. Our results show that even close to the Sun (∼15Rs), these non-thermal features are organized by collisionality. Moreover, observations at these unprecedented distances allow us to investigate the preferential heating of the alpha particles close to the source for both fast and slow wind streams. We show that the alpha-to-proton temperature ratio is positively correlated with the solar wind speed, which is consistent with Wind observations. Solar wind close to the Sun is less collisionally old than when it reaches 1 au. As such, observed differences in the temperature ratio between slow and fast streams near their solar source suggest causes that go beyond different Coulomb numbers. Our results suggest that slow and fast wind streams, originating from different solar regions, may have different mechanisms for the preferential heating of alpha particles compared to protons.