Coulomb collisions are believed to be a fundamental process governing the thermodynamic equilibrium of solar wind plasmas, yet their role within the Alfvénic slow solar wind remains poorly understood. In this study, we carry out a statistical analysis of proton core parameters measured by the Helios spacecraft to examine how its temperature anisotropy (T⊥/T∥) varies with collisional age for three types of solar wind, namely, fast wind, Alfvénic slow wind, and non-Alfvénic slow wind. Here, T⊥ and T∥ denote the proton temperatures perpendicular and parallel to the ambient magnetic field. Consistent with previous findings, we confirm that Coulomb collisions play a negligible role in the fast wind but strongly regulate the thermodynamics of non-Alfvénic slow wind. Remarkably, however, the Alfvénic slow wind displays two distinct regimes: a weakly collisional regime characterized by significant temperature anisotropy, T⊥/T∥ > 1, and a collisionally regulated regime characterized by T⊥/T∥ ≈ 1. In addition, a close examination of the radial evolution of several well-defined Alfvénic slow wind streams indicates that their thermodynamic behavior is strongly governed by several key plasma parameters, notably the solar wind speed. This in turn implies that the origin of the streams—either the cores of coronal holes or their over-expanded edges—determines the dominant processes governing the dynamics and thermodynamics of the Alfvénic slow wind.

Introduction
Data Analysis
    Helios Mission and Data Used
    Method
Results
    Statistical Analysis of the Three-Different Types of Solar Wind
    Thermodynamic Evolution of Alfvénic Slow Wind
Discussion and Conclusion
Acknowledgments
References

• Jimin Hong(Department of Astronomy and Space Science, Chungnam National University, Daejeon, Republic of Korea, Satellite Technology Research Center, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea, Hanwha Systems Co., ltd, Yongin, Republic of Korea)
• Kyungguk Min(Department of Astronomy and Space Science, Chungnam National University, Daejeon, Republic of Korea, Department of Earth, Environmental and Space Sciences, Chungnam National University, Daejeon, Republic of Korea, Institute for Sciences of the Universe, Chungnam National University, Daejeon, Republic of Korea)
• Jungjoon Seough(Korea Astronomy and Space Science Institute, Daejeon, Republic of Korea) Corresponding author