The evolution of the solar wind proton temperature anisotropy from 0.3 to 2.5 AU

We report an analysis of the proton temperature anisotropy evolution from 0.3 to 2.5 AU based on the Helios and Ulysses observations. With increasing distance, the fast wind data show a path in the parameter space (βallel p,T\perp p/Tallel p), and the first part of the trajectory is well described by an anticorrelation between the temperature anisotropy T\perp p/Tallel p and the proton parallel beta, while after 1 AU the evolution with distance in the parameter space changes and the data result in agreement with the constraints derived by a fire hose instability. The slow wind data show a more irregular behavior, and in general it is not possible to recover a single evolution path. However, on small temporal scale we find that different slow streams populate different regions of the parameter space, and this suggests that when considering single streams also the slow wind follows some possible evolution path.