First in-situ evidence of solar wind acceleration

Physical processes involved in the acceleration of stellar winds are essential in astrophysics as such winds are a mass loss process for stars. Hypothesis for stellar winds are difficult to check with experimental data in any other star but the Sun and even the processes of solar wind dynamics are poorly understood. Since the proposal by Parker of the stationary expansion of the solar atmosphere, the existence of the solar wind has been extensively demonstrated from data acquired from different missions in the Heliosphere. However, in-situ data showing that solar wind accelerates with the distance were still missing. Here we show the evolution of the solar wind speed distribution function with the heliocentric distance from 16 R⊙ to 172 R⊙ using Parker Solar Probe data. We observe that solar wind is accelerated until at least 60 R⊙, more than 20 R⊙ away than expected from previous studies. Below ∼ 25 R⊙ the subalfvenic wind dominates and is negligible from 35 R⊙ onwards. Here we find that the slow and fast winds are as mixed in the distribution function below 25 R⊙ as at 1 AU. Our findings provide empirical support for the models of solar wind dynamics.