Solar Wind Acceleration Driven by Velocity Space Diffusion and the Ambipolar Electric Potential

A global electric potential arises within the solar wind due to the mass disparity between electrons and protons, coupled with the constraints of charge quasi-neutrality and zero-current conditions on sufficiently large scales in the heliosphere. This so-called ambipolar electric potential may account for at least part of the solar wind acceleration. Recent findings from the Parker Solar Probe (PSP) reveal that the slow solar wind, with terminal velocities averaging around 250 km/s, could be entirely explained by the ambipolar electric potential. However, an additional, yet unidentified mechanism is required to explain the acceleration of the fast solar wind. 

Since the first in situ solar wind observations in 1959, neither magnetohydrodynamic nor kinetic models have been able to consistently account for the fast solar wind acceleration. Therefore, the processes responsible for this additional acceleration remain one of the most significant open questions in space physics. To address this challenge, we propose a pathway to account for the unexplained acceleration by incorporating velocity space diffusion of particles within the kinetic exospheric framework, which self-consistently determines the ambipolar electric potential. This was achieved for electrons by redistributing particles within regions of velocity space defined by the kinetic exospheric approach to account for a diffusion that would occur in the solar wind due to collisions or wave-particle interactions. These are therefore incorporated indirectly in the kinetic exospheric model through diffusion which inevitably fills regions of velocity space that would otherwise remain inaccessible and are thought to be the primary mechanisms behind the formation of the so-called halo population—higher-energy electrons that, unlike the strahl, are not predominantly directed an-sunward.

The recent discovery of the sunward deficit, predicted by the kinetic exospheric models, showed an anticorrelation between the electric potential and the solar wind terminal velocity, potentially implying that the electric potential is only a minor acceleration mechanism for the fast solar wind. We here find that even without the influence of velocity space diffusion, the same anticorrelation can be obtained by our kinetic exospheric model, from observationally derived input coronal temperatures, for a range of heliocentric distances typically sampled by PSP (above 13 Rs). This suggests that the electric potential might still be of major importance to explain the fast solar wind acceleration.