IPS observations of the solar wind velocity and the acceleration mechanism

Coronal holes are well known sources of high speed solar wind, however, the exact acceleration mechanism of the wind is still unknown. IFS observations indicate that the fast solar wind reaches an average velocity of 800 km s(-1) within several solar radii with large velocity fluctuations. However, the origin of the large IFS velocity spread below 10R. is unclear. We apply a previously developed coronal hole model with a more realistic initial state and numerically solve the time-dependent, nonlinear, resistive 2.5-D MHD equations. We find that nonlinear solitary-like waves with a supersonic phase speed are generated in coronal holes by torsional Alfven waves in the radial flow velocity. The outward propagating nonlinear waves are similar in properties to sound solitons. When these waves are present the solar wind speed and density fluctuate considerably on a time scale of an hour and on spatial scales of several solar radii in addition to the Alfvenic fluctuations. This is in qualitative agreement with the IFS velocity observations beyond 10R..