Solar Wind Acceleration by Solitary Waves in Coronal Holes

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Abstract
Coronal holes are well-known sources of the high-speed solar wind; however, the exact acceleration mechanism of the fast wind is still unknown. We solve numerically the time-dependent, nonlinear, resistive 2.5-dimensional MHD equations and find that solitary waves are generated in coronal holes nonlinearly by torsional Alfvén waves. The solitary wave phase velocity was found to be slightly above the sound speed in the coronal hole; for example, with the driving Alfvén wave amplitude vd ≈ 36 km s-1 and plasma β = 5%, the solitary wave phase speed is ~185 km s-1. We show with a more simplified analytical model of the coronal hole that sound waves are generated nonlinearly by Alfvén waves. We find numerically that these waves steepen nonlinearly into solitary waves. In addition, ohmic heating takes place in the coronal hole inhomogeneities owing to phase-mixing of the torsional Alfvén waves. When solitary waves are present, the solar wind speed and density fluctuate considerably on timescales of ~20-40 minutes in addition to the Alfvénic fluctuations. The solitary wave-driven wind might be in better qualitative agreement with observations than the thermally driven and WKB Alfvén wave solar wind models.