SOLAR-WIND ACCELERATION BY SOLITARY WAVES IN CORONAL HOLES

Coronal holes are well-known sources of the high-speed solar wind; how ever, the exact acceleration mechanism of the fast wind is still unkno wn. We solve numerically the time-dependent, nonlinear, resistive 2.5- dimensional MHD equations and find that solitary waves are generated i n coronal holes nonlinearly by torsional Alfven waves. The solitary wa ve phase velocity was found to be slightly above the sound speed in th e coronal hole; for example, with the driving Alfven wave amplitude v( d) approximate to 36 km s(-1) and plasma beta = 5%, the solitary wave phase speed is similar to 185 km s(-1). We show with a more simplified analytical model of the coronal hole that sound waves are generated n onlinearly by Alfven waves. We find numerically that these waves steep en nonlinearly into solitary waves. In addition, ohmic heating takes p lace in the coronal hole inhomogeneities owing to phase-mixing of the torsional Alfven waves. When solitary waves are present, the solar win d speed and density fluctuate considerably on timescales of similar to 20-40 minutes in addition to the Alfvenic fluctuations. The solitary wave-driven wind might be in better qualitative agreement with observa tions than the thermally driven and WKB Alfven wave solar wind models.