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Session 86 - Coronal Activity.
Oral session, Thursday, June 13
Coronal holes are well known sources of the high speed solar wind, however, the exact acceleration mechanism of the wind is still unknown. We solve numerically the time-dependent, nonlinear, resistive 2\frac12-D MHD equations and find that solitons are generated in coronal holes nonlinearly by torsional Alfvén waves. Initially, the ponderomotive force due to Alfvén waves excites longitudinal magnetosonic waves by coupling to the radial component of the momentum equation. Next, these waves steepen into solitons that accelerate the solar wind to supersonic speed in the radial direction even in a low-\beta plasma. 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 v_d\approx40 km s^-1, and plasma \beta=5% the soliton phase speed \sim 200 km s^-1. We investigate the parametric dependence of the soliton wavelength and frequency on the plasma \beta, and on the driving Alfvén wave amplitude and frequency. More simplified analytical model of the coronal hole leads to the Benjamin-Ono equation that predicts the generation of solitons analytically. The compressive dissipation of solitary waves may contribute significantly to coronal hole heating. In addition, Ohmic heating takes place near the coronal hole boundaries due to phase-mixing of the torsional Alfvén waves in the inhomogeneous regions. When solitary waves are present the solar wind fluctuates considerably on long time scales and on small spatial scales in addition to the Alfvénic fluctuations. This is in better qualitative agreement with observations than the thermally driven and WKB Alfvén wave solar wind models.
Program listing for Thursday