Solar wind acceleration by large‐amplitude nonlinear waves: Parametric study

Abstract

We investigate the parametric dependence of the solar wind acceleration by large‐amplitude nonlinear (LAN) magnetohydrodynamic waves. For this purpose we model numerically the self‐consistent problem of the solar wind with waves by solving time‐dependent, nonlinear, resistive 2.5‐dimensional (three‐dimensional with azimuthal symmetry) MHD equations driven by Alfvén waves. We find that when the Alfvén wave amplitude is above a parameter‐dependent threshold, LAN waves are generated in the model coronal hole. For typical coronal parameters the solar wind speed and density fluctuate considerably on a timescale of ∼10–40 min and with an amplitude of up to several hundred kmilometers per second near the Sun ( r ≲ 10 R S ) in agreement with recent interplanetary scintillation observations. The solar wind speed is inversely dependent on the driving frequency in the range 0.35–3 mHz. The amplitude of the velocity fluctuations increases with the amplitude of the magnetic field and the driving Alfvén waves at the base of the corona and decreases with the coronal temperature. We found that for the same typical solar wind and Alfvén wave parameters and an isothermal initial atmosphere, the WKB model predicts 30% higher flow velocities far from the Sun (32 R S ) than our self‐consistent wave model with high‐frequency Alfvén waves (ƒ = 2.78 mHz), conforming to the WKB approximation. However, our model predicts significantly higher average flow speed near the Sun. When low‐frequency non‐WKB waves drive the wind, our model predicts 25% higher solar wind speed than the WKB model far from the Sun. This result of our model is in agreement with linear studies of solar wind acceleration by Alfvén waves that take into account Alfvén wave reflection.