Evolution of Alfven Waves in the Solar Wind. Monochromatic Driver

We use a 2.5D magnetohydrodynamic model to investigate the propagation of azimuthally driven Alfven waves with different periods and their interaction with the solar wind. In the absence of waves, the dipole field is stretched into a helmet streamer by the solar wind. The wind speeds near the equator are lower than those in the mid and high latitudes. Magnetic reconnection in the equatorial plasma sheet regularly triggers a breakup and expulsion of a plasmoid. We next inject monochromatic Alfven waves with a moderate amplitude of 9 km s(-1) and a period of tau = 1000 s at the coronal base. A cavity showing features of forward and backward propagating modes is formed. The backward waves are able to accelerate the background plasma at mid and high latitudes through the nonlinear coupling to compressional waves. The size of the cavity increases with the period of the Alfven waves as long as the outer boundary remains in the sub-Alfvenic wind. When tau = 4000 s, we find enhanced acceleration and heating of the solar wind plasma as well as suppression of the reconnection in the equatorial plasma sheet. The amplitudes of the backward Alfven waves remain large inside the cavity and modify its size. The cavity ceases to exist as its outer boundary gradually moves into the super-Alfvenic wind and the large amplitude backward waves are swept away by the wind. Results suggest that Alfven waves with moderate amplitudes can modify the dynamics and the energetics of the solar wind plasma with the embedded magnetic field.