A Form of Nascent Solar Wind Outflow: Beam flow Generated by Slow-Mode Waves Through Landau Resonance in the Weakly Collisional Solar Atmosphere

Quasi-periodic disturbances of emission-line parameters are frequently observed in the corona. These disturbances propagate upward along the magnetic field with speeds 100 km s-1. This phenomenon has been interpreted as an evidence of the propagation of slow magnetosonic waves or argued to be a signature of the intermittent outflows superposed on the background plasmas. Here we aim to present a new "wave + flow" model to interpret these observations. In our scenario, the oscillatory motion is a slow mode wave, and the flow is associated with a beam created by the wave-particle interaction owing to Landau resonance. With the help of a kinetic model, we simulate the propagation of slow mode waves and the generation of beam flows. We find that weak periodic beam flows can be generated owing to Landau resonance in the solar corona, and the phase with strongest blueward asymmetry is ahead of that with strongest blueshift by about 1/4 period. We also find that the slow wave damps to the level of 1/e after the transit time of two wave periods, owing to Landau damping and Coulomb collisions in our simulation. This damping time scale is similar to that resulting from thermal-conduction in the MHD regime. The beam flow is weakened/attenuated with increasing wave period and decreasing wave amplitude since Coulomb collision becomes more and more dominant over the wave action. We suggest that this "wave + flow" kinetic model provides an alternative explanation for the observed quasi-periodic propagating perturbations in various parameters in the solar corona. Therefore, the compressible slow-mode waves, which can be driven and launched by magnetic reconnection, vertical piston oscillation, or periodic horizontal squeezing, is believed to play an important role in accelerating the plasmas into a form of the solar wind nascent outflows.