Alfven Wave Evolution in an Interaction System of the Fast and Slow Solar Wind

Large-amplitude Alfven waves (AWs) are often embedded in a high-speed stream of the solar wind. As the high- speed streams overtake the low-speed streams ahead, corotating interaction regions (CIRs) are produced in low heliographic latitudes. In this study, the nonlinear evolution of AWs swept into CIRs is numerically investigated by one-dimensional MHD simulations. Ulysses observations suggest that not only AWs amplified through the reverse shock but also magnetic depression structures (MDs) are found in the trailing portions of CIRs (e.g., Tsurutani et al., 1995). Our interest is the generation mechanism of MDs in the context of AWs-CIRs interaction system. While MDs are supposed to be remnants of the mirror instability (e.g., Winterhalter et al., 1994), we give alternative processes from a macroscopic view as follows. A large pressure gradient developed in CIRs results in intensifying the diamagnetic current, which reflects a portion of the incident AW energy in the opposite direction (from a plasma-rest frame) as AWs penetrate into CIRs. Since the reflected AWs also carry the current, the reduction of the background field intensity (i.e. MD formation) is simultaneously taken place in the area sandwiched between the forward-reverse pair of AWs. Further analysis will be given via hybrid simulations to show how these MHD processes are manifested in particle behaviors, such as an acceleration due to a ponderomotive force.