SH032-07
Surface Alfven Wave Reflection as the Slow Wind Formation Mechanism in the Alfven Wave driven Solar Atmosphere Model
Friday, 11 December 2020: 10:54
Virtual
Igor Sokolov, University of Michigan Ann Arbor, Ann Arbor, MI, United States, Bart van der Holst, University of Michigan, Climate and Space Sciences and Engineering, Ann Arbor, MI, United States, Charles Nickolos Arge, NASA Goddard Space Flight Center, Greenbelt, MD, United States and Tamas I Gombosi, University of Michigan, Department of Climate and Space, Center for Space Environment Modeling, Ann Arbor, MI, United States
Abstract:
The Alfven Wave turbulence based Solar atmosphere Model (AWSoM) employs the turbulent dissipation as the unified mechanism to explain both the coronal heating in the closed field region and powering and accelerating the solar wind in the coronal holes. The pronounced difference in the heating efficiency between the closed field region and coronal holes is attributed to the physical properties of Alfven wave turbulence, namely, that the nonlinear dissipation of the Alfven wave of the given direction is proportional to an amplitude of the oppositely propagating wave. With this regard, in the closed field region the efficient heating occurs, since the waves propagating from two ends of the closed line form a balanced turbulence with a high dissipation efficiency. To the contrary, within coronal holes the field lines are open, so that the outward propagating waves dominate. The only source for the inward propagating wave is the comparatively weak reflection of the dominant waves, resulting in gradually heating the solar wind as well its acceleration by the turbulent pressure.
This unified model explains the EUV images with a contrast between hot and dense (hence, bright) closed field region, and cooler rarefied (hence, dark) coronal hole plasma. However, the description of the solar wind speed is not perfect and worse than that provided by the WSA model.
Here, we incorporate one of the WSA model features, namely, the dependence of the solar wind speed on the angular distance from the coronal hole boundary to the footpoint of the magnetic field line connecting the observation point to the Sun. The closer is the coronal hole boundary, the denser and slower is the solar wind, within the WSA model. The present shortcoming of the AWSoM solar wind is that the closed field region proximity is not accounted for.
The heating near the coronal hole boundary is included into the AWSoM model via the surface Alfven wave present due to large difference in density between the closed and open field lines. Although the excessive heating on the surface wave is possible, we chose to parameterize the surface effect in terms of excessive nonlinear reflection proportional to the transverse gradient of density. The efficiency of such reflection is derived analytically and compared both with the WSA model prediction and with the solar wind observation data at 1 AU.
