We study coronal mass ejection (CME)-driven shocks and the resulting post-shock structures in the lower corona (2-7 R ). Two CMEs are erupted by modified Titov-Démoulin (TD) and Gibson-Low (GL) type flux ropes (FRs) with the Space Weather Modeling Framework. We observe a substantial pile-up of density compression and a narrow region of plasma depletion layer (PDL) in the simulations. As the CME/FR moves and expands in the solar wind medium, it pushes the magnetized material lying ahead of it. Hence, the magnetic field lines draping around the CME front are compressed in the sheath just ahead of the CME. These compressed field lines squeeze out the plasma sideways, forming PDL in the region. Solar plasma being pushed and displaced from behind forms a strong piled-up compression (PUC) of density downstream of the PDL. Both CMEs have comparable propagation speeds, while GL has larger expansion speed than TD due to its higher initial magnetic pressure. We argue that high CME expansion speed along with high solar wind density in the region is responsible for the large PUC found in the lower corona. In case of GL, the PUC is much wider, although the density compression ratio for both the cases is comparable. Although these simulations artificially initiate out-of-equilibrium CMEs and drive them in an artificial solar wind solution, we predict that PUCs, in general, will be large in the lower corona. This should affect the ion profiles of the accelerated solar energetic particles.
Collisions of gas particles with a drifting grain give rise to a mechanical torque on the grain. Recent work by Lazarian & Hoang showed that mechanical torques might play a significant role in aligning helical grains along the interstellar magnetic field direction, even in the case of subsonic drift. We compute the mechanical torques on 13 different irregular grains and examine their resulting rotational dynamics, assuming steady rotation about the principal axis of greatest moment of inertia. We find that the alignment efficiency in the subsonic drift regime depends sensitively on the grain shape, with more efficient alignment for shapes with a substantial mechanical torque even in the case of no drift. The alignment is typically more efficient for supersonic drift. A more rigorous analysis of the dynamics is required to definitively appraise the role of mechanical torques in grain alignment.
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