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Huirong Yan (Dept of Astronomy, UW-Madison)
Astrophysical fluids are magnetized with magnetohydrodynamic (MHD) turbulence playing a key role for various astrophysical processes. In my thesis I study different astrophysical implications of MHD turbulence for propagation and acceleration of cosmic rays, dynamics of dust as well as the new ways of observational studies of magnetic fields.
Using recently obtained scaling laws for MHD modes, we identified fast modes as the dominant agent for cosmic ray scattering for most of the interstellar phases. This conclusion was reached in spite of the damping of fast modes that I took into account. In addition, we found that the traditional picture of shock acceleration is incomplete, as it ignores the effect of preexisting turbulence in the surrounding gas. Our research revealed suppression of streaming instability, which is an essential component of first order Fermi acceleration in shocks, by the ambient MHD turbulence. This suppression limits the energy of cosmic rays that can be accelerated by supernovae and invalidates many conclusions reached on cosmic ray confinement for models of galaxies embedded in fully ionized plasma.
We found that dynamics of charged grains is dominated by MHD turbulence in a most of the interstellar environments. We introduced new mechanisms of grain acceleration and calculated shattering and coagulation rates of grains, as well as the rate at which grains can adsorb heavy ions, allow segregation of different grains and their alignment. The obtained insight into grain dynamics is essential for understanding dust physics, chemistry and evolution.
Another direction I have been working on is the observational studies of astrophysical magnetic fields. We studied alignment of atoms in the presence of anisotropic radiation and magnetic field. Atoms can be aligned in terms of their angular momentum by anisotropic radiation which is common in astrophysical environment. The alignment is modified by magnetic fields that cause precession of atoms. We have identified atomic alignment enables a new tool to study astrophysical magnetic fields. Since it allows also temporal variations of magnetic fields, atomic alignment provides a cost effective way to study MHD turbulence at different scales.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.