Hybrid Simulations of Wave Propagation and Ion Cyclotron Heating in the Expanding Solar Wind |
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Authors: | Paulett Liewer Marco Velli Bruce Goldstein |
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Institution: | (1) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109 |
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Abstract: | We present results from hybrid (particle ions, fluid electrons) simulations of the evolution of Alfvén waves close to the
ion cyclotron frequency in the solar wind, which take into account the basic properties of the background solar wind flow,
i.e., the spherical expansion and the consequent decrease in magnetic field and cyclotron frequency with increasing distance
from the Sun. We follow the evolution of a plasma parcel in a frame of reference moving with the solar wind using a 1D expanding
box hybrid model; use of the hybrid model yields a fully self-consistent treatment of the resonant cyclotron wave-particle
interaction. This model is related to a previous MHD model (Velli et al. 1992), which allows the use of a simple Cartesian
geometry with periodic boundary conditions. The use of stretched expanding coordinates in directions transverse to the mean
radial solar wind flow naturally introduces an anisotropic damping effect on velocity and magnetic field. We present results
for the case of a single circularly polarized Alfvén wave propagating radially outward. Initially, the wave is below the cyclotron
frequency for both the alpha partcles and protons. As the wind expands, the wave frequency (as seen in the solar wind frame)
decreases more slowly than the cyclotron frequencies and the wave comes into resonance. With only protons, heating occurs
as the wave frequency approaches the proton cyclotron frequency. With both alphas and protons, the alphas, which come into
resonance first, are observed to be preferentially heated and accelerated.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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