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Angular momentum transport and proton-alpha differential streaming in the solar wind

Feb, 2007
26 pages
Published in:
  • Astrophys.J. 661 (2007) 593-601
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Abstract: (arXiv)
The effect of solar rotation on the proton-alpha differential flow speed, vαpv_{\alpha p}, and consequently on the angular momentum transport in the solar wind, is explored. It is found that the force introduced by the azimuthal components plays an important role in the force balance in interplanetary space, bringing the radial flow speeds of the species considered closer to each other. For the fast solar wind, the model cannot account for the decrease of vαpv_{\alpha p} observed by Helios between 0.3 and 1 AU. However, it can reproduce the profile of vαpv_{\alpha p} measured by Ulysses beyond 2 AU, if the right value for vαpv_{\alpha p} is imposed at that distance. In the slow solar wind, the effect of solar rotation is more pronounced if one starts with the value measured by Helios at 0.3 AU. In this case, solar rotation introduces a relative change of 10-16% in the radial flow speed of the alpha particles between 1 and 4 AU. The model calculations also show that, although alpha particles consume only a small fraction of the energy and linear momentum fluxes of protons, they cannot be neglected when considering the proton angular momentum flux Lp{\cal L}_p. In most examples, it is found that Lp{\cal L}_p is determined by vαpv_{\alpha p} for both the fast and the slow wind. In the slow solar wind, it is also found that the proton and alpha angular momentum fluxes Lp{\cal L}_p and Lα{\cal L}_\alpha can be several times larger in magnitude than the flux carried by the magnetic stresses LM{\cal L}_M. While the sum of the angular momentum fluxes LP=Lp+Lα{\cal L}_P={\cal L}_p+{\cal L}_\alpha of both species is found to be smaller than the magnetic stress LM{\cal L}_M, for the fast and slow wind alike, this result is at variance with the Helios measurements.