770 research outputs found
MHD Simulation of the Inner-Heliospheric Magnetic Field
Maps of the radial magnetic field at a heliocentric distance of ten solar
radii are used as boundary conditions in the MHD code CRONOS to simulate a 3D
inner-heliospheric solar wind emanating from the rotating Sun out to 1 AU. The
input data for the magnetic field are the result of solar surface flux
transport modelling using observational data of sunspot groups coupled with a
current sheet source surface model. Amongst several advancements, this allows
for higher angular resolution than that of comparable observational data from
synoptic magnetograms. The required initial conditions for the other MHD
quantities are obtained following an empirical approach using an inverse
relation between flux tube expansion and radial solar wind speed. The
computations are performed for representative solar minimum and maximum
conditions, and the corresponding state of the solar wind up to the Earths
orbit is obtained. After a successful comparison of the latter with
observational data, they can be used to drive outer-heliospheric models.Comment: for associated wmv movie files accompanying Figure 7, see
http://www.tp4.rub.de/~tow/max.wmv and http://www.tp4.rub.de/~tow/min.wm
A Generalized Diffusion Tensor for Fully Anisotropic Diffusion of Energetic Particles in the Heliospheric Magnetic Field
The spatial diffusion of cosmic rays in turbulent magnetic fields can, in the
most general case, be fully anisotropic, i.e. one has to distinguish three
diffusion axes in a local, field-aligned frame. We reexamine the transformation
for the diffusion tensor from this local to a global frame, in which the Parker
transport equation for energetic particles is usually formulated and solved.
Particularly, we generalize the transformation formulas to allow for an
explicit choice of two principal local perpendicular diffusion axes. This
generalization includes the 'traditional' diffusion tensor in the special case
of isotropic perpendicular diffusion. For the local frame, we motivate the
choice of the Frenet-Serret trihedron which is related to the intrinsic
magnetic field geometry. We directly compare the old and the new tensor
elements for two heliospheric magnetic field configurations, namely the hybrid
Fisk and the Parker field. Subsequently, we examine the significance of the
different formulations for the diffusion tensor in a standard 3D model for the
modulation of galactic protons. For this we utilize a numerical code to
evaluate a system of stochastic differential equations equivalent to the Parker
transport equation and present the resulting modulated spectra. The computed
differential fluxes based on the new tensor formulation deviate from those
obtained with the 'traditional' one (only valid for isotropic perpendicular
diffusion) by up to 60% for energies below a few hundred MeV depending on
heliocentric distance.Comment: 8 pages, 6 figures, accepted in Ap
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