Three-dimensional MHD modeling of the solar wind with pickup protons

A. V. Usmanov, M. L. Goldstein, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

Using a 3D MHD steady-state solar wind model we have simulated the magnetic
field and plasma distribution throughout the heliosphere from the coronal
base out to 100~AU for a dipole magnetic field on the Sun tilted by
30$^\circ$ to the solar rotation axis. This represents the solar conditions
typical for a declining phase of solar cycle. The model assumes a uniform
distribution of neutral hydrogen throughout the heliosphere and accounts
for the processes of photoionization and charge exchange between the solar
wind protons and pickup protons. We present simulation results on the
formation of corotating interaction regions, their evolution with
heliocentric distance, and the effects of pickup protons in this evolution.
The simulation results are consistent with earlier studies in showing that
the pickup protons cause a deceleration of the solar wind beyond 10~AU.
Comparison of simulation runs with and without pickup protons shows that
the pickup protons act to weaken the corotating interaction regions. This
is presumably due to the deceleration of solar wind and additional pressure
from the pickup protons. The results show a significant latitude-dependent
compression effect of the deceleration on the interplanetary magnetic
field. The proposed solar wind model can be useful in studies of the global
heliospheric structure and the interaction of solar wind with the LISM.