3. Corona Expansion: Gravitation Excluded
[15] The earlier published three-dimensional MHD calculations
[Minami et al., 2002;
Podgorny et al.,
2004]
without regard
for gravitation ( G=0 ) showed that the heliospheric current sheet formed
at deformation of the solar magnetic field by the corona plasma flow is
not neutral. It has a nonzero normal component of the magnetic field.
The configuration of the magnetic field and position of the current sheet
(crosshatched region) are shown in Figure 2b. For comparison, Figure 2a
presents the lines of the initial (dipole) field. In all the figures, the time
and coordinates in the computational domain are given in dimensionless
units, as in Figure 1. The distribution of current in the sheet in the
X= const plane is depicted in Figure 2c. The values in Figure 2 are shown
in dimensionless and dimensional (in brackets) units. The
three-dimensional configuration of the magnetic field with the heliospheric
current sheet is presented in Figure 2d. To facilitate spatial perception, a
transparent virtual plane
P(Y=0.5 ) is introduced. The magnetic field lines
located in front of the plane and in the plane are shown by solid lines,
and the field lines behind the plane are shown by dashed lines. All the
field lines are extended along the
X axis as compared with the dipole
field lines.
[16] Figures 2e and 2f present distributions of the plasma velocity,
temperature, and plasma density corresponding to the steady state
regime. At a distance of ~3.6
radii of the Sun from its center, the plasma
velocity is
1.8
107 cm s
-1,
i.e., it reaches the velocity of sound
Cs =(gnkT/mi)1/2.
The point of transition to the supersonic flow is marked
by a cross. The plasma temperature at a distance of ~10 radii exceeds by
more than an order of magnitude its value corresponding to the adiabatic
cooling
T
rg-1.
The temperature corresponding to the adiabatic cooling is
shown in Figure 2f by a circled cross. The temperature distribution in the
plasma flow accelerated by the pressure gradient is governed by two
basic factors, i.e., cooling at expansion and thermal conductivity. The
role of radiation is negligibly small. Calculations have shown that the
flow velocity at corona expansion is the highest in the region with
nonzero density and temperature gradients. At first, the velocity
maximum is in the computational domain. Then, when moving away
from the Sun, the velocity maximum crosses the right boundary and goes
beyond the domain.
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