Expansion of solar corona in the Sun's gravitational field...

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 times 10⁷ cm s ^-1, i.e., it reaches the velocity of sound C_s =(gnkT/m_i)^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 sim r^g-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.