太阳风质子束流的演化

太阳神飞船观测表明，太阳风高速流中质子束分量相对于核分量的密度随日心距离增加而增加．提出解释这一观测现象的机理并给出二维数值模拟结果．由于阿尔芬波速随日心距离增加而减少，第二支左旋波将与更多的质子共振，把部分原来属于核分布的质子拉到束分布中来．用数值模拟方程方法求解回旋波共振导致的准线性扩散方程，数值结果与观测结果相符合．

THE DEVELOPMENT OF PROTON BEAM IN THE SOLAR WIND

In this paper a new mechanism to form the proton density distribution to explain the radial evolution of the relative beam density (nb/np) is suggested. It is found that it is the resonance between protons and cyclotron waves that drives the protons to form such a unique distribution. First, several possible cyclotron waves are calculated to give the cold plasma dispersion relation. Among those possible waves, the waves propagating outside will play an essential role in the beam distribution formation and the waves propagation both inside and outside will help the core distribution formation. With the axial symmetry assumption, the quasi-linear two-dimensional diffusion equation are solved numerically by using Lax-Wendroff and FTCS scheme combining cold plasma dispersion relation and wave-particle resonant condition. The boundary conditions are not the common ones. A specific explanation about the conditions is given. In the simulating process, an original isotropic proton velocity distribution and the plasma dispersion relations remain constant are assumed. Thus, with diffusion equation we simulated the transformation of solar wind proton density as it flowed from 0.35 AU to 0.9 AU. The contours of proton density distribution show explicitly that the core and beam parts of proton density distribution. In the solar wind plasma, with increasing heliocentric radial distance, the Alfven velocity decreases and more and more protons can be resonant with the second-branch left hand polarized waves, so a part of core protons can enter into beam distributions and become beam protons. From the numerical distribution the radial variation of proton density from 0.35 AU to 0.9 AU and is obtained the calculated results are compared with observed results. Although the cold plasma dispersion relation is assumed, which is not a self-consistent assumption, the simulating results are accordant with the observations. The result shows clearly that the ratio of proton beam number density to proton core number density increases with increasing heliocentric radial distance.