低色球层\ion{Ba}{ii} $λ4554$线碰撞去极化的证据
Evidence for collisional depolarization of the \ion{Ba}{ii} $λ4554$ line in the low chromosphere
摘要：
上下文对\ion{Ba}{ii} $λ4554$的形成进行严格的建模是潜在的有趣之处，因为这种强极化线形成于太阳色球层，而那里的磁场是相当不清楚的。目标。研究与中性氢的各向同性碰撞在极化\ion{Ba}{ii} $λ4554$线形成中的作用，从而在磁场测定中的作用。方法.计算了\ion{Ba}{ii}的$d$和p态与中性氢各向同性碰撞的多极弛豫和转移率。我们考虑一个平面平行层的\ion{Ba}{ii}位于低色球和各向异性照明从下面产生线性偏振的$λ4554$线的散射过程。为了计算极化，我们求解了包括碰撞、辐射和磁场效应的\ion{Ba}{ii}能级的统计平衡方程。结果推导了弛豫速率和转移速率随氢数密度$n_{\textrm {\scriptsize H}}$和温度的变化规律。$λ4554$线的极化明显受到中性氢各向同性碰撞的影响，尽管其上能级$^2P_{3/2}$的碰撞去极化可以忽略不计。这是因为\ion{Ba}{ii}的亚稳能级$^2D_{3/2}$和$^2D_{5/2}$的对准易受冲突的影响。在$λ4554$线形成的高度（$n_{\textrm {\scriptsize H}} \sim 2 \times 10^{14}$ cm $^{-3}$），我们发现忽略碰撞导致$\sim$在偏振计算上的误差为25%，在微湍流磁场测量上的误差为35%.
英文摘要：
Context. Rigorous modeling of the \ion{Ba}{ii} $λ4554$ formation is potentially interesting since this strongly polarized line forms in the solar chromosphere where the magnetic field is rather poorly known. Aims. To investigate the role of isotropic collisions with neutral hydrogen in the formation of the polarized \ion{Ba}{ii} $λ4554$ line and, thus, in the determination of the magnetic field. Methods. Multipole relaxation and transfer rates of the $d$ and p-states of \ion{Ba}{ii} by isotropic collisions with neutral hydrogen are calculated. We consider a plane parallel layer of \ion{Ba}{ii} situated at the low chromosphere and anisotropically illuminated from below which produces linear polarization in the $λ4554$ line by scattering processes. To compute that polarization, we solve the statistical equilibrium equations for \ion{Ba}{ii} levels including collisions, radiation and magnetic field effects. Results. Variation laws of the relaxation and transfer rates with hydrogen number density $n_{\textrm {\scriptsize H}}$ and temperature are deduced. The polarization of the $λ4554$ line is clearly affected due to isotropic collisions with neutral hydrogen although the collisional depolarization of its upper level $^2P_{3/2}$ is negligible. This is because the alignment of the metastable levels $^2D_{3/2}$ and $^2D_{5/2}$ of the \ion{Ba}{ii} are vulnerable to collisions. At the height of formation of the $λ4554$ line where $n_{\textrm {\scriptsize H}} \sim 2 \times 10^{14}$ cm$^{-3}$, we find that the neglecting of the collisions induces inaccuracy of $\sim$ 25% on the calculation of the polarization and $\sim$ 35 % inaccuracy on microturbulent magnetic field determination.
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