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arXiv 2402.00441astro-ph.SRastro-ph.IM
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太阳偏振谱线建模中的碰撞效应

Collisional effects in modeling solar polarized lines

Moncef Derouich, Saleh Qutub

备注：
Accepted for publication in Astronomy & Astrophysics
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AI总结：

本研究指出太阳偏振谱线建模中帕邢-巴克效应与碰撞效应结合模型的不一致性，通过密度矩阵方法求解K I原子统计平衡方程，明确碰撞退偏振的氢密度敏感区间，提示零场碰撞速率不适用于磁化介质的帕邢-巴克区偏振分析。

摘要：

为了对现有高灵敏度太阳光谱偏振观测进行符合实际的分析，在偏振太阳谱线形成的建模过程中严格纳入碰撞效应至关重要。实际上，即便观测结果看似与理论吻合良好，若建模过程中未正确纳入碰撞带来的重要效应，研究者仍可能对结果产生误读。我们指出，在针对各向异性辐射散射形成的太阳线偏振建模中，同时纳入帕邢-巴克（Paschen Back）效应与碰撞效应的现有模型存在不一致性。由于偏振对碰撞的响应越强，这些不一致性的影响就越显著，因此我们研究了偏振敏感的氢原子数密度$n_\text{H}$范围。我们采用了基于原子-辐射相互作用理论与原子碰撞理论发展而来的、张量不可约基下的密度矩阵形式体系。为评估钾I（K I）原子D1-D2线能级的碰撞退偏振效应，我们求解了具有超精细结构（HFS）的多能级原子的统计平衡方程。研究发现，碰撞效应作用显著，尤其是在氢原子数密度处于10$^{13}$至10$^{16}$ cm$^{-3}$区间时。迄今为止，针对存在太阳磁场条件下形成的偏振谱线的分析，即便纳入了碰撞速率，所采用的也都是假设零磁场条件下计算得到的碰撞速率。这种近似在汉勒（Hanle）区适用，但在帕邢-巴克区并不成立。对于典型的宁静太阳磁场，可能达到帕邢-巴克区的条件，且多个原子系统中会发生能级交叉。因此，在使用零场条件下计算的碰撞速率来解释磁化介质中形成的线偏振时，必须格外谨慎。

英文摘要：

Rigorous implementation of the effects of collisions in modeling the formation of the polarized solar lines is of utmost importance in order to realistically analyze the available, highly sensitive solar spectropolarimetric observations. Indeed, even when an observation seems to fit well with theory, one can misinterpret results if important effects due to collisions are not correctly implemented in the modeling process. We point out inconsistencies in the models adopted to implement the Paschen Back effect together with collisional effects on the solar linear polarization formed by scattering of anisotropic radiation. Because the significance of these inconsistencies increases as polarization becomes increasingly responsive to collisions, we investigate the range of hydrogen densities $n_\text{H}$ to which the polarization is sensitive. We used the density matrix formalism in the tensorial irreducible basis, which was developed within the theory of atom-radiation interaction and of atomic collisions. We solved the statistical equilibrium equations for multi-level atoms with hyperfine structure (HFS) in order to evaluate the collisional depolarization of levels of the D1-D2 lines of the K I atom. We find that collisions play a prominent role, particularly at hydrogen densities of between 10$^{13}$ and 10$^{16}$ cm$^{-3}$. So far, analyses of polarized lines formed in the presence of solar magnetic field have incorporated, if at all, collisional rates calculated assuming zero magnetic field. This could be a good approximation in the Hanle regime but not in the Paschen Back regime. For typical quiet Sun magnetic fields, the latter regime could be reached, and level-crossing takes place in several atomic systems. Therefore, one must be careful when using collisional rates calculated in the zero-field case to interpret linear polarization formed in magnetized media.

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