Collisional and magnetic effects on the polarization of the solar oxygen infrared triplet

TL;DR

The study addresses how elastic collisions with neutral hydrogen and turbulent magnetic fields shape the atomic alignment responsible for the solar O I infrared triplet polarization. It advances the field by computing a comprehensive set of depolarization and polarization-transfer rates for a multi-level O I model and solving the statistical equilibrium equations with both collisions and the Hanle effect. The key finding is that in the deep photosphere ($n_{ m H} \sim 10^{15}-10^{16}$ cm$^{-3}$) elastic collisions together with fields $B \gtrsim 20$ G strongly suppress alignment, while in the chromosphere the lower density weakens depolarization, allowing polarization to persist; this supports a chromospheric origin for the observed signals. The results underscore the need to couple accurate collisional rates with non-LTE radiative transfer to reliably diagnose solar magnetism from the O I IR triplet.

Abstract

Context: The scattering polarization of the infrared (IR) triplet of neutral oxygen (O\,\textsc{i}) near 777\,nm provides a powerful diagnostic of solar atmospheric conditions. However, interpreting such polarization requires a rigorous treatment of isotropic depolarizing collisions between O\,\textsc{i} atoms and neutral hydrogen. Aims: We aim to investigate the combined effects of collisional and magnetic depolarization in shaping the alignment of O\,\textsc{i} levels (and thus the polarization of the O\,\textsc{i} IR triplet). Methods: We compute, for the first time, a comprehensive set of collisional depolarization and polarization transfer rates for the relevant O\,\textsc{i} energy levels. These rates are incorporated into a multi-level atomic model, and the statistical equilibrium equations (SEE) are solved to quantify the impact of collisions and magnetic fields on atomic alignment. Results: Our calculations indicate that elastic collisions with neutral hydrogen, together with the Hanle effect of turbulent magnetic fields stronger than about 20 G, efficiently suppress the bulk of the atomic alignment in deep photospheric conditions where hydrogen densities exceed $n_{\mathrm{H}} \sim 10^{16}$ cm$^{-3}$. In the chromosphere, however, the lower hydrogen density weakens collisional depolarization, allowing polarization to persist. Conclusions: Our results are consistent with a chromospheric origin for the linear polarization signals of the O I IR triplet. Future studies should combine accurate non-LTE radiative transfer with reliable collisional rates in order to achieve fully consistent modeling.

Figures (5)

• Figure 1: Energy level diagram of the atomic model of the O i IR triplet adopted in this work. Blue arrows indicate the transitions of the O i IR triplet, while grey arrows represent the other radiative transitions included in the model.
• Figure 2: Normalized atomic alignment $[{\rho_0^{(2)}(n_{\mathrm{H}})}/{\rho_0^{(2)}(n_{\mathrm{H}} \!=\! 0)}]_{B = 0}$ as a function of hydrogen density $n_{\mathrm{H}}$ in the absence of a magnetic field ($B \!=\! 0$), showing the pure effect of elastic collisions on the fine-structure levels of the O i IR triplet.
• Figure 3: Sensitivity of the normalized alignment, $\rho^{2}_{0}(n_{\mathrm H},B=0)/\rho^{2}_{0}(n_{\mathrm H}=0,B=0)$, to uncertainties in the elastic O--H collisional operator $D$ as a function of $\log_{10}(n_{\mathrm H}/\mathrm{cm^{3}})$. For each level, the solid curve is the reference calculation ($D$), while the shaded band spans the range obtained by uniformly scaling all elastic rates to $D/10$ and $10 \times D$. The gray band and black line refer to the $S_{2}$ level (3s $^{5}\!S$, $J=2$), and the blue band and line to the $P_{1}$ level (3p $^{3}\!P$, $J=1$) shown in Fig. 2. Results are for $B=0$.
• Figure 4: Normalized alignment, $\rho^{(2)}_0(n_{\mathrm{H}},B)/\rho^{(2)}_0(n_{\mathrm{H}}\!=\!0, B\!=\!0)$, as a function of the magnetic field strength $B$ for several hydrogen densities $n_{\mathrm{H}}$, for four levels of O i. From top to bottom: $3p\,^5P_1$, $3p\,^5P_2$, $3p\,^5P_3$, and $3s\,^5S_2$. Each curve corresponds to a fixed value of $n_{\mathrm{H}}$, ranging from $0$ to $5 \times 10^{16}\,\mathrm{cm}^{-3}$, as labeled. The plots illustrate how both elastic collisions with neutral hydrogen and the Hanle effect contribute to the depolarization of each level.
• Figure 5: Same as Figure \ref{['OI_collisions_sensitivity_B0']}, but for a magnetic field strength of $B = 20$ G, illustrating the combined effect of elastic collisions and magnetic depolarization (Hanle effect) on the atomic alignment of the O i IR triplet levels.