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How Scientists study the Sun

The Sun, though the most familiar star to us, still holds many unsolved scientific mysteries, including the sunspot cycle and the coronal heating mystery. One such mystery is the enigmatic linear polarization of the sodium D1 line. Before exploring this issue, one may wonder how scientists study the Sun given that it is impossible to land on it. The answer lies in the sunlight that carries information about the Sun’s environment. By observing it, scientists can infer what is happening on the Sun.

Mysterious Spectral Line from the Sun

This blog post discusses the curious linear polarization of the sodium D1 line. The line originates from atoms transitioning from the 3p(J=1/2) level to the 3s(J=1/2) level, as illustrated in Figure 1. According to quantum theory, the Sodium D1 line should not carry polarization. However, in 1996, scientists observed that the Sodium D1 line from the Sun was linearly polarized, sparking interest in further research.

Figure 1: The energy levels related to the sodium D1 spectral line. This figure is adapted from the paper.

In 1998, , a theoretical explanation for the paradox emerged. The polarization of the sodium D1 line requires two critical ingredients: hyperfine structure and atomic polarization. The former arises from the interaction between nuclear spin and electron motion, leading to the original energy levels splitting into several sublevels with an additional label F (shown in Figure 1). The transitions with red dashed lines generate the sodium D1 line. However, the question remains of how to polarize the sodium D1 line from these levels. Atomic polarization suggests that having different atomic populations in these energy levels leads to a polarized line. However, this population difference is sensitive to magnetic fields, with the maximum allowable magnetic field strength being only about 0.01 G, much smaller than those from other observations (around 5~15G from other observations). Therefore, a satisfactory explanation for the mysterious polarized Sodium D1 line is still lacking.

Recent theoretical breakthrough

However, a recent study published in Physical Review Letters proposed another potential mechanism for generating a polarized line from hyperfine energy levels. Instead of atomic polarization, the authors focused on the difference in radiation fields pumping the atoms to higher hyperfine energy levels. When combined with the appropriate magnetic field strength, their simulation results demonstrated a strong agreement with the observed profile.

This new research provides a resolution to the two-decades-long sodium D1 line paradox and advances our understanding of the Sun to the next level. With ongoing or future observations, we can anticipate the resolution of more paradoxes about the Sun in the near future.