Astronomers have been able to resolve the solar sodium paradox, which has existed for over 20 years. They managed to theoretically prove that the linear polarization of radiation at the wavelength of the absorption line D 1 of sodium in the solar atmosphere also occurs in the presence of magnetic fields with an intensity of several Gauss, and the calculations agree with the data from observations of the Sun.

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Everything you need to know about the solved solar paradox

Solar Radiation

Most of the solar radiation that reaches us from its quiet areas in terms of activity (outside sunspots) is linearly polarized, especially close to the edge of the star’s disk. This is explained by the scattering of anisotropic radiation (the intensity of which depends on the direction) of the Sun in its atmosphere.

Polarization of radiation

If we talk about spectral lines, then the polarization of radiation occurs when an imbalance of the population of energy levels and quantum interference is created between the magnetic sublevels of the atoms of the solar atmosphere, which absorb the radiation incident on them (anisotropic optical pumping ).

Explanations

In 1998, it was discovered that there is a polarization of radiation at the wavelength of the absorption line of sodium D 1 (it is included in the well-known “sodium doublet” in the absorption spectrum of the Sun), which can only be explained by taking into account the hyperfine structure of the sodium atom and assuming that the lower energy the level corresponding to the ground state of sodium has significant polarization.

Contradictions

However, in this case, the lower chromosphere of our star should be practically non-magnetized (magnetic fields are at a level of several milligauss), which contradicts the results of observations, as well as theoretical considerations in the field of plasma physics, requiring the presence of fields on a Gaussian scale, which caused a paradox.

Solving the solar paradox

Solving the solar paradox is necessary to understand the physical processes in the solar chromosphere, which will be modeled on the basis of future detailed spectropolarimetric observations using next-generation telescopes such as DKIST.

New study

A group of astronomers led by Ernest Alsina Ballester of the IRSOL Research Institute in Switzerland published a paper in which they tried to resolve the solar paradox by simulating the polarization of solar radiation in the sodium doublet lines.

Method

Scientists used the recently developed theory of atomic-photon interactions, which made it possible to take into account the correlations between the state of photons before and after scattering (the phenomenon of partial frequency redistribution) in the presence of collisions of atoms and magnetic fields.

Model

Thus, the model was the first to take into account the details of the spectral structure of radiation together with the influence of magnetic fields of arbitrary strength and elastic collisions in a realistic atomic model that includes a hyperfine structure.

Results

The simulation results and calculations, which were compared with observational data with a ZIMPOL-3 polarimeter behind a quiet region close to the edge of the solar disk, showed that linear polarization of radiation at the D1 line wavelength can occur in the absence of any polarization of the ground state of sodium, even in the presence of magnetic fields with a strength of 15 gausses, which is required by the current models of the solar atmosphere.

Conclusions

The researchers note that the simulation was carried out for a somewhat idealized one-dimensional static model of the solar atmosphere – in reality, the effect of inhomogeneity of plasma parameters will be noticeable. However, the work still solves the solar paradox and, according to scientists, will help in studying the magnetic fields of the solar chromosphere in the current era of solar telescopes with large apertures.

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Sources:

• Ballester, E. A., Belluzzi, L., & Bueno, J. T. (2021, August 18). Solving the paradox of the SOLAR Sodium ${d}_{1}$ Line Polarization. Physical Review Letters.
• O’Neill, M. (2021, August 26). Solved: Mysterious Solar paradox that Puzzled physicists for 25 years. SciTechDaily.
• Physics World. (2021, August 27). Solar polarization paradox resolved at last.

Source: Curiosmos