A detection of the $2^{2}P_{3/2}-2^{2}S_{1/2}$ fine-structure transition of hydrogen in the radio spectrum of the Sun?

The hyperfine transition of atomic hydrogen at a wavelength of about 21 cm is an essential tool for studies of interstellar gas. It has been argued that also fine-structure transitions of hydrogen could be detected in astronomical sources. Our aim is to detect the fine-structure transition \(2^{2}P_{3/2}-2^{2}S_{1/2}\) of hydrogen at ∼10 GHz in the radio spectrum of the Sun, with a spectral resolution which enables a detailed study of the line profile. The Sun was observed with the 13.7 m radio telescope at the Metsähovi Radio Observatory, in Finland. We detect emission from two of the three hyperfine components of the transition. The width of the components is ∼15 MHz, much less than the expected natural line width of ∼100 MHz (broadened solely by the uncertainty principle). At red-shifted Doppler velocities, the lines show enhanced emission and possibly self-absorption. If the absorption happens at the chromosphere, our observations challenge the traditional view that chromospheric temperature increases gradually towards higher altitudes. Our unconventional results have to be confirmed by further observations. This transition would be the only known spectral line in the Sun at radio frequencies.