Methods and Results of the Search for a Neutral Hydrogen Line at the Frequency of 9.85 GHz in Solar Radiation

A. F. Dravskikh, N. G. Peterova, N. A. Topchilo

Transactions of IAA RAS, issue 48, 15–28 (2019)

DOI: 10.32876/ApplAstron.48.15-28

Keywords: Radio emission from the Sun, neutral hydrogen, radio line, magnetic fields, Zeeman effect.

Abstract

The only line of hydrogen that can be expected in the radio range of solar radiation is a line at the frequency of 9850 MHz (3.04 cm) associated with the $2^2P_{3/2}–2^2S_{1/2}$ transition between the levels of the hyperfine structure of a neutral hydrogen atom. It was predicted in 1952 by Wild J.P., who showed that with local thermodynamic equilibrium, its intensity was not great. And in 1958, at the very beginning of the search for the line, De Yager stated that the line should be greatly enhanced during chromospheric flares. These assumptions were fully confirmed by the full flux measurements of the Sun on small mirrors by A. Dravskikh. According to the 1959–1988 measurements, carried out with very low spectral (3-channel spectrograph) resolution, the line intensity in the quiet phase was ~ 1.55% relative to the continuum, and in the spectra of ~ 100 flares (1958-1962) some features were found that occupied ~ 20% of the event duration, which could be interpreted as due to radiation in the H3.04 line. Later, this result was confirmed by the observations on the RATAN-600 radio telescope with higher angular (28″) and spectral resolution (90 channels), which made it possible to select quiet areas with dimensions 40″ × 1370″ on the solar disk and an average line profile up to ~ 600 cases has been achieved. The obtained parameters of the line showed that in a quiet state the solar plasma is indeed in the thermodynamic equilibrium. However, for the objects with a fairly strong magnetic field (flocculi brightenings and sunspots), the observed profile of the $Н_{3.04}$ line turned out to be much wider and more complex than that calculated by Wild for a quiet Sun characterized by a very weak magnetic field. The calculation of the Zeeman effect for the hyperfine structure of the second level of the hydrogen atom showed that this effect results in splitting the $Н_{3.04}$ line into a number of additional components depending on the magnitude and direction of the magnetic field and generally leads to a significant change in the shape of the $Н_{3.04}$ profile. Comparison of the calculated profile with the results of spectral observations of the Sun on the RATAN-600 radio telescope made it possible to estimate the magnitude of the magnetic field, which turned out to be ~ 200 Gs for floccule brightening. Currently, RATAN-600 is the only tool which you can use to explore the $Н_{3.04}$ line with a fairly high frequency and angular resolution. But for the observations of non-stationary, insufficiently investigated objects and processes, such as coronal holes, prominences, coronal mass ejections, pre-flare depression of active region radio brightness, it is necessary to additionally increase the spectral resolution up to ~ 30 MHz with an analysis band of ~ 4 GHz, and at the first stage to use the observations with a high two-dimensional resolution of the order of the active region size (2–4 arcmin).

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