The solar corona, the outer layer of the Sun atmosphere, is a unique object for studying high-energy processes. Its temperature in a quiet state reaches 1–210⁶ K which is significantly higher than that of the photosphere (about 5500 K), this is a paradox and one of the main mysteries of solar physics. The corona extends for millions of kilometers forming structures such as magnetic field loops, coronal holes (sources of high-speed solar wind), and plasma ejections that directly affect the Earth’s space weather. Studying the corona is the key to solving many issues including an unexplained heating anomaly and the mechanism of solar flares. It can only be observed during a total solar eclipse or with the help of coronagraphs that hide the bright disk of the Sun. Modern missions such as Parker Solar Probe and Solar Orbiter have made it possible to come closer to unraveling the mechanisms of corona heating and its dynamics. However, key questions, including the nature of narrow-band absorption effects, remain open.

    Using a new high-speed spectral complex at the RATAN-600 radio telescope (an effective area of ≈ 1500 m² at 1.5 GHz) in the range of 1–3 GHz, previously inaccessible phenomena were discovered. In particular, strong absorption of radio waves over active regions with clear frequency boundaries in the decimeter range was detected. This discovery was made possible by a combination of high spectral resolution (up to 10^-5) and interference suppression methods, which allowed us to isolate weak signals (up to 10-3 s.f.u.) against the background of powerful radiation from the quiet Sun (50–100 s.f.u.) with a dynamic range of up to 90 dB.

    In this work, absorption in many fine structures in the coronal plasma was detected using a spectral complex with a relative frequency resolution of 10^-5 and a time resolution of 8 ms in the range of 1–3 GHz, in the radio emission spectra of individual active regions (ARs), prominences, and filaments. Similar structures in the form of “coronal rain” (Fig. 1, top) are studied in optics in the X-ray and EUV lines at large telescopes with diameters of 1.6 m and 4.0 m, but were detected for the first time in the spectra of the corona at radio waves (Fig. 1b). For comparison, Fig. 2 shows the spectra of the quiet Sun and the active regions of NOAA 13507(8). The dotted lines show the positions of the beam pattern at RATAN-600 at the moments of obtaining the spectra, and the dash-dotted lines show the width of the radiation pattern at a wavelength of 20 cm.

    The presented results offer great opportunities for further studies of the fine structure of the corona using the radio range. Combined data analysis allows us to obtain a more complete picture of the physical processes occurring in the solar corona and to come closer to understanding the mechanisms of its heating when measuring the temperature balance.

Fig. 1. a) Coronal rain on the solar limb; b) Absorption lines at frequencies of 1.2–1.7 GHz.

Fig. 2. Spectra of the quiet Sun (bottom) and the active region (top).

Published:
1. Bogod V. M., Lebedev M. K., Ovchinnikova N. E. et al. On a new concept of spectral radiometry at RATAN-600 // Izvestiya Krymskoy Astrofiz. Obs. 2023. Vol. 119. № 4. pp. 17–26.
2. Ovchinnikova Nina; Bogod Vladimir; Lebedev Mikhail, Observation at radio frequencies of the hydroxyl (OH) absorption line in filaments and prominences above active regions of the Sun, Solar-Terrestrial Physics, Vol. 10, Issue 3, pp. 18–24, DOI:10.12737/stp-103202403