Observation at radio frequencies of the hydroxyl (OH) absorption line in filaments and prominences above active regions of the Sun

Observations of AR NOAA 13614 on March 21, 2024: a — an SDO/AIA 304 Å image (the circle indicates the tracked position of the RATAN-600 BP cente); b — spectrogram of AR radio emission in transit mode; c — AR radio emission

spectra (from –166ʺ to +16ʺ from the solar disk center); d — averaged filament spectrum in AR NOAA 13614 with a 4 MHz resolution, obtained during tracking from 09:01:34 to 09:08:34 UTC; the achieved sensitivity can be assessed at 0.01 s.f.u.

Ovchinnikova Nina; Bogod Vladimir; Lebedev, Mikhail

When observing the Sun with RATAN-600 radio telescope using a spectropolarimetric complex in the range 1–3 GHz, as well as when observing coronal rain, absorption in the radio emission was discovered in the spectral region 1.5–1.65 GHz in active solar regions located under a cold filament or on the limb under a prominence. The observed line structure corresponds to the hyperfine splitting frequencies in the ground state of X2Π3/2 hydroxyl (OH) 1612–1720 MHz. When the observed active region passes through the knife-shaped beam pattern of the telescope antenna, the absorption band shifts in frequency due to a shift in the energy levels of the OH molecule in a magnetic field, which changes along the filament.