Hyperfine structure radio lines in spectra of hot interstellar and warm-hot intergalactic gas

Hyperfine structure lines of highly-charged ions may open a new window in observations of hot rarefied astrophysical plasmas. In this paper we discuss spectral lines of isotopes and ions abundant at temperatures 10^5-10^7 K, characteristic for warm-hot intergalactic medium, hot interstellar medium, starburst galaxies and young and middle-aged supernova remnants. Observations of these lines might complement soft X-ray observations with micro-calorimeters, but, in contrast to soft X-rays, they are not obscured by the Galactic interstellar medium and allow to study observed target bulk and turbulent motions with much higher spectroscopic and angular resolution. We estimate feasibility of hyperfine structure emission and absorption line observations from these astrophysical sources and gamma-ray burst afterglows using simple theoretical estimates of spectral line absorption cross-section and emissivity accounting for major processes populating upper hyperfine sublevel. Using modern and near-future instruments it appears possible both to observe absorption line of 14-N VII (rest wavelength 5.652 mm) from the warm-hot intergalactic medium and galactic halos at redshifts above z=0.15 and emission lines of several less abundant isotopes from young and middle-aged supernova remnants