  Both the standard theory of thermal radio emission and the synchrotron theory
encounter some difficulties. The most crucial for the former one is
nonpossibility to explain the radio spectrum of Venus in the decimeter range
(Ksanfomality 1985). The radio spectra of planetary nebulae at high frequencies
also are not comfortably consistent with the standard theory (Siodmiak &
Tylenda 2001). Here we show that the account for an induced character of
radiation processes sufficiently improves the predictions of the standard
theory.
  Moreover, the developed here theory of radio emission from non-uniform gas
gives the radio spectra of extended sources, such as supernova remnants and
radio galaxies, which are normally attributed to the synchrotron emission. It
is important, in this aspect, that the synchrotron self-absorption produces a
change in the polarization position angle across the spectral peak. No such a
change was detected in gigahertz-peaked spectrum sources (Mutoh et al. 2002).
Besides, the flat or slightly inverted radio spectra of low-luminosity active
galactic nuclei are not easily consistent with the synchrotron radiation
mechanism (Nagar, Wilson & Falcke 2001, Ulvestad & Ho 2001).
  Thermal radiation in a magnetic field is polarized (see, e.g., Lang 1978).
This is also true for radiation considered in the present paper, which can be
described as incoherent induced radiation (Prigara 2001b).
