  Solar observations with the Atacama Large Millimeter/submillimeter Array
(ALMA) provide us with direct measurements of the brightness temperature in the
solar chromosphere. We study the temperature distributions obtained with ALMA
Band 6 (in four sub-bands at 1.21, 1.22, 1.29, and 1.3 mm) for various areas
at, and in the vicinity of, a sunspot, comprising quasi-quiet and active
regions with different amounts of underlying magnetic fields. We compare these
temperatures with those obtained at near- and far-ultraviolet (UV) wavelengths
(and with the line-core intensities of the optically-thin far-UV spectra),
co-observed with the Interface Region Imaging Spectrograph (IRIS) explorer.
These include the emission peaks and cores of the Mg II k 279.6 nm and Mg II h
280.4 nm lines as well as the line cores of C II 133.4 nm, O I 135.6 nm, and Si
IV 139.4 nm, sampling the mid-to-high chromosphere and the low transition
region. Splitting the ALMA sub-bands resulted in an slight increase of spatial
resolution in individual temperature maps, thus, resolving smaller-scale
structures compared to those produced with the standard averaging routines. We
find that the radiation temperatures have different, though somewhat
overlapping, distributions in different wavelengths and in the various magnetic
regions. Comparison of the ALMA temperatures with those of the UV diagnostics
should, however, be interpreted with great caution, the former is formed under
the local thermodynamic equilibrium (LTE) conditions, the latter under non-LTE.
The mean radiation temperature of the ALMA Band 6 is similar to that extracted
from the IRIS C II line in all areas with exception of the sunspot and pores
where the C II poses higher radiation temperatures. In all magnetic regions,
the Mg II lines associate with the lowest mean radiation temperatures in our
sample. These will provide constraints for future numerical models.
