  The radio spectral index is a powerful probe for classifying cosmic radio
sources and understanding the origin of the radio emission. Combining data at
147 MHz and 1.4 GHz from the TIFR GMRT Sky Survey (TGSS) and the NRAO VLA Sky
Survey (NVSS), we produced a large-area radio spectral index map of ~80 per
cent of the sky (Dec > -40 deg), as well as a radio spectral index catalogue
containing 1,396,515 sources, of which 503,647 are not upper or lower limits.
Almost every TGSS source has a detected counterpart, while this is true only
for 36 per cent of NVSS sources. We released both the map and the catalogue to
the astronomical community. The catalogue is analysed to discover systematic
behaviours in the cosmic radio population. We find a differential spectral
behaviour between faint and bright sources as well as between compact and
extended sources. These trends are explained in terms of radio galaxy
evolution. We also confirm earlier reports of an excess of steep-spectrum
sources along the galactic plane. This corresponds to 86 compact and
steep-spectrum source in excess compared to expectations. The properties of
this excess are consistent with normal non-recycled pulsars, which may have
been missed by pulsation searches due to larger than average scattering along
the line of sight.
