  The ratio of H$\alpha$ intensity to 1.4 GHz radio continuum intensity in the
North Polar Spur (NPS) is measured to be $\lesssim 50$, two orders of magnitude
smaller than the values of $\sim 10^4$ observed in the typical shell-type old
supernova remnants, Cygnus Loop and S147. The extremely low H$\alpha$-to-radio
intensity ratio favours the Galactic-Centre explosion model for NPS, which
postulates a giant shock wave at a distance of several kilo parsecs in the hot
and low-density Galactic halo with low hydrogen recombination rate, over the
local supernova(e) remnant model.
