  Although spectroscopic observations of RR Lyrae stars have now been carried
out for almost a century, it is only recently that it has been identified that
the hydrogen line exhibits three successive emissions in each pulsation cycle.
The purpose of this study is to clarify the physical origin of these three
emissions and their connection with atmospheric dynamics and the influence of
Blazhko modulation on their intensity. From the value of the blueshift of the
main H$\alpha$ emission, the velocity of the hypersonic shock front was
estimated between 100 and 150 $\pm 10$ km$\,$s$^{-1}$ (Mach number between 10
and 15). It has been established that the shock velocity increases from the
minimum Blazhko to its maximum, and after that, gradually decreases to the
Blazhko minimum to start growing again.
  This observational result is consistent with the shock model proposed in 2013
to explain the Blazhko effect. The intensity of the H$\alpha$ emission
increases with the shock velocity up to a maximum value around 137
km$\,$s$^{-1}$ and then decreases as the shock velocity increases further. This
effect would be the consequence of the more and more important ionization of
the atoms in the radiative shock wake. The second (blueshifted) H$\alpha$
emission, is the consequence of an approximately constant supersonic
compression (Mach number between 2 and 3) of the upper atmosphere falling on
the photospheric layers, during 3 to 16% of the pulsation period. Finally, the
third H$\alpha$ emission (P-Cygni profile) would be the consequence of the
expansion of the high atmosphere induced by the shock wave during its final
weakening.
