  The analysis of X-ray observations suggest an ultrastrong ($B\gtrsim 10^{14}
\,{\rm G}$) surface magnetic field at the polar cap of pulsars (Szary, 2013).
On the other hand, the temperature of the polar caps is about a few millions
Kelvin. Based on these two facts we use the Partially Screened Gap (PSG) model
to describe the Inner Acceleration Region (IAR). The PSG model assumes that the
temperature of the actual polar cap is equal to the so-called critical value,
i.e. the temperature at which the outflow of thermal ions from the surface
screens the gap completely. We have found that, depending on the conditions
above the polar cap, the generation of high energetic photons in IAR can be
caused either by Curvature Radiation (CR) or by Inverse Compton Scattering
(ICS). Completely different properties of both processes result in two
different scenarios of breaking the acceleration gap: the so-called PSG-off
mode for the gap dominated by CR and the PSG-on mode for the gap dominated by
ICS. The existence of two different mechanisms of gap breakdown naturally
explains the mode-changing phenomenon. Different characteristics of plasma
generated in the acceleration region for both modes also explain the pulse
nulling phenomenon.
