  Observations of the thermal X-ray emission from radio pulsars implicate that
the size of hot spots is much smaller then the size of the polar cap that
follows from the purely dipolar geometry of pulsar magnetic field. Most
plausible explanation of this phenomena is an assumption that the magnetic
field at the stellar surface differs essentially from the purely dipolar field.
We can determine magnetic field at the surface by the conservation of the
magnetic flux through the area bounded by open magnetic field lines. Then the
value of the surface magnetic field can be estimated as of the order of
$10^{14}$ G. On the other hand observations show that the temperature of the
hot spot is about a few million Kelvins. Based on these observations the
Partially Screened Gap (PSG) model was proposed which assumes that the
temperature of the actual polar cap (hot spot) equals to the so called critical
temperature.
  We discuss correlation between the temperature and corresponding area of the
thermal X-ray emission for a number of pulsars. The results of our analysis
show that the PSG model is suitable to explain both cases: when the hot spot is
smaller and larger then conventional polar cap. We argue that in the second
case structure and curvature of field lines allow pair creation in the closed
field lines region thus the secondary particles can heat the stellar surface
outside the actual polar cap.
  We have found that the Curvature Radiation (CR) plays dominant role in
avalanche pair production in the PSG. We studied dependence of the PSG
parameters on the pulsar period, the magnetic field strength and the curvature
of field lines.
