  Thermal radiation from hot polar caps is examined in radio pulsars with
drifting subpulses. It is argued that if these subpulses correspond to sparking
discharges of the inner acceleration region right above the polar cap surface
then a simple relationship between the observed subpulse drift rate in radio
and thermal X-ray luminosity from the polar cap heated by sparks should exist.
This relationship is derived and tested in pulsars for which an appropriate
good quality data is available.
