A precondition for the radio emission of pulsars is the existence of strong , small-scale magnetic field structures ( ‘ magnetic spots ’ ) in the polar cap region .
Their creation can proceed via crustal Hall drift out of two qualitatively and quantitatively different initial magnetic field configurations : a field confined completely to the crust and another which penetrates the whole star .
The aim of this study is to explore whether these magnetic structures in the crust can deform the star sufficiently to make it an observable source of gravitational waves .
We model the evolution of these field configurations , which can develop , within \sim 10 ^ { 4 } – 10 ^ { 5 } yr , magnetic spots with local surface field strengths \sim 10 ^ { 14 } G maintained over \gtrsim 10 ^ { 6 } yr. Deformations caused by the magnetic forces are calculated .
We show that , under favourable initial conditions , a star undergoing crustal Hall drift can have ellipticity \epsilon \sim 10 ^ { -6 } , even with sub-magnetar polar field strengths , after \sim 10 ^ { 5 } yr. A pulsar rotating at \sim 10 ^ { 2 } Hz with such \epsilon is a promising gravitational-wave source candidate .
Since such large deformations can be caused only by a particular magnetic field configuration that penetrates the whole star and whose maximum magnetic energy is concentrated in the outer core region , gravitational wave emission observed from radio pulsars can thus inform us about the internal field structures of young neutron stars .