We show that r -mode oscillations distort the magnetic fields of neutron stars and that their occurrence is likely to be limited by this interaction .
If the field is \gtrsim 10 ^ { 16 } ( \Omega / \Omega _ { B } ) G , where \Omega and \Omega _ { B } are the angular velocities of the star and at which mass shedding occurs , r -mode oscillations can not occur .
Much weaker fields will prevent gravitational radiation from exciting r -mode oscillations or damp them on a relatively short timescale by extracting energy from the modes faster than gravitational wave emission can pump energy into them .
For example , a 10 ^ { 10 } G poloidal magnetic field that threads the star ’ s superconducting core is likely to prevent the \ell = 2 mode from being excited unless \Omega exceeds 0.35 \Omega _ { B } .
If \Omega is larger than 0.35 \Omega _ { B } initially , the \ell = 2 mode may be excited but is likely to decay rapidly once \Omega falls below 0.35 \Omega _ { B } , which happens in \lesssim 15 ^ { d } if the saturation amplitude is \gtrsim 0.1 .
The r -mode oscillations may play an important role in determining the structure of neutron star magnetic fields .