Using Ginzburg-Landau theory, we consider the effect of a magnetic field on the anisotropic (${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$) neutron superfluid. In a strong magnetic field the pairing state has an angle-dependent energy gap with a node along the field direction. Methods of homotopy theory are used to determine the topologically stable line defects allowed by the ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ order parameters. Our free-energy analysis of line vortices gives a qualitatively different structure to the single vortex, outside of the core region, from that obtained by other authors.

• Received 10 October 1979

DOI:https://doi.org/10.1103/PhysRevD.21.1494