code: def spin2y_from_mass1_mass2_xi2_phi_a_phi_s(mass1, mass2, xi2, phi_a, phi_s):
      
      chi_perp = chi_perp_from_mass1_mass2_xi2(mass1, mass2, xi2)
      phi2 = phi2_from_phi_a_phi_s(phi_a, phi_s)
      return chi_perp * numpy.sin(phi2)
docs: Returns y-component spin for secondary mass.