def METHOD_NAME(self, simulation):
    """Primary magnetic field
    Parameters
    ----------
    simulation : SimPEG.electromagnetics.frequency_domain.simulation.BaseFDEMSimulation
        A NSEM simulation
    Returns
    -------
    numpy.ndarray
        Primary magnetic field
    """
    # Project ePrimary to bPrimary
    # Satisfies the primary(background) field conditions
    if simulation.mesh.dim == 1:
        C = simulation.mesh.nodal_gradient
    elif simulation.mesh.dim == 3:
        C = simulation.mesh.edge_curl
    bBG_bp = (-C * self.ePrimary(simulation)) * (1 / (1j * omega(self.frequency)))
    return bBG_bp