def METHOD_NAME(self, simulation):
    """Primary electric field
    Parameters
    ----------
    simulation : SimPEG.electromagnetics.natural_source_simulation.BaseNSEMSimulation
        A NSEM simulation
    Returns
    -------
    numpy.ndarray
        Primary electric field
    """
    if self._ePrimary is None:
        sigma_1d, _ = self._get_sigmas(simulation)
        self._ePrimary = homo1DModelSource(
            simulation.mesh, self.frequency, sigma_1d
        )
    return self._ePrimary