def make_subtitle(rho_rms_aurora, rho_rms_emtf,
                  phi_rms_aurora, phi_rms_emtf,
                  matlab_or_fortran, ttl_str=""):
    """
    
    Parameters
    ----------
    rho_rms_aurora: float
        rho_rms for aurora data differenced against a model. comes from compute_rms
    rho_rms_emtf:
        rho_rms for emtf data differenced against a model. comes from compute_rms
    phi_rms_aurora:
        phi_rms for aurora data differenced against a model. comes from compute_rms
    phi_rms_emtf:
        phi_rms for emtf data differenced against a model. comes from compute_rms
    matlab_or_fortran: str
        "matlab" or "fortran".  A specifer for the version of emtf.
    ttl_str: str
        string onto which we add the subtitle

    Returns
    -------
    ttl_str: str
        Figure title with subtitle

    """
    ttl_str += (
        f"\n rho rms_aurora {rho_rms_aurora:.1f} rms_{matlab_or_fortran}"
        f" {rho_rms_emtf:.1f}"
    )
    ttl_str += (
        f"\n phi rms_aurora {phi_rms_aurora:.1f} rms_{matlab_or_fortran}"
        f" {phi_rms_emtf:.1f}"
    )
    return ttl_str