def METHOD_NAME( z, r ) :
    """Returns relative density at position z and r"""
    # Allocate relative density
    n = np.ones_like(z)
    # Make sine-like ramp
    n = np.where( z<ramp_length, np.sin(np.pi/2*z/ramp_length)**2, n )
    # Supress density before the ramp
    n = np.where( z<0, 0., n )
    return(n)